[sdiy] Clavinet C Preamp Problem

Tom Wiltshire tom at electricdruid.net
Wed Oct 10 22:42:32 CEST 2012


Two things occur to me:

Germanium transistors vary *a lot* - is the circuit sensitive to gain variations? Several famous germanium circuits need certain gains or certain ratios to get the sound right.

You've replaced most/all the components in the circuit, so we could assume those are ok. That only leaves the joints and the tracks, so check those. On a old board, the tracks can crack quite easily, especially if it's a paxolin board. I'd look carefully for hairline fractures.

HTH,
Tom


On 10 Oct 2012, at 20:32, <tprivitera at nc.rr.com> <tprivitera at nc.rr.com> wrote:

> 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
> 
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