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hitachi 3d7 noise

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#1 IVH

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Posted 16 September 2016 - 03:59 PM

Hello,

I recently bought a hitachi 3d7 MKII from ebay, it is in great state: everything works and it looks amazing, scratches are hard to be found. I'm going to install a bluetooth module and a lipo battery (while keeping it original, of course).

When I received it, I noticed all sliders were stuck, a little cleaning and contact grease/cleaner fixed that. But the buzz/noise was still there...

 

I did some research here on Boomboxery and found what has to be known to isolate the problem:

 

-it happens on the left and right channel + with headphones

-noticeable when on batteries

-the noise gets louder when turning the volume up

-it can be filtered out using the equalisers

-it is even noticable when nothing is connected (tape deck disconnected from main board, no line-in connected)

-clearly hearable when playing music through the cassette deck or line in.

-the resistance between the ground of the power supply is 0 Ohm to all 'grounds' I measured on the board 

-If I touch the ground when it is powered on, there is much more noise (this might be normal?)

 

Please tell me if I'm wrong:

The issue lays after the power supply and before the equalisers. And maybe even before the 3 different amplifiers (?).

Unfortunately I can't find a service manual to download, does anyone know where to find one ?

Where do you think the problem is most likely to lay ?

 

Thanks in advance,

ivh



#2 Fatdog

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Posted 16 September 2016 - 04:18 PM

Welcome aboard, IVH!  Before any technical advice from other members, I would suggest sending a PM to member isolator42.  He is pretty much the goto guy with all things Hitachi 3D.



#3 isolator42

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Posted 16 September 2016 - 04:33 PM

Hi there,
I might be a big fan of the 3Ds, but I'm no electronics expert, unfortunately. It sounds like you already know more than me - from what you say it sounds like trouble in the pre amp stage somewhere. Maybe worn out capacitors somewhere?
My advice, for what it's worth, would be to look out for another 3D7 with good sound but cosmetic damage, etc. (so it'll be v cheap) & do a transplant job.
Anyway, good luck! :)

#4 IVH

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Posted 16 September 2016 - 04:50 PM

Thank you for the replies and the welcome guys !

I am the one who send you (isolator) an email a few weeks ago asking if you had a 3d7 for sale, well, I found one ! :)

That's a good tip, but I'm afraid I'll have 2 3d7's with bad sound then, this one was supposed to be functioning 100% correctly too :/

Yes !! Pre amp sounds logical to me, do you know where it's location is on the board ?



#5 jimmyjimmy19702010

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Posted 16 September 2016 - 05:31 PM

I've got an original service manual for the 3D7. (The original MKI version which i assume is the same electronically).

I'll have to try and scan it to a PDF.

#6 IVH

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Posted 16 September 2016 - 05:44 PM

That would be amazing ! It would make the job so much easier :)



#7 IVH

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Posted 18 September 2016 - 08:11 AM

I got a small update, When on a clear frequency, the hum is not hearable when on radio !! What does that mean ? The radio is preamplified too, right ?

I'm lost ;)



#8 jimmyjimmy19702010

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Posted 21 September 2016 - 05:36 AM

Below are high resolution iPhone5S pics of an original Hitachi 3D7 service manual. Hopefully, you should be able to
zoom in for details:

Attached File  IMG_2886.JPG   180.72K   14 downloadsAttached File  IMG_2887.JPG   213.69K   12 downloads
Attached File  IMG_2886.JPG   180.72K   14 downloads



#9 IVH

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Posted 21 September 2016 - 06:30 AM

Perfect ! Couldn't thank you enough jimmy !! It's all clearly readable =D   

I'll let you guys know what the issue was (if i ever find it).

When it's all finished I'll maybe start a how to boombox modernisation topic (bluetooth and lithium po installation). Even though you guys do already know how to do that for sure.

greetings,



#10 Superduper

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Posted 21 September 2016 - 04:35 PM

The 3D7 is the same as TRK-7620H?  Well, heck, why didn't you just say so to begin with?  I was looking through my archives for a TRK-3D7 manual and found NONE, which was a bit suprising since I have most Hitachi manuals.

