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921B circuit question

System 55 • System 35 • Model 15

921B circuit question

Postby analogmonster » Thu Apr 05, 2018 12:54 pm

Hi owners, technicians and other knowing people, one can find three trim pots within the sync part of the circuit, one is labeled "Phase Lock Offset" in the service manual (the one which adds a voltage to the OTA output IC13, zero adjust I assume), one is located between the outputs of the 796 Balanced Modulator (100r) and one (10k) which adds a negative voltage at the summing node feeding the sync (Q5). In the service manual adjustment location diagram of the 921B one can find two sync related labels, "Phase Lock Filter Cutoff" and "Phase Lock Filter Balance" beside the previously mentioned "Phase Lock Offset". Does anybody know which trim pot is what and how they have to be adjusted? Thanks in advance, Carsten
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Re: 921B circuit question

Postby WZOLL » Wed Apr 25, 2018 11:29 pm

The 10k trimmer appears to be the phase lock filter cutoff in the service manual by looking at the pcb traces on the 921b board. I would assume that the phase lock filter balance is the 100r which it seems to be by also looking at a picture of the board. I'm not sure how you would set them though. Sorry for the late reply if you've already figured that out.
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Re: 921B circuit question

Postby analogmonster » Thu Apr 26, 2018 8:35 am

WZOLL wrote:The 10k trimmer appears to be the phase lock filter cutoff in the service manual by looking at the pcb traces on the 921b board. I would assume that the phase lock filter balance is the 100r which it seems to be by also looking at a picture of the board. I'm not sure how you would set them though. Sorry for the late reply if you've already figured that out.

Thanks for your reply. What you say is what I assumed according to my understanding of the circuit diagram, and you confirmed that now, so thanks again for your analysis.
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