Question about biasing SS amplifier and blowing output transistors, ONKYO TX-SR508

Thread Starter

Yami

Joined Jan 18, 2016
335
Hi guys,
Just wanted to clarify some doubts I had with biasing issues on SS amplifiers. I am really interested on working on amplifiers and anything related to audio. I have been reading on amplifiers.

So here is the series of events on the project I am working on:
  1. Output transistor Q6050 was shorted and one half of the emitter resistor R6100 was open (left channel) and the B+ fuses were open.
  2. So I borrowed the two output transistors and the emitter resistor from a working channel which was not used. Replaced the fuses however I didn’t replace the driver transistors(I should have!).
  3. Powered on the unit and measured the biasing voltage at the test points and it was within specs given on the SM. However I should add that I didn’t monitor it long enough.
  4. Played some signal through it and it was working perfectly fine.
  5. So assembled the unit and returned it to the owner to be used until the replacement parts arrived.
  6. Checked it at his place, was working fine and after I got back he called me back saying that the amp had failed and that there was no output.
  7. I checked the amplifier again the same left channel had shorted. The two output transistors were shorted and the B+ voltage fuses were blown. The emitter resistor was intact this time though.

As per what I have read online regarding what could blow the output stage (some reasons):
  1. The speaker being shorted - I checked the speakers and I didn’t measure a short across the terminals. However it had a crossover circuit and only what I could read was a capacitor charging and both speakers showed similar behaviour. The L and R speaker sounded same to me when it was working.
  2. Cold/broken solder joints - I have checked for these and didn’t come across any obvious points.
  3. Other components failed - I have checked almost all the components on the left channel and it measures ok.
  4. Biasing issues.

I would like to clarify about the biasing circuit used on this amp. I have done some research on Vbe multiplier circuits but most of the circuits I came across the variable resistor is connected to the base of the Vbe multiplier. However in this unit the VR is connected to another transistor’s base Q6010. I am finding it a bit hard to grasp how this works. Could someone kindly explain.

Q6010 is placed next to one of the drive transistor and it has got what looks like thermal compound applied to it. What is the reason for this?

If it was a biasing issue and somehow it caused the output transistors to short out, note B-C-E were shorted - why didn’t it take out the drive transistors? The drive transistors still checks out ok.

All of the components related to the biasing part of the channel checks out ok but hypothetically speaking what component failure for e.g.: the potentiometer’s contacts loosing connection, some kind of thermal effect messing up with the biasing circuitry’s transistors?.

If its some kind of an issue which only causes the fault when power is applied what further troubleshooting should I be doing on this circuit? At the moment I am waiting for the replacement parts to arrive but meanwhile I would like to figure out what exactly caused the fault - could I power up the amp without the output transistors and see if the proper voltages are preset which is shown on the SM and lets say if the drive transistors are faulty, ‘leaky’ for instance would I be able to measure that?

Based on what I have detailed above, what’s your opinion for the reason for the fault to occur?
Thanks in advance for the help.

P.S - I couldn’t find replacement for some of the transistors and resistor so here are the equivalents that I have ordered
2SC1740S-S ——— 2SC2240
2SA1930 ——— A1837
0.22 Ohm 2W emitter resistors —— 0.22 Ohm 5W
Also note that there are some film capacitors which looks a bit black, I am not sure it was like it to start with. It measures ok especially when compared to a working circuit. Hope you can see it in the photo
 

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Martin_R

Joined Aug 28, 2019
27
Hi, Q6010 transistor is the one doing the thermal tracking of the output transistors, that's why it has thermal paste applied to it. As regards to the output transistor failure, a possible area for concern would be the bias adjustment potentiometer R6040, possible noisy wiper could cause bias fluctuations. Have you got an oscilloscope? The amplifier could be oscillating at high frequency, worth checking.
 

Thread Starter

Yami

Joined Jan 18, 2016
335
Hi, Q6010 transistor is the one doing the thermal tracking of the output transistors, that's why it has thermal paste applied to it. As regards to the output transistor failure, a possible area for concern would be the bias adjustment potentiometer R6040, possible noisy wiper could cause bias fluctuations. Have you got an oscilloscope? The amplifier could be oscillating at high frequency, worth checking.
Thanks @Martin_R so only purpose of Q6010 is just to track temperature? Sorry for a such a noob question but how does it effect the bias lets say when the driver transistors get hot? I thought it was doing some sort of 'variable resistive'(I hope I made sense) thing parallel to the Vbe multiplier.
Unfortunately haven't got access to a scope at the moment :( , I have also read about this potential oscillation - would that be seen on the output when viewed on a scope?
I have checked R6040 for proper operation and measured and the needle rises and falls steadily without any erratic movement on my ohms meter when the wiper is moved. Maybe should I hit with Deoxit?
 