 

I would suggest you look at the bias voltage/current levels for Q401 R/L.  Excessive hiss is a symptom of improper biasing of transistors and both are fed through a single resistor, fed by IC401.  Good luck.



#11 IVH

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Posted 21 September 2016 - 05:08 PM

Yes! forgot that too, i looked through all your manuals too but couldn't find the 3d7 ;)

Thank you for the tip super!! I had no idea where to begin.
Is it possible that this only produces a hiss in tape mode, and none in tuner mode?

#12 Superduper

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Posted 21 September 2016 - 05:37 PM

(1) If you ever need a service manual and it's not listed in my archives, just send me a inquiry.  Although most of the other equipment listings is pretty accurate (98%), boomboxes are a different matter since I culled many from the list and have them stored separately due to my personal interest in them.  Of course if you send me an inquiry, I would need the "accurate" model number, and  not a "popular" model number. 

 

(2) Yes, because if you look at the schematic where the function selector switch is, you will see that in tuner mode, the other modes are shorted to ground, which isolates any signals produced by the tape preamp section.  Likewise, if in line/tape mode, the tuner signals are shorted to ground.  This shorting also prevents crosstalk which I think you will find is not unusual in other boomboxes.  For example in tape mode, during moments of silence or low volume, you might be able to hear the radio ever so slightly.  Also, note that both tape and line-in functions are amplified by Q401 R/L.  I would start with the resistor that feeds the collectors of those 2 preamp transistors.

 

(3) Not saying this is or is-not where the problem is, but if it was my boombox, that is where I would start.



#13 IVH

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Posted 21 September 2016 - 05:46 PM

Your help is much appreciated. I should have looked om the schematics before asking, sorry for that. My exuse is that it's 1:44am here so I'm already in bed.
I'll post a more in depth reply tomorrow after measuring.
Thanks again!

#14 IVH

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Posted 22 September 2016 - 08:57 AM

Today I checked the resistance of resistor R407 (this is the resistor you mentioned, right?). It gives a resistance of 0.985K, this is within the allowed range of 5%.

I also checked R406R, which gives 6.69K, so no luck here either...

R405L gives 6.69K too, and is therefore in perfect order.

I measured the potential between pin 4 and 5 (ground) of IC401,that gives a voltage of 8.05. A 4.4% difference with the bias voltage.

 

I don't know if this is useful, but I tested the voltages on the outer pins of the Q401R and Q401L transistors. Those gave 0.649 and 0.643v. I can't read the voltages clearly but those should be 1.6v, if I'm not mistaken.



#15 Superduper

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Posted 22 September 2016 - 10:50 AM

I don't know if this is useful, but I tested the voltages on the outer pins of the Q401R and Q401L transistors. Those gave 0.649 and 0.643v. I can't read the voltages clearly but those should be 1.6v, if I'm not mistaken.


Depending upon whether you are speaking of the schematic or board diagrams, the outer pins differ on pin assignment. Based on what I see on the PWB diagram, the center pins are the collectors and those should read 1.6v. The outer pins will be base and emitter. The emitter should be be 0v and the base should be 0.7v. This is important since .7v coincides with the "turn-on" region of most silicon transistors. It's important to keep a transistor "turned-on" and in the "active-region" to maintain a Class-A-B mode. Go ahead and check all: Base/Emitter/Collector. If those are all "normal" then you'll need to keep looking. Your problem is most likely fore of IC401. All signals tape/line/radio goes through IC401 and since you already know that the radio function performs fine through IC401, then that should be excluded. There isn't that much circuitry fore of that IC. Can't tell you much more than that but at least you know where to focus. Also make sure to consider and check all the usual stuff which includes deoxidizing the function and record/playback switches, and bad ground/solder joints. You may want to go ahead and just reflow and/or replace the solder in that area of the board since it's so small. Additionally, while you are there, it can't hurt to replace the handful of electrolytic caps in that part of the circuit. Again, not that many parts, and not that hard to do.

