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K200B-2 Both Preamps bad? [message #26786] Mon, 09 July 2018 12:18 Go to previous message
Saransk is currently offline  Saransk
Messages: 20
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
 
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