Hitachi TRK 8190 - no stereo reception in FM

[Superduper]

"............I briefly grounded the negative motor terminal and afterwards the tape started working properly - no motor turning in
stop mode. Now plays cassettes fine...BUT the radio is broken again. It's reverted to the briefly decaying sound......."

"...... when I touched the black motor wire to ground as I was looking for stray injections I heard a small pop and then the cassette side of things started working correctly. I was dismayed when I discovered the radio broken as a consequence.............."

And I'm very curious "exactly" how did you ground the motor terminal, and I mean "exactly." And why oh why would you do that? And are you sure that the "pop" wasn't the sound of a voltage regulator going up in smoke when you grounded something that maybe perhaps wasn't intended to be grounded? By the way, decaying sound and power is the classic symptom of poor connection somewhere, perhaps in the power switch. During low current demand with a poor connection, enough current might be passed for initial operation but the poor connection develops heat which has a cascading effect and cycle of increasing resistance in the connection, which results in more heat and a corresponding reduction in voltage, etc... until where power might even seem to disappear until the connection cools down. Failing caps might cause this too, as well as a failing/damaged regulator which might work initially but as it heats up, loses capacity. This failure can be confirmed by using freeze spray on the affected regulator. If power seems to be restored temporarily as the transistor or zener diode is cooled, that basically confirms that this regulator is suspect.

 

[beamrider]

And I'm very curious ...

Thanks, yes. I thought that maybe the motor wasn't grounded properly so tried grounding the black wire to the large heat sink around the regulator. I realise now that maybe I shouldn't have done that.

But if the voltage regulator was bad/damaged, I would think it wouldn't play cassettes would it (?) which require more power than radio, yet cassette playback is now fine.

 

[beamrider]

Ordered some freezer spray. Will spray a few things and see if it makes any difference.

 

[beamrider]

Tonight discovered the AM radio is also working.

So to summarize:

- Cassette works
- AM Works
- FM Works (including FM Stereo light) only briefly

 

[Superduper]

Thanks, yes. I thought that maybe the motor wasn't grounded properly so tried grounding the black wire to the large heat sink around the regulator. I realise now that maybe I shouldn't have done that.

But if the voltage regulator was bad/damaged, I would think it wouldn't play cassettes would it (?) which require more power than radio, yet cassette playback is now fine.

You're making some presumptions here that heatsinks are grounded. That is often, most likely even, the case but it's also not uncommon for large transistors to have the collector connected to the transistor case or die and if it was bolted to a heatsink without an insulator, that would make the heatsink hot. Regardless, wasn't the motor wires soldered? I mean HOW did you ground the motor? Did you desolder the motor lead and then take that wire and probe for ground? Or did you take a cut wire and jump the motor lead terminal to the heatsink? I'm not saying for sure you did something wrong, but when you do things like that, it often results in unpredictable results when loose stuff accidentally touches something even for a microsecond, especially in a live circuit. If you suspected or wondered if the motor was properly grounded, then you could always check and verify with a DMM but honestly, for a motor that runs all the time, a poor ground isn't really something to be suspected, it's almost always a switching issue and in this case, it most certainly is. And you asked whether it's possible the cassette back accidentally shorted to something when you reassembled it... you didn't do that live did you?

As for a damaged regulator.... indeed if the regulator was blown, the cassette deck might not function but only if we are talking Q501. There is more than 1 regulator, Q502 is the regulator that supplies power to the tuner. And are you sure that it was the regulator that you tried to ground onto? Regulators rarely have a large heat sink unless this was something like a large amplifier or receiver, or regulated lab power supply. Most likely, a large heatsink in a boombox is going to be the main power amplifier. Anyhow, you ought to seriously consider what I said about poor connections. Discount that are your own peril, but that is one of the most common problems with symptoms as you described. In fact, it's not uncommon for regulators to just have a clip or screwed on smaller heatsink that is not grounded through the PCB. In that case, if there was no insulator used to secure the heatsink, there is a very good chance that it is "live".

