Advice: H/K AVR 365 Amplifier adj/mods

[b4nd1t0]

New guy here. Tell me if wrong forum, etc.


I recently purchased a Harman Kardon AVR-635 from the factory refurbished store on Ebay.


Here's what I need some help/advice with:


First, it seems this model/design has had some historic issues with idle -- 7 daughter boards (one for each channel) contain a trim pot and transistor (a simple adjustable vref, apparently?), and are wired to the amplifier boards with connectors that may fail.

After looking at the Schematic, it seems to me that an open circuit on these connectors could cause the idle to increase (to the point of causing the protection circuit to kick in?). An earlier service manual (for the AVR630) suggests that these connectors be replaced with soldered hard connections if there is a customer complaint re: protection mode.

Second, the amplifier idle spec is 23mv. My back of the envelope calc suggests that will cause about 3-4 watts to be dissipated in each of the output transistors at idle -- about 42-56 watts total for the 14 (7x2) output transistors. The power consumption for the AVR at idle on the manufacturers spec sheet is 59 watts-- so sounds about right if all the channels are adjusted properly.


Unfortunately, the AVR is idling closer to 100 watts total, so I'm thinking its time to check all the idles to cure that extra 40 watts of consumption -- I'm concern it could all be in one channel. Does that make sense?

Here are my questions:

(1) Hard-wiring all the idle adjust connections -- I can share the schematic, do you think its worth doing just in case?

(2) The the idle test points are awfully small and hard to reach. Is there a trick or do I need to buy a specific H/K test tool to reach and connect to them? Can I make something?

(3) I'm using this amplifier in stereo mode, and intend to continue doing so for the foreseeable future. Any reason I shouldn't un-bias the unused amplifier channels so they don't draw power? Save power and keep the AVR running cool?

(4) Anything else I might do modification-wise while i'm inside this thing?


AJ


PS, I have a schematic of the main amp board+daughters, its a 600k PDF -- anyone want to see it? what's the best way to share?



FWIW, I bought the HK because I couldn't resist the ebay price and thought it would make a simple 2.0 TV/Music solution with lots of digital audio inputs (cable box, Airport Express, WDTV all have optical.) I have good memories of HK gear -- and recall liking their amplifier design philosophy in my early EE days....not sure if that still applies; don't EE much anymore

[dogsbark26]

Heck yes, I'm interested in seeing that pdf! I've got an AVR um... 745? which I assume is similar. Where did you find the schematic?

I'm just learning about electronics, so it's unlikely I'll be able to read much of it - let alone help you; but I'd love to see it all the same.

Thank you.

[b4nd1t0]

You can get a bunch of service manuals, w/ schematics for H/K stuff here: That's where I got it. 745 is there, too.
Harman/Kardon Receiver Service Manuals | All Things IT Blog