Home » VintageKustom.com » Repairing Kustom Amps » K200B-2 Both Preamps bad? (Low - No output from preamps)
K200B-2 Both Preamps bad? [message #26786] Mon, 09 July 2018 12:18 Go to next message
Saransk is currently offline  Saransk
Messages: 11
Registered: February 2017
Location: Northern Virginia
Junior Member
Found a nice K200B-2 head for sale. Description lead me to believe it would be a simple fix.
"Low or No output"
Preamp Boards 105 and 303
Got the amp, pretty loud hiss from Power Amp. someone had tried some troubleshooting - found disconnected blue output wires and a jumper wire in cabinet.
Repaired the blue wires
Plugged in to both inputs from both boards - very low sound from Normal (105) and none from Bright (303) channel.
Voltages on both input transistors looked off (Q100/Q300) - just happened to have some. Replaced both #1 transistors - hope for the best. No joy.
The rest of the voltages on the boards seem pretty good, I get the 2.3 volts on Q104's emitter.
Possible that the Tantilium caps are bad - replacements on the way but the front end voltages seem to be the culprit.
Here's what I see. Same issue on both boards. First, the Emitter of Q100/Q300, instead of the -.5 volts the schematics read, I'm getting @ +.9 volts. That causes the Q101/Q301 Collector to be off. moving the volume control doesn't change the reading. Unlike the usual failure it appears that the 2N3638 PNP is bad on both boards - is that a reasonable deduction - or could something else be causing the Emitter of Q100/Q300 to be off.
The amp, other than the loud hiss, seems to be in good shape - voltages are where they should be. I jumpered from the input jack to the input side of R700 (33K) on the Amp board and got a good output - low, but good.
This is not the first Kustom amp I've redone but it is the first that both preamp boards have the same failure.
Any ideas on some line of troubleshooting I've missed
Thanks
Re: K200B-2 Both Preamps bad? [message #26787 is a reply to message #26786] Tue, 10 July 2018 12:03 Go to previous messageGo to next message
chicagobill
Messages: 1860
Registered: April 2003
Senior Member
It sounds like you are on the right track. I often find that I need to replace both input transistors to get a channel working.

The volume control is dc isolated from the circuit by the two caps C103/C104, so unless one of them is shorted, changing the volume control should not change the dc voltages on the input transistors.

What are the other voltages on Q100 and Q101? Are both 8 volt supplies working?

I would resist changing any caps until you prove them to be bad or until you have the channel working and want to upgrade for better sound or reliability. Too many changes can lead you in the wrong direction, which may make the repair harder.
Re: K200B-2 Both Preamps bad? [message #26788 is a reply to message #26786] Wed, 11 July 2018 22:01 Go to previous messageGo to next message
Saransk is currently offline  Saransk
Messages: 11
Registered: February 2017
Location: Northern Virginia
Junior Member
Replaced the 2nd transistor in the input darlington pair - 2N3638 on both boards - sound - good sound.
Normal channel sound just like it should - normal
The "Bright" channel sounds like there is a an overdrive boost attached.
But the volume control doesn't seem to change the it much
The Reverb and Tremolo barely work so I suspect it will take some sleuthing to find the parts that are not working.

The couple of darlington pairs are suspects as well as the tremolo diodes.

Any other suggestions welcome

Mike
Re: K200B-2 Both Preamps bad? [message #26789 is a reply to message #26786] Thu, 12 July 2018 05:58 Go to previous messageGo to next message
stevem is currently offline  stevem
Messages: 4064
Registered: June 2004
Location: NY
Senior Member
There are many revisions of that PC 303 board so how is yours stamped?
Re: K200B-2 Both Preamps bad? [message #26790 is a reply to message #26789] Thu, 12 July 2018 22:26 Go to previous messageGo to next message
Saransk is currently offline  Saransk
Messages: 11
Registered: February 2017
Location: Northern Virginia
Junior Member
The Board is marked KEI-1 PC303
Is this the first revision?
The circuit seems to be actually either revision 5 or 6 as there is a 2200 resistor on the collector of Q302.
Looks like Q302 might be the next candidate for replacement.

Mike
Re: K200B-2 Both Preamps bad? [message #26791 is a reply to message #26786] Fri, 13 July 2018 13:30 Go to previous message
chicagobill
Messages: 1860
Registered: April 2003
Senior Member
So the problem is that the FX channel is distorted and the volume control is not working?

Divide and conquer. Turn off the reverb and the trem/vib and turn off the bright switch, any change? With the reverb on, shake the tank to hear the springs rattle. Is the sound of the reverb springs distorted? Is there background hiss? If there is, do the tone controls have any effect on the sound of the hiss?

Start reading voltages through the channel and compare to the schematic values. Now is where you start to look for leaky caps that cause voltage disruptions to the circuit.
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