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Kenwood KA-9100 Bad Right Channel

Homebrook

Homebrook

Member
This is my first post on Audiokarma. What a great help Audiokarma is! Without all of you I would be clueless. Thanks!
I bought a 9100 for cheap that had the right channel not working at all. The first thing I did was to thoroughly clean the Normal/Separate switch. That got a signal through the right channel (yeah!), but it is raspy and somewhat attenuated and when I press the low filter button it completely cuts out. I have cleaned it the best I can without complete disassembly. Am I on the right track that the problem is the subsonic filter button and that if I can just get it thoroughly clean I'll fix it, or are there other things that might cause this problem? I am thinking that since the button causes the channel to entirely cut out when pressed, that it might also cause the degraded sound when it is unpressed. I know about the service bulletin Kenwood issued to address intermittent output. Could that three-prong connection for the Tone Amp output cause a raspy channel? I have cleaned and worked all the other switches and pots.
 
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Hi there,

Welcome to AK. Most likely it is because of dirty controls. Had one last time and its a very good amplifier. I had the same issue but only sometimes.Did you listen tru headphone?? Can you hear sound from both channels?

Raj
 
Raj, Yes - dirty controls, no doubt. I don't have headphones with the large headphone jack. I do hear sound from both channels.
This morning I separated the preamp from the power amp and connected it to my other 9100. First I used the preamp from my good 9100 and the power amp the bad 9100. I was surprised to find that the right channel is still scratchy! So the scratchy right channel issue is in its TA-100w IC. Am I thinking correctly? Would a bad capacitor on that circuit cause the scratchiness?
As far as dirty controls in the preamp, the subsonic filter still causes the right channel to cut out, and the Tone Defeat/On switch causes the right channel to diminish and degrade significantly, so I've still got some work to do cleaning those controls. I'll need to disassemble that section to do a decent job.
 
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Had the same or similar issue with my 9100... its over 30 years old now and a "favorite"!

So I decided to play "the Scientist".., disassembled it, attempted to replace everything on the right channel signal path, and still had drop out issues.. :dunno: !?
So, I back tracked, "de-oxed" the pre-amp path and after using the switches several times, did get some stability in the circuit.

To this day, not sure what I fixed.... :idea:? But I wouldn't attempt it again!

Well.... not until I get another "favorite amp"...
 
Thank you for the suggestions. I cleaned the subsonic filter button and Tone Defeat/On switch again, but to no avail. I am beginning to wonder if there might be a problem in the tone and filter circuits themselves, or with the switches. It really is quite a job to get at those boards! Has anybody disassembled the amp to get at those boards who could give me a couple pointers in doing it? It seems like I'll have to pull that whole main bundle of wires out to get at them. Also I am a little intimidated at getting at the TA100W IC for the right channel. That board is also fairly buried. As it is I can't see the side where the soldering is to look for any bad soldering joints. Sigh... I'm screwing up my courage to go further in taking the amp apart to see what is going wrong with it. I'll have to proceed very carefully and methodically. I feel like I'm going spelunking.
 
Excuse me as I am recalling this from memory... and over 55... o_Owell you start to "bundle".

Yet, here are (3) three suggestions I would make at the out set:

  • Make sure you download the PDF schematic for the amplifier! There is also a users manual and in it are the instructions for "tuning" the amplifier circuit at some later time.
  • Label everything.... as there are a lot of boards and associated connectors. You DO NOT :no: want to have confused these at reassembly.
  • Examine the amplifier circuitry turning the amp chassis "up-side-down". Yes, this is one time where getting another perspective will help you reach many hard to get to test points.
Unfortunately in my disassembly the first time out, I had not realized this. Several troubleshooting tasks would have proven much easier.

Lastly.... and perhaps firstly..?

Reapply the "Deoxit" to those suspect switches liberally....
Why, you may ask? Think of the age of this amplifier.. those switches have been subject to years of entrenched oxidation and particulate matter.
Deoxit will run off if you give it a path to fall. If you turn the chassis up side down and towel underneath it, you will find most will end up on the towel. And hopefully the nuisance contaminant will end up there as well.

Finally... be patient, take your time, and know that if you do remedy these issues.. you will have an amp that serves you well for many years to come :thumbsup:.
 
50nstillhifi, over 55? I'd rather have "bundling" than inexperience! :-) Well I'm 60 this year, but I still like to learn. I am thoroughly enjoying getting into vintage electronics.
Thank you for your excellent counsel!! I do have the service manual, but not the owner's manual. I have not been able to find a KA-9100 Owner's Manual anywhere, although I have it for the KA-8100, which helps. Yes I will clean the controls again. I will take your advice to label everything, that and take pictures all along the way, and take exhaustive notes as well. I do have the advantage of having a working amp with which to refer.
When you mean "tuning" the amplifier, what do you mean? I have already adjusted the voltage offset. The left channel is at about 1.5 mV and the right channel is about 8.5 mV. One thing I did notice as I was adjusting the right channel that I have not seen before (not that I have any great amount of experience!) is that once I got to 8.5 mV it would not go any lower even if I kept turning the adjustment potentiometer quite far past where it stopped changing. I set it at the point where it got to 8.5 mV and left it there. What I have experienced on the other channel and my other 9100 is that if I keep on turning the voltage goes from positive to negative or negative to positive. I don't know if that means anything or not.
 
Hmmm.... that's interesting... the fluctuation? As that may be an indicator of the earlier issue you had and I also, in the past.