 

[beamrider]

You're making some presumptions here t.....

Many thanks for your detailed reply.

I did indeed use a piece of cut wire and when fitting the cassette I may have forgotten to turn off the bench supply - both something I won't be repeating.

I checked and there is a 0.8v difference between the heatsink and the black wire motor terminal on the main board. I'll replace Q502 again and insulate the back of it from the heatsink with a piece of greaseproof paper. Hopefully the "motor boating" won't return, but if it does I'll take it from there.

I've checked all the connections and switches and re-cleaned them.

Still puzzled why the AM radio works, perhaps it draws less current than FM?

 

[Superduper]

Ok, so.... AM Works and FM works briefly? Hey, I think you really need to clean the heck out of the band switch, and until you do, I'm gonna boycott this thread. Here's why: I already mentioned to you before about bad connections. You may not know this but inside that band switch is 10 (count em, 10) separate switches gangs that all throw at the same time. I'll bet you that is almost certainly the issue and those type of switches are not easy to clean. The other reason is that if the tuner is getting power and AM works but FM does not, and it is not a poor connection issue, then the repair is going to involve a skillset that is beyond the DIY and I would suggest you seek the services of a technician.

About that regulator Q502... I believe that is a TO-126 package transistor. It has a metal die on the backside. You need to check to see if that die is electrically connected to any of the 3 leads. If it is, and you decide to insulate it, don't just use paper. Not only is that not the proper insulator, it will probably hinder heat transfer. You need to use a proper TO-126 mica or other acceptable insulator. Google or search on eBay or something. It's a common item. Oh and you probably want to install heat sink compound between the transistor and the insulator and the heatsink to ensure proper heat transfer.

 

[beamrider]

Once again - thank you. I appreciate you taking the time to give detailed advice to DIYers on this forum.

 

[beamrider]

So it's not the band switch - I've left it in the FM position and I have it working (kind-of). Radio & Tape operate, but the tape motor seems to be running constantly when in the cassette position (but no "motor-boating").

I can't clean the band-switch any further without disassembling it.

I think it's related to the main heatsink somehow. I've freeze tested the capacitors etc and it makes no difference.

The thermal compound I was using is non-conductive. Q501 & Q502 appear to require the metal plate to be in contact with the heat-sink for the radio to operate. Move the transistors away from the heat-sink and it reverts to the fading behaviour. Attaching them however, leaves the heatsink at + ~13.5V. I'm not sure if this is normal or not.

The 2SC1162 data-sheet and empirical tests shows the collector is tied to the metal plate. The service manual shows the collector of Q501 as 13.5V and Q502 as 12.6V however this can't be if they're tied together by their plates?

There is also a small riser coming out of the main board (soldered to ground) with a hole in it that looks like it's meant to be screwed the heatsink, but I can't connect it as everything stops working when I do. It also has what looks like the remains of an plastic insulating washer , but again, I'm a bit confused as to the purpose of it if it's insulated from the heat sink.

I also can't see any TO-126 mica's listed in the parts list of the service manual.

So working kind of, but something isn't right.

 

[Superduper]

Dude! Think about what you're doing! Look at the schematic diagram. What you've effectively done is taken a jumper and connected it from the collectors of Q501 to Q502. You already knew the datasheets states that the transistor dies are hot, I warned you and you looked it up. Look at the schematic and now draw a line connecting those 2 points, THAT is now your new schematic diagram, . Yeah, you are right that one can't be 12.6v and the other 13.5v anymore, since you've now shorted those 2 points. If you have a meter, I think you'll find that to be true, you don't need to guess.

