KA-7100 Questions...

I recently picked up a KA-7100 from Goodwill with the left channel sounding full and powerful and the right side is small and much lower volume. I have tested this with phono, tuner and aux inputs through speaker A, B and A+B and get the same results. Also tried switching from Tape A source to Tape B source. No change.

Should I start to look for bad components... maybe I can test the output transformers? I've got a multimeter and I'm not afraid to learn how to use it! Will a recap of the preamp section potentially fix such a problem? Maybe something else completely?

After doing some research here on AK I have decided I can probably clean and recap this amp as a way to learn troubleshooting and repair. I've been a sound engineer for 20 years, so I have some idea of how to proceed but still have lots to learn. I have already changed the power supply capacitors but since I've never done a recap before, I'm taking it slow.

Also, on another thread about a KA-7100 recap, it is recommended to change bypass capacitors. For this, the parts list references 4 x1uF film, 5989-100v1.0-F capacitors which I sourced locally. Problem is, I'm not sure where to find them! I have included photos with my guess as to where two of them are...

Thanks for looking!

Spark Arrestor

Your pictures show the spark arrestor for the power switch contacts.

Your symptoms (reduced volume in one channel) may be caused by something as mundane as oxidation in the balance potentiometer.

^ ^ ^ What he said...^ ^ ^

Those Kenwoods have notoriously intermittent switches and pots. This includes speaker, inputs, tone control/mute/loudness switches... you get the idea.

One quick check you could do is this. You see those yellow/blue wires on your attached picture? Unsolder and switch their positions. These are the signal inputs to the amp. Don't unwrap the wires, just move the posts. If the weak channel is the same, it's the amp. If the weak channel moves, it's somewhere ahead of the amp. Odds are that it's not the amp, it will be dirty switches.

Good Luck...

The by-pass capacitors of which you speak are soldered across the main power caps.

That was leesonics thread. Lots of good info to be had.

[thread]462314[/thread].

ovaeasy said:

^ ^ ^ What he said...^ ^ ^

Those Kenwoods have notoriously intermittent switches and pots. This includes speaker, inputs, tone control/mute/loudness switches... you get the idea.

One quick check you could do is this. You see those yellow/blue wires on your attached picture? Unsolder and switch their positions. These are the signal inputs to the amp. Don't unwrap the wires, just move the posts. If the weak channel is the same, it's the amp. If the weak channel moves, it's somewhere ahead of the amp. Odds are that it's not the amp, it will be dirty switches.

Good Luck...

Click to expand...

I will chime in here too and reiterate ovaeasy's advice. First thing to do is deoxit the switches and pots. it took me three times to fix mine.

The 1uf bypass caps you refer to are not installed on the KA-7100 by Kenwood so they will not show up in the schematic. And, they will not fix your problem. If you follow Leesonic's rebuild thread you can restore yours, but it seems like it would be best to first fix the problem.

And step 1 is as above!

Thank you very much for the answers to my questions...

I have now cleaned the pots and switches with DeOxit two separate times, taking them from stiff and gummy feeling to a silky smooth operation, especially noticeable with the volume pot.

I repeated the audio test as before, through all available inputs and routing choices, but the imbalanced output volume remains exactly the same.

I will be testing the front board next this afternoon (after kids hockey) as described by ovaeasy by switching the blue and yellow posts to determine if the problem switches sides. For what its worth, I am suspecting an amplifier problem.

Thanks also for the pointer towards Leesonics' thread... that is where I got the parts list for the recapping and I'll be sure to look it over again to absorb any other pertinent advice...

Sure seems like a lot of us here on AK looking to prolong the life of the KA-7100... interesting statement on the quality found there.

Thanks again... your answers are helping me to gain a clearer understanding... I'll be back with more questions I'm sure!

The low volume channel moved!

So, I switched the yellow and blue leads and the low right channel volume switched to being low left channel volume... thank you for the suggestion! Feels like I'm getting somewhere now...

I will get at the pots and switches again with DeOxit... but if that makes no change for the better, would the next step be to check or replace those parts? Solder connections look ok...

I already have the necessary capacitors to recap the whole front board, but since no one has mentioned those as something to investigate yet I assume it probably won't help with this problem...

thanks!

A third DeOxit cleaning...

did not change the problem...

Thanks for the suggestions. I have a KA-3500 that suffers from similar issues and I miss using it as it was a clean, smooth, reliable power supply. I'd like to bring it back into service so will follow some of this guidance.

Check DC voltage to ground at pins 7 and 8 in the power supply section (on my Ka-7100 these are white and red wires) These pins are in the right rear section of the amp and right on the outer edge or the PCB. This is the regulated power feeding the front end (tone section, etc)

Based on your finding of low output moving R to L when you reversed the yellow and blue wires (these wires are the processed signal that feeds the power amp) there is something amiss in that front-end section that receives its power from pins 7 and 8 mentioned above.

What DC voltage to ground do you have at these pins?


PS I agree that it would be best NOT to recap until after you have solved any other issues. New, unnecessary work opens up potential for new issues, and trouble shooting gets far more difficult when you have multiple issues as opposed to one single issue. Tired caps is not the most likely cause of your problem.

