Help with Koch Studiotone 20w Guitar Amp

[sonolink]

Hello everybody

A friend has asked me if I could repair his Koch Studiotone. The Mid was noisy and the Reverb pot was broken and sometimes the amp would loose volume. So I ordered 2 B20k Alpha 16mm pots and replaced them. Now I don't have any sound at all...

I tried replacing all the tubes with new ones but no go. I've checked that I have replaced everything as it was before disassembling it and inspected carefully in case some cable had gone loose but I can't find anything.

Any ideas for helping me troubleshooting please?

Cheers
Sono

 

[PRR]

NO! sound? Not even hiss?

Bad speaker connection?

Can you work inside without getting killed?

Are there DC voltages? Higher (300V-400V) at the power supply. Half that at most Plates?

Signal tracer. Find where the signal stops.

 

[CJ]

going to be tuff without a schematic and no joy searching,

also a cramped build,

look for a solder short or lifted trace near the pots you soldered,

what kind of test equipment do you have?

start from the back and work your way forward, touch the grids of the EL84's with a screwdriver and see if you hear anything, if not, it might be the output transformer, measure the ohms across the primary (plate to plate) with the amp unplugged, look for an open winding,

if you get noise by touching the pwr tube grids, work back to the phase inverter, probably a 12AX7 or AT7, touch the grids, pins 2 and 7 with a screw driver and check for noise, do the same thing with the preamp tubes, check the input jack, does it have an effects loop? try putting a patch cord into the send and return jacks,

 

[sonolink]

First of all THANKS for your replies
I will check everything you suggest as soon as I get back home (I went on tour for a few gigs just this morning...)

NO! sound? Not even hiss?
Bad speaker connection?

Ok. I'll check that but I doubt it since there was sound before I replaced the pots. I've checked input connections and all seems ok except for a resistor that should be 1M according to its color code and reads 1.23R on my multimeter. In any case even if I´m not seeing the colors right I don't think that would be a reason for no sound at all.

Can you work inside without getting killed?

I can indeed sir
I'm not an experienced EE or anything like that but I'm not a novel builder. The problem is I don't really KNOW this amp. I'm more of a Plexi p2p builder but hey, it's a friend right?

Are there DC voltages? Higher (300V-400V) at the power supply. Half that at most Plates?

Ok. I haven't checked voltages since I thought this was going to be a simple pot replacement but I'll check that.

going to be tuff without a schematic and no joy searching, also a cramped build

I know. I hate these and that's why I'm crying out loud for help

look for a solder short or lifted trace near the pots you soldered

I already did but will do again

what kind of test equipment do you have?

Multimeter. Scope.....what else do I need?

start from the back and work your way forward, touch the grids of the EL84's with a screwdriver and see if you hear anything

I'll try that and report back
Thanks CJ and PRR

 

[sonolink]

Hello again

I've got sound now. This amp has a recording output with a switch that connects a dummy load and disconnects the speakers...... :

So anyway I do have sound BUT very low volume. Something is not right.
Tubes are new so I'll try to read voltages on them and report back. All input is very welcome

Thanks
Sono

 

[sonolink]

Found schem!!

http://www.larsmusik.com/ST.pdf

 

[sonolink]

Voltage at EL84 anodes is about 390VDC.

I tried plugging the guitar in the FX Return and it sounds fine.
So my guess is that the problem is in the preamp section.

Any ideas?
Cheers
Sono

 

[sonolink]

Did some chopsticking. The lead between the input jack and the board in funky. I'm going to resolder it and report back

Cheers
Sono

 

[sonolink]

Problem solved

The lead between the input jack and the board was funky and probably went even worse when I disassembled half the amp to be able to replace the pots. I had to disassemble the rest of the amp to be able to replace the shielded cable and it was a bit of a PITA but now it's rocking.

Thanks for your help guys!!
Cheers
Sono

 

[CJ]

awesome!  thanks for the schemo!

what kind of speaker do yhey use in that?

cj

 

[sonolink]

This one wears a Celestion G12 Vintage 30 but originally it comes with a c..ustom designed VG12-60 Koch 12" speaker.

It sounds ok. It's a small club no hassle to cart around amp, versatile (3 channels). Personally I prefer my 18W single channel handmade Plexi. I seldom find versatile amps that have character, hehe, but again it sounds ok