Re: DC503 Not Working

Roger Evans

When you measure R331 in-circuit there is a parallel current path through U330 and R330 (1k) so you might well see a low value of resistance and it would probably be different if you reversed the polarity of the probes. R331 being low wouldn't be a problem since it is only required to establish a logic 1 level at pin 6. The problem would arise if for instance there was some damage to the PCB giving a low resistance from pin 6 to ground or one of the 'optional' connectors on the TM506 backplane that someone had previously set for a different plugin. As long as the voltage on pin 6 is more than about 2.5V that is read as logic '1'. It will probably measure very close to 5V.

The regulator for the -22V supply is not an IC, it is the discrete circuitry around Q540, Q545, Q548 and the PNP pass transistor in the mainframe.


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