Checked the voltages at pin 7 + 8

pin 7 - 29 volts
pin 8 - 27.4 volts

amp was on with no load / volume...

thanks for your help

Ok. Well those values being what they are, I am not knowledgeable enough to say whether that is the cause of your channel imbalance. Maybe it is, maybe not.

Ideally they should read around +/- 28v. On my KA-7100 they are around -28.3v and +28.4v. The voltages that you are seeing are not ideal, but whether they might indicate the cause of your problem I do not know.

Hopefully EchoWars will pop in with guidance...

Too bad the deoxit didn't fix this. I see you've tried all of the inputs. Other preliminary things to rule out, I imagine you've already swapped your left and right inputs for testing as well as the left and right speakers. If you have headphones they can be useful for testing. When you switch to mono, do you get full sound on both channels?

Have you measured the DC offset (center voltage in the SM, p.16) and Bias voltages (p. 17)? (Edit: no need to check right away, as Leesonic points out below the amp section is OK).

That's about all I can offer for the moment, just basic stuff, hopefully one of the experienced techs will weigh in.

If you find the tape and source selectors difficult to turn, there are some brass bushings the shafts pass through just behind the face plate (the SM labels it the sub-panel, p.5). A small drop of sewing machine oil will loosen them up.

OK, so we've already established the problem is in the preamp section of your amp by switching the yellow and blue wires. Plus you've already tried DeOxit and that didn't work. You probably have a bad cap or a bad transistor somewhere. Do you still have the same problem through the phono input compared to the aux or tuner input? How about with the tone controls switched out? Bypassing the tone controls takes out quite a bit of circuitry.

You could pull caps and test them, or you could replace them. If you plan on keeping the amp then it makes sense to replace them anyway. You can cut down on the cost quite a bit by not replacing the four main caps, but this is a false economy IMHO. You could also pull the transistors and test them, or replace them. Replacements are cheap enough.

Hope this helps,

Lee.

Audio through the headphone output...

... follows the same pattern as through speaker outputs: Left side normal / good, right side low / bad. Stereo and mono output.

Engaging (or dis-engaging) tone controls does not affect the problem and seem to work fine.

All inputs (tuner, phono, tape and aux) are the same at output.

I already have the caps for the front board ( and the rest of the amp) so I will re-cap it later and see what happens... also, I have sourced a replacement transistor for the Q34 position as mentioned in Leesonic's original post on this subject (thanks for that!) and wonder if replacing that could affect this problem, too.

A few weeks ago I tested the DC offset and it was well within acceptable range, as I remember ... have not tested Bias voltage, but will look at the info you point to and learn to do that, too...

Thank you!

Wow!

I just did a quick cut and paste from my spreadsheet into a Mouser parts list importer. It came back with $75 worth of components, and this was just caps, trimmers, relay, couple of transistors, plus a few other parts. It did not include any small signal transistors, or the outputs. Kinda makes you wonder, when you restore these things like I do, if it's really worth it for the small profit you make.

Lee.

leesonic said:

Wow!
Kinda makes you wonder, when you restore these things like I do, if it's really worth it for the small profit you make.

Lee.

Click to expand...

True. When you consider the time put into it, it's more of a public service.

danny1968 said:

also, I have sourced a replacement transistor for the Q34 position as mentioned in Leesonic's original post on this subject (thanks for that!) and wonder if replacing that could affect this problem, too.

A few weeks ago I tested the DC offset and it was well within acceptable range, as I remember ... have not tested Bias voltage, but will look at the info you point to and learn to do that, too...

Click to expand...

I think Q34 is a relay driver. In Leesonic's post he replaced some of the tone board transistors, but they may not be listed on the BOM. You can use these as replacements if you go that route: KSA992 for 2SA640 and KSC1845 for 2SC1400.

I have restored 2 KA-7100's, they are relatively easy to work on. I was very pleased with how good they sounded afterwards, so I am sure you will find it rewarding. One of them had bad output transistors on one channel and was blowing fuses. The other had either very low, or no output out of one channel, and the fix was deoxiting the switches, hence my recommendation. I thought it was hopeless, but eventually it did work.

Results! The good and the bad...

... and the downright mysterious.

Good: Re-capping the front board has brought the low channel up to proper volume!

Bad: Re-capping the front board has somehow caused the amp to go into 'protect' when the bass/treble filter engage switch is engaged. That did not happen before the re-cap.

So, the front board had clearly been 'repaired' and generally treated roughly during what must have been a previous attempt to re-solder the original caps in place. Maybe a previous owner tried to look for a cold solder joint as a solution to the problem? There were burns and some of the conductive 'printing' on the pcb was missing at the connection of cap c4, so I made a 'jumper' to R18, where I followed the original connection to (see photo). Maybe that was a mistake, the board connections are hard to see and I kinda suck with schematic reading - translating. Hope you can see it in photo at the left lower side.

Maybe this is my problem now? Otherwise, any help on troubleshooting the bass / treble filter section / switch would be great... I suppose I should De-Oxit the switch again just to be sure...

Thanks for looking!

Check your caps. I had a 7100 do exactly the same thing after I recapped it, it turned out I'd put one of the caps in the wrong way. Failing that, maybe a bad transistor. If it's not a cap problem, I'd replace them all anyway.

Lee.