#16 IVH

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Posted 25 September 2016 - 04:11 PM

Here is my way too late response, I've been busy with lithium and bluetooth part the last few days. But why put bluetooth and a lipo in a box with a huge hiss ?

The part about the transistors is really interesting, that will definitely help me fixing this box !

Today I have rechecked all resistors that are (I think) before IC401. This time I did it right, by marking each checked resistor on a photo of the board. There were some shocking ones: R414R/L reads 0.9 kOhm instead of 27 kOhm, and that is just one of the many. Although I'm not sure they are all before IC401, I'll be replacing those.

Tell me if I'm wrong, but if I'm going to recap. I'll recap the whole thing, as shipping costs tend to be more expensive than the caps here.

When replacing those resistors, I'll reflow the part before IC401 like you said.

Hope that fixes it !

Edit: Wouldn't it be curious that R414L and R failed both and read almost the same resistance (0.905kOhm and 0.902kOhm) ?
I'm not a specialist but maybe another one failed, altering both these readings.
In the suroundings those Resistors, there are:
- R422L and R, reading 16k92 instead of 33k
- R433L and R, reading 0.805 and 0.811k instead of 1k
- R413L and R are both fine.
- R420L and R are both fine.
- R421L and R are on the limit, reading 2.06k and 2.05k instead of 2.2k. The tolerance limit of those is 2.09k

I will be removing the apparently failing ones from the board this evening and measuring them without being connected. I'll let you know!

#17 IVH

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Posted 26 September 2016 - 12:28 PM

Unfortunately but as expected, R414R reads a nice 26k unsoldered. I shouldn't try to check resistors in circuit...



#18 Superduper

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Posted 26 September 2016 - 02:09 PM

All of the readings you provided were predictable.  I saw your post earlier but didn't get time to respond.  Unfortunately, I'm out and I'm still not in a position to provide a comprehensiive response so I'll have to do that later tonight when I get home and have the service manual in front of me.  BTW, what is the B/E/C voltages I asked for.  Did you get those, and how accurate is your DMM?  Also, have you thoroughly cleaned the record/playback as well as function selectors yet?  It sounds simple enough but it's very important because these switches can perform some switching to ground, and if poor connection, the connections left floating can act like mini antennas and picks up interference which gets amplifed by the preamp into hum or buzz.



#19 Superduper

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Posted 26 September 2016 - 03:19 PM

OK, here are a few tips that I hope will help you and others in the future for troubleshooting.

 

Let's just use your example about R414 R/L.  Since both R/L circuits are parallel and identical, and should behave in likewise fashion, let's just deal with R414R.  The schematic is shown below and you are testing for resistance of that resistor:

Attached File  3d7-2.jpg   75.57K   2 downloads

 

Now, let's see what happens.  Unfortunately, when a resistor is tested in circuit, there is always the possibility of multiple resistance paths.  The first possilble path is the most obvious one, and we expect 27k ohms.

Attached File  3d7-3.jpg   78.52K   7 downloads

 

However a quick study shows that there is at least one alternate path.  Pin 8 of IC IC401 is connected to one leg of R414R and may return thorugh some circuitous path to the other leg of R414R, however any effect is probably small.  Let's just consider the more obvious alternate path, as seen below.  It goes through 2-resistors of 1k and 100R, combined of which we expect 1.1k ohms.

Attached File  3d7-4.jpg   86.96K   1 downloads

 

Now if we consider only those 2 circuit paths, we can use the following formula to come up with the actual resistance if R414R is tested in circuit:

 

So using the following formula:
 
1/Rt = 1/R1 + 1/R2,   where Rt = Total Resistance, R1 = 27k and R2 = 1.1k ohms.
 
or  1/Rt = 1/27k + 1/1.1K
 
or  1/Rt = 1.1k/29.7k + 27k/29.7k
 
or  1/Rt = 28.1k/29.7k
 
or  1(29.7k) = Rt(28.1k)
 
or  29.7k/28.1k = Rt
 
or 1.057k = Rt

 

You arrived with .902k ohms instead of 1.057k ohms.  However since there may indeed be additional parallel resistances which we did not account for, and the fact that we are presuming perfect values through the 3 resistors we did calculate, and accounting for possible meter errors and variances, this is an acceptable result.