And as for the mica insulators, those are almost never listed in a service manual (unless they are unique and propriety type), they are incidentals such as grease, zip ties, deoxidizing solvents, and such. Service manuals are intended for use by technicians and not novices. This is also why there's generally no handholding step by steps listed for rebelting a deck, they presume you know this stuff. If you don't know what mica insulators are, as I mentioned to you previously, "GOOGLE" it. Every technician is intimately familiar with those. Mica isn't just an electrical insulator, they have special properties and are excellent heat conductors simultaneously, which is why they are specifically used when mounting transistors to heatsinks.

As for why everything stop working when you install the heatsink screw...... IT'S BECAUSE YOU ARE SHORT CIRCUITING EVERYTHING TO GROUND owing to the collectors of those 2 regulators which were hot (now zero volts as everything is shorted to ground). It's a wonder you haven't blown a fuse or your transformer or worse.

Lastly, you do not use freeze spray on capacitors. It's useless, that's not their failure mode. You typically use them on semiconductors. That would be like your IC's, transistors, diodes, etc.

Ok, nobody is born knowing electronics, you must learn them. But to learn, you must absorb knowledge. If you're gonna ignore and discount advice that takes times for them to formulate for you, then keep in mind that nobody likes to feel like they're wasting their time.

Good day to you sir, I'm sure you'll figure it out.

 

[Tinman]

Like SD said, you have to insulate the transistors from the heatsink.
It's not enough that you're using non conductive grease.
If the heatsink is grounded, you're grounding out both transistors.
If the heatsink isn't grounded, you're still shorting the transistors out on each other.
I recently had a heatsink with two transistors and an amplifier on it.
I removed the heatsink to replace some capacitors and forgot to replace the transistor insulators.
I blew two resistors twice before realizing I forgot to reinstall the insulators.
After replacing the insulators, all was back to normal.
This might not be your only problem but it's one of them.

Some pics might help.

 

[beamrider]

Good day to you sir, I'm sure you'll figure it out....

Thank you. I appreciate your advice am not deliberately ignoring it. I have attempted everything you have suggested.

I acknowledge that I shouldn't be shorting these transistors together. However without the heatsink in place it still doesn't work (and no I only try it briefly and wouldn't allow any time for heat to build up - fact I touched the transistors and they didn't even feel warm). So I need to figure out why. I've removed and retested Q502 several times and it tests fine on the DMM. The output form Q501 looks correct at 13.3V. I have a current limiting bench supply set to 0.25 amps, so that's why I probably haven't blown anything.

If what you're suggesting is I disassemble and clean the band and function switches, I think that is probably beyond my abilities as you say they are full of tiny parts.

Can I ask why the heatsink is grounded if nothing is supposed to be connected to it?

 

[beamrider]

Like SD said, you have to insulate the transistors from the heatsink.
It's not enough that you're using non conductive grease.
If the heatsink is grounded, you're grounding out both transistors.
If the heatsink isn't grounded, you're still shorting the transistors out on each other.
I recently had a heatsink with two transistors and an amplifier on it.
I removed the heatsink to replace some capacitors and forgot to replace the transistor insulators.
I blew two resistors twice before realizing I forgot to reinstall the insulators.
After replacing the insulators, all was back to normal.
This might not be your only problem but it's one of them.

Some pics might help.

Thanks for the advice. I'll post some pics in case it helps.

Yes, I blew Q502 but only once so far.

I'll order some Mica kits as SD suggest in the meantime.

 

[Superduper]

The output from Q501 should be 12.6V, not 13.3V.

Your suspicions that the reason nothing blew because you had it powered up by a lab supply is correct if you actually are limiting current to 0.25A, but that also means that the boombox is limited to under 3.4 watts. While that is good for checking certain things, you have introduced another variable into the mix. Is 3.3 watts enough for everything to power up properly? Maybe, maybe not. That might also explain why the regulators aren't heating up, although honestly, most regulators in boomboxes don't even have heatsinks so they aren't really designed to handle all that much power. The most power hungry device is going to be the power amp and that chip gets connected directly to the power rail so the regulator isn't even involved in that. And if the power amps begin to ramp up, the power supply will start to cut the power. Could this be the reason your tuner is cutting out? Who knows, unless you give it the power it is supposed to receive , it could definitely be a factor. The specs says the unit consumes 24 watts. You are providing it only 15% of it's normal available current.

