Kenwood KA-5500 catastrophe - please help

I replaced the failed 2SA721 that Kenwood used for Q10 (the manual lists 2SA620 for Q10 but a 2SA721 was what was really installed). So I subbed it with a KSA992 as apparently that has been done before and it works fine. Just wondering if that different (newer) transistor would cause the difference in DC offset for that channel as it is closest to the differential pair? Any idea where else I might look?
 
I did measure R6 and R5 and they are slightly different but not by much! I'm talking 56.1 vs 55.9 or something like that. Perhaps I will try to match the good channel and replace R6 with an equivalent value on R5 and see what happens.Thanks.
 
That's still going to be a lot of trial-and-error matching of the pairs of resistors in each channel An offset adjustment circuit would probably have a couple of fixed resistors and a trimmer in parallel to one of them to tweak that in, like a 33k in series with a 47k paralleled to a 50K trimmer.
 
Hi guys, update on this thread, I was able to add mods that allowed me to balance the DC Offset on both channels. I replaced various static resistors and ultimately subbed pots for R17 and R18 that allowed me to bring the offset to 5Mv on each channel. It sounds pretty good. However....another problem has resurfaced. I say resurfaced because I remember this symptom / problem happening 6 years back and I thought it was a dirty volume POT. It isn't. The problem now is that the left channel (the channel I did not blow up!) now is problematic, as I have to raise and lower the volume control and mess with S-10 a bit to get the sound to kick in. At first it starts up sounding distorted and much lower in volume but eventually after messing around with S-10 and the main volume control it kicks in and sounds fine. I have sprayed Deoxit on all controls, I've also recapped the entire control board with no changes. It was easy, so I figured why not. I messed around and cleaned the famous S-6 tone control defeat switch but I was not having issues with that so it didn't fix the issue. I am going to try to localize the issue today by signal tracing. I hope it is not one of the opamps on the control board. I didn't want to start messing around with subbing those unless I know for sure that is the issue. Has anyone heard of the protection circuit causing an issue on only one channel? It doesn't seem like that's possible if the relay works properly, which it certainly appears to.
 
If the protection relay contacts themselves are slightly resistive or even a bit corroded or pitted, your symptom can occur. Once the volume is raise sufficiently to "burn through" the resistive layer, the unit will play normally until the relay is cycled again. It may be worthwhile to pull the relay cover and burnish the contacts.
 
Has anyone heard of the protection circuit causing an issue on only one channel?

You could easily verify this by injecting a signal through the aux and looking at the input of relay with your scope. I had a similar problem on Sunday with an old marantz. My case one side of the relay wouldn't engage even when the relay coil was energized.

Since you are going to do some signal tracing anyway, start at the relay and work back.
 
Well I attempted to localize the trouble to a particular board, I injected test signal at the aux input and measured the output of each channel of the control board (measured at the inputs to the power amp) and the left channel signal looked a bit strange but the wave height was the same as the right while I moved the volume control. I then injected directly into the inputs to the power amp board and the sound volume was the same on both channels.(Leading me nowhere) During the test for the control board I had to move the volume control up and down and when I do that sometimes the problem is not noticeable anymore. It comes back every time I shut the unit off, so I need to inject directly to the power board when the problem is evident and without moving the volume control much. Also, the power board is installed properly and there is not much room to probe with the board installed that way. I have not yet devised a good way to support everything safely with the power amp board open, the heatsinc and finals are heavy and those leads are easily shorted, etc. Any ideas would be appreciated, if anyone has worked on this particular amp before. Thanks. By the way, I have decided to upgrade the opamps in the tone control circuit anyway. I found good converter boards and ordered the usual replacement - the OPA604AP. I've also replaced Qi1 and Qi2 with no changes to this problem. The overall sound of this amp seems to be deteriorating as I replace components, which is disappointing. Looks like I will need to continue to recap and replace components as I learn more about what is causing this problem with the left channel.
 
I quickly looked at the 5500 schematic and the aux goes through a few switches -> balance -> volume -> tone stack -> amp. since you said the amp is fine, looks like the tone stack might be the problem. I see a couple of couping caps (ci1, ci9, ci23 for one channel) that might be good test points. I've had good luck looking at the op amp output on a scope to verify it is a good signal. Low filter switch (s7) needs cleaning?

Sometimes a simple wire soldered to a test point allows easier access to a scope probe. I try not to do this but sometimes you don't have a choice.
 
Thanks Rick. I am going to do that. Yes, I suspect the tone control circuit somewhere. Thing is I have recapped that board already (replaced all lytics) and it sounds slightly worse in both channels in addition to the problem not going away. Less Bass, less punch, perhaps a bit more distortion in the mids. Perhaps I will look further into cleaning S-7 or bypassing it. The problem seems like an intermittent cap I have not found yet or worse, the opamp is going bad which I of course now have a solution for. However I would indeed like to make a positive ID on that problem before I upgrade any more components.
 
You should be listening to Watthour with regard to this issue. Your symptoms sound exactly like a relay problem to me. Best way to quickly test this is to play the unit then tap the relay lightly with a plastic item (toothbrush or whatever you've got) and see if the sound is affected. I don't recommend burnishing the contacts. In my experience if a relay gets bad enough to start cutting out, its time to just replace the thing. If not, often the problem just keeps rearing its head again.
 
Thanks hopjohn, I plan on cleaning those contacts soon. It just didnt make sense as to how the relay would be affected by raising and lowering the volume control, I thought that once it kicked in, it was "in".
 
Updates to this ongoing thread- I worked on the amp this past weekend and decided to replace the opamps even though I still need to clean the relay. I found and fixed many cold solder joints and poor connections on the control board. That took a while as I had bad solder joints on the brand new converter boards I purchased for the opamp upgrades! I am not too keen on turning this thing on and off with the boards just hanging there so back and forth I went with removal and re-mounting. What I did find with the servicing of the control board was an amazing increase in fidelity and sound quality though. With my signal generator connected and using a very low signal and my scope I was able to fix the problems I introduced myself and some that were present before I took the unit apart! Some were ground issues between the board and power amp input, etc and were causing noise and degradation of sound. I still have to clean the protection relay but even that issue is not as evident anymore. I got to play the amp for an hour at high volume and the results were encouraging. As I've turned a "repair" into a "refurbishment" I will undoubtedly need to replace more caps and when I do that I will remove and clean the relay. I could not see an easy way to take the cover off without removing the power amp board though. Thanks all, for the encouragement and help.
 
It appears that the outputs are socketed. If so, you might pull the outputs on the questionable channel and try your 60W in the DBT again. Apparently, the outputs are not grounded, but in case something is biasing them on hard, this will potentially indicate that.
 
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