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
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.
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.
Heathkit HD-1250 grid dip meter
Nasty case of 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.
Sticky foam residue on the coils
Sticky foam residue on the coils
Corroded contacts
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.
Goo and paint scraped off
Goo and paint 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.
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
Much better looking now, and much more stable.
Heathkit IG-102 schematic from https://www.vintage-radio.info/
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
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
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 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.
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.
