Heath kit GC-1000 Most Accurate Clock

The Heath kit GC-1000 Most Accurate clock (well, mostly anyway)

Power supply reliability upgrade information. Here's how to do it, and it's pretty easy too!

The Heathkit GC-1000 clock was the first consumer-grade digital clock offering disciplined accuracy performance by means of radio reception. We have one of these venerable old WWV clocks, and being 20+ years old it was in need of some work to restore the clock to good operation.

I'm an EE, and I also dabble in designing and manufacturing GPS disciplined Nixie Tube clocks as a side line hobby. Hoping to shortcut the time spend under the cover of This Old Clock, I did a search on the internet for published information about GC-1000 failures and common problems.

I came upon a couple of internet based businesses offering repairs and upgrades for this clock, as well as a great article about accuracy improvement. Our clock really was not in need of a new CPU board, or expensive tuneup work. I already knew the problems were related to the power supply, even though I could not find anything published about it. I did find that the GC-1000 is certainly a beloved item from back in the glory days of Heathkit Inc.

OK, I had some spare time, and it was time to have a look at the clock. It certainly was dusty inside after opening it up for the first time in 20 years. The clock always ran hot. And heat kills electrolytic capacitors. I looked over the power supply section with a meter and scope. Really nasty ripple. Twenty years running in that heat had dried out the capacitors. Plus, the ripple was very bad for that vintage CPU, and there's no cheap replacement.

In 1983, there were few choices for voltage regulation, and none of them very efficient. Note the discoloring of the FR4 PCB material in the power supply section -too much heat for too long. I decided to replace the vintage 7805 series regulator with an Integrated Switching Regulator module, as well as replace many of the electrolytic capacitors within the power supply section. The new regulator would dramatically reduce the heat generated from the transformer and other related components in the power supply through improved efficiency , as well as lower the overall power consumed by the clock.

The voltage regulator U202 and it's heatsink were removed, and replaced with a Power Trends (TI) PT5101 5V 1A Integrated Switching regulator. I had to remove the test board that was originally mounted on top of the regulator heat sink, and remount it with a stand off spacing it from the board. This PT5101N is currently available from DigiKey for $12.25ea. The part shown on my example images will be different from the part you receive. Three 1000uf 25v radial Electrolytic capacitors on the main board required replacement at locations C203, C205, and C206. One 330uf 25v radial Electrolytic capacitor on the main board required replacement at location C208. One 22uf 25v radial Electrolytic capacitor at location C212 on the main board (adjacent to the new regulator) required replacement, and was upgraded to a 33uf 25v radial Tantalum capacitor for improved reliability. A 3.9 ohm 5w resistor at location R201 was removed, and replaced by an insulated jumper wire. Originally this was required to limit the incoming voltage to the 7805 regulator, and is an unnecessary heat generator with the new switching regulator installed in place of the vintage series regulator.

Few other capacitors are involved with regulation, and I decided they did not require replacement.

Image showing C205, C206, C212 and the new Integrated Switching Regulator module already installed on the board.

Image showing C203, C205, C206, C208, and C212. Note the toasty FR4 board material at the voltage regulator and rectifier areas.

The original 7805 Series Regulator at U202 was replaced with the PT5101N

Integrated Switching Regulator module.

Resistor R201 was removed and replaced with an insulated wire jumper.

Image showing the GC-1000 reassembled, with a spacer installed to support the test board over the new regulator module.

You will immediately discover the GC-1000 is operating cool, with barely any detectable warmth from the clock cover. I highly recommend completing the calibration procedure in the manual after you have completed the power supply upgrade.

Update: April 2005

I recently noticed the fellow who offers the expensive GC-1000 tuneup work has added a "cool clock" blurb to his web site. Apparently this new blurb is in response to my posting of the GC-1000 information -enabling you to perform the PS reliability upgrade work yourself at little or no cost. Don't be misled into purchasing an aftermarket CPU, or other unneeded components. The problem is simply an inefficient series regulator design and very dried-out electrolytic capacitors. There are no cuts needed. And no RF interference problems either. The information I have provided is simple to understand, with no irreversible changes.

Update: June 2006

From all the email; the Heathkit GC-1000 Power Supply reliability upgrade procedure has clearly been an outstanding success. One expert in the GC-1000 field has been kind enough to share an image the original GC-1000 program software running in the Mostek MK3870 MCU. If you are lucky enough to own a piggyback ROM version of that vintage MCU, you can program a ROM with the object HEX data in this ZIP file.

Jeff Thomas