Thanks Fred for the input.

I replaced the transistors and fired up the amp with the lightbulb tester.

I found that Q329 and Q331 get warm. After some checking around I found that R347 which is in series with the IcQ pot is bad. I'll replace it and I'll be back shortly with updates.

Good to hear that.

The light-bulb tester has saved my shop hundreds of hours and thousands of dollars. Before I had one, I would gradually run the amplifier up on the variac. That was effective, but it took a long time, and variacs don't have current limiting, so from time to time I'd see smoke.

I used to work for Technics (Panasonic). From 1985-1991, I managed one of their company-owned factory servicenters. It paid very well, but it was an ultra-high-stress job. We didn't service much audio though. Mostly VCR's, camcorders, and TV's.

Fred
owner
Classic Audio Repair

Fred, it is very interesting to hear a bit of your background in this field. I always like hearing people who know their stuff well speak.

Okay here is a new update for you guys.

After replacing the resistor I no longer have the strong heating of the left channel transistors Q329 and Q331. However I still notice that these two get warmer in the same period of time than the corresponding right channel pair. I haven't left the amplifier operating for a very long period of time because I fear damaging it. But the transistors are no longer getting anywhere as hot as they used to in the same period of time, now I have just a very slight heat. Nevertheless again Q329 and Q333 are getting slightly warmer than their right channel pair in the same period of time.

I injected a 1khz tone and connected speakers. I tried to do the Icq adjustment (voltage adjustment is already ok) but the pots did not seem produce any different in the voltage reading and while the amp is now amplifying again both channels produce a hum. Sounds like 60hz and is more prominent on the left channel. This hum is not controllable by balance or volume controls. I'd say the right channel has about a rating of 60% audio output and maybe 40% hum. The left channel (the one we're working on) I'd say maybe 40% audio 60% hum. It's strange because at one point I had no hum on the left channel at all. I do no not suspect the audio cable feeding the amplifier.

I would very much appreciate some more help. I very thoroughly went through the left channel circuitry as in everything that comes before the driver transistors right up to around Q302 and Q304 just to verify and control all of that section. Everything there is fine.

Thanks again everyone for your patience and again your excellent help.
-Carlos

Okay so i found another bad resistor (totally missed that little sucker) and the heating problem is now gone. But I do still have that humming issue.

Still looking for suggestions everyone. Thanks.

I would recheck the transistors. That resistor may have caused damage to them.

It wasn't R363 (220 ohm), was it?

Hi Vince,

Nope it was R361, I had an extra replacement for Q329 so I decided to swap it out and see if that made any difference but that is a negatory, so it's not there.

Will have to see about the rest. Would this make sense for a bad transistor on the left channel to affect both channels? Also when I mean at one point I had no hum in that channel it was during one of the tests after I had discovered hum. In other words on a second or so try everything was perfectly clear in that channel. On another attempt for some reason it was back. Why do I always get the weirdest problems.

If one channel is drawing excessive current, it will cause the power supply to load down. This will induce a buzz in the normally operating channel due to feedthrough from the power supply (since they share the same power supply.)

This is a common phenomenon where someone has cranked up the bias in one channel, or where one channel is malfunctioning such that it draws excessive current.

However, excessive current is nearly always accompanied by heat!

* * * * *

Assuming you have a standard Class-AB complementary symmetry amplifier, or similar, and assuming that the output transistors and driver transistors are neither shorted nor leaky, the amount of "drive" to the driver and output transistors is a function of the forward voltage drop between the bases of the complementary driver transistors. This should be checked, and compared to the value from the "good" amplifier.

There is commonly a bias element located between the bases of the driver transistors, and the bias element is there to provide thermal feedback. This bias element is either a diode or a transistor, and is often called a "bias compensator." This compensator usually works in tandem with a potentiometer, that is used to "dial in" a specified bias current. Both the compensator (whether diode or transistor) and the potentiometer have substantial failure rates -- and should always be checked.

Fred
owner
Classic Audio Repair

Fred, once again let me tell you I very much appreciate your expertise and help. I'm learning a lot here. EICO you too.

I will control the components mentioned.

The bias diodes you mention must be 319 and 317 on my machine on this channel. I don't think I'll be finding the original part here they are MA27A2, "1.2V Bias Supply" according to another technics service manual. I can't find any other information on them beyond that. However they tested fine. I'm suspecting one of my driver transistors may have been damaged before I repaired the faulty resistor problem. I will re check the diodes however. Do you know a suitable replacement?

