Re: DC503 Not Working


On Mon, Jul 27, 2020 at 10:52 PM, Roger Evans wrote:

Interleaved answer.


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.
You mean that when the unit is on, only 1 “zero” should show?
I’ve never had one of these. I thought 4 should be show (from picts I’ve seen).

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)
I have changed ALL the IC’s on that board but the 2 that I mentioned in an earlier post.

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.
Will do.

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.
When I measured 0V across that capacitor, the top “Trigger Board” was removed. Hence the cap was not
connected to the coil.

Here, I at least, am confused as to
whether there is too much current draw or a fault in the voltage regulator.
Both Voltage regulators were also changed. I really changed EVERY IC’s on that board but 2.

If you can measure the voltage across R546 it should help to narrow down the
Will do and report back.

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.

Thank you Roger.

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