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hitachi 3d7 noise

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#31 Superduper

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Posted 29 September 2016 - 05:32 PM

1)  Like I said, your boombox does not have a conventional line-in feature.  If it did,you would have that option available in your function selector and you would be able to play that just like as if you selected "tuner."  That you need to depress the record button indicates that you are monitoring the line-in during a recording session, unless you tell me you have a "special" record button lever that allows you to operate the record button without the deck working or without the need to simultaneously record-pause.  In any event, it is irrelevant to the issue at hand.  The only thing that is important is that you have this issue and it appears related to your tape section.  Or...  is it possible that it is also happening during tuner mode too but due to a higher signal to noise condition, you are not hearing it?  Please tune your radio to a quiet location on the bandwidth and see if is still making the noise?  

 

2) Reflow does indeed mean to remelt the solder/tin.  However, you need to observe and recognize if the connection is a good solder joint or not.  This takes some experience to recognize whether the parts to be joined are wetting or balling the solder.  If the solder is balling, then I would desolder the entire connection by removing old solder and doing it over to ensure that the solder is actually sticking.  A bunch of solder balled around a lead is not what you want to see, although I suspect from your posts here that you have sufficient experience to know the difference.

 

3)  Quick test is exactly that.... it's quick but since the results aren't always conclusive, what you are looking for are numbers that make no sense and obviously wrong.  For example, you never want to see capacitors or resistors shorted or open although resistors usually open rather than short.  You never want to see resistors that tests higher in-circuit  than the printed value since that is almost a guarantee that there is an issue.  Through experience, you'll learn to recognize that low value resistor readings can generally be trusted more than high value resistor measurements and when they are off, usually a quick examination of the circuit diagram tells you whether that off-spec reading makes sense or not.  You don't really need to "compute" the actual expected value since you can usually see at a glance whether a reading is about right based on the circuit diagram.  The "quick" test is something we do simply to see if there is something obvious.  

 

4)  How "off" is "off."  There is almost always some amount of deviation, not only in the circuit itself but also in the equipment you use to do the testing.  The multimeter itself has some inherent inaccuracies and that's before you add the probes and the surface oxidation of the parts tested.  Resistor drifting really isn't critical.  In fact, if you have a 1k ohm signal resistor in the auido circuit and it has now changed to 10k, your boombox will likely still operate sort of normal although the audio might be a bit attenuated.  However, an 100R resistor that has changed to 130R in a bias circuit could cause enough problems to become noticeable or problematic.  V/R (voltage/resistance) measurement tests are useful to see if something in a circuit is amiss since it gives us a good picture of the overall health of a circuit, but the V readings posted usually is only accurate when the power supply is at spec.  So if the schematic shows 12V at the power supply and it is currently powered by AC or batteries, then the voltage could be different and the voltages in any non-regulated section will all be globally off.  

 

5)  A 68pf capacitor will charge almost instantaneously.  Any small capacitor will charge fast enough that, depending upon the voltage applied to the circuit under test by the particular meter, be too fast to see.  A resistance test on capacitors isn't a very good way to test them anyhow unless they are either shorted or open, and with small capacitors, even that is usually inconclusive.  This is where a higher end DMM is useful since they usually have a cap testing function, or an LCR meter.  Some of the cheap transistor testers I mentioned earlier can test small caps accurately.  Out of curiosity, I just now stuck a brand new 82pf ceramic cap into one of those testers I got off eBay and after about 3 seconds, the reading came back at 82pf.

 

6)  Replacing all electrolytic caps is certainly an option.  Ideally, I'd like to see where the problem is first, but lacking some more sophisticated instruments than a cheaper DMM, then it's certainly a service that one could try.  Some folks don't like the shotgun capacitor approach but if you ever want to recap a boombox someday, might as well be now since a partial replacement today means that in the future, you will end up replacing the same ones again?  Would I recap a boombox like this?  Absolutely without a second thought.  The service manual only shows 12 transistors total for this boombox so it's pretty simple and doesn't have that many components.  By comparison, the Marantz CRS-4000 boombox, about similar size to this one which I recapped about 10 years ago had around 100 electrolytic caps, if I recall correctly.

 

7)  I don't agree that ceramic caps do not go bad.  I have definitely seen them go bad.  The difference between electrolytics and non-electrolytics is that electrolytic ones have a more predictable failure rate based on age and usage and heat, and stress whereas non-electrolytic ones fail randomly and therefore are not replaced unless they go bad.  That's why you test them if you suspect them.  

