AC30C2 repair help

[goldstache]

Fixing my roommates AC30c2 and I can't obtain a schemo.  The grid resistors have 334VDC on them and there obviously should be any positive DC there. I'm trying to follow upstream to PI's coupling caps but can't seem to locate them!  Damn PCB!!!!!
I'm assuming they are .01uF but am not sure. Anyone got a schemo or have tackled such an issue?
Thanks

[sluckey]

The grid resistors have 334VDC on them and there obviously should be any positive DC there.

Huh??? What does that mean?

Which grid? What kind of tube?

[goldstache]

Sorry for the vagueness.
The grid stoppers to the el84's have 340VDC which makes me want to check the coupling caps off of a 12ax7 LTP PI.  Trouble is its PCB and I can't find a schemo.  To make things worse it's on PCB. It's got some "molex" -like connectors and I have found the feed to the el84 grid resistors.  Just having trouble discerning which box style caps to check.

[pompeiisneaks]

This site has a ton of great schematics, is this the AC30C2 you're looking for: http://el34world.com/charts/Schematics/files/Vox/Vox_AC30C2.pdf

There are a ton on the vox page of different kinds cc2 etc

http://el34world.com/charts/Schematics/files/Vox/Vox_Schematics.htm

[sluckey]

Are you talking about control grid (pin 2) of the EL84s? If so, there should be zero volts on them. Having 334v on them would mean that two coupling caps are shorted (or some other similar problem). It seems unlikely to me that two caps would fail at the same time.

If you are talking about screen grid (pin 9) of the EL84s, that voltage is perfectly normal.

That is what I was getting at with my vague questions. Exactly which tube pin are you measuring 334v on? Working on a complicated pcb can be nearly impossible without a schematic and a good pcb layout.

I'm looking at a schematic for AC30C2 and the phase inverter is V3. The coupling caps (C50 and C51) are 100nF. I'm thinking more and more that you mistakenly measured 334v on the screen grids.

     http://el34world.com/charts/Schematics/files/Vox/Vox_AC30C2.pdf

[goldstache]

Wow, thanks Gents!

Sluckey, you are correct.  EL84 pin 2 control grid.
My pal had me look at it because it crapped out.  Saw a burnt 3k3, traced it to EL84 pin 2.  Pulled the tubes, swapped out the 3k3.  Started looking for coupling caps before it.  That schemo should really help.  I'll give it a look and see where the voltage is similar and check for shorts.

[goldstache]

Well Im experiencing some strangeness.  I changed the coupling caps from the PI to the control grids of the EL84's (they were not blocking DC).   Once swapped out, put in some test tubes and was monitoring DC voltage on the EL84 grids, no more DC on grids and amp was starting to come up in the speakers.  But then..........  LIGHT SHOW in one of the tubes.  And I don't mean red plating, Im talking Lightning in a bottle!!!!!!!!!


Immediately turned off the Variac, which unfortunately doesn't have a current meter.


I then unhooked the CT and Plate taps of the Output Transformer and brought her up.  Everything seems cool with voltage and no light show or red plating. 
Leaning towards a BAD Output Transformer. 


Does anybody have recommendations about the best procedure to check it or diagnostic process. 
Thanks!

[PRR]

I'd be thinking about a bad EL84.

The stress on the tubes is the same whether the PT works right or is shorted.

[goldstache]

Turns out on the model I'm servicing.  The Connector for the OT primaries runs right up against the control grid rail.  When the previous set of tubes shorted.  It burned that trace and though it metered fine both in continuity and resistance (even with OT not connected).  I scraped the trace where the arc probably occurred and it solved all the problems.  Back to sounding great. 
Thank you!

[boogieWoogie]

I just fixed the same Vox AC30C2 failure: arcing from OT transformer spade lug to the grid.  My symptoms were no audio, buzz, and finally ~240VDC on the output tube grids.  I removed all coupling caps (suspecting that they'd died & were leaking the plate's B+ DC into the grid), removed cabling.  Finally I was down to bare bones of a circuit & there was 56k ohms between the grid & plate.

There is another thread on this board of someone who's fixed the exact same problem.  That makes 3 of us.  Vox shoulda done a recall.

My fix was to grind out the arc'd pile of carbon & metal using a dremel wheel, and pack the ditch w/ hand-molded epoxy.

Arc, pre-fix
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Post-fix (grinding + epoxy)
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