Author Topic: Pianet L Preamp Stopped Working  (Read 483 times)

Offline theseacowexists

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Pianet L Preamp Stopped Working
« on: May 13, 2018, 08:37:10 AM »
I really should stop poking around in these things...

The amp tech who initially replaced all the e-caps when I bought the Pianet L added a small film cap between the zener diode and ground (I think), since he thought it might help with the noise. It didn't really, so at one point I disconnected it by just disconnecting one of the leads from the board.

Well that came back to bite me today when I was poking around trying to see something, the clipped lead of the film cap came into contact with where it had been clipped off, made a tiny spark, and no more sound.  Now, in the past when I've done work on the preamp, sometimes it doesn't work when I turn it back on.  The solution to this has been clipping a lead from the pickup itself to the tip of the output jack.  This was a suggestion from the same amp tech who worked on it, and it worked every time.  But not this time.  Tried it several times, and no sound ever came back to either the speakers or the output. 

The Pianet is definitely getting power, since the transformer buzzes like it always does, and the pickup is getting 140 volts as usual.  But there doesn't seem to be any voltage going to the preamp section.  The irony of this is that the reason I was in there was to draw a diagram of the preamp so that I can try to have a better understanding of how it works!

Offline OZDOC

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Re: Pianet L Preamp Stopped Working
« Reply #1 on: May 13, 2018, 08:08:52 PM »
Sounds like you may have temporarily exposed the transistor that follows the Zener diode to the 280V of the reed bar.
Try replacing the Zener first then replace the transistor next to it?
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Offline theseacowexists

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Re: Pianet L Preamp Stopped Working
« Reply #2 on: May 14, 2018, 09:37:18 PM »
I was looking at it wrong - the cap he added was between the E300/C5 diode and the ground.  And that diode is reading OL in both directions.  Still no voltage coming from the transformer to the E300/C5, but I think it's safe to assume that the diode is shot.  Any ideas on locating an equivalent? The zener and the AC107 are measuring normal. 

Offline theseacowexists

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Re: Pianet L Preamp Stopped Working
« Reply #3 on: May 30, 2018, 10:14:45 AM »
Turns out that a wire near the voltage regulator pot was crimped against something and shorting out the whole circuit.  Uncrimping the wire brought the Pianet back to life!  Odd thing is that this wire was nowhere near the area where that film cap sparked and started the whole problem.  This was discovered by the tech who originally worked on it for me, after he determined that the transformer was OK, and that the problem might have been in one of the transistors.  He eventually started looking for a bad connection or short and found it.

It's no wonder they switched to PCBs starting on the N models.  The L preamp just seems downright primitive compared to the later preamps.  It does have a different sound than the N, which I can't quite describe, but I do appreciate the difference between the two.  Now if only I could solve the buzzing problem, this L would be perfect...