1178 meter calibration

[salomonander]

Hey there,
Im calibrating a 1178 and im not certain if my results are ok.
Im talking about the meter calibration and tracking of the gain reduction.
The service manual wants me to feed 0dB 1k sinus and adjust to 5dB of gain reduction on the +4 meter setting. I then have to adjust the meter tracking trimmer to get the same result in the GR setting of the meter. Works just fine and everything under 5dB of gain reduction is shown properly on the meter when comparing the +4 and GR setting.
But when i go above 5dB of gain reduction the tracking goes off. What the +4 setting shows is not exactly what the GR setting is showing. Now i can adjust everything to 10db of gain reduction (as opposed to the 5db) and the tracking gets better in that region. But it then gets worse in the lower settings.

Is this normal? Or am i missing something?
Here is a link to the service manual:
https://thehistoryofrecording.com/Manuals/UREI/UREI-1178_Manual.pdf

 

[gyraf]

Exact monitoring of gainreduction is really not that important. And the meter is pretty slow anyway...

 

[salomonander]

Thanks Jakob,
just wondering if this could have anything to do with the FET biasing. I did have some problems there.
The manual asks to set the output to +1dB and then turn the trimmer till -2dB (615mV) is reached.
But both trimmers seemed to have some sort of decay. Whatever value i set it on, the voltage kept on climbing up (extremely slowly) for about 10 minutes. So the best value i was able to set was 630mV instead of the 615mV. Could this contribute to the issue? And is the decay in the trimmer normal?

 

[salomonander]

I did figure things out by now. Close the window was the secret. The setting is extremely temperature dependent. I was now able to have both channels settle at 615mV. Thanks