MA27A2 might be a variant of MA27A, which matches up to NTE601. NTE601 is listed as:

NTE601
Silicon Varistor
Temperature Compensating Diode

Here is the link to the NTE601 stats, so you can see if it matches the info you have:

http://www.nteinc.com/specs/600to699/pdf/nte601.pdf

However, the first part number listed in the manual, was SVDMA62-1, which NTE cross-references to NTE109, which is:

NTE109
Germanium Diode
Fast Switching General Purpose

Shown here:

http://www.nteinc.com/specs/100to199/pdf/nte109.pdf

BTW, just posted the section in question, since I keep checking your service manual:


Diode list:

Silicon varistors are actually bias diodes. The term "varistor" is often incorrectly applied. The devices in your machine are NOT variable resistors (hence the term "varistor.") They forward bias the driver and output stages to the correct value, and they also provide thermal feedback. Their variability is not in their resistance, but rather in their forward voltage characteristics, which decline by about 2mV for every centigrade degree rise in temperature.

Unfortunately, these diodes are getting harder and harder to find. In a pinch, you can use a 1N4148 to achieve about six-tenths of a volt, or two 1N4148's in series to achieve 1.2 volts -- although the 1N4148 is only an approximate match to a bias diode.

Fred
Classic Audio Repair

Thanks guys, the diodes are not bad and I still suspect the drive transistors being damaged during the time I had the overheating problem. Unfortunately I've been a bit busy (and a bit broke) and I've yet to get them. I will probably go tomorrow if not Saturday for sure.

Alright, update you guys.

I replaced those transistors and since I had a spare set of the power transistors I changed them to in case something was wrong with them. I noticed a couple of the driver transistors looked like cheap Chinese brands, replaced those with a different brand.

Audio clarity has improved but the crazy hum remains.

But alas, what on earth is this noise? Could the amp just simply be picking up line noise? I believe the hum might be 120hz or 60hz.

I asked the guy I got it from, original owner, if he had this problem while he used it and he says that his memory is a bit fuzzy but he seems to recall that it did do this before. But who knows how reliable that info is. It's only there and annoying at 0 volume and after you turn it up at around 25% it starts to get covered up.

Another thing to note is that the hum now seems to be equal on both sides. I'll get one more listen at that and report back. Perhaps the previous transistor problems where the ones causing the hum to be amplified differently in each channel making it seem like it was a left channel issue.

I think this is going to turn out to be the big bad supply capacitors C408 and C409. I resented swapping them out from the beginning with new ones because they are pretty expensive cans! And with the strange problems I was having I was unsure and fearing the worst I suppose.

Sorry I am kinda running around with a chicken with it's head cut off sometimes, so I apologize for the gaps in communication my friends.

For a moment I thought that it could be dirty DC bias~via Tone switch on the front panel. When I pressed it a few times it seemed to make the hum go away and at one point it went away completely until I pressed it again. I took it out of the board and cleaned it up but had no results. I don't know what the heck that was all about!





-Carlos

Alright guys here is one more update.

I changed out the filter cans and that made a HUGE difference. There is still a slight hum but it's been reduced by I'd say 90%. I'm attributing the remaining hum with maybe some other dried out electrolytics in the power supply and/or all the interference coming from the AC lines.

I'm still in test mode but I've had the device playing for an hour and a half or so at low volume and so far so good.

Thanks everyone for the great help, I'll get back to you when I've tested it and over viewed thoroughly.

Okay update number 2.

Amp is operating great with the exception of the hum. I have not been able to locate the source. I checked out the capacitors related to the power supply but they are okay. I don't see any signs of excessive current draw anywhere. It's the last little detail left. Anyone have any ideas?

I got done adjusting the biasing the other day to which I forgot to mention. 2mv 10 minutes after initial power up as stated in the manual.

Other than that again I must say it works and sounds great. The hum gets covered up at around 10% volume.

Apologies for the duplicate replies but it looks like the party is over once again. I've been trouble shooting this thing to try and find the fault but so far no luck. Unfortunately last night while I had the amp running for a while for testing I shut it off and it didn't come back alive. Q334 went between collector and emitter. Ugh great. Now I really need to figure out what's going on with this hum. I've been going over this thing with a FINE toothed comb. I've tested and verified about 85% of all the components and so far I've found nothing bad. This includes every single electrolytic.

For some reason I no longer have any rail voltage getting to the transistors. I assume though that this is possibly circuit protection kicking in due to the short. Please correct me if I'm wrong.

