Quad 303 calibration issue after revision

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02-Jun-2019 02:49 AM

jonintaiwan

jonintaiwan

Posts: 10

Just did a revision on a 303, but ran into a problem during calibration. I can get the 67 volts across pins 1 and 9 on both boards, but trying to get 33.5  volts across pins 4 and 6 failed on one board, while succeeded on the other. I just get about 11 volts across pins 4 and 6, and the RV100 doesn't seem to alter this.

Is there an easy solution to this, or do I need to refer to someone with more expertise?

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02-Jun-2019 06:13 AM

EJP

EJP

Posts: 1644

I hope you mean you get 11 volts across pins 5 and 9? 11 volts across 4 and 6 would be catastrophic bias runaway. Anyway check/replace the 150K resistor and everything else leading to or from the DC offset pot. Also check Tr102/3/4, and C100.
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02-Jun-2019 10:21 AM

jonintaiwan

jonintaiwan

Posts: 10

Yes, I meant across pins 1 and 5, not 4 and 6.

Funny thing, I just went to check it all again, and I cannot reproduce the fault this time, so I am not sure what's happening, maybe I have an intermittent bad connection somewhere. I shall monitor the calibration a few times, and check the components you mentioned.

Many thanks for the valuable prompt help, it is much appreciated. Although I have successfully done 3 Quad 303 revisions, I confess that I am not really much beyond the 'electronics by numbers' approach.

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