Equipment Fix

Heathkit IP-2718 Fix

A few weeks back I acquired a Heathkit IP-2718 triple output power supply. I fired it up and discovered that only the fixed 5 V output worked, and the two variable outputs were stuck at 0 V. So I did the traditional dance of my people.

This particular power supply is from that magic time between vacuum tubes and ICs when transistors reigned supreme. So I didn’t have to try and find a replacement for an obsolete IC or a binary for a microcontroller.

Inside the Heathkit IP-2718
The inside

After opening the case it didn’t take long to spot the problem. Six power resistors had gone bad.

Since resistors don’t tend to fail in isolation. That is frequently some other component fails and causes the resistor to dissipate power in excess of it’s rating and thus release the magic smoke. I checked the capacitors and diodes for shorts and they appeared ok. The transistors appeared ok as well. Since all of these parts were probed while attached to the board it’s hard to say for sure. So I got some replacement resistors. Pulled out the bad resistors and replaced them. Fired up the power supply and everything seems to be working fine.

In this close up of the bad resistors it’s easier to see the cracking and discoloration. Also note that the new green resistor on the right is smaller and has a higher power rating. I’m not exactly sure how that trick was managed. I’m guessing it’s 40 odd years of materials and process tweaks.

Bad resistors (left) next to a replacement (right)

One of the most interesting parts of this repair for me was seeing the traces on the PCB.

PCB Close up

The copper tracks on the board are wild, almost artistic. compared to modern PCBs where CAD software has made all the tracks uniform width and fine pitch packaging has caused them to get packed as close together as possible. Like this one

Modern PCB from Wikimedia,
Modern PCB, Courtesy of Wikimedia