Let's Get Technical > Troubleshooting Problems

Viper probs, info moved from Schematics section

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guitardude57:
Howdy all,
Thanks for the quick response with the schematics for the ADA Viper!

After going over the amp yesterday, and finding most of it not working well...
took out the chassis, and discovered the cut pot for the speaker emulator was broken
by whomever messed with it last.  And the wrong value by what the schemo shows.

R166 is supposed to be a C50K (reverse taper audio) according to the drawing.
There was a A100K in there.  Since I don't see any ECO's (Engineering Change Order)
for a revision of the pot value, I am assuming the last guy couldn't find the correct value,
or it was placed by the factory.  In any case the phenolic card between the pot body and the terminals was broken, skewing the wiper contact.
The emulator IN/Out switch was stuck, hence no audio...

Now with the pot turned full open, and tweaked to make a contact internally, emulator works
and passes audio.  Switch to turn off now works.

Both channels function, after cleaning all the pots with Blue Shower.
There is a noticeable hum on Red and Green channels with and without audio inserted in input.
Plugging audio into power amp in (FX Return) Audio is clean, and dead quiet of any hum.
So somewhere around the mixer, is a bad filter cap.

My Techtronix O-scope shit the bed on my last move... so finding the hum will be a bear...

Any others out there find a similar hum issue in these...in the front end, or mixer circuit?

Much Thanx!

Trying to find a US supplier for the R166 cut pot.  C50k value.

Update:  CE Distribution has a snap on pot that will work for R166, just have to remove the snap bracket first.

MarshallJMP:
What happends if you turn the gain pots down, do you still have hum?

guitardude57:
Hi,

Yes, with the gain pots on either channel turned all the way down,
the hum is there.  Since the lossy nature of the tone stacks, with the TMB turned down,
the hum is gone.

With the drive controls up, the hum is there as well as expected hiss with no input.
Masters turned down, it is dead quiet.  Either channel.  So problem is before the Master Volumes.

So either the mix circuit before the FX loop, (B4 Masters) or after the shared input where the signal separates to the 2 different channels is a bad cap...perhaps.
I would think if the problem was in the PS filters early on, the hum would be in the FX Return...where it is not.

My O-scope is fried, so hunting for hum and hash is problematic the easy way.

FX return input to amp with audio...clean and quiet.

Whatcha think?

MarshallJMP:
I would start with the high voltage PS. There is a testpoint TP 5, it should be 220Vdc, can you measure it and let me know?

guitardude57:
212.5V

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