Amp hot enough to melt its own solder?

'restorer John's post about same just to add more than just the 'red squared' noted joints. I'd almost bet if the solder pads were cleaned (protective stuff from supplier removed) the solder would have bonded to the solder pad an not leave the 'rings'. note too that most older soldering was manual and took some skill. (get the damned thing out the door!)

If not capable of installing some heat sinks... simple fan cooling does wonders, too.
 
It may be the case that the TO-220's are the ones producing most of the heat. Before going to the trouble of adding heatsinks on any of the TO-92's, I would suggest verifying that they are indeed sources of significant heat.
 
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Does this count as highly localized?

No, that isn't what I was thinking about. Yes, it is localized to that general area, but what I meant by highly localized was more like this, which is what I found on the TO-220s on the 18V circuit on my SR-2A (Q405, which corresponds to Q207 or Q210 in your TA-3A). I was able to scrape some of that off (might have been some glue or other compound applied at the factory, or just blistering?), but the brownish stain you see around the edges remained. In any case, this would be consistent with the heat from Q405 being carried down to the board by the legs, and the board damage being highly localized around the legs. There was barely noticeable staining on the underside of the board at these spots, and solders (or surrounding ones) were not ruined, as yours seem to be.Q405.JPG

I don't have a TA-3A so I can't compare directly, but my two TA-2As do not have what you show on the boards or even more local staining, and I continue to be a bit perplexed as to why your nearly black stain extends so far and so evenly from the devices. Perhaps someone with more repair experience can shed some light on this.

In the TA units, as you know, there is an logic board overlying the area of the 18V circuit, which does not help heat dissipation, but still, it seems odd. Looking at the schematics, I also note that in the TA-3A circuit there are 4 TO-220 devices, whereas in the TA-2A there are 2 TO-220 and 2 smaller, lower-power TO-92 or related devices in the equivalent positions. So there probably is more heat being dissipated by this circuit in the TA-3A, but I am reticent to push the analogies given the differences (perhaps I've gone too far already).

Nonetheless, I don't think what you are observing is normal (I think the Nak engineers of that era were better than that). That is why I was wondering if there might have been an episode of overheating, such as the air vents on the top cover being temporarily covered at some point by some papers, etc., or some other event that caused that. Just a thought.
 
@northpaw, you raise several points:
- there is a logic board above heat-affected area on my main PCB, and it shows the effects of having been heated from below. I agree that having the logic board directly above does no favors to the cooling processes of the components on the main board.
- my stain doesn't look so much like "these two transistors got hot at these six wires" but rather "a pretty hot thing radiated heat for some time, over this entire are" -- it looks like what would happen if you applied a heat gun to the area for an hour. It makes me wonder whether R269 (the 2 Watt, 220 Ohm resistor parallel to the speaker relay) got really hot and just radiated heat over the board. That resistor is on long legs and sits .5 to .75 inches above the board, and it seems like it is in position to radiate heat in such a way as to produce the scorch mark I have. Plus, check out the "shadow" on the side of the speaker relay.
eW6lqGg.png

- I am not aware of the machine having had obstructed cooling, but it literally ran continuously for more than a decade in my living room (we'd just leave it on).
 
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Oh, and I just noticed while taking that photo that Q211 is loose on all three legs; it's just being held in by friction. Same with one side of R268.
 
I agree with your thinking -- something getting hot and spreading some serious heat -- but, boy, if it was that resistor, I would have thought that the plastic case to your speaker relay would be a pile of goo. But if that "shadow" isn't a shadow, but some melting, then it is certainly a candidate. It would be a good idea to check its resistance. Also check out the general situation with R262 (should be off to the right in your photo, with one leg showing), as it seems to play an identical role for Q407.

As an aside, if this is the first repair to this unit, there are two other things that have been reported here a few times for the TA series. The first is that one channel or the other goes intermittent, which could be the speaker relay going, but Freddimac here has argued persuasively that this is due to failed solders on the relay pins on the underside of the board (probably the result of the repeated vibration and perhaps a contribution from less-than-great wave soldering). While you are having your solder party, it would be good to proactively hit those.

The second may just apply to Q525 on the TA-2A, although the same device with the same number is present in the TA-3A. A Nak factory service bulletin from 8 Feb 1990 indicates that Q525 on the logic (top) board (a 2SA733) is apparently a bit undersized for what it does and will cause some mayhem if and when it goes. A suitable and currently available replacement is PN2907A-AP (recommended in an older post here somewhere, and the one I used); it just needs to have the pins folded to get the right ECB pin pattern.

However, I also just looked at the TA-3A service manual (last page), and it has a different service bulletin from 8 Feb 1990 addressing the same type of problem. It notes that Q208 and Q209 on the main board should be upgraded (one of them is a 2SA733, and one a 2SC945). Now, you want to guess where on the main board Q208 and Q209 are located? Yep, the same area you are working, next to the speaker relay.

So perhaps the root of your issue is with those two transistors. If you don't have access to that factory bulletin, let me know and I will make a copy and post it, but it should be the last page of the download of the TA-3 service manual from HiFi engine.
 
