Repairing old Heathkit GC-1000 Most Accurate Clock

[joeqsmith]

This clock was given to me in non-working order.  It was released in 1983 and assembled by the original owners father.  A quick check,  there was a bad 60Hz hum.  When I pulled the main filter cap, one lead fell out.  Suspect it corroded.  I checked some of the other alum. elecs. and a few had high ESR or very low capacitance.  One 22uF read 2uF.   I didn't find anything else wrong with it and went ahead and realigned it.  It's been running now in this current state for a few days and maintaining a lock. 

Pretty cool bit of history.

[floobydust]

That's really nice, a lot of advanced engineering for the time. Dec. 1983 $250 kit price is $790 in today's dollars. That was a big purchase.

It seems to have a Mostek 3870 MCU driving a DAC controlling a varactor diode (VCO) on a 3.6MHz ref. oscillator. Like an early GPSDO but WWV instead. I wonder if anyone else did this back then? On an odd ball MCU with just an accumulator, also doing the LED mux. Pretty good for a few KB.
"Propagation delay can be set for up to 18.75 milliseconds (3600 miles from WWV in Ft. collins, Colorado)."

A History of Early Microcontrollers, Part 6: The Fairchild F8 and Mostek MK3870
TEST pin can be exploited to read out the firmware https://hackaday.com/tag/mk3870/

[Haenk]

I'd like to mention

https://www.mikrocontroller.net/attachment/108482/Elektor_10_1980g.PDF

That's 1980, and an even more proper "most accurate" clock

[joeqsmith]

Notice in this video that the bottom of the unit has various decals.  Also notice the test board is completely different.  It looks like someone had swapped in a SMP in place of the linear regulator on theirs as well.    You can see the damage to the PCB on mine from the heat.  Because their father had worked for Heathkit, it is very possible this was one of the pre-released units.  This is how I ended up with the service manual.



A site where there are swapping in a SMP. 
https://www.thomasnixie.com/thomas-nixie-history/heath-kit-gc-1000-most-accurate-clock

A video where they are swapping in a SMP.


Manual for it
https://www.scribd.com/document/695099016/Heathkit-GC-1000-H-Manual

Another decent site for one.  Similar, I realigned the tone decoders and the receiver but made no other changes to it beyond cleaning up some of the old flux.  The goal was to leave it as original as possible.
https://www.smbaker.com/heathkit-gc-1000-most-accurate-enigma