 

 



#20 Superduper

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Posted 26 September 2016 - 03:39 PM

Alright, so with results like this, you might ask yourself what's the point of checking resistors in circuit, since the answer is basically useless, right?

 

Look, when you do "quick" resistance tests of resistors in circuit, the results are not always useless.  As the name implies "quick" check, means that you are looking for results that might tell you what type of readings would definitely show something amiss.  You were looking for 27k but got less than 1k.  Again, you are asking yourself what possible resistance measurement results is more wrong than that, and will tell you that something is definitely amiss:

 

Remember this rule, parallel resistance ALWAYS provides a sum of which is the same or lower than the expected value.  Given this rule, then one can come to the conclusion that if you were expecting 27k and got 35k, or 350k or 1M.... anything more than 27k will indicate that there might be a problem.  

 

Use these quick rules and tips when doing checks:

 

  • If your check results in a measurement in line with expectations or LOWER, then test result is inconclusive and closer examination of circuit diagram is indicated to see if parallel resistances exist.  Confirmation by retesting after lifting one leg of the component is the only way to positively know for certain.
  • If your check results in a measurement HIGHER than expected, there is likely a problem with that resistor.
  • High value resistors tested in circuit will very frequently test lower because parallel circuit paths usually have a greater affect on the total circuit resistance.
  • Low value resistors are usually not significantly affected by parallel resistance paths.  For example, a 27k ohm parallel resistance to a 100 ohm resistor being tested will have neglible effect on the reading.
  • When testing any component, I always test from a solder pad further up and down in both directions.  For example, when testing R414R, I might probe at the solder pads of resistors R413R and R433R.  The reason is that if the resistor R414R is suffering from a cold solder joint, or break in circuit, then this could be brought to light since they are part of the circuit as well.


#21 IVH

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Posted 26 September 2016 - 05:14 PM

Ouch, i should have seen those resistors in parallel earlier ! Your clear schematics really helped !

I'm using a small wurth digital multimeter, the advertised precision is 1.2% for potential measurement.

I forgot to mention I cleaned all sliders and switches thoroughly with contact cleaner when I received it, and moved the rec and tuner/tape switches ~50 times (as said by you in another thread).

 

I don't know why I forgot to measure the transistors, but here are all related to the tape audio part:

Q401L:   E = 0v   B = 0.698v   C = 1.9v

Q401R:   E = 0v   B = 0.704v   C = 2.28v

Q801L:   E = 1.123v   B = 0.496v   C = 4.27v

Q801R:   E = 1.175v   B = 0.55v   C = 3.65v

Q502:   E = 6.76v   B = 0.653v   C = 9.04v

Q402:   E = 13.71v   B = 13.4v   C = 13.80v    ??? 

Q801L:   E = 13.25v   B = 12.34v   C = 13.9v  " " "

 

I didn't mention the output of the AC/DC circuit is pretty high: between 14 and 15.5v (it might not be made for france's 250v supply...) according to the service manual, it can be rewired for 250v. But in another thread I read you saying that the voltage from acdc's is often higher while not under load. 

 

I'll have to remember the formula you used in your 2nd post, it's very useful !

 

Wow ! Those 'quick rules' are good to know !! could be a sticky if you ask me. Tomorrow I'll recheck the whole circuit, but with those rules applied, especially the last one.

