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H/K receiver repair: looking for help with diagnosis

I was given a free Harman/Kardon AVR 40 receiver, with a caveat: only the center channel works. Also, the unit puts itself into standby after a short time. (The first time you turn it on, it'll last for maybe two minutes; after that, it generally goes into standby after less than a second. Cycling the power repeats this cycle.)

It's not under warranty, so I cracked the thing open. Didn't see any obvious blown fuses or leaking caps, but that's all I know how to look for. What other components might be the culprit? I'm interested to learn how to check the various components and figure out where something's gone wrong.

Is there an easy way to determine whether this is, for example, a toasted power transistor vs. some other component? (As a physics student, I have access to all sorts of equipment--I just don't know what to look for.)

Thanks,

Scott Morrison snipped-for-privacy@mit.edu

Reply to
sdmorr
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Grab the service manual at:

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Manual/AVR40 sm.pdf

There's not much that's easy about these amps, sorry. If you don't have a bit of diagnostic skill and background in electronics theory, you're likely in for a tough time. You may want to go pay $20-30 for a shop to diagnose it for you. Depending on what they come back with, it may be quite inexpensive to repair.

As far as the amp shutting down, there are two protection circuits in this amp which will shut it down completely, one monitors current through Q215/Q216 and powers down via Q217/Q218/Q219 if current is excessive. You may wish to check R227 which determines this cutoff current.

There is also a thermal protection circuit with two posistors, one in the transformer winding and one on the heatsink. Either of these going high-temp (or being faulty) would shut down the amp.

This amp looks like a bastard to diagnose. I count about 30 IC's and several hundred passive components.

You ARE using 8-ohm speakers, and only one pair, right? When you say only the center channel works, do you mean that neither the front L/R nor the rear L/R outputs function?

Dave

Reply to
Dave

. You may want to go pay $20-30 for

Man. I charge 55.00 for diagnosis on surround receivers, and I'm about to raise it to 75.00 , at least on the flagship models.

Mark Z.

Reply to
Mark D. Zacharias

Sounds like maybe 2 separate problems.

The center channel only problem relates to the DSP board in my mind anyway. The problem of going back to standby reminds me of a couple HK's I've seen which had a signal sized diode open up on the display pc board. The diode was supposed to drop .6 volts in series with the 5 volt line to the micro. When it opened up, the micro lost power. One of the involved units was intermittent like yours.

Mark Z.

Reply to
Mark D. Zacharias

And you probably still lose money on at least half... people just choke at spending upwards of $50 for "nothing". They feel better about spending $500 for a new unit than $200 to repair an "old" (1 or 2 year-old) unit.

Reply to
Dave

Yeah, people aren't even bringing in that many regular sized surround receivers anymore. It's pretty amazing the features and relative quality you can get in a new "mid-fi" HT receiver in the 250.00 to 350.00 range. Yamaha and Onkyo, and other middle range models all seem to be adequate for any but the real tweakoids out there. And when they do come in, it's often the result of a lightning strike which may be simple or a real can of worms and often no way to tell for sure until you are way too deep into it...

I'm the audio guy in my shop but more and more I'm working on TV's.

Mark Z.

Reply to
Mark D. Zacharias

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