 

8)  By the way, this is not a very sophisticated boombox.  One look at that PCB shows this.  Unfortunately, I find that simple designs like this often aren't always as immune to noise as they can/should be.  



#32 IVH

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Posted 30 September 2016 - 11:06 AM

1) It's exactly the way you describe it, line in is part of the tape function. I didn't understand a phrase you wrote a few post ago about 'monitoring the tape' that explains my misunderstanding (I think).  

Not in any way against you, as this thread is getting really long and messy :) But somewhere near the beginning of this thread I mentionned that the hiss is not appearing on a (near) clear frequency.  It's possible that I'm completely wrong, but to me the hiss is not the same in tuner mode than in tape. 

On tuner mode the hiss appears to come from the antenna picking up some noise.

It's difficult to discribe, but the hiss in tape mode sounds more 'excited' and it takes a sec to appear/get loud after switching from tuner to tape, it's like the noise has to 'charge up'. 

What i'm saying here is surely bull****, but that might be because the caps are charging up. The tuner does not do this, indicating the noise is coming from the outside.     

Edit: The same happens when the unit is powered up, the hiss takes a second to get loud.

Here is a video:

The down position/ the position near the little blue switch is tape mode. the center posistion is tuner mode.

 

 

 

 

2) There were indeed a few of those bad solders where the solder formed a ball instead of something resembling a tipi tent. Those have been fixed.

 

 

3) I understand a quick test isn't the best thing. Although I cannot see if something is correct in a glance, I do now know, thanks to this website, that when 2 resistors are in parallel. The total resistance is, depending on the difference of resistance between R1 and R1, between a half of and a little less than the lowest R. 

It's a beginning ;)

 

 

4) Hmm, interesting. Maybe I should do some more serious testing of those important resistors. There were some just past the 5% tolerance.

 

 

5) That's a pity, I was hoping this was the source of the noise. Those cheap testers are surprisingly accurate to measure that 82pf cap correctly ! 

 

 

6) It's true that it would be good to know where the problem is coming from, maybe to help someone else someday with the same issue, or for yourself if the same issue reappears years later. (yes, I plan to keep this one for a long long time :D ). You're right, I've till now never done a partial recap, I like it better to do all at once. They aren't that expensive and once you have the thing open, why not do everything ? 

It does me good that you too you'd do the recap on this boombox, it might be low end, it certainly wasn't expensive at the time, nor it is now. Still I really like it's design and isolator42's adoration for the 3d7 made me love it's sound without ever hearing it myself. Anyways, thanks for that little motivation !

That must've been a big job, and if ever something goes amiss, a lot to troubleshoot. Why does it have so much more components ? Is that because the quality is way better (which is obviously true. I still want the Philips/Marantz FA-930 (MK2). Which combines my countries best brand and a pioneer) or because it has more functions ?

 

7) Wow, that's interesting ! learned something today. 

 

8) What a pity, it would have saved us loads of time if hitachi did a better job in the 80's ! You should start a company producing simple, quasi fail proof boomboxes with all the modern stuff, but still based on old designs. I would buy one for sure ;)

 

 

Just a question, is/are there any cap(s) that is/are most likely to cause such a problem ? I have a couple laying around for my fathers speakers, I could solder those in and buy him new ones. If that fixes it the move to buy all caps necesary a whole lot easier !



#33 Superduper

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Posted 30 September 2016 - 05:40 PM

Quality is dictated by design, not by the number of components, however, some designs are purposely designed to reject noise, and provide cleaner sound.  The power of an amplifier isn't due to the number of components.  In fact, the output section typically has very little components compared to the preamp section.  But an amplifier is exactly that, it amplifies and it doesn't care what it is.  So if you feed it garbage in, you will get BIG garbage out.  If you look at a high fidelity device, you will see that it typically has a lot of components.  All are there for a reason and I assure you that if there was no purpose, they wouldn't waste money to put it in.  In a circuit, you might omit bypass caps.  It may work fine, or the circuit might become unstable and oscillate.  Or they can pick up hum or RF or other externally introduced signals which gets amplified by the amplifiers.  Adding bypass caps into strategic locations ensures a stable operating circuit.  As well, you will find that in may quality devices, that strategically placed stiffening caps exist.  If you remove them, the circuit might still appear to operate fine, but under the right conditions, performance will suffer.  And on some amplfiiers, you'll see output capacitors that are placed in series with the speakers.  These are often large, 1000uf to 2200uf caps.  They are not cheap.  IF you remove them and jumper them, you'll likely notice NO degradation of sound.  That is, until you crank the system and it begins to distort.  Under certain conditions, the amplifier could output DC in which case, you will blow your woofers.  The tweeters might remain OK and this is because the tweeters are protected by the high-pass caps.  What I'm saying is that manufacturers don't just complicate a design just to earn the higher price tag of the device they are selling, those extra components are there for a reason.  Some for protection of certain components, some protect against catastrophic avalanche failures that occur when a single component fails, some to clean up sound, and etc. etc. etc.

