[sdiy] Clavinet C Preamp Problem

[tprivitera at nc.rr.com]

Thanks to all for the recommendations and things to try.  Something Tom mentioned about a break in the circuit got me thinking and I finally found the problem.

I de-soldered each pickup from the rocker switches and soldered them individually into the input of the preamp.  Turns out, the lower pickup has an internal short to ground.  It measures the correct DC resistance across it's leads, but had next to no output, compared to the isolated upper pickup, which sounded great.  I lift the wire mesh shield on the preamp from the lower pickup's lead and it starts working (although with a VERY loud hum).

Both pickups are always in the circuit when connected to the rocker switches (just different combos of parallel, series, out of phase) and this internal short was causing the bias problem on the transistors and was being masked, because the resistance test across the pickup showed it as good.

Got a new one on order.  Looking forward to this last step in the restoration, as this Clavinet actually has a pretty cool history.

Thanks again,
Tim

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Paul Perry
Sent: Thursday, October 11, 2012 8:41 PM
To: synthdiy diy
Subject: Re: [sdiy] Clavinet C Preamp Problem(more)

And, note what happens when the power is reversed:
the 25uF cap connected to the T2 emitter charges via leakage through T2.
When the power is put the right way around, this cap provides foward bias to 
T2, which then amplifies.. until the cap discharges through the T2 emitter 
resistor.

Maybe I am right - maybe I am not - but the thing to remember is this: every 
circuit always works exactly as the laws of physics say it should. It may be 
that the circuit you are working on, isn't the circuit you think (or wish) 
it is, though!
That's advice from the late Bob Pease, and it's helped me a lot.

Think about it this way: before you can restore your gear to the circuit it 
WAS, you have to determine what it is NOW.

paul perry melbourne australia



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