Harman Kardon AVR audio when warm

Harman Kardon AVR no audio when warm

All,

Harman Kardon AVR 254 upon power up, audio comes from both speakers and headphones. All sources seem to work fine. After 2-3mins, audio grows crackley and disappears replaced by a low hiss/rumble.

IC 36 and 37 are the 8 and -8v linear regulators respectively. These get very hot, nearly burn fingers. A shot of freeze spray, and the audio comes back. Some darkness to the PCB below the regulator.

Could this be bad regulators, or maybe a bad design? Should I replace with higher amp voltage regulators or add heat sinks? Is the heat normal?

Should I look for another fault that is creating a high current condition? Bad electrolytic capacitors?

Electrolytic capacitors in the voltage regulator circuit are with 20% of their value, but have ESRs in the 6Ohm range.

thank you!
 
Last edited:

ikonw8

Member
2016-11-03 12:58 pm
Toronto
Continuing the troubleshooting

Some more troubleshooting.


I checked the capacitors in the supply circuit, and none seemed to be bad.


Then I supplied +15v and -15v to the board and monitored the output of the 7808 and 7908 regulators.


When cold, the 7808 puts out exactly 8v and draws 85ma. The 7908 puts out -7.2v and draws 85ma.



Continuing to apply +15v and -15v, the 7908 regulator output drops gradually, while the current drawn by both supplies remains the same. When the output of the 7908 reaches about -2v, it collapses quickly to 0v and the +15v supply draws 420ma. Of course, the 7808 gets very hot.


I am thinking now that I have a flaky 7908 regulator, since both in and out of circuit, the 7908 puts out -7.2v.


Of course, I suppose there may be something else in the circuit that is creating an unusual load for the regulators as they warm up... I have never seen anything like this before.


Nothing else gets hot on the board.


Going to swap out the regulators next and see what happens!
 
Looks like the 7908 is toast. Dropping 7 V at 85 mA (0.6 W) is really not too much to ask for even from a bare TO-220... it should normally be running at about 35 K above ambient, like the positive counterpart, so maybe 60-70°C - and these are rated to 125 or even 150°C die temp.

It's odd to be seeing the reg itself going bad, usually they'll just be cracking their solder joints from thermal cycling and resoldering will sort things out for a good while.
 
Success!

All,


I replaced the 7908 voltage regulator, and success.



I now have a stable -8v drawing 85ma. 7808 also puts out +8v and remains stable since the 7908 is not lowering voltage under load. Both voltage regulators get warm but not anywhere close enough to burn.


The voltage regulators supply the DSP and Opamps, which is why I was losing audio after a few minutes of operation.



As I mentioned, I also replaced a few 100uf / 16v electrolytic capacitors that were on the verge of being out of spec.(or were already!). i.e. for 82uf, ESR 7ohm, vLoss 6.1%.


I should also point out that the receiver first presented itself with "Protect" mode on: which I troubleshooted to a pair of shorted power transistors and a failed emitter resistor on the Centre channel.



Thanks all for your input.


ikonw8