Re: DC503 Not Working

Roger Evans


You have (at least) two distinct issues with comments coming in which are relevant to one or the other and are randomly interleaved.

1. The LCD display does not scan so typically only one LED is lit. This you have traced to U330. One thing that hasn't been mentioned here is that the output from U330 pin 10 also goes to U332 pin 6 so a fault in U332 (which I don't think you have changed) would stop the multivibrator from working. If U332 is socketed, just remove it and see if you can get the square wave on U330 pin 10.

2. The -22V supply is somewhere around 0V. This completely changes the bias of the analogue input stages and makes it less likely that any inputs will actually trigger the digital circuitry. Here, I at least, am confused as to whether there is too much current draw or a fault in the voltage regulator. If you can measure the voltage across R546 it should help to narrow down the problem. If the voltage across R546 is more than about 0.6 - 0.7V it will turn on Q540 and shut down the regulator as intended - then you have to track down what is drawing too much current. If the voltage is much less than this then there is likely to be a fault in the -22V regulator and you could measure the voltages on the base, emitter and collector of Q548. It is also worth checking that R546 is close to its nominal value, if it has drifted high it will shut down the regulator prematurely.


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