Heathkit HD-1250 clean up

The Heathkit HD-1250 grid dip meter (SN 05706) I acquired at the hamfest a few months ago looked in decent condition aside from some pretty major foam rot.

Finally got around to getting it cleaned up, which turned out to be more of a mess than I anticipated. Unfortunately, the foam rot wasn’t as easy to clean up as I thought it might be.

The foam ended up disintegrating into an ugly sticky mess. I was able to brush most of it off, but it left a gluey residue on everything.

I was able to clean off the sticky goo on the coils, but unfortunately whatever it was that the foam disintegrated into also had the same effect as paint stripper. When I tried to wipe the residue off the case, the paint underneath was coming off as well.

Since it didn’t look like I was going to be able to get any of the goo off the case without taking the paint off as well, I decided to just scrape it all off. The case is in three pieces: face plate and two side pieces. Undo four screws at the bottom and one each in the front and back of the case and the case comes apart pretty easily.

The insides look in pretty good shape, aside from a leaky battery that also took the battery connector with it (easy enough to replace). A few more pieces of disintegrating foam that were originally to cushion the battery, but it was just dry and crumbly fortunately.

Scraping the goo off the face plate was easier once it was freed. Pretty much all of the paint on the top of the face plate ended up coming off. Some of the paint along the top of the side pieces where the foam stuck also had to be scraped off.

There was also a good bit of pitting in the lower part of the face plate from the goo.

Mostly cleaned up and back in the case

The HD-1250 is a little more bare, but mostly cleaned up. The labels on a couple of the coils have faded away, so there’s no indication of what frequency range they’re for. I’ll have to hunt down a manual and see if the coils are described in it. Aside from some surface corrosion on part of the RCA plug shell (easily sanded off), the coils still look in good shape. The electrical repairs and testing will have to wait for another day.

Heathkit IG-102 capacitor replacement

Replacing the DC blocking disc capacitor (C20, 0.01 μF)  just before the Fine Atten pot helped clean up the output signal quite a bit.  I didn’t get a picture of the wave form before, but it was pretty ugly looking and not very stable.

IG-102 wave form, about 500 kHz
IG-102 wave form, about 500 kHz

Much better looking now, and much more stable.

Heathkit IG-102 schematic from https://www.vintage-radio.info/
Heathkit IG-102 schematic from https://www.vintage-radio.info/

Heathkit IG-102 diagnostic

Fortunately, my IG-102 came with the assembly manual.   The IG-102 is roomy enough inside that it’s pretty easy to get in and probe around checking things.

Things I’ve found so far:

  • Audio frequency (AF) output, which the manual says should be 400 Hz, is around 260 Hz.  With the AF dial turned all the way up, the AF waveform measures about 20Vpp.
  • I can reliably get RF output when the Coarse Atten switch is at the Hi position and the Fine Atten dial is all the way clockwise.
  • Setting the Coarse Atten switch from Hi to the middle position drops the frequency by about a factor of 10 (according to my M3 frequency counter).
  • Setting the Coarse Atten switch to the Lo position makes the RF output drop out.
  • Turning the Fine Atten dial counter-clockwise ends up changing the frequency (unexpected) instead of changing the signal level.
  • The band select dial is kind of fiddly.  Sometimes when I switch to a different band, RF output drops out.  Switching back and forth will usually bring the RF output back.
  • Band A (100 – 300 kHz) is the most accurate.
  • Bands B (310 – 1100 kHz) and C (1 MHz – 3.2 MHz) are about 50 kHz low.
  • Band D (3.1 – 11 MHz) is low by 0.5 – 0.7 MHz.
  • Band E doesn’t work.
  • Band F (32 – 110 MHz) is reasonably accurate up to 50 MHz, but maxes out around 75 MHz.

V1 12AT7 pin voltages

46.3 VAC7 VAC
56.3 VAC7VAC

V2 6AN8 pin voltages

46.3 VAC7 VAC

IG-102 AF signal
IG-102 AF signal around 260 Hz

The RF signal sampled at pin 2 of the 12AT7 tube looks decent enough.

IG-102 RF signal
IG-102 RF signal around 370-ish kHz

Need to do some more poking around with the oscilloscope to check the RF waveform at other places along the output path to see what’s happening before I try to figure out what bits I might need to replace.

Back on the bench: Heathkit IG-102

Back on the workbench is the Heathkit IG-102 signal generator I acquired a few years ago.

Heathkit IG-102 signal generator
Heathkit IG-102 signal generator

The last time the signal generator was on the bench was to have a new power cord and BNC connectors installed.

This time, I’ll replace that big orange capacitor with something newer (along with any other components that look like they need replacing).  The signal generator doesn’t seem to be putting anything out at the BNC connectors anymore, so that will be another thing I’ll check.

Heathkit IG-102 20 uF capacitor
Heathkit IG-102 20 uF capacitor

Another Heathkit IG-102 mod

Spent another evening poking around inside the Heathkit IG-102 signal generator, this time with schematic in hand and probing around with my DMM to check voltages in various places. I wanted to make sure everything was working ok, and it seems to be. The voltages I was getting were all pretty close (within 10%) of what was printed on the schematic.

I also replaced the original 60s or 70s era power cord with a polarized power cord (harvested from I don’t remember what now). It was a simple replacement and probably not critical but I figured having a new(-ish) cord might make it a little more electrically safe. The hard part was unsoldering the old cord and getting the plastic strain relief thing out of the chassis.

I was able to set my HT to a few frequencies in the 10-30 MHz range and heard tones when I adjusted the signal generator to those frequencies. That seems like a good sign it’s working.