Ed, on page 11 of my PL1000 Service Manual (Prepared 6/76) it states, the B+ voltage on the positive side of the Log Amp C13 is ".25V to 3.5V
typically 2.85V". I understood this to mean adjust R20 to read 2.85V at C13.
Mark,
Yes I saw that as well. I should have included some verbiage (as noted below) in order to avoid comments (gotcha's) that might follow.
There were (2) different IC's used used over the years (and revisions) of the PL10 log amp pc board in Phase Linear 1000 series 1's.
The earlier version IC (Z1) was a MFC6040 and is discussed in the Phase Linear 1000 service manual. The later version IC (Z1) was a MC3340 and is not discussed in the 1000 series 1 service manual and also happens to be what I have in the 1000 series 1 I am working on. You will see the pc board parts layout and schematics are slightly different in the 1000 series 1 service manual versus the one that appears on
page 29 of the 6/81 revision of the 4000 series 1 service manual.
NOTE: The 6/81 version of the 4000 series 1 service manual also has a copy of the schematic and parts layout of the earlier version of the pc board on
page 30 like that shown in the 1000 series 1 service manual.
Comment: This can all get a bit confusing when you look at the schematics and parts layouts and comparing them to which version you may actually have in your 1000.
The 6/81 revision of the 4000 series 1 service manual states:
"Log Amp adjustment. For earlier log amp circuit boards with MFC6040 for Z1, drive one channel of any line level input with 200mV at 1kHz. Rotate the correlation threshold fully clockwise and adjust log amp trim pot R20 for 95mV at pin 8 of the log amp molex connector. For later log amp boards utilizing an MC3340 IC for Z1 the above procedure also applies, although due to the tolerances of the MC3340 the voltage setting may have to be set higher."
"NOTE: A listening test should always be performed after log amp adjustment to verify proper operation. The best listening test method is to connect a turntable to either of the PHONO inputs and select a recording having a particular high surface hiss passage or lead-in groove. Monitor the signal source (ideally through headphones for increased hearing sensitivity) and verify that, at fully clockwise threshold position, there is virtually no difference in signal when the correlator switch is thrown in and out. With the switch in, counterclockwise rotation of the threshold control should yield a significant decrease in hiss at the 11 to 2 o'clock threshold position range. Rotating log amp trim pot R20 CCW increases sensitivity while CW rotation decreases sensitivity."
By the way, the 1000 series two noise reduction system also used the MC3340 IC.
The 1000 series two service manual has similar verbiage:
section 6-2.7 states:
"
Log amp adjustment and correlator sensitivity.
The log amplifier trim pot R25 can be set by driving one channel with a 200mV, 1KHz sine wave. With an AC voltmeter measure the voltage and the high side of correlation threshold pot R27 and adjust R25 for 95mVAC. Turning the trim pot counterclockwise will increase the output (sensitivity) of the log amp. Any fine tuning of the correlator such as setting the log amplifier for a specific phono cartridge output should be done by ear. Set the log amp trim pot R25 such that the noise from the signal source attenuates in the 11-to-2 o'clock range of the correlation threshold control."
Dean always performs these series of tests when he works on a 1000 or 4000.
By the way, Dean said that he liked the earlier log amp IC and that Bob Carver liked the later IC. If my memory serves me correct, Dean told me the reason why the change was made was because the earlier IC was no longer available and they had to use something else. Perhaps Bob wanted to show or state that any change was always made for the better?
Ed