Martin_R

Joined Aug 28, 2019
27
Thanks @Martin_R so only purpose of Q6010 is just to track temperature? Sorry for a such a noob question but how does it effect the bias lets say when the driver transistors get hot?
Unfortunately haven't got access to a scope at the moment :( , I have also read about this potential oscillation - would that be seen on the output when viewed on a scope?
I have checked R6040 for proper operation and measured on the meter the needle rises and falls steadily without any erratic movement on my ohms meter when the wiper is moved. Maybe should I hit with Deoxit?
Switch cleaner on R6040 wouldn't hurt at all. Q6010 isn't 'just' to track temperature, it's function is to compensate for the output stage when it gets hot. As the output transistors warm up the vbe of the transistor reduces, this in turn causes the transistor to pass more current, get hotter,vbe gets less, more current flows, gets hotter, if you're unlucky they short circuit - I think you get the picture. The vbe multiplier Q6010 job is to divert some of the output transistor drive current by reducing the voltage across the output transistor push pull pair.
 

Martin_R

Joined Aug 28, 2019
27
Oh, while I think about it, are there any parts on the PCB that look to have run hot and discoloured the PCB? It doesn't necessarily means there's a fault, some components may run warm / hot in normal operation, but can be the cause of dry joints ( I think you refer to them as cold joints) Run the soldering iron over them if that's the case, and remake the joint.
 

Thread Starter

Yami

Joined Jan 18, 2016
335
From looking at the board it seems the heat has affected it, slightly more brown on the amplifier section compared to other sections. I have looked for bad joints couldn't find any but I'll look again.
 

Dodgydave

Joined Jun 22, 2012
9,637
As said Q6010 is the thermal transistor, that when it gets hot pull the two bases of Q6030 and Q6040 together and thus lowers the base biases and shuts the current down,

As for what caused the fault, speaker shorts, blown output capacitors, over voltage on the PSU are some reasons.
 

Thread Starter

Yami

Joined Jan 18, 2016
335
As said Q6010 is the thermal transistor, that when it gets hot pull the two bases of Q6030 and Q6040 together and thus lowers the base biases and shuts the current down,

As for what caused the fault, speaker shorts, blown output capacitors, over voltage on the PSU are some reasons.
Thanks @Dodgydave, by output capacitors do you mean the cap across the base of the output transistors C6040 in this case? and would PSU issues affect only one channel?

I'd like to add one more point for completion sake, when I was first testing out the amplifier on my bench after installing the borrowed transistors and resistor from the unused channel I heard a relay click while I was playing music, nothing ill happened the amp was working fine. I turned off the amp and back on again was playing music and after couple of seconds I heard the relay click. I didn't figure out what it was. Its not the speaker relay which clicks when the amp is first turned on. What could this be? The amp is an Onkyo SR508
 

Martin_R

Joined Aug 28, 2019
27
Hi again Yami,
Did a search on this amp, it's quite a beast, isn't it? 7 Channels to play with! I noticed on the circuit diagram there are some zobel compensation components that you should check, for the left channel check R6130 and cap C6030. Also check the soldering to these. A fault with these would possibly cause the amp to be unstable on a load. The relay you heard click was probably the speaker protection relay dropping out, just before the transistors went into meltdown.zobel components.JPG
 

JoeJester

Joined Apr 26, 2005
4,390
On another thread, there is a suspect of counterfeit transistors. Where did you get your stock? Both your part numbers showed up in a search as potential counterfeits.

If you need to test the channel to see if anything preceding is causing the problem, you can replace the base-emitter junction with a diode like a 1n4148 so you can do some tests to ensure nothing unusual is overdriving the outputs. Just dont expect to adjust it to the normal current setting.
 

Thread Starter

Yami

Joined Jan 18, 2016
335
Hi again Yami,
Did a search on this amp, it's quite a beast, isn't it? 7 Channels to play with! I noticed on the circuit diagram there are some zobel compensation components that you should check, for the left channel check R6130 and cap C6030. Also check the soldering to these. A fault with these would possibly cause the amp to be unstable on a load. The relay you heard click was probably the speaker protection relay dropping out, just before the transistors went into meltdown.View attachment 219700
Thanks so much @Martin_R, thanks for the advice I am going to closely look at that pcb and components. Regarding the relay click - the music kept on playing even after I heard the relay click. I have a suspicion that it was the 'voltage shifting' relays. RL6091 and RL6902. Whats the reason for this B+ voltage change? and when is it suppose to happen maybe at certain temp or volume? When I heard the relay click I wasn't changing the volume. The SM shows that the control signal for this is 'SEC1H' I can't figure what this is. Also one more important thing I just realized is that the two fuses which blew were F6901D and F6902D those are on the low voltage path.
PS Switch .jpg
 

Thread Starter

Yami

Joined Jan 18, 2016
335
On another thread, there is a suspect of counterfeit transistors. Where did you get your stock? Both your part numbers showed up in a search as potential counterfeits.

