IG-102 – AB4UG Radioblog

Tuning up the Heathkit IG-102

After spending some time figuring out how to use the Digilent Analog Discovery 2 USB oscilloscope I picked up at Hamcation earlier this year, I was able to use it to look at the output from the Heathkit IG-102 signal generator.

The Digilent AD2 is pretty cool. The software lets you use the AD2 as an oscilloscope, signal generator, spectrum analyzer, and several other things.

Using the spectrum analyzer function made it pretty easy to set the IG-102 to a frequency, see what the actual frequency was, and then tweak the variable inductor coil for each band setting to tune the output to the dial setting.

All the coils got tuned a bit and now it's calibrated reasonably accurately. Output signal is only around 130 mV (peak), which seems pretty low to me. pic.twitter.com/Ql4YreW0Ee
— imabug (@imabug) October 30, 2022

The output of the IG-102 is pretty low though, less then 0.3 V peak-to-peak. I’m not sure if it’s supposed to be that low, but I expected it would be in the 3-5V range.

Well, at least it outputs something that’s reasonably well calibrated now. I suppose if I had to, I could build an amplifier to feed the signal into. I’ll see if there’s anything else I can do to get the signal amplitude up to where it seems like it should be. Maybe the IG-102 needs the tubes replaced as well.

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.

Much better looking now, and much more stable.

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 20V_pp.
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

Pin	Spec	Measured
1	65	62
2	-4	-5
3	0	0
4	6.3 VAC	7 VAC
5	6.3 VAC	7VAC
6	62	59.6
7	-0.75	-0.38
8	0	0
9	0	0

V2 6AN8 pin voltages

Pin	Spec	Measured
1	70	62.8
2	-0.2	-0.5
3	1.5	1.57
4	6.3 VAC	7 VAC
5	0	0
6	125	127
7	130	130.9
8	0	0
9	2.5	2.64

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

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.

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.

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.