Scope Troubleshooting Weak Channel

[Cygnus X1]

Hmmm.
This one has me scratching.
A Cygnus 7 with the usual Gain 1 and Gain 2 setup.
Symptom is the signal is weak and distorted.
First looked for anything obvious like loose connections, bad grounds, bad tubes, and voltages.
Nothing stood out as too unusual.

So I set up a reference 1K signal on one channel of the scope, and put the probe for the other channel at the speaker and started working backwards.
I was getting the same distorted wave all the way back to the tone stack.
So I placed the probe at the master volume and changed where the signal was being injected by the signal generator (1K sine).
Distorted through the first gain stage, then it would clean up as soon as I got to the second gain stage.
So pin 7 input is funky, but pin 2 is relatively good.

Here's a pic to show it.
Bottom signal is the clean reference, top is gain stage 1, then 2 respectively.

Now to hunt it down.
I'm going to put the probe closer to the stage 1 functions and probe around there.
Don't know what I will find but it shows what a great tool a scope can be for troubleshooting what our ears can't discern.

Don, don't worry, your Tek scope is still here.

Just that Channel 1 is flaky.
I just need to get into it and clean it up and repair that fine tuner on the voltage divider.
I wanted full use of both channels for this experiment.
This Hitachi was dirt cheap ($22 local surplus), 20Mhz, and has a nice clean signal.
The display is skewed at an angle though...something I'm sure there is adjustment for somewhere.

Also got a spare scope probe for $3.00 (!!!) at a thrift store.
Brand new in the package.
I was shocked to see it and they certainly had no clue what it was for!

 

[Soul Tramp]

Cyg, you are the master of bargain hunting! I don't know how you keep coming up with all these deals.

The top trace in your first picture is normal for the signal path after the tone stack. That's what TMB does to the signal.

Does yous scope display amplitude and frequency. That's what you really need to case things.

For a gain recovery stage the waveform on the plate (after the coupling cap) shouldn't look any different than the waveform on the grid. Only the voltage should change.

 

[Cygnus X1]

Cyg, you are the master of bargain hunting! I don't know how you keep coming up with all these deals.

The top trace in your first picture is normal for the signal path after the tone stack. That's what TMB does to the signal.

Does yous scope display amplitude and frequency. That's what you really need to case things.

For a gain recovery stage the waveform on the plate (after the coupling cap) shouldn't look any different than the waveform on the grid. Only the voltage should change.

Yes, plus I can play with frequencies with my HP 3112A function generator.
You don't want to know how much that Cadillac cost.
<cough $25.00...cough>
Same symptoms across the audible spectrum.
You may have missed the part where I was injecting signal into different stages to narrow it down?
So the top trace first pic is ugly where it's supposed to be clean.
Bottom pic top trace is the next stage, showing what it's supposed to be.

I went ahead and plugged a guitar into that second spot and sure enough the signal was strong and clean.

So it's somewhere between the instrument input and the second gain stage.
I could shotgun everything there, but that's no fun!



And checked for DC on the coupling cap.
Nothing.
Replaced it anyways. No change.

 

[Soul Tramp]

Without the schematic I can't really visualize the circuit. Are you leaking the signal to ground. Could be a tone stack issue, but I don't know where you have it in the circuit (before or after the problem).

 

[Cygnus X1]

Oh, I'll find it.
Just venting a little, and hopefully sharing some troubleshooting tips along the way.

99% of issues I find are indeed grounding.

 

[Soul Tramp]

I would be lost without a scope. Good luck and have fun with it.

 

[D-Stagg]

For a minor tidbit... you mentioned the skewed trace on the scope. The "trace rotation" screw just below the power button will correct that.

 

[Cygnus X1]

For a minor tidbit... you mentioned the skewed trace on the scope. The "trace rotation" screw just below the power button will correct that.

 

[elektrik]

Just my quick two penneth, check for positive voltages on the grids, if you find any (other than on a cathode follower) then its a fair chance the tube has gone faulty. Happened to me a couple of times at Hiwatt, not so obvious at first

 

[Cygnus X1]

Changed the tube twice.
I'm starting to think it's a cracked ceramic socket.
Actually I have thought that from the start, but didn't want to admit it.