If you need to test the channel to see if anything preceding is causing the problem, you can replace the base-emitter junction with a diode like a 1n4148 so you can do some tests to ensure nothing unusual is overdriving the outputs. Just dont expect to adjust it to the normal current setting.
Thanks @JoeJester, I still haven't received the ordered parts. I also came across what you mentioned. That's why I ordered from Digikey.
Thanks for the tip about replacing the base-emitter junction with a diode. You mean the output transistor's B-E junction right? Could you please elaborate on the procedures/measurements I could do. Thanks
 

JoeJester

Joined Apr 26, 2005
4,390
Remove the output transistors. Insert a diode in the B-E connections ensuring to maintain proper polarity. No use blowing up a transistor when a 10 cent diode will do and you don't add excessive stress on the high voltage supply. On this schematic, you see a voltmeter Labeled TP501. That is the same topology as your biasing test point. You are biasing less than class-B operation so you don't add cross-over distortion.

Here is where we did it on a different amplifier. After all the B-E junction acts like a diode. Place your scope at VF1 and VF2.

diode insertion.jpg
 
Last edited:

Martin_R

Joined Aug 28, 2019
27
Thanks so much @Martin_R, thanks for the advice I am going to closely look at that pcb and components. Regarding the relay click - the music kept on playing even after I heard the relay click. I have a suspicion that it was the 'voltage shifting' relays. RL6091 and RL6902. Whats the reason for this B+ voltage change? and when is it suppose to happen maybe at certain temp or volume? When I heard the relay click I wasn't changing the volume. The SM shows that the control signal for this is 'SEC1H' I can't figure what this is. Also one more important thing I just realized is that the two fuses which blew were F6901D and F6902D those are on the low voltage path.
View attachment 219722
Thanks so much @Martin_R, thanks for the advice I am going to closely look at that pcb and components. Regarding the relay click - the music kept on playing even after I heard the relay click. I have a suspicion that it was the 'voltage shifting' relays. RL6091 and RL6902. Whats the reason for this B+ voltage change? and when is it suppose to happen maybe at certain temp or volume? When I heard the relay click I wasn't changing the volume. The SM shows that the control signal for this is 'SEC1H' I can't figure what this is. Also one more important thing I just realized is that the two fuses which blew were F6901D and F6902D those are on the low voltage path.
I hadn't spotted that relay, and yes it does look like it changes the supply voltage to the amp. Not seen this arrangement on any amp before, so I'm making an assumption here, that the supply voltage is reduced during no signal and/or low volume levels to reduce heat build up in the output stages, though in the latter case the amp wouldn't handle transient power very well. It's odd that only one channel out of 7 seems to be the problem, very strange, so my attention wouldn't be on the power supplies, though I may be wrong.
As said in post #11, the output transistors could be replaced with forward biased diodes connected between E - B. Bias adjustment should alter the voltage measured across R6070, but don't adjust it above about 1.5v for fear of putting too much current through the driver transistors. Output voltage across the speaker terminals could also be measured for stability, with no input signal. Really need to 'scope the output to make sure it's not oscillating.
Oh, just for good measure, see attached image for the output inductor and relay, and check to see if they're ok too.
 

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Thread Starter

Yami

Joined Jan 18, 2016
335
@JoeJester & @Martin_R I wonder what would happen if I put a signal through with the diodes in? As I have read that people play a signal through with the output transistors removed and it would be something like a 10 Watt-ish amplifier. So I wonder what would happen with the diodes in (asking out of curiosity).
Such a brilliant tip with temporarily replacing the OT with a diode and troubleshooting issues, gonna come in handy for the future as well.

Thanks so much
 

Martin_R

Joined Aug 28, 2019
27
It will work, but be very cautious about over driving the driver transistors, normally they would provide the base current to the output transistors, maybe a couple of hundred mA on signal peaks, but if you missadjust the bias setting the driver transistors could be destroyed, same with cranking the volume level up.
 

JoeJester

Joined Apr 26, 2005
4,390
Well, nothing will happen. The point of those are for you to see if there is an oscillation or spike going into the finals to cause a failure. There is no current amplification going on, so at best, you can cause the diodes to fail with sufficient drive. All those diodes do is provide a path to the load so the voltage readings will be closer to normal and allow you to do some testing without blowing up another set of output transistors.

Set the bias potentiometer to the same physical position as the other one. You can do the measurements, but you will find the test point voltage (measured similarly like the TP501 connection in the schematic above) to be much lower than if there were current amplifiers in the output. In the set we initially did that test in was the TP was about 20% of the normal value with the potentiometer set at the same physical position as the other channel.
 

Thread Starter

Yami

Joined Jan 18, 2016
335
One more question :) I am going through Bob Cordell's book - Designing Audio Power Amplifiers. I am trying to figure out stages of the onkyo amplifier and trying to find similarities from the examples given in the book.(I haven't gotten far into the book yet)
Please see the attach photo, the pink dotted line I have drawn is the negative feedback path right? R5030,R5230 (why have two resistors in parallel would a lot of current be flowing in this path)&R5200 forms the voltage divider for this path right? I read from somewhere that issues on this path could also take out the output transistors. I have tested the components on this path but would an issue on this path cause the output transistors to blow - or an issue this path would only introduce DC offset?

Thanks for the help
 

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Dodgydave

Joined Jun 22, 2012
9,637
It's not uncommon to have same value resistors in parallel , mainly due to pick and place machine space , there is limit as to how many components they have , and so doubling up on components they already have is easier.
 
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