 

Forget investigating those resistors for the moment, as the deviation isn't significant enough to make me think they are problematic.  If it were a bias issue, it would be more distortion or other issue instead.

 

As for the 68pf capacitor, I'm not saying it's bad or not bad, you would have to either test it or replace it, or tack another one behind it to see.  You simply can't test it with a ohm meter since a properly working one charges up so fast it will behave like O/C and even if you can see it charging, what value would you assign to a capacitor that is capable of charging?  Would you guess based on how long it takes to charge up?  Also, what if it were leaky -- it may still charge.  Or how about if the ESR is unacceptably high? 

 

As for using your Dad's speaker caps to do this job, don't bother.  Just order a proper set and be done with it.  Firstly, home speaker caps are usually higher voltage and very large compared to the signal caps you will likely be using.  Speakers also don't use large values, they are almost all low value caps.  You only need about 3-dozen caps anyhow which really isn't that many for a boombox.  In fact, I could go into my inventory right now and collect the whole lot for you, but the shipping costs alone to France probably exceeds what it will cost you just to buy them (caps & tax & shipping) from France.  Probably a lot faster too especially if it gets caught up in Customs.  Anyhow, since your issue is global (not limited to L/R), I would suspect more a cap that is tethered to ground rather than a coupling capacitor (series).  The purpose of some caps is to shunt hf noise to ground before it gets amplified.  

 

I asked you to recheck the hiss sound and the reason I said to make sure that it's not also coming from the tuner is because if it is, then the issue won't be isolated to only the tape preamp section.  You certainly can expect it to have a slightly different sound when you switch modes because the circuitry is taking a different path.  I'm more concerned with the level of the hiss and at least when I listen to your video on my smartphone, it sounds like the hiss is about the same level in either mode.  That could suggest that the issue is not isolated to only the tape section fore of the IC401 preamp chip.  Based upon what I'm hearing, I'm not convinced that this is the case.  You say you don't hear it on a clear radio station -- I don't know but that could simply be because of the higher SN ratio from a strong station overwhelms the hiss so you don't hear it.  You are the best judge of that since you can listen to it live.  I almost have to listen to it in front of me to decide since audio recorded in a video recording is often adjusted up/down automatically by the recording device's ALC.  

 

I'd admit that I really didn't listen to your videos previously since my office computer does not have speakers.  I'd always thought that you had more of a hum issue than a hiss issue.  This really sounds more like a grounding or poor shielding issue to me, but could also be due to noisy semiconductors.  Do you have access to an oscilloscope or an audio signal generator, or audio signal tracer?  They could help you narrow it down.  Also, you might be able to strategically isolate some sections one at a time to see if you can rule out certain circuits.  For example, if you lift one leg of R501 R/L, you will cut out the audio signal to the full range amplifier and hopefully, your speakers will be silent.  Or if you isolate C523, you'll hopefully remove the SW circuit from the equation in case it's somehow injecting some hiss.  These are just some techniques that you could employ to try to narrow down or rule out certain circuits as potential contributors.  You can also jumper the entire EQ section just in case that section is somehow overamplifying the high frequencies in the frequency of the hiss.  



#34 jimmyjimmy19702010

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Posted 01 October 2016 - 02:52 AM

This thread is very interesting - anyone who has had a 3D7 apart will know they are a very basic box but the stuff they did include works well for its intended market - heck, the deck doesn't even have auto stop but it does sound really good!? All the money went into the 3 amps, the centre speaker and the sensitive tuner. My 3D7 needed everything serviced to make it work right but it ended up one of my most reliable boxes to date - the deck & tuner have worked perfectly since the major overhaul.

