The distorted waveform mystery wrapped into the multiple schematic versions enigma! Quite the detective story, and much more interesting than just a busted power supply rail repair.
If the PIN diodes have been replaced with ordinary diodes, or if the bias to the PIN diodes is screwed up, or the diode is damaged, that could cause the nonlinear distortion. The PIN diodes have long reverse recovery time so they act as resistors, conducting over both halves of the cycle, when presented with sufficiently-high frequencies.
What a lost cause! There was a guy had truckload of those Wavetek signal generators at the Electronics Flea Market. All looked like someone had tried to fix them, and was unsuccessful. You might have one of those. Likely all the modules were swapped out 25 years ago and are nfg.
There is one main amplitude control loop from the diode at the output over the amplifier in the middle to the pin diodes at the input. This loop gets a control signal from the lower right. It seems to work. There is the other circuit with the two transistors. The one at the output can short the output signal via 1µF to ground. These two transistors get no further control signal. Maybe they act as a limiter or fuse and they take care that the signal does not get to high to avoid damage. For a test you may take the 1µF capacitor out (or whatever value is in this place in the current version). In the 3 stage amplifier you could also check the intermediate signals with a non connective probe to check whether just the output stage is distorted to verify that the clamping transistor at the output makes the problem. (... if it is the case, ... if not ... go to start ...)
I would suspect that the input attenuator diodes are actually PIN diodes, and they are frequency dependent. They are only effective above certain frequencies.
did you break the AGC loop yet and try with a pot connected to diode junction to adjust gain manually? Also probe each collector in amp to see where the distortion starts
The distorted waveform mystery wrapped into the multiple schematic versions enigma! Quite the detective story, and much more interesting than just a busted power supply rail repair.
Yup.
"I have to scratch my head some more" oh my, I have that so often 😅
If the PIN diodes have been replaced with ordinary diodes, or if the bias to the PIN diodes is screwed up, or the diode is damaged, that could cause the nonlinear distortion. The PIN diodes have long reverse recovery time so they act as resistors, conducting over both halves of the cycle, when presented with sufficiently-high frequencies.
The "strange" diodes may be PIN diodes.
maybe Bob Moog designed that module? 😄
What a lost cause! There was a guy had truckload of those Wavetek signal generators at the Electronics Flea Market. All looked like someone had tried to fix them, and was unsuccessful. You might have one of those. Likely all the modules were swapped out 25 years ago and are nfg.
Maybe one of the 2 pin diodes in the attenuator is not behaving well. I think they should be matched to not distort the signal.
Modules work independently... Don't work together... As easy as a bad connecting wire?
There is one main amplitude control loop from the diode at the output over the amplifier in the middle to the pin diodes at the input. This loop gets a control signal from the lower right. It seems to work.
There is the other circuit with the two transistors. The one at the output can short the output signal via 1µF to ground. These two transistors get no further control signal. Maybe they act as a limiter or fuse and they take care that the signal does not get to high to avoid damage.
For a test you may take the 1µF capacitor out (or whatever value is in this place in the current version).
In the 3 stage amplifier you could also check the intermediate signals with a non connective probe to check whether just the output stage is distorted to verify that the clamping transistor at the output makes the problem. (... if it is the case, ... if not ... go to start ...)
Hmm as there are still 3 episodes left. Do I think he will get it?? ;)🤔
I would suspect that the input attenuator diodes are actually PIN diodes, and they are frequency dependent. They are only effective above certain frequencies.
did you break the AGC loop yet and try with a pot connected to diode junction to adjust gain manually? Also probe each collector in amp to see where the distortion starts
Ceramic caps can create non-linearity. Another reason not to blindly use them replacing others.
Have you checked your connections/cables?
Clear as mud?
At 2:33 that's probably the most horrible distortion of all time. 🙂👎 The mystery deepens.
What is the lowest frequency according to the specs please ?....cheers.
1MHz