[sdiy] Clavinet C Preamp Problem

[tprivitera at nc.rr.com]

All my analog synths are working great right now and I’m hoping there is someone here who can help me with a Clavinet C that is giving me fits.

 

This is a very simple circuit with only 4 caps, 2 germanium transistors and 6 resistors.  See: http://www.gti.net/junebug/clavinet/Clavinet_C_schematic.pdf 

 

The preamp is only putting out a very minimal signal.  Weird thing is, when I reverse the battery terminals, holding the battery on backwards for a while, then flip it back right, the Clavinet will work perfectly for about a minute or two, then a little static comes and the volume drops WAY back down again.  I can reliably repeat this process.

 

I believe the pickups are good as I can hear sound out of each of them and they measure about 180 ohms DC resistance across their leads.  The rocker switches were initially making a lot of static and crackling noise in certain positions, so I disassembled them and carefully cleaned the contacts.  Now there is only a slight pop when I switch them back and forth.

In the preamp itself, I replaced all the electrolytics with new, good quality caps and swapped multiples in to make sure a cap wasn’t bad.  I removed every resistor and tested to make sure the values were right and I sourced NOS AC151R transistors and swapped new ones in.  None of the electrolytics are in backwards.  I also replaced the one non-polarized cap where the pickups connect with a new one.  I also tried swapping in 1.5k resistors for the 620 ohm resistors, which dropped the voltage down a little on the transistors.  That didn’t work either and I put the 620’s back in.

I'm pretty much at my wit's end with trying to figure out the problem.   Anybody seen this before or know where the issue may be?

 

Thanks in advance,

Tim