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

Martin_R

Joined Aug 28, 2019
26
R5040 is the other part of the voltage divider, not R5200. The other resistors are parallel connected probably because to have a resistance that's 1/2 the value would probably require a higher wattage component, at greater cost than 2 lower power resistors.
 

Thread Starter

Yami

Joined Jan 18, 2016
332
Some further developments/discoveries, after some more digging around. I have soldered the diodes as suggest (1N4148)

I noticed that C6040 and C5050 (caps across the base’s of OT & drivers) reads slightly higher ESR value compared to a good channel. It was around 1.9Ohms. My meter indicates that cap is good <47 uF cap. Its sort of in the good zone so to speak. It says ‘audio’ on the capacitor’s label - I am pretty sure that I would not be able to source one of those from the local shop. First of all could this be an issue enough to blow the OT if so could I replace it with a non-‘audio’ capacitor?

I took out the driver transistors out of circuit and measured the resistances and compared it to new transistors and they were identical. So I am planning to keep the old one, mainly because I am not sure whether the new transistors I got from the local shop is a genuine part. What are your thoughts on keeping the old drive transistors as they are measuring ok.

I took out Q6010 out of circuit to measure it - there was thermal compound on it but it had more of a silicone like consistency. The compound had dripped onto the leads of the transistor. There was no dripping on to the leads of the same transistors on other channels. The compound when measured was not conductive like how some glues becomes conductive after time. Since 2SC1740S are obsolete the replacement suggest by people were 2SC2240. I measured resistance measurement with the one I took out and compared it to a new 2SC2240 and they were identical as well except for one measurement. Measured across the C and E using an analog meter at x10k (the highest my meter goes to) red probe on C and black probe on E - the old one appears to be more ‘conductive’ than the new one. When the probes are reversed there is no ‘conduction’ on either one. I did clean the compound off the leads before taking the readings.

I haven’t powered the amp yet to measure the voltages as I have to partly assemble the unit to do so. Will continue tomorrow.

Thanks again for the time.
 

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JoeJester

Joined Apr 26, 2005
4,361
I measured resistance measurement with the one I took out and compared it to a new 2SC2240 and they were identical as well except for one measurement. Measured across the C and E using an analog meter at x10k (the highest my meter goes to) red probe on C and black probe on E - the old one appears to be more ‘conductive’ than the new one.
What is the model number of your analog meter?
 

JoeJester

Joined Apr 26, 2005
4,361
Thanks for telling me about your meter. The only concern I would have is if you used it to measure voltages in a higher impedance area as the could be loaded down by the meter causing an incorrect reading.

My other concern with the meter, which was proven unnecessary, was the current you were applying when testing the transistors. For instance the classical SIMPSON 260 analog, 20k/volt meter could supply something like over a couple hundred milliamps on one of the resistance settings. I wouldn't want to guess how many transistors that were made defective till the found out that the meter was the culprit.

Its just a reminder to me to be mindful that not everyone will be using a 10 M ohm input impedance digital multimeter.
 

Thread Starter

Yami

Joined Jan 18, 2016
332
Thanks for telling me about your meter. The only concern I would have is if you used it to measure voltages in a higher impedance area as the could be loaded down by the meter causing an incorrect reading.

My other concern with the meter, which was proven unnecessary, was the current you were applying when testing the transistors. For instance the classical SIMPSON 260 analog, 20k/volt meter could supply something like over a couple hundred milliamps on one of the resistance settings. I wouldn't want to guess how many transistors that were made defective till the found out that the meter was the culprit.

Its just a reminder to me to be mindful that not everyone will be using a 10 M ohm input impedance digital multimeter.
Thanks @JoeJester, I use a Fluke 179 DMM. The before mentioned voltages were measured using it.
I was advised by two local techs on two different occasions that I should be checking transistors on an analog meter - then only would I be able to measure what they called 'leakage'. I tried searching online as to why that was the case but couldn't find any resources. I have been meaning to do a post on it for sometime. I thought it might have been a trade secret or something.
Here is the procedure I used for measuring the above mentioned transistors in this case:
1. Used diode mode on the fluke = all the results were ok.
2.Used ohms mode on the fluke = couldn't read any values when probes were attached reversed biased.
3.Finally used the analog meter = results seems to be identical compared to a good transistor and what the techs had showed me before.
I used that procedure when I checked all the transistors on this channel.
 

