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

MrChips

Joined Oct 2, 2009
22,505
Locate the protect detection signals and measure the voltages at turn on and 5 minutes after when the rely clicks.

GNDDG2
+3.3VMPU
THERMAL

VOLH
SEC1H
VPROTECT
IPROTECT


ONKYO TX-SR508 Protect Detection.jpg
 

Thread Starter

Yami

Joined Jan 18, 2016
337
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.)
The owner's manual says
"If you use speakers with a lower impedance, and use the amplifier at high volume levels for a long period of time, the built-in amp protection circuit may be activated."
I have a feeling it's wrongly detecting this 'lower impedance' load, the relay switching does a funny 'wobble' on the biasing voltage. But could under biasing cause high current draw enough to take out the OT and the fuse?

Thanks @MrChips
 

JoeJester

Joined Apr 26, 2005
4,390
"If you use speakers with a lower impedance, and use the amplifier at high volume levels for a long period of time, the built-in amp protection circuit may be activated."
Did you get the model of the client's speakers to check on their published impedance? Of course this doesn't mean they are that value, but you can replicate it with a similar value of resistors for testing purpose.

The other option is to carefully, inspect the path from the output transistor emitters to the speaker terminal to ensure it is correct.

Then you can confirm or reject that portion from your hypothesis.
 

Thread Starter

Yami

Joined Jan 18, 2016
337
Finally I have figured out the why the relay randomly clicks - I was wrong, my speaker actually is rated at 4 ohms. When I changed the settings for 4 ohms from the amp the relay issues was solved. When 4 ohms is selected the the B+ voltage remains at the lower voltage setting that is 52V across the rectifier.
The owner uses a KEF Q900 the impedance rating is stated as 8 ohms and the amp was set at the proper setting for that, but I do have a feeling the relay would have clicked at his place too because the fuses which were blown was on the low voltage line.
I was able to get my hands on a scope and I measured the outputs of the DAC since the problem seemed to affect the left channel only and the HDMI inputs were used when the amp failed. I was hoping that there was an issue on the LPF and some high frequency content was going into the amplifier section. But this was not the case a clean signal was seen on the outputs of the DAC-damn! Can't think of what else to look for :S
 

JoeJester

Joined Apr 26, 2005
4,390
Does that clean audio signal go all the way to the output of the amplifier ... or at least to where you have the finals removed?

I can tell you in the other thread where counterfeits were suspect, it also didn't show any unusual activity at the inputs so, we are back to blaming the counterfeits.

You only checked it, to confirm or deny an earlier problem. So far you denied it came from the DAC. Move towards the output and check some more.
 

Thread Starter

Yami

Joined Jan 18, 2016
337
Does that clean audio signal go all the way to the output of the amplifier ... or at least to where you have the finals removed?

I can tell you in the other thread where counterfeits were suspect, it also didn't show any unusual activity at the inputs so, we are back to blaming the counterfeits.

You only checked it, to confirm or deny an earlier problem. So far you denied it came from the DAC. Move towards the output and check some more.
Yep, sounds like a good plan. Will do so.
 

JoeJester

Joined Apr 26, 2005
4,390
If all is well, put in a known good pair of transistors, from a known good source, adjust the ICQ and monitor for whatever time the service manual recommends.

Then comes a music test at a low power.
 

Thread Starter

Yami

Joined Jan 18, 2016
337
I have received the spares from digikey, going to solder them in. The amp threw another curve ball today. Discovered this charring near to the +/- 15V regulators. The +/-7V for the R2A15218FP IC (used for switching between sources) is also derived from here. Couple of capacitors around that area had gone out of value and reading high ESR value. This was hidden under all the boards had to disassemble everything to discover this. Going to try and replace them all before assembling.
 

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Thread Starter

Yami

Joined Jan 18, 2016
337
Hi everyone, just wanted to thank you guys alot for the help I have learned so much.
Just and update - I installed the new spares and replaced the capacitors which have gone out of spec. The random relay clicking has gone but I guess it was to do with the back cover not being attached while I was powering it up as it serves as a grounding path from one board to the other. I have been running the amp all these while and it seems to be holding up :)
Appreciate the help again.
 
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