Onkyo amp repair, could the speakers be the culprit.

Thread Starter

Yami

Joined Jan 18, 2016
335
Hi guys,
I was working on this Onkyo amp which had a shorted channel. One of the power NPN transistor had shorted out and one half of the noble resistor was also open. (I made a previous post about transistor replacement). Anyway I took the two power transistors and the noble resistor from a working channel that is not used. Amp was working perfectly when I tested it. I should ask is it ok to leave the unused channel without the two transistors and the resistor?
Ran it for sometime I heard a relay click after a while not sure which one it was - no error message nothing and the amp kept on working. Turned off and on again no problem was working.
So I took it to its owners place and I wanted to check whether the speakers had shorted as this was my suspicion for the original failure.
He had two tower speakers - I measured across the terminal (it has got a filter circuit) and here is something peculiar I found:
1.Left speaker measured in the mega ohms range right away.
2.Right speaker showed the numbers crawling up in other words I saw the capacitor charging, and I heard the woofer move when touched with the DMM.

The owner was using it for sometime and he called me back saying the amp had died again. From what he described I know that the fuses on the B+ voltage rails had blown and the amp didn’t have time to go into protection mode.

So guys am I right in thinking the speakers might at be at fault here. I am going there tomorrow to see whether the same channel had shorted out.

Any adivce/tips on the matter would be greatly appreciated.
 

JoeJester

Joined Apr 26, 2005
4,390
What is the model number of the amplifier and what are the circuit designators of the failed components?

If it didn't sound right the first time you returned it, you would have had complaints.

You can sweep the speakers for a frequency response to see if there are any problems with them. If you never done that before, read this article from Elliottt Sound Products

When you did that resistance check, did the ohm meter settle down on near the speaker impedance.
 

Thread Starter

Yami

Joined Jan 18, 2016
335
So I had a look today. Checked out the speakers and they both does in fact behave the same way numbers crawl up like a cap charging and stops at around 0.7Mohm and when I short the two speaker leads together I can hear the woofer moving (possibly the cap discharging).
This time too the same channel had shorted out, but for a change the two power transistors (NPN,PNP) has shorted. The noble resistors remains ok. Looks like something upstream from the power transistors had shorted damn!
I did not swap the driver transistors from the good channel I should have done so!
I didn't do that as it tested ok. Even now its testing ok. I shall order them and change all when I'm getting the new spares.
Could temperature detection circuit cause this kind of failure? I noticed that the thermal compound has become very crusty with the sensors attached to the transistors.
What else should I keep in mind to further troubleshoot? Also I'd like to ask what other components you suggest I change considering this unit is around 6 years old to not run into further trouble down the line. I did go around with my ESR meter checking the electrolytics. Every capacitor I checked seems to be ok.
Something else I should remark is that I noticed that B voltage rails are around 54 to 55V. Schematic says it should be 51V, I'm assuming that this is ok.
The model of the Onkyo SR508.
Appreciate the help.

Contact @R!f@@
He lives where you are. He might be able to lend a hand. He is good at these things.
Thanks @MrChips , Hello there @R!f@@
 

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