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Thread Starter

Yami

Joined Jan 18, 2016
332
Right so I powered up with the amp, with the didoes in place. Left it for maybe half an hour so nothing blew :)
So I replaced them with transistors borrowed from yet another unused channel and I wanted to monitor anything happening on the bias. So I connected some jumper wires from the test point and had series 100 ohm resistors on each wire. Powered up the amp - and it seems to follow what the SM described. However at one point my heart sank when I saw 254mv! I quickly turned off the amp. I touched the 'test' resistors and they seem to be slightly hot as well :S. But after thinking about it for a while and checking whether anything had shorted I realised that it was due to the fact that my test probes weren't making proper contact with the test point. PHEW!
So I powered back the amp again and monitored the biasing and it followed almost exactly what the SM describes. 2.5mV on startup and after couple of minutes goes to around 9 to 11mV and stabilizes. So is it safe to say that the biasing is ok?
Another thing I want to remark is I measured something around 55mV of DC offset on the right channel and something around 30mV on the left channel. There are no adjust points for the DC offset. My question are:
1.Does DC offset got anything to do with the biasing or biasing circuitry?
2.Are the above mentioned values acceptable for the DC offset?
3.Could the DC offset be taking out my output transistors? Maybe something in the crossover circuitry of the speaker shunting to ground putting a short across the terminals. (just thinking out loud)
 
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JoeJester

Joined Apr 26, 2005
4,361
What did you monitor when the diodes were in the circuit?

There will be a DC voltage because you adjusted the bias to have the diodes conducting. It should be consistent with the readings on the service manual at the emitter of the preceding transistor near 0.6V.

If you monitored with an oscilloscope ... did you see any thing more than the dc bias that got the diodes to conduct? If you didn't, do you have access to an oscilloscope?

Those transistors failed for a reason. Counterfeits could be one reason, unless they were the originals.

It appears to be consistent with the service manual.
 

Thread Starter

Yami

Joined Jan 18, 2016
332
What did you monitor when the diodes were in the circuit?
Basically I was monitoring across the emitter resistors. I just wanted to see whether it goes out of whack, and since it remained stable I took output transistors from an unused channel and replaced the diodes.

If you monitored with an oscilloscope ... did you see any thing more than the dc bias that got the diodes to conduct? If you didn't, do you have access to an oscilloscope?
I haven't got access to an oscilloscope at the moment :(

Those transistors failed for a reason. Counterfeits could be one reason, unless they were the originals.
The output transistors are still holding on. It seems to be working at my place it was the case the first time as well. The output transistors shorted out at the owner's place last time - I did check at his place when I took the amp it did work, it was only after when I got back he phoned me to say that the amp had failed. The problem was on the same left channel. I did check whether the speakers had shorted, but it had a crossover circuit and probably a series capacitor. So I couldn't measure whether there was a driver shorted. When we were playing music through I didn't remark that there was any difference in the Left and Right speakers.
I still haven't received the spare parts that I have ordered. Every component in the amp at the moment is still what it came with - I haven't put in any new components.
My plan is to bench test and try to make sure that there are no problems.
In that process that's when I measured the DC offset values mentioned above.
 

JoeJester

Joined Apr 26, 2005
4,361
Do you know the circumstances surrounding the failure? What device was the source audio, how loud was the volume setting? Etc.

Well, you can test every input and ensure the.output reaches the power specified.

I certainly would do that so you don't have a second return for repair.

Abuse the hell out of it .....
 
There are no adjust points for the DC offset. My question are:
1.Does DC offset got anything to do with the biasing or biasing circuitry?
2.Are the above mentioned values acceptable for the DC offset?
3.Could the DC offset be taking out my output transistors? Maybe something in the crossover circuitry of the speaker shunting to ground putting a short across the terminals. (just thinking out loud)
Hi Yami,
To answer the above points
1. The DC offset is due to Vbe mismatch between the differential input pair Q5000 and Q5010. If you put your finger on either transistor the heat from your finger will cause the offset to change.
2 Accepted offset is less than 50mV generally, so you're in the right ballpark.
3 No, the DC offset won't blow the transistors, massive DC offset in the region of volts may destroy your speaker though. To this end manufactures add relays in the output stage to disconnect the speaker under fault conditions. You have checked the zobel compensation components haven't you?(post 10)
 

Thread Starter

Yami

Joined Jan 18, 2016
332
Do you know the circumstances surrounding the failure? What device was the source audio, how loud was the volume setting? Etc.

Well, you can test every input and ensure the.output reaches the power specified.

I certainly would do that so you don't have a second return for repair.

