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Old 31st July 2005, 01:45 AM   #1
beady is offline beady  United States
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Default Help troubleshooting older solid state amp please >>

Hi all,

I was cleaning out the room over the garage and dug out the old Kenwood KA-7002 amplifier my dad gave me as a kid.

I used to use it as a sub amp, but I haven't used it in a long time, but it used to work fine on both channels. I hooked it up and sound only comes out of the right channel. But, if I turn the balance all the way left and turn the volume up 2/3-3/4 of the way it crackles some and then the music will come in, following the bass hits (but it's crackly). If I rotate the volume knob back and forth with the bass it seems to help a little. Sometimes it will break into working full-time on the left channel for a few seconds, but then it drops out again.

If I press the "null balance" button, sound comes out of both speakers OK.

I have tried switching inputs (moved RCAs from input 1 to input 2), and tried different speaker outputs (tried on A and B independantly), but nothing changes the problem.

I tried some contact cleaner spray on the volume/balance control, but that didn't seem to help.

The heatsink on the left side is barely warm, while the right side is more so (normal temp?).

Thanks for any hints/suggestions,

Mike
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Old 31st July 2005, 02:06 AM   #2
beady is offline beady  United States
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Hmmmm, I have noticed that when the left side is playing it is not as "bright" sounding as the right, by a very noticeable margin. Could this be part of the problem, or is it separate? Any way I can fix this?

Thanks again,

Mike
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Old 31st July 2005, 03:55 AM   #3
AJT is offline AJT  Philippines
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without powering on, try wd-40 on all pots, and switches, turning them left to right and back as you spray, get some cotton cloths to catch the extra wd40, do the same with the switches. be generous with the wd40 but wipe off any excess....

then you can try again...
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Old 31st July 2005, 04:05 AM   #4
beady is offline beady  United States
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Any reason WD-40 would work better than a dedicated electronics cleaning spray I tried? It didn't seem to help :-(
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Old 31st July 2005, 04:09 AM   #5
AJT is offline AJT  Philippines
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Quote:
Originally posted by beady
Any reason WD-40 would work better than a dedicated electronics cleaning spray I tried? It didn't seem to help :-(

i am into restoring audio gears and wd-40 works for me all the time, freon based contact cleaners can be used first, then wd-40 for its lubricating effect, loosens dirt also...
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Old 31st July 2005, 04:09 AM   #6
Eva is offline Eva  Spain
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Both balance and 'bright' faults are probably due to worn out potentiometers and/or bad solder joints.

Concerning contact cleaning sprays, I would like to mention that I have never got any positive result from them in worn-out potentiometers
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Old 31st July 2005, 04:11 AM   #7
AJT is offline AJT  Philippines
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yes, i forgot to mention, if cleaners don't help, do the next best thing, change those pots...
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Old 31st July 2005, 04:13 AM   #8
beady is offline beady  United States
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OK, I may try the WD-40.

Something else I noticed earlier is that if I move (wiggle firmly) the RCA input of the left channel it affects the output. Seems to happen on at least aux 1 and aux 2 inputs (only ones I tried).......

Thanks,

Mike
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Old 31st July 2005, 04:16 AM   #9
AJT is offline AJT  Philippines
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contacts on those jacks must have tarnished, that is why cleaning it very well can help...
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Old 31st July 2005, 07:23 AM   #10
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Default I use WD40 for this case, and moving potenciometers up and down many times.

This is really moving, all the way up and down, equipment off...and inserting more WD40 and going again and again...i use to repeat that movement for 3 minutes (around).

I could have good results, not to eliminate all noises, but to reduce them to a reasonable level... to avoid replace the potentiometers...if the noise return.... i think they return, but not so bad as before the "treatment".

Check all trimpots too.... in advance, make marks of their position...one dot in the board and another aligned painting over the trimpot.... and go moving from left to rigth,as oxide will create a lot of problems..do it dry when starting...and them use the WD40 or a better contact cleaner.

First of all, clean all board with a brush.... remove the old board resine... something factory used, to old sets, covering the boad in the solder side, alcohol will dissolve,also Kerozene,that is Jet plane fuel, and many solvents used to dilute paint will help (do not use thinner!) the material used to cover the board, in the copper side, use to absorb watter, and i could fix noises just using a hair drier under the PC board..... clean all dust, do not use watter or any other material over transformers..and take care of alcohol that can destroy plastics...check if alcohol can destroy your painting.... passing it in some corner to check.... so, use it to clean your boards, both sides, as alcohol do not erase silk screen parts numbers and values...but.... to have guaranteed results..test alcohol in those parts too before start with a brush..
Avoid alcohol in the front panel, in special the transparent plastic... more safe to use Kerosene (the aviation jet plane fuel)

All switches need to have WD40 in long jets during the time you will be pushing buttons on and off.

I made that hundred of times.... had one or two troubles, because humidity penetrated the transformer, producing leakages and electrical shocks.....had to keep it under the Sun for one entire morning only..

The WD40 and those movements, are not a guarantee to fix the potentiometer.... but normally reduce the problem to an acceptable level, and also, turns clear when the only possible solution is the replacement of the unit.

I do not want to shock you too much guys, but normally i remove transformer and the unit goes into a small childreen plastic swiming pool, with a lot of strong soap and brush... and it stay there, as a submarine for hours...... after dry, i will start the WD40 and the rest of cleaning..... of course, wooden cabinets must be removed.....oh!..i love to wash those things.

But really, in this thread you have Eva, good to follow her advises...she is very competent.

regards,

Carlos
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