Could D405 and D407 have something to do with this? Meaning the hum problem. I've been over pretty much the entire power supply itself as well, with the exception of a few resistors I need to double check, and have found nothing wrong. I don't really have the appropriate equipment to test out D407 and D405 which are SCRs, I tried using a technique I found online but it doesn't seem to work for me. It could either mean these things are bad or I just simply cannot perform the test with what I have.

I also have a question concerning the biasing. I set the amp to the manual specified voltage on the specified taps for IcQ but doing some monitoring I notice the value shifts up as the temperature rises. The amplifier has not really gotten very hot at all (still in test mode) but I'm concerned about what the biasing is supposed to act like during normal operation. This shift I notice happens after I return the volume knob back to "0". Is it normal for it to increment during use? The biggest shift I've seen so far is from the 2mv to around 5mv. It doesn't stay that way for very long once the volume is turned down. Is this normal or is it supposed to be constant? I do take into account what Fred has told me about the biasing diodes and thermal feedback, but perhaps I need a little bit more information.

Thanks everybody for your patience.
Advice, as always, is much appreciate it.
The challenge continues!

I think there are better qualified people who can answer your questions much better, but if you have a scope, you can use that to trace the 60hz noise. I am not sure if I am reading this right, but Q334 looks to me to be a PNP transistor, with the emitter going to ground and the collector going to -B voltage? How was the voltage there? There is a Test Point neat by (TP304) which is related to the right channel's icq. Perhaps there is a connection there, as you mention the voltage drifting? I'd double check C318, R374 and C342.

On page 4 of your service manual, it states "if protection relay turns OFF due to overload, the circuit will not restore unless power supply is once turned OFF and then again turned ON.". I believe this simply means if you turn it off, wait a sec or so, and turn it on, it resets. Perhaps it is not resetting and this is causing the lack of voltage? Kinda sounds like it. See page 4 on info for setting the supply voltage plus detection and overload circuits.




Did you follow the instructions on page for for adjusting the voltages with a resistor, followed by setting the icq, 10 minutes after the power has been on? The drift could be from not waiting for 10 minutes, or could be the power voltages are not set correctly?

Is there heat sink grease on the 2 diodes you mentioned? Hopefully it's not dried up. How did you test the SCRs? Using a voltmeter? I think there is also a way using a flashlight, but I'm not sure what the wireing for it is. You can check on youtube for that. Try "SCR flashlight".

I'm not sure what else I can say. Hope you get it working!

Vince, thank you VERY much for your kind support.

I have made some advancements. I should clarify that when the amp went dead I didn't notice until the next night. It turns out someone "borrowed" the amp without my permission. A younger relative of mine. So due to this I don't know if the damage was due to some messing around from his part or a technical issue. I do know he wanted to test some speakers. HMM!!!!

I followed the instructions correctly for the biasing adjustment and did wait the ten minutes recommended by the manual for setting the IcQ. All the voltages are okay.

Most importantly I believe I have discovered the source of our hum. It's quite interesting. The problem I believe was R371. Load resistor? While going over the amp I discovered that this thing was not correctly soldered to the board. When I measured it's resistance it measured correctly but I ignored the fact that if it was soldered on the board correctly it would read at around 1 ohm or less due to it being parallel to a coil. Since it read the whole 10 ohms of it's value this meant that it wasn't properly connected. The tricky thing was that both on top of and underneath of the board the darn thing looked like it was correctly soldered. It must have slipped out during the solder cool down because it left the indent of it's lead in the solder pad making it look like the lead was sticking out and covered with solder. You REALLY have to pay very close attention sometimes don't you?? So I have read that shorted capacitors and bad resistors in this section can cause hum and since I've practically been over the entire thing without finding any other bad components, I'm pinning it on this one.

I have yet to re-power the amp to see if it will work without the hum.

I've still got my doubts on the bias drift. While I did my testing after about half an hour or less of playing music at around 45% volume when returning the knob back to "0" the biasing read 5mv or so. It usually would not take very long for it to drop back to normal though. Still long enough to cause me some concern however. What I don't know is if the thermal feedback circuits/components are there to compensate during the heating of the amplifier or if they're supposed to help it always stay at 2mv.

The SCRs where tested with my multimeter. I found a circuit for testing SCRs using a flashlight lamp and I'll apply it to test these things out. I'll let you know what happens there as well. Oh and yes, everything that needed it got a nice fresh dab of thermal paste.

I will report back to you with any more information. I'm very willing to get this thing going because I'm very interested in learning about these circuits and how they work. To me there's no better way of learning something than to be right there meddling with it hands deep in resistors and other components. So It's not so much about restoring the amp as it is about learning something new.

Thanks very much my friend!