I can confirm that the deck should have very little hiss even at maximum volume with play depressed with no tape inserted. The tuner should also have no hum when off station and lock on solidly with a very quick FM stereo lock on.

The highs and mids are super crisp on my example. The bass from the BTL 3rd amp is fantastic too - not as bassy as the bigger 3D8 but you can crank it without the centre 8 ohm driver distorting (unlike the 3D8).

I hope you get your example working right - it will be worth it as they really are a super loud and solid performing early eighties boomer.

#35 IVH

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Posted 01 October 2016 - 07:02 AM

Another really interesting paragraph. I believe it would be so cool to know what component does what in a circuit, like you do. Good to know my vintage philips amp has some 'signs of quality', I found a few of those huge output caps in there, the biggest being 9*4cm (!) Clipping is my biggest fear when I bring it to a party, most people won't believe when I say that turning the volume too loud might kill the speakers.

ok, I will forget about those resistors !

Can't argue against that ;)

As for the caps, I'll follow your advise, it will save me some hassle too. I'll make a list and order asap. Is it a good idea to replace the power supply one too ? if so, would a higher voltage mean a longer lifespan?

Ouch, sorry if I did not describe the issue well! About the hiss sounding different because it takes a different pad, It sounds logical now, but I didn't think of it :/
And thanks to jimmy, we know it shouldn't make the noise it does when off station in tuner mode. So I agree it surely is the same noise. But there is some good news to come !

I did the tests you asked for, and I think I got some good news !
After unsoldering one feet of R501L, the left channel was DEAD silent !
If I read the schematics well, this won't exclude the equaliser circuit, you mentionned bypassing the equilisers, but I'm a bit scared of doing something wrong.
Is soldering a wire between R421R and C810R the correct and a safe way to bypass the equalisers for the right channel ?
Unsoldering C523 unfortunately didn't change anything to the hiss.

The hum/hiss confusion is because in the beginning I thought that a hum is a hiss. Well I was wrong.. I hope all the troubleshooting we did is applicable for a hiss issue too.

#36 IVH

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Posted 01 October 2016 - 07:16 AM

This thread is very interesting - anyone who has had a 3D7 apart will know they are a very basic box but the stuff they did include works well for its intended market - heck, the deck doesn't even have auto stop but it does sound really good!? All the money went into the 3 amps, the centre speaker and the sensitive tuner. My 3D7 needed everything serviced to make it work right but it ended up one of my most reliable boxes to date - the deck & tuner have worked perfectly since the major overhaul.

I can confirm that the deck should have very little hiss even at maximum volume with play depressed with no tape inserted. The tuner should also have no hum when off station and lock on solidly with a very quick FM stereo lock on.

The highs and mids are super crisp on my example. The bass from the BTL 3rd amp is fantastic too - not as bassy as the bigger 3D8 but you can crank it without the centre 8 ohm driver distorting (unlike the 3D8).

I hope you get your example working right - it will be worth it as they really are a super loud and solid performing early eighties boomer.

 

Thank superduper for the interesting part of this thread ;)

 

I'm so glad to hear your positive feedback about your 3D7 ! I do really hope we will get this thing working as it should, I'm looking forward to it. Especially now I heard how great it can be from yet another person ! 

 

I confirm the good tuner, It was the first thing I noticed. It's really easy to lock it on a station compared to some other tuners that are laying around here.



#37 IVH

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Posted 01 October 2016 - 03:47 PM

I tried counting the caps, but there were a few surprises.

There are some differences between the schematics and the board itself. The mk2 version isn't possibly only a visual upgrade. But also a technical upgrade.

As an example, on the mk2 board, I count 3 3.3uF caps. According to the schematics, there should be 5 (excluding C903, which is part of a seperate board for the led level indicator). 

 

The opposite goes for the 22uF ones; There are 3 on the board, and a single one in the schematics.

 

C515 has a capacitance of 1000uF, the schematics show a 220uF one. The voltage remains the same though (16v). This one might have been replaced with a different one though, as it's color is slightly different than the others.

 

In addition to this, there are loads of additional components in the tuner section: C163, C164, C159, C158, C157 and L156, just to name a few.

 

 

Unfortunately, it appears someone has been messing with this box, luckily only the tuner section appears to be affected though.

There are many components missing, those include C160, R155, C165, C166, C211 and a 'bridge' named j001. C160 has been ripped off, as the remainings of its 'feet' are still soldered on. I will not be able to replace some of those, as the mk1 service manual does not include all of the missing parts...

