VOX AC15 reverb issues...I'm stumped.

[tim]

Here are the schematics:






No Reverb...Reverb pre-send sig is present on grids of V3 but not the plates, unless I unplug Conn 2 (which goes to Conn 1 on the power amp Board)...then I see signal on V3 plates. Plate voltage is found all the way from the power supply through R7, through the primary of the reverb drive tranny to the plates of V3, but no signal. Something is loading down the signal (I'm running 1khz through the amp) but not the HV to the tube plates...

Any ideas???

I really am stumped on this one. Hopefully it's not the driver tranny as these are no longer available. It seems fine reading DC resistance through each winding and the primary and secondary windings are not shorted.


Thanks for any insights...

[The Dude]

Maybe I'm missing something, but if you unplug CON2, don't you remove plate voltage? How could you then have signal on the plate with the tube not conducting? What are you measuring for plate voltage on V3?

[tim]

Maybe I'm missing something, but if you unplug CON2, don't you remove plate voltage? How could you then have signal on the plate with the tube not conducting? What are you measuring for plate voltage on V3?

And I call myself a tech...

I have been working on this for several hours and I think I'm approaching burnout (fortunately it's mine and not a client's)...I've tried to locate the fault off and on for several months and can't seem to locate it...plate voltage all the way to V3 but no signal of the plates...I think I need to do some poking around V3 and see if there is any current flowing through the tube. I replaced the tube already (easiest thing to try first) but no go.

I guess I just needed a reality check...crap, I can't believe I didn't think of that...I'm putting this down for the evening. I have to be at my 9-5 early tomorrow.

[The Dude]

Well, you're not the first, and won't be the last, to be bitten by that bug.

If you wanna post voltages on V3 when you get back into the thing, that might help us figure it out.

[tim]

Well, you're not the first, and won't be the last, to be bitten by that bug.

If you wanna post voltages on V3 when you get back into the thing, that might help us figure it out.

Yeah, I'll probably be on it again tomorrow evening with a fresh pair of eyes (and fresh brain). I need to get this thing finally fixed so I can sell it and get it out of my shop.

[tim]

Well, you're not the first, and won't be the last, to be bitten by that bug.

If you wanna post voltages on V3 when you get back into the thing, that might help us figure it out.

OK...back at it again...

Here are the voltages around V3

Pins 1, 6 177vdc (no 1 khz sig seen where I would expect to see it)
Pins 2, 7 0ish with a 1khz signal injected into input jack
Pins 3, 8 20ish vdc
Pins 4, 9 6.3 AC filament
Pins 5, 9 6.3 AC filament

Thoughts?

Does the voltage on 3 and 8 seem a bit high?

EDIT: I think I found the problem...C26 is reading about 36Kohms out of circuit...Hopefully I have one in my stash (or something close).

OK... I found one and replaced and it seems we're good to go...I'll hook everything up tomorrow evening and do a full test on it. I'll let you know how I make out...thanks for setting my head back on straight.

[Enzo]

THAT transformer may not be around, but why wouldn't a plain old Fender reverb transformer work? it is just a pair of triodes driving a transformer.

But we are jumping ahead. Is the pan conected? Unhook it. Does the signal appear on those plates now? The transformer itself is easy to test, just connect it directly to something and see if it "transforms". Or go over to RG's Geofex site, and under tech tips is a simple transformer tester. In any case a short across the secondary would swamp the primary. There could be an issue with the pan or the cable, so get them off of it to find out.

How did you determine the windings were not shorted? Your meter cannot do that. Two turns shorted together will change the resistance by very little yet will kill a transformer. Shorted windings do not short end to end as a rule.

You can unplug the transformer and clip a 100k 0r 220k from those plates to B+. Does the stage amplify now?

How large is the signal at the grids, pins 2,7? Inject a strong signal directly into pins 2,7. ANything?

Unplugging the transformer removes B+ from the plates, so I suspect any signal you then get there is capacitively coupled inside the tube. You say you get signal that way, but how large a signal?

[tim]

THAT transformer may not be around, but why wouldn't a plain old Fender reverb transformer work? it is just a pair of triodes driving a transformer.

But we are jumping ahead. Is the pan conected? Unhook it. Does the signal appear on those plates now? The transformer itself is easy to test, just connect it directly to something and see if it "transforms". Or go over to RG's Geofex site, and under tech tips is a simple transformer tester. In any case a short across the secondary would swamp the primary. There could be an issue with the pan or the cable, so get them off of it to find out.

How did you determine the windings were not shorted? Your meter cannot do that. Two turns shorted together will change the resistance by very little yet will kill a transformer. Shorted windings do not short end to end as a rule.

You can unplug the transformer and clip a 100k 0r 220k from those plates to B+. Does the stage amplify now?

How large is the signal at the grids, pins 2,7? Inject a strong signal directly into pins 2,7. ANything?

Unplugging the transformer removes B+ from the plates, so I suspect any signal you then get there is capacitively coupled inside the tube. You say you get signal that way, but how large a signal?

Hi Enzo,

It has been so long since I've been on here I wasn't even sure if you were still on the forum...

Anyway, I'm pretty sure I fixed it....the 470pf 500v cap going from the plate to the cathode was "mostly" shorted (63Kohms), leaking 20ish volts to the cathode putting it in hard cutoff. I replaced it with what I had (a 470pf disc ceramic instead of what was in there, a silver mica cap) and all seems well; signal is now flowing through V3....I'll know for sure tomorrow when I put it somewhat together and run it with the reverb tank installed.

I did check out the transformer basically the way you said to...I put a signal from the sig gen I have onto the primary and was able to see the smaller signal on the secondary so I was pretty sure the tranny was fine.

The Dude set me on the straight and narrow...I don't get to play with amps like I used to and sometimes I forget the simplest things....I've been doing this off and on for over 35 years and the simplest things still get me sometimes.