 

I don't know if I'm making your day, but what I know is that you"re making mine  :yes:



#22 Superduper

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Posted 26 September 2016 - 10:12 PM

I'll offer the following notes:

 

  • When doing voltage checks, it is important to note that the values indicated in schematic diagrams are based upon a constant prescribed source voltage.  Generally speaking, one should hook up the system to a lab adjustable power supply to ensure that the system is getting the voltage that is indicated.  In this case, 12V.  It is not 100% requirement to use a regulated power supply and if you do not and use the AC transformer instead for a power source, then some values will change and you'll need to account for that.
  • You have Q801L listed twice with 2 different sets of voltage measurements.  This is probably a mistake but regardless, Q801 L/R are aft of IC401 and probably not where we should be focusing at this time, if for no other reason than that we know that part of the circuit should be capable of operating normally when a radio circuit is applied.  So, for now, you should probably ignore that portion, and focus on the other part of the circuit.
  • You are getting different collector voltage for Q401L and Q401R.  They should be the same.  You should probably check R404R for possibly higher than normal resistance and R405L for possibly higher than 470k, .
  • B-Q502 seems to be low compared to schematic.  Are you sure that is a correct measurement and does your super-bass woofer appear to be working normally?
  • Q402  These measurements are off, which I presume you already noted.  You may want to check R424/426 (possibly too low?), R427(possibly too low?). and R428 (maybe open).With these measurements, I presume that you are not able to record to cassettes?  If these measurements were accurate, and the resistor failures are true, this might well be the source of your problems.

Your DMM is not very accurate so you need to understand that your measurements aren't 100% accurate.  The 1.2% you mentioned is probably the best case scenario.  Add probes, and other factors and the deviation could be very noticeable.  By comparison, the Fluke meters I use have lower than 0.1% accuracy.



#23 IVH

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Posted 27 September 2016 - 12:38 AM

I'll post a longer reply this afternoon when I am free. From what I read, there is some hope for a simple fix.

I forgot to replace the last Q801 with Q501, the ripple filter transistor. Doesn't sound too illogical if there is something bad too. 

When I measured the whole circuit a few days ago, I marked R428, 426 and 424 as being a bit off. I'll remove those from the circuit this afternoon. There's still some hope left !



#24 Superduper

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Posted 27 September 2016 - 09:37 AM

You may want to recheck Q501 (readings previously indicated for Q801L correct?).  The emitter voltage should not be higher than the base, although I suspect that you have those two figures reversed.  If not, you may want to check the transistor out of circuit to see if it is working properly.  A cheapo eBay transistor checker will help simplify your diagnostics.

 

172320564326

 

One similar to the one above will serve you just fine, although I don't personally recommend any particular one, but there is a whole bunch of different versions.  I think that they are mostly all based on a custom arduino platform.  Actually for all you guys, I recommend you get one.  For less than the cost of acquiring an arduino and the associated parts, you can get one completely custom programmed for this purpose.  It's not going to replace a $5000 specialty lab instrument, but for the crude testing we do on boomboxes, you can't beat it for the money.

 

Anyhow, worse case is that Q501 is not working properly, which means that it's not filtering and supplying excess voltage to your tuner (bad, maybe future issues).  Currently, it is clearly passing current otherwise your tuner won't be working.  But when operating properly, the emitter should be controlled by the base, and with the emitter greater than the base, that does not seem to be the case based on your measurements.  Not sure this will have any affect on your issue at hand, still it's something worth noting since your measurements indicate that this is a potential issue and as you already have the boombox open......

 

================

 

Unrelated question:  IVH, you are a french-person eh?  You seem to have an excellent grasp of the english language, even my occasional typos (happens when I type fast to try and save time) don't seem to cause you to miss a beat.  Does your country teach english to the general population or how is it that you come to understand english so well?



#25 IVH

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Posted 27 September 2016 - 01:21 PM

Maybe I should connect it to a 12.6v lipo, that's a way smaller difference than the transformer's output.

Here are the values of all the resistors you asked for:
R424: 269Ohm, 270 indicated
R426: 390Ohm, 390 indicated ! this one was measured off circuit as parallel resistors were affecting the readings.
R427: 56.2kOhm, 56k indicated
R428: 7.1 Ohm, 6.8 indicated. This one has been measured between one leg of Q402 and a leg of C417 like you said.
R405R: 464kOhm, 470k indicated
R405L: 456kOhm
R404R: 38.2Ohm, 39 indicated
R404L: 38.6Ohm

The woofer seems to work properly, the difference between 3d on and 3d off is clearly noticable and bass is good. It's not performing very well at very low frequencies, but that might be normal for a woofer in a non airtight enclosure/box.

