KB100 low and distorted volume

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Ted
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KB100 low and distorted volume

Post by Ted » Sun May 07, 2017 3:30 pm

Hi all,

I recently bought myself a third or even more (who knows :D ) hand kb100. Ever since I was a teen going into the local music store, that peavey logo had a mesmerizing effect on me. So I just had to buy one. If only I would have checked it out a bit more :| It seemed to play ok, although I noticed they used the power amp in. That should have rung a bell right. Well it didn't, I was just to greedy to get my hands on a Peavey.

Back home I noticed that all three channels have low volume and sound gets distorted if you turn op the level (or raise the level on the input source). Tried several keyboards and other sources (cd amongst others). all gave the same 'thin' over compressed sound. Since the power amp works fine I concluded the problem must be in the pre-amp section.

So I started looking around for info on the net. I found earlier threads on this forum talking about similar problems with a KB100. Reading up on advise given that probably one or more op amps had gone bad, i've tested the +15 and -15 volt on all op amps (all have 15 volts on pin 8 and -15 volt on pin 4). I also followed Enzo's tip to check on several volts being present on the outputs. none worth mentioning (all either 0 or very low, like 0.15). Since those op amps are readily available and are quite cheap I dedecide to go ahead and exchange them. Just to be sure. No luck. all (except the 87478) have been exchanged but the problem is still there. So I started to test the condensators, lifting one pole from the board and my DMM using the ohm and then DC 'trick'. all seemed to work, seeing a rapid increase in resistance and a decrease in voltage while measuring DC. I even went out decoupling the zener diodes and testing those. They work, of course they do :). The pre amp section however, unfortunately, still isn't.

I did discover something odd. My particular KB100 is a 1989 model, at least the circuit board states 801989 and bd no is 98851152, the schematics I found of an 1988 KB100 are for bd no 98851151. That's not odd of course but what is, schematics and diagram for the *1* version both show a 10k resistor (R145) but it's not there on the board. holes are though. Humm, strange. Is it left out on purpose in the *2* version? Can anyone tell me if this was left out on purpose?

Kind regards,

Ted

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Enzo
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Re: KB100 low and distorted volume

Post by Enzo » Sun May 07, 2017 6:54 pm

The VERY first thing I'd do would be plug a spare cord from PREAMP OUT to POWER AMP IN. Especially if they were using the power amp jack. If the jumper cord restores sound, the return jack (power amp in) has a bad cutout contact.

Otherwise isolate the problem. Plug a signal into one of the input jacks, now run a cord from the PRE EQ PATCH jack over to some other amp and speaker. Does that sound OK?

Plug your signal INTO that patch jack, but only to the first click. (It is a TRS jack) Does that come out OK or does it have the issue?

Turn up the reverb and rock the amp to crash the springs. Does that noise come out strong and clear or does it still have the problem?

There is a remote footswitch jack for the reverb. Plug a signal into that jack and turn up the reverb control. Does that signal come out OK or has the issue?

Each of those tests will narrow down the area the problem lives in.

R145? The one I see is in channel 3. Channel 1 also lacks one in that position, while channel 2 shows one. In any case, it would only affect channel 3 if it were really "missing", so use the other two channels for now until you fix the main problem, then you can go back and see if channel 3 really has a problem.

Ted
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Re: KB100 low and distorted volume

Post by Ted » Mon May 08, 2017 3:45 am

Hi Enzo,

thank you for your help on the R145 mystery. Agreed it won't cause the problem but it did intrigue me to be sure :). I did perform the Pre-amp to power-amp patch. Didn't resolve the issue.

Since I got the amp out anyway I took timer to clean all contacts and pots. They all check out ok. Did not try to connect the pre-eq out to a different amp however. Will try that for sure!

I've been crunching my rusty old gears on this one for days. It doesn't seem to be a very complicated board, but it sure is challenging enough. Problem is that I do not own a scope so that complicates things a bit I guess :)

I'll let you know how it went with the pre EQ out. Thanks again for your help!

Kind regards,

ted

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Enzo
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Re: KB100 low and distorted volume

Post by Enzo » Mon May 08, 2017 3:41 pm

All those tests I described involve only connecting signals, scopes are great, but not needed yet. What we do need is to isolate the problem to a section of circuit.

Ted
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Re: KB100 low and distorted volume

Post by Ted » Mon May 08, 2017 4:13 pm

Hi Enzo, Fair enough :);

I checked the pre EQ out. Good signal. Then I rechecked the pre and power amp connections at the rear. I must have overlooked previously but I found a clear suspect in a blown 22uf 25v condensator (C200). Must have been focussing on the pre-amp a bit to much, still can't fathom how I overlooked this previously :S

Ordered a new (actually also a couple of spare ones for future use). Makes sense that it might be the culprit?

Again thanks for your help and tips!

Kind regards,

Ted

Ted
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Re: KB100 low and distorted volume

Post by Ted » Sat May 13, 2017 9:34 am

And it's fixed. It turned out to be the condensator. :D thank's again for your help Enzo!

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