Heathkit IG-18 continued ...
Now that the power supply is working well I replaced the obvious - the 100 uF cap with corrosion that was across the meter. Then I reviewed the alignment procedures in the assembly manual and proceeded to investigate the distorted sine wave. When I attempted to turn the "feedback" and "bias" pots I could see the distortion vanish at points, but the pots were so dirty that I couldn't keep the adjustment stable.
I applied Deoxit to both of the pots and turned them back and forth several times. After that - presto - I could set the adjustments properly and was rewarded with a good sine wave on the output. I also tried to adjust the other two pots on the main board - "meter cal" and "symmetry" (for the square wave) but were as dirty as the others, so same treatment and again, worked fine after cleaned.
Anyone want to take up smoking now?
Facing the last major issue, I turned to the sine amplitude - the meter amplitude indication only went halfway and the owner stated this had not been the case until lately. He stated that this problem had appeared slowly over some time. After probing the generator circuit I found that the input amplitude to the output stage (fine adj. pot and attenuators) was good. However, we have a serious problem.
At some point in time the original fine adj. pot was replaced by another one - why I have no idea. But ... the job is an abomination (read: Gorilla Joe got to it before me).
As I stated before, by observation, the pot is mounted cock-eyed and it's coming apart at the seams. The original shaft was cut off and a slot filed into to mate with the wire link, it but it's not even centered. The pot is supposed to be 5k. The thing had to be removed.
After checking, it was presenting 4.7 k max - and here's the main thing - only 1.3 k at minimum on the center or variable pin. This is why the amplitude is not going full scale any longer. I took it apart to see what's inside. The shaft stuck at a certain point so that explains the reason it won't go to zero. The rear of the shaft is not on-center and there is nothing at the back inside it to keep it aligned there. A Gorilla Joe mod again?
I lashed up a test with a new 5k pot I had on hand, and with that setup, the output - min to max - worked perfectly. However, it's not the same shaft size as the old one, the shaft is too long to fit, and too small for a good tight fit in the plate hole. It's connected to the variable shaft by an odd "U" shaped wire link that fits in a hole on the main shaft and then in the cut-out of the pot shaft.
So, we're looking at some cutting, filing, Fusion 360 / 3D printing for a customized adapter or bushing.
Be back soon ...