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.

So much for the oscilloscope

My dead oscilloscope is still dead.  After spending some time poking around the innards, I’m starting to think the issue is on the high voltage side that provides power to the CRT.  I’ve checked the voltages at connectors where the PCB is labeled with voltages, and those match up.  It seems like the rest of the scope is probably working except for the CRT side of things.

I think at the moment, the scope is beyond my ability to fix.  Guess I’ll shelve it for now and maybe go back to it when I’ve gained a few more skill levels and higher level items.

Off to the next project!

Diving into the oscilloscope

With the power supply repaired, the next item to go on the bench is the dead oscilloscope.

Hitachi V-1060 oscilloscope
Hitachi V-1060 oscilloscope

Undoing four screws on the sides of the scope let me slide the top cover off to reveal the innards.

Under the cover
Under the cover
Side view
Side view

The top circuit board looks to be the power supply board, and probably a few other things. Undoing a bunch of screws holding the top board down and a bit of fiddling around (discovered the board is on a hinge) let me lift it up to reveal more of the scope’s innards.

Beneath the power supply board
Beneath the power supply board
Beneath the power supply board
Beneath the power supply board
Power supply
Power supply

Three screws hold the cover of the power supply section.

Power supply
Power supply
Power supply
Power supply

A few dust bunnies inside, but overall everything looked to be in fairly decent condition (aside from not working).

First look around the inside didn’t reveal anything obviously wrong. No blown caps or scorched areas.  Whether this is something I’ll be able to repair or not is still up in the air. This scope is probably going to be spending a while on the bench.

Power supply: Fixed!

Yay, the power supply is fixed!

Thomas/LA3PNA suggested I look at the input and reference voltages going into the μA723 regulator, so while I was at it I looked at all the pins in the empty socket. The only unusual thing I found was about 0.1V where the output pin of the regulator went. I’m guessing the voltage regulator probably didn’t like having that much voltage on its output pin, and that’s probably what was killing them.

Pin 10 went to the anonymous red transistor type thing, which in turn was connected to the pass transistors and Vcc.

Mystery transistor
Mystery transistor

Consulting with Thomas again, he said it was probably a Darlington transistor or something similar that failed. The μA723 output turns on this transistor, which is then able to provide more current to turn on the pass transistors than the μA723 alone would be able to.

Took the transistor thing out and connected the μA723 output to the base of the pass transistors and everything worked! Got a stable 13.7V at the output of the power supply, and I could turn it off and on again without any problems.

Found a TIP31 power transistor in my collection of parts and put that in place of the dead red transistor and it looks like this power supply is back in business.

TIP31 replacement transistor
TIP31 replacement transistor

It’s a pretty ugly soldering job, but I think it will hold up. Still need to test it  under a load though.