PSN-L Email List Message

Subject: RE: Filter/Amplifier
From: steve hammond shammon1@.............
Date: Sun, 14 Apr 2002 10:31:33 -0700  are you talking about the amp at 
the bottom of this page or like this one this is the steps I follow? Check 
the seismograph for spider webs. Check the connections for dampness and 
water damage. If you can, try to split amp into stages without 
disconnecting the system. Pre amps , filter and leveling. I use jumper 
wires and connect the stages into the A/D.

These are the steps I follow trying to get an amp back to life.

 (1) Check the power supply by running the amps on battery for awhile. 
Check all the resisters with a meter looking for damage. The 100 ohm power 
resisters feeding the +/- 12V to the op amps will crack once in a blue 
moon. Look and probe for loss or bad solder connections on the board. Time 
and moisture damage will cause all sorts of errors.

Did the error go away? NO--

(2) Op 27 have a tendency to respond as you described below when they are 
going out. Use a jumper wire to skip around each op amp stage and see if 
the error goes away. See if you can isolate the error by splitting the pre 
amps from the filter/AC/level gain stage.
Did you get it down to one of two stages? Replace the OP amps in that 
Did the error go away? replace op amps/ repare {} NO--

(3) Replace the two op27's in the pre amp section. If there is any 
question, replace them a second time. This is the most common source of 
noise. Try using a different pair of op-amps. Make certain this is not the 
source of your error.
Did the error go away? replace op amps/ repare {} NO--

(4) Jumper around the 3-poll filter and the AC coupling cap op amp(s) 
connecting the input wire of the 3-poll filter to the input of the leveling 
and gain section.
Did the error go away? replace op amps/ repare {} NO--

(5) Replace the op-amp for the level/gain.
Did the error go away? replace op amps/ repare {} NO--

In the next step you will use fixed value resisters to parallel (or jumper 
around) the two pots. Don't just use a jumper wire.

(6) spot solder the 50K pot with two 25K resister across the wipers of pot. 
(Common in most pots is the center connection)  You won't be able to level 
the output but if the error goes away the pot is bad.

(7) In the gain stage I use a 100k pot. Spot solder a 100k resister across 
the wiper to common of the pot setting the pot to max gain. If the error 
goes way the pot is bad.

(8) Replace the AC coupling cap.

(9) Go back to the pre-amp and start replacing the caps.

     Steve Hammond
PSN Aptos, CA

-----Original Message-----
From:	Randall Pratt [SMTP:randallpratts@...........
Sent:	Saturday, April 13, 2002 8:10 PM
To:	psn-l@..............
Subject:	Filter/Amplifier


Does anyone have troubleshooting experience with the filter final amp 
section of the PSN circuit?  After 2 1/2 years of operation I began to 
notice strange signals in my records.  Now I can only get a full scale 
reading either + or - with a very small change of the centering 50K pot.  I 
removed the pot thinking it was bad but I can't detect any shorts or bad 
spots over the range of adjustment.  If I remove the OP27 from its socket 
the AD reads a steady 2 and responds properly to various voltages applied 
directly to the output pin of the OP27 socket.  I have the input shorted to 
ground and I have tried removeing the LF 353 with no change.  I have also 
swapped the OP27 with another with the same result.

Randy << File: ATT00001.html >> 

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