I have that service bulletin and have ordered the upgraded transistors to replace Q208/9, along with most of the rest of the elements in that area of the board.
 
I have not read the whole thread. But in general, you should not have burn like this in normal amp with good design. I don't have schematic, so I can't comment. But just changing to an updated transistor of the same package is not going to solve the problem, if the transistor is hot, mount a tiny heatsink for TO-220 on the original transistor.

But the burn area is much bigger than the transistor, you might have other components that got heat up. You really need schematic and go through the voltages to determine whether it was designed like this OR something gone wrong that cause the problem.
 
Alan, those should be Q210 and Q211.

The service bulletin indicates that Q208 and Q209 may cause the circuit to fail due to inadequate power handling and cause transistor damage (no specifics). You're a talented EE so you can probably figure what the truth is; my suspicion is that the entire circuit in this area was misbehaving due to the problem with Q208/9, and either caused or interacted with the solder failures (illustrated in the first post, but additional ones commented on in post 26), and probably the entire region got hot.
 
Hi

I read the schematic a little. Q210 and 211 supply -18V. Q210 supply -18V to phono stage, Q211 just general -18V. From calculation shown, current through R269 is 73mA, voltage drop across R269 is 53-37=16V. I would first measure to verify about 16V drop across R269 to confirm it is working normally according to the schematic. If more than 16V, something else is drawing too much current.

But even it is normal. Power dissipate in R269 = 16V X 0.073A=1.16W. It's going to get really hot. The only answer is to replace with two 2W resistors, one 100ohm and 120ohm in series to get 220ohm.

Q210 Q211.JPG

I would put a small heat sink on Q210 and 211. They have really small heatsink.


Of cause, you can increase the value of R269 a little. Right now, the voltage drop Q210 and 211 is 37-18V= 19V. You really don't need that much voltage overhead. If you increase R269 to say 330 ohm, use 150ohm in series with 180ohm 2W resistor, you drop 24V across the resistors. Then you only drop 10V across Q210 and 211, you don't need to do anything on the transistors. But it's up to you.





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R262 is the counter part of R269, same idea applies.
 
Regarding to Q208 and 209, R265 and R266. It's not too bad. Check the voltage according to the schematic, if the voltages are correct, it should be no problem. I did the calculation as long as R265 and 266 is 1/2W resistors, you are ok. Q208 and 209 dissipating 0.3W each according to the voltages shown in the schematic, they will get warm, but it's ok.

Q210 Q211.JPG
 
Alan, thanks for your quick analysis of this circuit. It reminds me that if I want to develop a better understanding of these circuits, I need to be more analytical, more willing to stop looking at a screen and get out a pad and paper and do the simple math. After all, this is nothing more than what could be in a question in the first exercise in an introductory course in electronics. I never had one, beyond what I got in the Electricity and Magnetism part of Intro Physics, which was more theoretical and a long time ago.
 
IMO and 'restorer john's' observations.. granted those transistors in question and heat sinks on order, but I've seen equivalent soldering on other models as I'm sure John has, too because the op pics are pretty good but all the associated circuits to the transistors solder pads don't look 'healthy'.
 
Alan, thanks for your quick analysis of this circuit. It reminds me that if I want to develop a better understanding of these circuits, I need to be more analytical, more willing to stop looking at a screen and get out a pad and paper and do the simple math. After all, this is nothing more than what could be in a question in the first exercise in an introductory course in electronics. I never had one, beyond what I got in the Electricity and Magnetism part of Intro Physics, which was more theoretical and a long time ago.
It is actually very simple if people stop and do calculation. I did not even verify how much current drawn from the other circuit, I just take the voltage reading given by the schematic and calculate to find out the resistors dissipate over 1.1W and that will get piping hot already.

I really think people that are interested in restoring and get into trouble shooting should spend a little time to read the transistor basics. Get the book by Malvino.https://www.amazon.com/Electronic-P...e=UTF8&qid=1502990378&sr=1-3&keywords=malvino

It is a very easy book to read, it shows how to separate the transistor circuits in DC equivalent and AC equivalent and how to calculate the voltages. I started my career with this book, I never go to school on electronics at the time, just bought the book and studied and got my first job as technician, then I got promoted to EE in 1980. I did studied back all the rest, but this book got me into my career and I give full credit to this book. The link I gave, you can buy a very good condition used book for less than $10, it's a must have book if you are interested in amplifiers. Everything I did in this circuit is covered in the book ( in early chapters if I can recall).
 
Thanks for the suggestion. I have been reading through "Mastering Electronics" by Watson, but am finding it only partially satisfying (some explanations lack clarity and/or thoroughness; thin in many areas of interest because it tries to cover so much ground). I've give the Malvino text a go.

Apologies to Jupitersspot for the diversion, while you await parts...
 
Malvino is very clear and easy to understand. That was my very first book on electronics and I had no prior knowledge of SS electronics at the time. I got the reference from Heald College as they were using this book for their class. I even got a job designing analog IC using the knowledge from the book.

Hey, the used ones are so cheap!!! I have very good luck buying used books on Amazon, they all are in super good condition. I never buy new books because I'll mess them up fast by putting all my own notes in it.
 
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