Harmon Kardon HK770 amp problem

[taj]

Hi,

I'm a semi-newbie at electronics.

I have an old HK770 amplifier. It sounds okay (not great) but the heatsink on one channel get scorching hot within a few minutes. So I shut it off before any output devices fail.

Does this sound like it might be oscillating, or thermal tracking transistor failure, or ??

I've also noticed the speakers make a slight popping noise when connecting them to this amp. Too much DC offset? I'm not 100% sure how to measure that. Across the output terminals with no input?

Does anyone have a service manual or schematic for this amp?

Thanks!

-- Todd

[anatech]

Hi Todd,
It does sound like you have some DC offset. That channel could be oscillating, or there may be a fault causing high bias.

-Chris

[taj]

Originally posted by anatech
Hi Todd,
It does sound like you have some DC offset. That channel could be oscillating, or there may be a fault causing high bias.

-Chris

Thanks Chris,

Across the speaker terminals I measure -6.6v on the hot channel, and +.09v on the cool channel. I assume it's DC voltage I'm measuring here right? Not current? I measure no current on either channel.

And I can probably also assume -6.6v is not expected or appreciated?

Looks like I'm going to have to open it up and start looking for differences between the two channels.

Is there a way to test for oscillations without a scope?

..Todd

[Gigapod]

A highly innacurate test for amp HF oscillations is to just put a resistor across the speaker terminals and check if it heats up. Or a small lamp and check if it lights up (the lamp will integrate the HF). Or use a small signal diode and capacitor...

6.6V across the speaker terminal? TURN IT OFF quickly!!!!!!

Read this: http://audiokarma.org/forums/showthread.php?t=5634

[taj]

Originally posted by Gigapod
A highly innacurate test for amp HF oscillations is to just put a resistor across the speaker terminals and check if it heats up. Or a small lamp and check if it lights up (the lamp will integrate the HF). Or use a small signal diode and capacitor...

6.6V across the speaker terminal? TURN IT OFF quickly!!!!!!

Read this: http://audiokarma.org/forums/showthread.php?t=5634

Thanks gigapod. Hey, that thread you linked to by EchoWars is outstanding. Highly recommended reading for other newbies here. And Thanks EchoWars if you're tuned in.

..Todd

[taj]

Thanks Chris, for the email (on both counts!) My email link should be live now. I thought I fixed that before. Apparently not.

So, this amp has bias adjust but no offset adjust, and it sounds like it's far beyond adjustability anyway. I'll just follow the service manual's voltage measurements until I find something awry. I hope it's not those funky dual diff amp transistors. I suspect they may be hard to find, and replace.

..Todd

[taj]

I was wrong again. (I thought I fixed that too ).

It does have DC offset adjustment. Somehow I doubt it will move 6 volts though.

I should probably swap out all the small electrolytics first right?

..Todd

[anatech]

Hi Todd,
Look up the specs on the dual transistors and substitute single ones in there that you will hand match. Thermally couple them.

Before getting into those (you can always simply measure the hFE on each section), check transistors 410,412,414 and 416 for leakage. Often HK will run a transistor close to it's limits both thermally and voltage wise. The result is odd faults. On many HK amps I just replace the lot of transistors with parts having higher ratings. Look for bad solder joints also. Dark brown areas indicate heat. Fix the fault first, then go over the whole thing. Look up each transistor's rating and compare it with your supply voltages.

-Chris

[taj]

Sounds like good plan to me. Thanks again Chris. I'll keep you posted.

..Todd

[anatech]

Hi Todd,
I guess we won't hear from you for a while then .............

-Chris