As for tuning... my term for the voltage offsetting. However, I have found the vintage amplifiers do lend themselves to "tuning" when in separate mode. Not too many current day amps seem to have this "separate" feature. I balance and tone my 9100 with an Audio Control... vintage model with subsonic filtration... yes, I played a lot of vinyl back in the day.. :rockon:part of the "Club scene"!

One peculiarity with the DC amp is the feedback loop used for the preamp section... supposedly to deliver cleaner "sound". I believe the goal was to keep independent signal paths uncorrupted by left/right channel distortions (please don't quote me). In the 9100, the power supply path and its delivery to the amp section is quite apparent. When I went in to test my amp, I would detect via a multimeter peculiar voltage flux at the tone control points. At the time, I thought a capacitor along the signal path to be responsible..... See how many you count?

Well, needless to say, I abandoned the replacements, particularly after researching some of the non OE specs.... not exactly matching.. size or value.

My "bundling" refers to mentally cramming 30 hours of thoughts into a 24 hour day.... I just beginning to ..... SLOOOOOwwww it down :(
 
Thanks, 50nstillhifi! I'm not sure what the "fluctuation" is you mentioned...

In an attempt to determine the precise symptoms exhibited by the problems in my new KA-9100 I connected it again to my other good 9100 using one for the preamp, the other for the power amp, and then reversed the positions. I listened with my headphones, which I found much easier to do than constantly fiddling with my speaker connections. Plus I could hear more carefully what was going on. Here is what I came up with:

Preamp symptoms:
1. Pressing the FILTER, SUBSONIC push-button ON cuts out the right channel about 80% (can hear a tiny level of music) introducing a constant level hum, unaffected by the volume level, and not overcome until the volume is at 50%. The FILTER, SUBSONIC push-button position does not negatively affect the left channel.
2. Flipping the TONE switch ON diminishes right channel sound level about 50% while introducing a hiss. The hiss volume stays constant despite the volume setting.
3. Flipping the TONE switch ON introduces a hum into the left channel which is most noticeable when the volume is all the way down.
4. If FILTER, SUBSONIC push-button is OFF and TONE switch is in DEFEAT position, then sound is OK on both channels.

Power Amp symptoms:
1. Left channel sounds OK
2. Right channel is scratchy and diminished
3. Can hear some scratchy noise out of right channel even when balance is 100% left.

I really do want to see if I can fix this amp myself. I hope in the process to learn a lot more about how it works and just to test my logic and troubleshooting skills. I am in awe at the level of knowledge and skill constantly on display at AK! I enjoy the challenge, but I am well aware of what I don't know. In order to get my ducks in a row for when I take my amp apart I have a few questions, which, I am sure, will demonstrate my ignorance all too clearly. I have also listed what I plan to do.

So here goes:
First, my preamp issues:
I have cleaned the FILTER, SUBSONIC push-button again with DeoxIT, but it still does not work. I have also cleaned the TONE switch. In each case I need to determine whether the problem is with the switch or in the circuit itself. I plan to disassemble the preamp boards to see what I can figure out:
1. I'll thoroughly clean the problematic switches one more time with DexoIT. Is it possible the problem lies elsewhere, or do the symptoms I described above definitively point to the FILTER, SUBSONIC push-button and TONE switch?
2. How can I test whether the FILTER, SUBSONIC push-button and the TONE switch work? Do I just test for continuity in the on-off positions? Where do I put the probes? (maybe that will be obvious when I have the board exposed.)
3. If they do work, then I know the problem is on the board itself somewhere. I can examine them carefully, trace the signal through it to see where it is lost or compromised and replace capacitors, or whatever I find is wrong.
4. I am assuming I can trace a signal through the preamp boards with them disassembled, but still connected. Is that true?

Then, more significantly, about the power amp circuits:
1. What, exactly, is the TA-100WA integrated circuit? Is it the black metal enclosure alone (labeled, "TA-100WA") attached to the large, finned heat sinks on each side, or does it refer to BOTH the metal enclosure AND the power amp circuit board to which it is attached?
2. What is inside the black metal enclosures labeled TA-100WA, transistors, or what? I'm curious. Can I take them apart to diagnose them, or are they to be regarded as literal black boxes?
3. What strategies can I use to isolate the problem between the left power amp circuit board and the TA-100WA metal enclosure attached to it? Should I go through the trouble of removing the TA-100WA metal enclosures from each channel and swap them? Then if the problem switches channels I would know that the problem is with the TA-100WA enclosure. If not, the problem is in the board. Is there an easier strategy? I'm getting myself in pretty deep here.
3. I know that the power amp circuit boards are not interchangeable from one side to the other, right to left, or vice versa, but is that also true for the TA-100WA metal enclosures? For example could I remove the TA-100WA metal enclosure from a left channel power amp board and attach it to the power amp board for the right channel and have it all work? It seems to me they should fit on either side, because on the right side the pins are labeled 1-10, from back to front, then on the left side the pins are labeled from 1-10, from front to back, which means, in my view, I should just be able to take one from one side to the other simply by flipping it around and the pins would be in the correct order. Plus, they're labeled the same too. Am I missing something?
4. If it is the power amp board, then I can go through it with a signal tracer, or simply replace the capacitors. That is my thinking.

Is that a good plan? I appreciate any answers, guidance or information you might have, before I have my amp apart in pieces. I would really like to understand what I am doing, otherwise I don't have a hope of fixing anything.
 
I'm going to move this thread to the Kenwood-Trio specific forum, which is where I probably should have posted it to begin with. Check there for the rest of this thread.
 
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