You are right, I just noticed i reversed B and E loads of times... sorry for that. I've remeasured all those just to be sure.
Q502: E = 6.44v B = 6.69v C = 8.95v
Q501: E = 12.26v B = 13.10v C = 13.68v
Q801L: E = 0.490v B = 1.117v C = 4.22v
Q801R: E = 0.545v B = 1.173v C = 3.64v
Q402: E = 13.37 B = 13.7v C = 13.81v

I understand and will take the accuracy into account. My father is electrician and got some professional multimeters, I think I can use them if necessary



About the transistor tester: I'll look into it, as I have an arduino laying around programmed as a low voltage cutoff for the lipo that i'm going to build into the hitachi.
Even though the transistors are is possibly good now after having remeasured everything properly. It's always handy to have a transistor tester laying around, especially with all those vintage amps laying around ;)



Answer related to an unrelated question: Thank you for the nice compliment! It's a bit complicated as I'm dutch and moved to france at the age of 9. Dutch have the reputation to be quite good at english. Unfortunately, english teaching is pretty bad in france, pupils (and most grown men) think english isn't important and refuse to learn it. This slows down lessons for the ones willing to learn it. I learned your language thanks to the internet ! Without it, my english would've been deplorable.



#26 Superduper

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Posted 27 September 2016 - 02:46 PM

You even know the word deplorable?  I would say even many people in USA probably can't fully articulate the word deplorable, much less the phrase basket of deplorables.  

 

Ok, it's getting too hard for me to flip back and forth now from your readings and this reply text box, so I'm going by memory here and making some presumptions.  Firstly, I believe your readings are all now (with some amount of deviation), relatively close to the specs, right?  So with that, nothing that I'm seeing in your readings suggest they would be the source of your problems except Q402 readings which are totally off but that is because you made measurements in PB mode, and those readings (in parenthesis) should be taken in Record mode.  I suspect that if you retake the measurements in Recording mode, that those readings will come in line, approximately with specs.

 

 The L to R imbalance in your readings disturbs me since they should be identical but even with the deviations, "probably" isn't the source of your problems.  There is a reason for the Q401 L/R imbalance and I don't know, I might even take a resistance measurement between R407 and the C/B of both Q401's.  Perhaps there is some additional resistance in the circuit path that is the cause.  If not, then it may be due to a leaky (C406 L/R) capacitor.  I would also check the emitters of Q401's to ground to see if they are equal.  

 

At this point, you've nearly exhausted the V/R test readings which are the measurements I use to be able to give you an analysis and opinion from thousands of miles away.  You haven't checked all resistors yet and they might be important (see the thread below for an example).  After that, then next up is capacitors which are harder to check since they can't really be checked in circuit reliably.  Additionally, better and more capable equipment is also required such as an LCR meter but that cheapo ebay tester I mentioned earlier might work in a pinch.  I always do a quick test by checking all caps and see if any are shorted.  If yes, they should immediately be considered bad.  This simple test can be done in circuit.  Since it sounds like you only have a DMM, you'll have to see if your meter can test caps too.  If so, then you can test them by lifting one leg.

 

Other tips.  Bad solder joints is the source of many problems that are not obvious at first glance.  It does not cost anything except a bit of time to redo those connections.  Also, any caps that has one leg tied directly to ground should be checked more carefully.  Many are ceramic and not necessarily electrolytic.  Hum issues can be a pain in the butt to figure out, especially with ground loops but on verified circuits (not in prototype or new designs) that worked fine before, ground loops would probably only be introduced by poor connections compromising a good ground circuit.

