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EIP 545B Frenquency counter

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Dave Cawley:
Thanks HP, I'll try that later but we are entertaining Jill's Mum and a Church lady today !

BTW I missed the keyboard test page.

More later

Thanks

Dave

Dave Cawley:
.
Well, I checked all the Tantalums and they were all just fine, apart from the one I already replaced that had a dead short.

I have cleaned all the interconnect pins between the two boards and with some new pins exercised the sockets with switch cleaner.

So far it is the same as before,  displays the correct frequency on Band 3 but unable to switch to any other band or to switch the power meter on, only the power meter offset and a frequency limit switch seems to do anything.

U12 pin 2 the keyboard interrupt stays low until any key is pressed.

U11 the MC68B21P, Figure 111a shows the clock on PA1 pin 3 to be continuous, I cant's see anything there.

Any help greatly appreciated.

Thanks  Dave

Dave Cawley:
The clock waveform/drawing isn't scaled or accurate in the service manual.

It turned out to be :

I had another look, this time with my elderly but lovely HP Logic Probe.  When I probed the 74LS195 pin 10 the clock input, and this chip is the keyboard scanner, it started working and kept working.  I had a look and only on that chip nearest the top cover edge,  had corroded pins, so I put it in an IC runner/straighter and cleaned one side of all the pins with very fine emery as well as switch cleaning the IC socket that looked OK anyway.  It's now stable as a rock, even the limited power meter function works !

So just one tantalum capacitor and a clean of one IC's pins.

Thanks for the help and inspiration here.

Dave

HighPrecision:
Sorry for late response, I'm living very sad days in this period, just lost my beloved father   :'(

Good, I've suspected the KB row counter U3, well done.

From your first post I expect a 548A model, the PIA 68B21 is present in newer units (548B), based on the new A105 microprocessor board clocked @2MHz, check the A105 if it's a newer board, also installed in 548A CCN# 2309

Dave Cawley:
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.
Very sorry to hear about your father.

Yes, I said it was an "A" when in fact it is a "B", not sure why I said that.

The good news is that now it is perfectly responsive whereas years ago,  when it last worked,  it was often difficult to drive, maybe the corrosion had already started to set in.

Attached is a photo of the Micro board with 3 chips missing !  But it works and I believe it has never been touched as I know the original purchaser.  BTW if you right click and copy the image it is 3K wide.

Regards

Dave

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