Someone must have had a reason for doing this, but it remains unclear to me.



#38 Superduper

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Posted 01 October 2016 - 06:00 PM

If you are going to recap the whole box, then you should probably just order and replace whatever is in there.  I'm not sure about the 1000uf one though.  On the diagram, it shows that it is a stiffening cap for the cassette motor.  That sounds awfully large, almost like it is there to compensate for some issue.  220uf sounds about right for a motor of that purpose.

 

As for the tuner component deviations, if your tuner is working right, probably shouldn't worry about those too much.  The cap that is ripped off is definitely concerning though.  Maybe it got damaged by a novice and they just ripped it off so it wasn't hanging and didn't know what to do?  However, as it is in the tuner, and not likely the cause of your immediate concern, perhaps just ignore them for now.

 

As for missing components, that might not necessarily true.  That is, if I understand you correctly.  Are you saying that there are vacant places on your PWB without components?  That is not unusual.  Many boards are designed with more than one configuration.  That might've been intentional and for additional features such as SW etc. that may be included in other regions.  The bridge you speak of sounds like a jumper.

 

I'm got my hands busy right now with cooking dinner as we have guests and some household chores but when I finish and get a chance, I'll give you some tips on how to isolate or jump certain parts of the circuits so you can test and see whether there is any improvements, which can help us narrow down where to focus.



#39 jimmyjimmy19702010

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Posted 01 October 2016 - 07:19 PM

The 'missing parts' on the tuner board may only be used when it's configured as a 4 band tuner as this model was also fitted with a 3 band tuner - notice the right hand switch with one less switch option:

 

The much more common 4 band tuner:



I was concerned that the 7620 service manual may not be entirely correct for the 3D7 Mark 2 that was made several years after the original 1984 TRK-7620.

 



#40 IVH

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Posted 02 October 2016 - 06:11 AM

@jimmy It's cool to see the differences between both versions. Your version seems to lack long wave, and mine has only a single short wave. Could that be related to the region where it was sold or just because you got the mk1 and I the mk2 version ?

 

Attached File  20161002_140542.jpg   151.61K   2 downloads

 

Attached File  20161002_140639.jpg   108.93K   2 downloads

 

@superduper Ohh, didn't see it is for the tape motor, in that case I'll not replace it as I'm not planning to use it anyway. 

That might be the explanation for the 'missing' components ! Here is a photo to respond to your question:

 

Attached File  20161001_225512.jpg   97.83K   3 downloads

 

I would really like to replace the ripped of cap, but I'll have to leave it as it's not present in the schematics, but as you say, as long as it works fine...



#41 Superduper

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Posted 02 October 2016 - 03:31 PM

If I had to take a guess, I am going to say that the cap was probably installed in error at the factory.  When the assembler realized this, he simply cut the un-needed cap off.  Now that I know that 3 and 4 band versions exist, I'm pretty certain now that it's like I said... the extra components are due to changes in the circuitry for the optional tuner configuration.  I doubt that installing a cap where it isn't needed is going to have any effect, so if you really want, you can go ahead and install them.  The value is probably same or similar to the adjacent ones.



#42 crazygamer

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Posted 03 October 2016 - 11:45 AM

I´m owner of 2 of those 3D7s- both are TRK-7620E´s, one is the earlier version without 3D7 logo, second one is with 3D7 logo. Both of them are 4 band radios, FM/AM/SW/LW, without fine tuning knob. Only minor differences inside and outside. These look fairly simple from inside, i´ve never gone into those depths of it, but i got to know about them a lot more.  :yes:

 

 



#43 Superduper

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Posted 03 October 2016 - 12:50 PM

So what is 3D7?  Is that not a model number?  



#44 jimmyjimmy19702010

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Posted 03 October 2016 - 04:33 PM

So what is 3D7? Is that not a model number?

The 7620 was the first of the 3rd speaker (3D) boomers. After the huge success of this model, they started making bigger versions and needed to give the 1st edition its unique model number whilst identifying it as part of the expanding 3D series.

