2 trim pots added?

[mobilefm]

Iv'e just finished a C84 kit and found no mention of what to do with the 2 added trimmers.
The online instructions refer to R68 and R72 for setting voltage, what should I do with trimmers R33 and R36.
It seems like you could use these to turn off the OPAMPS and set voltage with the OPAMPS installed?

Anyone else have revised C84's

[tpryan]

First use R68 and R72 to adjust the power supply rails to +/-18V to +/-24V. The higher the voltage, the more headroom you'll have, up to a maximum output of about +32dBu at +/-24V. Set the gain to maximum and adjust R36 for minimum DC between the outputs of U1. An easy place to measure is at R26 and R30. Then use R33 to adjust for minimum DC at the output (CONN2, pins 2 and 3).

[mobilefm]

Tim,
Thanks for the fast response.
Setup was easy enough, I adjusted to +24V and -24V, and yes the Burr browns do get hot, but where do they sound best, or will the extra heat be a problem?
Thanks, Fred

[LewKellogg]

A couple of questions about the added trim pots...

First, should the Op Amps be in or out for the "low DC"measurement/adjustment? I know they are out for the voltage adjust - That worked like a charm - I am at 18V per the instructions.

When you say "minimum DC" at outputs of U1 as measured between R30 and R26: What is a reasonable number? I'm reading very low (like 0.09 volts).

For the second reading at CONN2 what is a reasonable value: I'm reading 4.5V between pins 2 and 3.

[tpryan]

The op-amps need to be installed before you can make meaningful measurements at R26/R30 and CONN2. In each case "minimum" means as close to 0 as you can get. Look for values below 25mV at both test points.

[LewKellogg]

Thanks Tim!

[Padded Cell Studios]

I am a little confused about this step. Do I put the negative probe on J3 pin 2 and where do I put the positive probe.

[Padded Cell Studios]

Also U1 gets hot while U2 (U3 on board) stays cool.

[tpryan]

I am a little confused about this step. Do I put the negative probe on J3 pin 2 and where do I put the positive probe.

To adjust R36:

Apply power
set gain to maximum
Set DMM to measure DC voltage
Connect probes to the outputs of U1. A good place to probe is at R26 and R30. Polarity is not important.
Adjust R36 until the voltage is as close to 0 as possible.

To adjust R33:

Apply power
set gain to maximum
Set DMM to measure DC voltage
Connect probes to CONN2, pins 2 and 3. Polarity is not important
Adjust R33 until the voltage is as close to 0 as possible

[tpryan]

Updated assembly instructions are up.