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

PinSpecMeasured
16562
2-4-5
300
46.3 VAC7 VAC
56.3 VAC7VAC
66259.6
7-0.75-0.38
800
900

V2 6AN8 pin voltages

PinSpecMeasured
17062.8
2-0.2-0.5
31.51.57
46.3 VAC7 VAC
500
6125127
7130130.9
800
92.52.64

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.

Heathkit IG-102 capacitor

The Heathkit IG-102 signal generator has this big 20 μF 150VDC electrolyic capacitor that has 2 leads on the positive side and one lead on the negative side. It looks like an axial capacitor, but I’ve never seen one with two leads on one side before.

20 uF capacitor
20 μF capacitor

20 uF capacitor
20 μF capacitor

Although it doesn’t seem to be showing any signs of failing, given the unknown age of the signal generator I thought it might be a good idea to replace it with something new. I’m thinking I should replace it with two caps, one for each lead on the positive side. Don’t have any axial caps in the parts bin, but there are plenty of 20 and 22 μF radial electrolytic caps. No new 20 μF caps though.

Might have to go shopping.