Category: Electronics

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.

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 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.