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Author Topic: Viper probs, info moved from Schematics section  (Read 4170 times)

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guitardude57

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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.
« Last Edit: Time Format by guitardude57 »
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MarshallJMP

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What happends if you turn the gain pots down, do you still have hum?
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guitardude57

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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?
« Last Edit: Time Format by guitardude57 »
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MarshallJMP

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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?
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guitardude57

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212.5V
« Last Edit: Time Format by guitardude57 »
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MarshallJMP

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Ah that's a bit low, usually they go up in value and then you start getting hum.Or something is drawing more current. Maybe you should replace the high voltage caps. Could be one is leaking. There are 3 10µF/350V caps.
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guitardude57

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Already ordered them... :)  In my cart anyway...

Funny, whoever was in here before, changed the 2 - 10 uF 450's between the mid and treble pots C28 ...and treble, and master, C33 ...and c19.  So I think someone tried to fix this hum, and replaced some caps...
« Last Edit: Time Format by guitardude57 »
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MarshallJMP

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Does the voltage go up if you pull the tubes out? Since this combo has a regulated high voltage I find it strange that it's too low. So it seems somethings is either drawing too much current or there is something wrong in the regulator.
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guitardude57

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Weird...pulled tubes expecting the voltage to go up considerably higher...
Reads 183.7

WTF...lol
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guitardude57

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C19 and C33 are two of the 220V filters...They were replaced with 10uf 450V...those are right off of the tube power entry for pin 1 and 6...

 C108 which is right on the power supply area, that is the 1 of 3 filters on the PS, is original.
Subbed in a big ole 16uF 450 axial, that barely made contact...got 220.4V  YAY!

Ordered some extra 10 and 22uF radials for the bin, and new 12ax7's...the ones in there are original...this thing has been pretty beat up.

Should have parts by Tuesday.

Thanks for your assistance, my friend!
« Last Edit: Time Format by guitardude57 »
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MarshallJMP

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Re: Viper probs, info moved from Schematics section
« Reply #10 on: Time Format »

No problem, glad you got it fixed !!!
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guitardude57

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Re: Viper probs, info moved from Schematics section
« Reply #11 on: Time Format »

Customer loves the amp, as it functions well, even up to now.

Does anyone know if someone has a foot switch available for this beast?
Or maybe the circuit board I can retrofit into a new switch box?

I suppose I could make one from scratch... but prefer to at least find the foot switch board.

Thanx!
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Kim

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Re: Viper probs, info moved from Schematics section
« Reply #12 on: Time Format »

I have access to a footswitch for the Viper.  I'll check into it this weekend and provide some info for that.
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guitardude57

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Re: Viper probs, info moved from Schematics section
« Reply #13 on: Time Format »

Thanks KIM!
Maybe get a close up shot of it, and any numbers on the circuit board.

Depending on what they have going on, may be able to etch a board...
« Last Edit: Time Format by guitardude57 »
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MarshallJMP

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Re: Viper probs, info moved from Schematics section
« Reply #14 on: Time Format »

I think I have a circuit board of the switch.
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