Harman Kardon AVR 25II Receiver Repair

[Arty30]

Hi all, I just picked up a broken Harman Kardon AVR 25II receiver that does not work. I quickly found an interesting issue where if I move the volume knob, but only while moving the knob, the voltage at the speaker terminals spikes, tripping the protection relay and shutting itself down. If it doesn't get high enough to trip the relay the voltage settles down once the knob stops moving. I briefly had it working and it appeared to be otherwise functional. I cleaned the volume potentiometer and re-flowed about a hundred cracked solder joints, and no luck. so, I'm wondering if anyone here has any advice, I found a service manual on  https://elektrotanya.com/the_harman_kardon_avr25ii.pdf/download.html, not the best quality but it's what I was able to find. Thanks.

[djsb]

Maybe make a list of what works and what doesn't on the front panel, so you can isolate the problem to a particular section before delving into component level repair. Having said that manual says to check C304 and C305 and IC301(under heading "Noise volume control" on page 23). If you have a signal generator, you can feed an audio signal into the volume pot and check for an output (either from a speaker or an Oscilloscope). The protection circuit is controlled by the main CPU, and it appears that the pot is motor controlled, not that that is relevant.

[Arty30]

Thanks for the tip, I'll check those components and functions and see what I come up with.

[Arty30]

Ok, so I checked those three components and found no issues, IC301 is pretty well isolated from the audio circuit and is only in charge of the volume motor, the two caps were also okay. as for functionality, I can hardly touch the volume knob before it shuts off. I can get the tuner to work and all the buttons appear to be working properly. there is also an awful amount of static and popping noises whenever the amp is on, I suppose that's the voltage across the speakers that I'm hearing, which I've found to be jumping all over the place. unsurprisingly if I take out the preamp to main jumpers, it is quiet and stays on.

[poot36]

It sounds like you my have leaky coupling caps in the pre-amp stage that is letting DC get to the volume control.  If you measure DC output from the pre-amp stage on the pre-amp stage to the amp stage jumpers this will confirm that the issue is in the pre-amp stage.  You should take a look at this video series on how to repair amplifiers with basic tools:   Also check that the power supply's are putting out the correct voltages and that there is no AC ripple on them.  Good luck in the repair.

[Arty30]

Thanks, I have verified that I have the same voltage issue on the preamp jumpers.

[poot36]

That is sort of a good thing.  At least the main amp section should be ok.  If this unit has a digital based volume control that could be part of the issue.

[Arty30]

Ok, so I had some time today to tinker and here's what I found, (in no particular order): Left channel is distorted, right may be very slightly. Touching the top of a cap on right channel makes a loud buzz (none others do). I found a coupling cap on left channel with 78 ohms ESR in preamp section. It has four "modes", Dolby pro logic, 3CH logic, Theater, and Stadium. both Dolby and 3CH logic are distorted while theater and stadium are not. The DC offset issue on the speakers is still there, but only when I switch it to Dolby and 3CH logic settings. so far where I think I'll start is with the preamp section, which is a patch of about 25 transistors and a dozen or so caps. The board is discolored from heat and I can see it with my thermal camera getting to almost 80c is some spots, that is near the temp rating of the caps. I would not be surprised if they are dried out after 25 years of that, this is where I found the coupling cap with high ESR (red top), and the one that doesn't like being touched (gold mark), in the picture attached below. Perhaps replacing the several small caps in that area will help a few of these issues. Also, I can't really find many coupling caps, is there an area I should look other than the beginning of the preamp circuit?

[poot36]

If you get a buzzing sound from touching the top of a cap that means that some part of the circuit is not connected correctly.  The cap you are touching is connected to the ribbon cable that goes to the tuner section.  I suspect that either the cap is open or there is a bad solder joint between the cap and the tuner board.  Could also be a bad connection on the pre-out jumper links as it also connects there as well.  I assume that you have checked that the power supply is providing the correct voltages and that there is not excessive ripple on the outputs?

[Arty30]

all of the caps in the area I took the picture are heat roasted, probably dried out. I found more that are either a fraction of their rated capacitance, and others that have like 150 ESR, or both, so I think I should start there with an issue that I know exists and can fix relatively easily. The power rails all seem fine.

[poot36]

Good point.  Also check for cracked solder joints from the heat as well.

[Arty30]

yeah, there were dozens of cracked joints in that area, there was a transistor that was just plain loose. I re-soldered several dozen joints in that area.

[Arty30]

Alright, sorry for being quiet, I replaced the cooked caps in the amp section. It did not necessarily fix the issues, but I definitely cleared up an unknown and it may even sound better. I have basically narrowed it down to two faults. Firstly the left channel is distorted, I tracked ti to the main amp board, if I inject a signal past the pre-amp jumpers, it is still distorted. secondly, the dc offset issue is still present, on the "Dolby Pro Logic" and "3CH Logic" settings only, when I turn the volume knob, it trips the protection. This is present at the pre-amp jumpers so it's not the main amp. I think I'll start with the left channel distortion.

[poot36]

If you have a oscilloscope you should check the base of all the transistors in the left channel amp section.  This will let you see where the distortion is getting introduced into the system.  If you are lucky it is a noisy transistor.  If all you have is a voltmeter you can check the voltages listed in the schematic.  The darker line with the arrow in the schematic indicates signal flow.

[Arty30]

ok, I feel dumb now. the left channel distortion has been my speaker this whole time!. At some point one of my speakers got damaged, probably from the voltage issue I'm chasing, and it somehow made the speaker itself distort the sound. I noticed it when I swapped speakers from left to right and now the left was fine and right was distorted, found another pair of speakers and low and behold, both channels sound good. Although after several minutes of running the right channel cut out, and when I turned it off and let it cool, it was back, maybe a failing transistor?. I'm starting to think it may not be worth my time to try and fix, it may be completely cooked and as soon as I replace one bad part another one kicks the bucket.

[poot36]

If you watched the video (and its follow up videos) I sent you it is a very bad idea to connect speakers to a amp with unknown faults.  You can at very least damage the speakers and worst start them or the amp on fire.  If you must connect speakers to an amp with unknown faults always use a capacitor in series with the speaker as this will block any dc on the output.  I would start by replacing all the small signal transistors in the heat damaged part if they are not to expensive (making sure to use matched pairs of transistors where needed).  Good luck with the repair.