#45 IVH

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Posted 07 October 2016 - 02:49 AM

That's some interesting history lessons Jimmy
Here is a small update about the repair, norm helped me by-passing components like the equaliser, the pre amp,..
From that we concluded that the pre amp might be faulty, I've ordered one, it costed me 5€ plus shipping (!). He also recommended me to buy one of those 16 pin dip sockets, the IC will snap right in.
I'll report back to you as soon as it's soldered in, can't wait :D

#46 isolator42

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Posted 14 October 2016 - 11:28 AM

AFAIK, Hitachi first tried the extra, separately amplified bass speaker in a boombox thing with the TRK7620 (Can't say for sure if it was the first as the Sanyo BigBen may well pre-date it). It was a success (particularly with me), leading to Hitachi's 3D series in the mid 80s, which included a twin-deck (3D8), a 3pc (3D9), a sort-of mini (3D2) & even a CD boomer (disappointingly named CX-W800). The original Hitachi 3D (TRK7620) stayed in this first-gen 3D line-up, becoming the 3D7 somewhere along the way. 
If anyone can adequately explain the numbering of the 3Ds (beyond the higher the number, the bigger it is), I'd be very interested.

 

I've always felt that the success of the Hitachi 3Ds was mainly that previously, a decent amount of bass was only possible with the more expensive boomers. This concept brought the more impressive sound into the mainstream budget, so people like me as a teenager could afford one.

Pretty much every boombox manufacturer very quickly had their own version available - Sony's Megabass, Sharp's X-Bass, JVC's Hyper-Bass, Panasonic's XBS, etc. More bass became a fashionable feature in the late 80s, which, at the budget end of the market, led to some rubbish 'extra-bass' buttons added. Without any extra amplification or decent woofers these just muddied the sound at lower volumes & distorted the sound at high volumes.



#47 IVH

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Posted 16 October 2016 - 10:55 AM

Fortunately the 3d7 does not have a button like that ;) What's the point of such button anyway, playing with the equilisers achieves the same thing, doesn't it ?

 

And you must be happy cause another 3D7 has been 'saved' !!   :w00t:

Thanks to norm, the hiss is gone !

 

He did loads of troubleshooting, it ended up being 2 pages long. Way too long to post here, but here is a sum up:

- We started with bypassing components and 'cutting the circuit' starting with the power amp and going up to the line in RCA jacks. This resulted in the pre amp and anything before it being suspected.

- With a brand new pre amp installed, the hiss persisted.

- We checked if the voltages measured at the different legs of the pre amp were as indicated, that seemed to be OK.

- Then norm came with the (according to me) genius idea to bypass the tape preamp transistors, and installing smaller value resistors on the left and right channels. The old one (680k) was heavily attenuating the signal. A smaller one would make the tape pre amps unnecessary. The 680k resistors (R412 R and L) have been replaced with 288k ones.

 

This did the trick !!  The sound is even louder now, and it sounds way cleaner to me too.

I'm impressed with this boombox (and norm's knowledge), it is much louder and sounds much nicer than I ever thought !

 

I am sooo grateful to norm, without his help I would've given up long ago.



#48 IVH

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Posted 16 October 2016 - 01:57 PM

Ps: here is a photo of the fix:

 

Attached File  20161016_190358.jpg   271.03K   7 downloads



#49 isolator42

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Posted 17 October 2016 - 09:26 AM

You have certainly put in a lot of effort, good for you.

I made my 3D7 look the part again - restoring back to original from a sticker-tastic 80s, but when I found the tape motor was clapped out after many years of use, I went with a Digisette & never regretted it:

https://www.amazon.c...r/dp/B0000A1O6K

http://www.stereo2go...isetterevisited

I swear the 3D7 never sounded so good as it did with the Digisette. With a following wind, it really did make a decent, rich sound, despite it's lowly status compared to the top flight boomers.

 

 

...these days, of course the likes of the Bose SoundLink Mini have tempted me with their outstanding sound to convenience ratio, but let's not dwell on that  ;-)



#50 Superduper

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Posted 17 October 2016 - 04:36 PM

We had concluded that the noise was being introduced at the tape preamp stage.  With nothing more than a voltmeter, there was no way for the OP to narrow it down further except to do parts swap.  While the plan had always been to restore the tape preamp section, when IVH clarified that there were no plans to use the tape deck, ever, and that it was to be modified for other purposes, then that is when we decided that it was best to bypass the tape preamp section entirely.

