Jung-Markell Pre Trouble Shooting

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skipwave

Well-known member
Joined
Jul 8, 2005
Messages
1,416
Location
Los Angeles
This pre has been 90% complete and sitting around for over a year, due to a couple failed attempts at building a power supply from scratch.

The thread that started it all: CLICK

I finally powered it up, and it's not passing a signal. My only clues at this point are a couple of strange voltage readings. -9V at the input of the LT1115. -18V at the output of the LT1010 buffer amp. -18V at the output of the LT1115, which I thought might be normal given that the current source is connected between this point and the -18V rail.

I've carefullly inspected my work for proper connections to each IC pin and for solder bridges, and it looks clean and correct. Any thoughts?

Thanks!

Brian
 
I assume the -9V is at the + input of the 1115? Pull the servo amp and see what kind of DC voltages you get now. Set the pot for minimum gain (pot at maximum value).

Still screwed up? Remove the current-source FET. It may no longer be a FET, but a piece of wire.

Still screwed up? Check all ground connections in addition to the voltage rails.

Peace,
Paul
 
Paul,

Thanks for the reply, especially for reminding me to break the circuit into sections to troubleshoot, instead of staring at the whole thing and scratching my head.

I'm convinced its the servo that is configured incorrectly. I didn't socket the ICs, and I don't want to desolder the LT1097 if possible. Is it safe to simply disconnect that chip from the power rails, to see if the rest of the circuit is working?

Thanks again!

Brian
 
[quote author="skipwave"]I'm convinced its the servo that is configured incorrectly. I didn't socket the ICs, and I don't want to desolder the LT1097 if possible. Is it safe to simply disconnect that chip from the power rails, to see if the rest of the circuit is working?[/quote]

I dunno, but if you pull one end of the 10k resistor that's connected to pin 6 of the servo amp, the servo loop will be opened as effectively as if you'd pulled the servo amp.

Peace,
Paul
 

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