Heatsinks are large chunks of metal that is prone to absorbing interference. That's one reason why most all large metal parts are grounded. Electrically connecting power rails to a large heatsink will cause it to act as an antenna, not at all desired if you are trying to design a quiet circuit.

Connecting both transistors to the same heatsink is not a problem IF you had connected them properly which is to electrically isolate them from each other, and from the grounded heatsink, but mechanically connected to enable proper thermal transfer. With mica insulators. When have we heard that before?

A photo is probably helpful. You speak of heatsinks and what nots but we are just picturing (and guessing) what is conventional use. If the heatsink is located in the proximity of the regulators, then it's almost a surety that it is intended to dissipate heat from the regulators.

As for the switch... if you are convinced that you've cleaned it properly, then maybe that isn't your problem but part of the issue is that you are introducing factors into the mix and that skews everything. We presume that when you speak of symptoms, everything is in normal use. However, you are grounding things, shorting things, limiting power, etc.

Lastly, as for your motor running all the time... I will tell you right now that unless the motor switch (S13) is actuated, it is impossible for it to run unless the circuit has been modified based on the schematic. The exception is if capacitor C514 is shorted, in which case, the motor will run every time the function switch is set to P/L/R but not tape. The tape function REQUIRES S13 motor switch to be actuated. You can easily confirm whether the capacitor is shorted and if it is, either replace it or take it out of circuit. But if I recall, you claim the motor runs continuously if the mode is set to tape (and not radio?). If this is the case, then for certain, the switch needs to be looked at.

 

[beamrider]

The output from Q501 should be 12.6V, not 13.3V. ..

Sorry, yes you are correct. 13.3V was measured at the base next to ZD501.

As for the current limiter, it makes a click and displays a red light when active. This did not occur during the above tests.

Once again thanks for your reply. I will act on your recommendations (with no shorting/grounding) and feedback.

 

[beamrider]

So still battling with this...

Q502 is temporarily out of circuit.

Have 12.6V on output of Q501 which is also present on the input to R444L and R444R, however when I put them back in circuit, each resistor induces a 2V reduction at the output of Q501.

I wouldn't suspect identical faults on L/R channels so would this indicate that Q501 is suspect and dropping voltage under load?, even though it tested good before I soldered it in?

 

[beamrider]

okay, so turns out the replacement Q501 I got from E-bay had failed after being put in-circuit either it was dodgy or something is causing it to blow. Replaced it and now stable at 12.6v at emitter of Q051. Have a few more bits of the downstream circuit to connect back up again and then but Q502 back inline (with insulators!). Watch this space

 

[beamrider]

so Q502 back in and the voltage stable at 12.6V. Unfortunately, with moving the board around so much for investigation, all the brittle ribbon cables broke off, so I had to replace them all with individual wires.

So this weekend's job is to connect everything back up and do a test run. Then if it works, refix the heat-sink with mica insulators.

 

[beamrider]

so I replaced all the ribbon cables with JST connector blocks and new ribbon cables (the wires were too messy) and basically got the unit working again radio/tape. Then whilst working on adjusting the DPRS solenoid, the radio (FM)and DPRS just stopped working again. I hate this thing it's like snakes and ladders, every time I think the end in sight it throw another curve ball at me. Reading back through this thread, it looks like Q502 is suspect again - if so I've no idea why it keep failing.

 

[beamrider]

so it was Q502 again! - must have replaced this 4 times now, I didn't want to connect it back to the heatsink (with micas!) until I was confident the unit was fully operational as it's a real pain to get the heatsink off it it fails again during repair, but looks like I don't have a choice.