 

There are 2 reasons for this:  1 ) this boombox does not have a true line-in feature.  Instead the line-in jacks are intended for those signals to be recorded to tape.  Therefore, the line-in signals are highly attenuated to suit the recording circuitry, and then re-amplified via the tape preamp for monitoring.  For obvious reasons, this is not nearly as desirable as having the line-in signal treated as a line-level source.  Without a working tape deck, there was no point in restoring that faulty circuitry because it would never be used.  Additionally, some of the circuit could be disabled to reduce idle current consumption since they really are just going along for the ride without being used.  2) Additionally, since the OP's priority was for listening to line-in programs, bypassing the recording circuitry will allow the line-in program to be played without the need to put the boombox into "record" mode, especially since that would involve the signal path to traverse several switches including the dreaded oxidation prone record-bar, and also require the use of the record button to be locked down, either by modifying the tape deck mechanics, or by inserting an appropriate tape into the chamber.  And of course, that would mean that both the tape and erase heads be energized with AC bias current, and that the tape motor be operating (unless of course that were to be cut out of circuit).

 

So with careful bypassing of the audio signal, and by disabling the (now) parasitic current draw of the 1) tape preamp, 2) mic mixer, 3) tape bias oscillator and 4) if desired the internal mics, the boombox now can playback line-in signals by simply putting the unit into tape function.  All of this was done without any parts needed except replacement of the line-in attenuating resistors with a more suitable value.  Based on IVH's observation of the results, it sounds much better than before now so I suppose this is a good modification for anyone that has one of these guys and wants a line-in feature, without need of future use of the tape deck.

 

I will note that for those with a good working tape deck that wants to have a (true) line-in/aux feature without having the circuit traverse the tape circuitry, this model is a good candidate for another method which I documented in a line-in mod on a Wards Airline.  It looks like it could be done the same way with this model.



#51 Superduper

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Posted 17 October 2016 - 05:19 PM

AFAIK, Hitachi first tried the extra, separately amplified bass speaker in a boombox thing with the TRK7620 (Can't say for sure if it was the first as the Sanyo BigBen may well pre-date it). It was a success (particularly with me), leading to Hitachi's 3D series in the mid 80s, which included a twin-deck (3D8), a 3pc (3D9), a sort-of mini (3D2) & even a CD boomer (disappointingly named CX-W800). The original Hitachi 3D (TRK7620) stayed in this first-gen 3D line-up, becoming the 3D7 somewhere along the way. 
If anyone can adequately explain the numbering of the 3Ds (beyond the higher the number, the bigger it is), I'd be very interested.

 

 

After flipping through some of my service manuals, I found one for a 3D2, 3D5, the 7620, the 3D8 and even a 3D30.   Of interest is that the 3D8 appears to not only include a double deck (compared to the single deck of the 7620) but studying the diagram, it appears that it also has a real line-in feature too.



#52 Helmar

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Posted 20 October 2016 - 01:24 AM

I also have one of the TRK 7620 retrofitted with a bluetooth receiver.

I added a switch in the battery compartment, which enables the receiver and disables the tapedeck motor (and vice versa).

In order to upgrade the power supply like in in old days of analog audio I added a bigger capacitor (decoupling Capacitor 1,69€, 6800uf/35V www.conrad.de).

 

Now I read this interesting post and I would like to know if the gurus could add a more detailed instruction in order to achieve better audio quality (which is not bad in the original state of the boombox like it is now) with regard to this:

 

QUOTE

So with careful bypassing of the audio signal, and by disabling the (now) parasitic current draw of the 1) tape preamp, 2) mic mixer, 3) tape bias oscillator and 4) if desired the internal mics, the boombox now can playback line-in signals by simply putting the unit into tape function

UNQUOTE.

 

That would be an interesting upgrade.

 

A detailed report of the restoration and some more photos will follow once I find the time ...

 

Best regards

Helmar

 

 

Attached Files



#53 IVH

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Posted 20 October 2016 - 02:34 AM

Hey helmar,

Good to see another 3d7 fan in europe! Yours seems to be in great condition.

You mention you made a switch to switch between the tape motor and the module, does this mean you're still using it to play tapes ?

You won't be able to after this mod. 

 

If you don't have one, I recommend buying the 3d7's service manual from analogalley, it makes the job much easier. You can also use the service manual posted here, but it can sometimes be a bit difficult to read.

 

To bypass the tape preamp, you'll have to lift a leg or remove R412R/L, C405R/L and R417R/L. The last one is not a must, but I had completed the bypassing when superduper told me so, so I left it like this. As for removing them or lifting a leg, I recommend just lifting a leg so it can be restored at any time.

