Re: Advice for a sick DC505a- discovered a mainframe issue as well.


On 24 Dec 2015 18:08:56 -0800, you wrote:

Ok, I got the unit sorted out. Took my time and dismantled it and separated the PCB sandwich, reseated the connector in question, which was quite loose, and did continuity from transistor driver to common anodes.

All digits are ok now.

Upon power up the odd gating and counting had stopped and all digits were stable. This turned out to be an issue with the TM503 vs the TM501 mainframe. It seems the TM503 power rail makes the DC505A counts with no signal. With a signal it counts by about 15% off. I tried it by itself in a good. Different slots gave same issue. Yet an SG503 and PS503A have no probs in the TM503.

I note that the DC505A manual (pg 7) mentions a ripple capacitor upgrade (11k to 18Kuf) when used in a low power slot IF there are multiple DC505A units in the mainframe. Well, I just have one.
I will have to remember to take a look at this on mine. It is touchy
after it warms up and I am still working on it. It never occurred to
me that the problem might be related to the power supply mainframe.

DC505A installed in a TM501...all is stable and counts are perfect.

Did some basic tests...with a sig. from the SG503....Frequency counts via Chan A were good and period measures on B are good as well. Trigger level adjustment pots are a bit touchy, but I guess with some use that may clear up.

Can't see how to service those pots with Deoxit.
One problem I had on mine was false counts between the 4th and 5th
digit from the left caused by noise from the display multiplexing
getting into level shifter Q680 shown on schematic 3.

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