 

If you have time, you can review this thread by Joe (BluFuz) where he had a very hard time fixing a hum issue.  You can skip to the fix by going directly to approximately post #255 +/-.

http://boomboxery.co...ixing-a-legend/



#27 IVH

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Posted 27 September 2016 - 03:21 PM

Don't flatter me too much ! But indeed, many americans are quite bad at english, and tend to not even know the basics of their counties history. That surprises me a lot.

 

You are right, the hiss is not any different between channels. But volume may be, I don't know, from what I understand it would be best to fix the imbalance too.

 

I checked almost every resistor on the tape part (didn't post everything), but I marked loads as being off because of my lack of understanding parallel resistors. I will soon recheck all with the new service manual and do the necessary math.

 

As usual, it's getting late, I'll reply to and do the things you said in the 2nd to 5th paragraph tomorrow. Thanks again for all the tips, I could never have thought those by myself.

 

I did read page 3 of that topic a few days ago, it was an interesting read. I immediately started looking for those stupid printed resistors, I couldn't find one luckily, hopefully there really aren't. But yes, I might learn a lot from that thread, so reading the whole thing might not be that bad of an idea ;)



#28 Superduper

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Posted 27 September 2016 - 07:40 PM

It's good that there's no printed resistors as that would add another dimension of craziness to troubleshoot.  Much of the problem with Joe's boombox was not only that the printed resistors were all drifting or open, but also just the printed traces were going bad too.  Since they could flip up/down the board through vias many times while traversing the board, it could get tedious trying to track them all down.  Some even go under components making it very hard to follow.

 

I just noticed that your boombox does not have a true line-in.  The diagram didn't make sense to me since I was sure you mentioned listening to music via line-in.  This must be while monitoring the tape during record, right?  I don't have and never had one of these so I'm not sure.  Also, you didn't test the mixer circuit.  That is on the small headphone jack and includes Q403.  Highly suggest you investigate that as a bad ground wire or emitter resistor could certainly introduce hum.  In fact, R437 on the mixer board was exactly the equivalent resistor on Joes boombox that caused the hum.  More things for you to check, LOL.  



#29 IVH

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Posted 28 September 2016 - 05:32 AM

It's complicated enough like this ;) I can't imagine how it would be with those printed resistors.

 

I don't exactly know how it works, but line in works without the tape deck connected to the board, you just have to ''press a slider'' that is normally pressed by the record button. This can be seen in the video.

 

I wanted to rule out that the issue is related to the mixer board. So I connected the board to my father's MFB through line out without the speakers and mixing board connected. The hiss is still there but it's not noticable at low volume. (maybe related to noise cancellation in my father's amp)

Line in seems to not work through line out, as the rec switch just cuts line out off, even while in tuner mode.

There will be 2 videos demonstrating this.

This should rule out the mixer board. I'm now going to do all the testing you mentionned in your second last post ! 

The more things to check, the more possible fixes to be found :P

 

 



#30 IVH

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Posted 29 September 2016 - 02:39 PM

Today I found some time to do all the measurements you asked for. Unfortunately I found nothing too staggering...
I did also reflow the part before IC401 (if by that you mean remelting the existing tin).

The resistance between Q401L (E) and ground is 38.9Ohm, a mere 0.4 Ohm less than the one between the emitter of Q401R and ground.

 

I did measure the resistance between R407 and both the collector and base of Q401R and L

The resistance between both the collectors is the same: 7.68k Ohm

Between Q401L's base and R407 the resistance is 463k, the R side gives 472k. That doesn't sound too off to me.?

 

I finished measuring all resistors beginning with 'R4' I could not find anything off.

 

The same goes for the capacitors, except for a minuscule ceramic one (C426) that appears to be open. It's resistance is  from the beginning, it doesn't slowly nor rapidly rise. Is it broken or does it charge up instantly because of it's small rating (68PF) ?

 

Like you said, there isn't much left to measure. Is it a good idea to just replace all electrolitic caps ? Someone once told me, when ordering a recap set, that ceramic ones don't go bad . I was thinking of buying a bunch of nichicon FG caps, The 3D7 might even sound better than when it was fresh from the factory with those soldered in ;)





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