Be careful to lift the correct leg, as you'll need some 'holes' to solder the bypass wires.

After this, you'll have to solder bypass wires between the former R412 hole (the one that shows zero resistance to the RCA socket) and the hole from C405's desoldered minus pole.

R412 does, as learned from superduper, an attenuation resistor, without it, the signal the preamp receives will be too high and it will distort all the time, even at low volume. But, as we bypassed the tape preamp, you can replace it with a lower value. The value depends on your bluetooth module's volume. 

 

I replaced it with a 288k one, that suited if I connected my phone through the rca ports. But my bluetooth module has a much higher line out signal, so I had to add another 160k on the bluetooth module's audio cable.

 

I recommend starting with a resistor about the same value as mine, turning the bluetooth volume to max and test it out. If it distorts, add 100k, if the volume is too low, remove 100k, then repeat.

 

Here are some instructions from superduper regarding the disabling of the other components:

I took a look at the schematic and I think you should be safe to remove the following components (or lift a leg).  But if you do, you should do them in stages and check for proper operation between each stage.  Otherwise, if you do them all and then there is a problem, which do you restore?

 

Mic Mix:  R434 R/L, R435

Tape Bias Oscillator:  R424, R425, R426

Tape Preamp:  R407

Internal Microphone:  R430

 

 

Hope this helps



#54 Helmar

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Posted 21 October 2016 - 01:12 AM

Thanks for the instructions - good to know.

Currently I will leave the 7620 as much as possible in its original and genuine state. I also have a 3D88e waiting for its restoration, which I prefer over the 3D8 and 3D80. However the build quality of the 3d7 is the best and was thinking about changing the loudspeaker chassis to better ones and making a closed box resp. bass reflex box out of it. But this approach is only an idea which might consume more time than available for that.

 

Helmar



#55 IVH

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Posted 26 October 2016 - 06:06 AM

I finally put it together a few days ago, it played fine for a few hours. And the lipo battery lasts for hours !!

But then the left speaker started making a 'farting' noise when there is some bass. I thought the speaker was dead, but I was hoping for some bad wiring. After getting the top off, and reconnecting some connectors, the noise was gone !

Being happy the speaker is fine, i put it back together only for the farting noise to be there again ??!!

It had to be related to the top cover pushing some wire, right ?

Well, after some time I noticed it only occurs when the VU led board is plugged in:

 

Attached File  20161026_133937.jpg   112.63K   1 downloads

 

I don't quite understand how this affects the sound, and even less only the left speaker's.

The only explanation I see is that the vu meter is only monitoring the left channel. But that isn't logic.

 



#56 Superduper

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Posted 26 October 2016 - 12:34 PM

The problem could be that the preamp module (BA343) is unable to supply enough current during heavy bass transients.  This could be due to either the current dropping resistor values have changed in the signal path, or that the LED chip is drawing too much signal current, or that the preamp is not an OEM original part, rather it might be a counterfeit or a poor performing generic replacement.



#57 IVH

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Posted 01 November 2016 - 05:47 AM

Thanks for the information, I removed the vu leds for now. I'll look into it some day but first I'll enjoy it!

The first possibility sounds right, as I noticed that all other frequencies are mute when there is some bass, this happens even when the low frequency equaliser is at 0 db

#58 isolator42

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Posted 02 November 2016 - 12:18 PM

After flipping through some of my service manuals, I found one for a 3D2, 3D5, the 7620, the 3D8 and even a 3D30.   Of interest is that the 3D8 appears to not only include a double deck (compared to the single deck of the 7620) but studying the diagram, it appears that it also has a real line-in feature too.

 

Yup the 3D8 does have a 'proper' line in & a very similar sound to the 3D7 too. The downside is the EQ on the 3D8 is only 3 band...



#59 IVH

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Posted 22 September 2017 - 02:21 PM

The problem could be that the preamp module (BA343) is unable to supply enough current during heavy bass transients.  This could be due to either the current dropping resistor values have changed in the signal path, or that the LED chip is drawing too much signal current, or that the preamp is not an OEM original part, rather it might be a counterfeit or a poor performing generic replacement.


Hey superduper,
I just wanted to let you know that my 3d7 is still working great!
I am still extremely with it and your help fixing the noise issue.
Once I get a better camera, I'll post a video on YouTube of it in action.
Hope you are doing well!



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