Abuse the hell out of it .....
Hi Yami,
To answer the above points
1. The DC offset is due to Vbe mismatch between the differential input pair Q5000 and Q5010. If you put your finger on either transistor the heat from your finger will cause the offset to change.
2 Accepted offset is less than 50mV generally, so you're in the right ballpark.
3 No, the DC offset won't blow the transistors, massive DC offset in the region of volts may destroy your speaker though. To this end manufactures add relays in the output stage to disconnect the speaker under fault conditions. You have checked the zobel compensation components haven't you?(post 10)
Thanks @JoeJester & @Martin_R, yes I checked the zobel network and it seems to be in order.
Just realised something very interesting after reading Joe's comment (the plot thickens), when I checked the amplifier last time I used all the analog inputs to input my signal and even this time I have been using the analog inputs to test the amp.
The owner however uses the HDMI ports. He has got his DVD player, TV and some other devices connected to the HDMI ports and the amp has got some kind of automatic function of detecting which device is currently being used - so I guess its switching from one to another. The owner asked me even at that time could that have caused the amp to blow up and I thought that its highly unlikely, since the fault is on the output side of things.
Reading online about Onkyo amps many have had issues with the HDMI board, but none have reported of the outputs blowing. Do you guys think that its a possibility? the input is saturating the outputs during switching? I was under the impression that these amps would be designed in such a way that there would be some sort of 'limiting' if the input is driven hard.
Thanks so much for the help guys. Really do appreciate it.
 
Hi Yami gotta be honest here - no idea what signal hdmi puts out, or how it's decoded. It's possible that there's a fault on the left channel in the decoding process. Really need to scope the channel and look for HF oscilation. Check on all inputs.
 

JoeJester

Joined Apr 26, 2005
4,361
I have no idea either, however, I'm sure an earlier signal fault, could cause that type of problem.

Typically, you verify the client's complaint. Of course if the client says, there is no output, and you don't have a clue what they use for inputs, you fix that problem ... then test all functions. In this case, I would disable the outputs; by any means ... to test all the inputs to ensure there is no strange stuff happening at ANY of the inputs.

Of course if you don't disable the outputs, and the original problem still exists, you will will discover the earlier function that blew the second replacement set of output transistors.

The extra time (money) is why clients choose buy new than repair.
 

Thread Starter

Yami

Joined Jan 18, 2016
332
Right guys, so I may have discovered what potentially has caused the amp to fail -
I had the amp turned on was monitoring the bias test points, the voltage measured was exactly what the service manual states 2.5mV at start-up and goes to around 9mV. After about 5 minutes in I heard the relay click I have mentioned about this in a previous post. The moment the relay clicked the voltage at the bias test point varied a bit (it didn’t go over 9mV) and it started going down! It went down till 2.5mV and remained there.
I have now confirmed which relay it is too! Its the relay which switches the B+ voltage. Here is what I measured across the bridge rectifier for the B+ voltage -when the amp is turned on the relay clicks and it measures around 110V after about 5 minutes the relay clicks and the measurement now is 54V. I turned off the amp and immediately turned it back on again. The same thing happens it starts with 110V and exactly around 5 minutes relay clicks and the voltage is 54V. The relays are controlled by a signal from the controller ’SECH1’ not really sure in what case this is suppose to turn on and off.
There are people who have reported about hearing random relay clicks from Onkyo receivers in AV forums. No one has identified the issue but someone has suggested that messing around with the settings (software) - specifically changing the impedance to 4 ohms gets rid of the clicking even though the speaker that he was using was stated at 8ohms.

Could I be on the right track do you think that this might have caused the amp to fail at the owners place? He has got Left and Right tower speakers as well as the centre and the rear speakers too. While I have only one 8ohm speaker connected on my bench which I’m using to test the amp.

Thanks for the help.
Edit:- PS. I have also realised an interesting link - when I was testing the amp at my place I heard the relay clicking on and off and it didn't have any detrimental effect. When I took the amp to the owner's place, I didn't run it for 5 minutes or long enough for the relays to click. He would have used it for that long only after I got back. However I can't imagine as to why the outputs should blow when the voltage level is shifted to a lower value :S

PS Switch .jpg
 
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MrChips

Joined Oct 2, 2009
21,869
By following the circuit schematics I can see that when the controller sets SEC1H to logic HIGH, Q6707 is turned on energizing relays RL6901 and RL6902. These relays switch from the low voltage transformer secondaries (approx. +26V and -26V) to high voltage (approx. +52V and -52V).

Who knows the conditions for switching. It could be for higher power output or cut back to lower voltage on a fault detection.
(For testing purposes, you could ground the base of Q6707 and force the supply voltage to the lower voltage setting.)
 
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