Vox AC30-CC2X Reverb Return Problem

[nevetslab]

I have a Vox AC30-CC2X on the bench with no Reverb Signal return. I had found the high voltage MosFET Q102 type LND150N3 wasn’t passing signal. The Drain voltage was 276VDC and the Source voltage being 0.71V. Injecting signal by way of the Return RCA jack to it’s gate got there, but no signal on the Drain. Powered down, the gate measured 70 ohms to ground. Ordered parts, installed a fresh one when they arrived.
With the signal fed into the amp, I still had no Reverb. So, in checking the return path, I had signal passing thru it, though the level seemed quite low. After looking at the schematics, the signal from Q102 passes up to the Reverb level and tone circuit by way of a ribbon cable, then comes back to the Reverb PCB and is routed to Ch B of IC103, a 2120D Dual Op Amp…switched part like that 5201 IC used on Marshall amps. I wasn’t seeing signal at the input @ Pin 7.



I unplugged the ribbon cable, and jumpered the send/return pins to bypass the Reverb pots, and now I had signal at Pin 7 of the 2120 IC. But, no signal on Pin 5, which is the common output of the two op amps. Pin 1 is the control voltage port on this op amp. I had 12VDC with no foot switch plugged in. With the F/S plugged in, Pin 1 would change from 15VDC to 12VDC. No output signal in either case.

I swapped out the 2120 for a new one, but with the same results. Then, I swapped out the switching transistor Q103, being a 2SC2910 hi voltage med hfe part. Closest I had to it was a BF422 and tried that. No change….still no output from IC, with Q103 switching fully on and off. The output from the 2120 is DC coupled to the summing input of IC1B (a 2147 hi voltage IC via 56k summing input resistor). The Switching IC 2120 is running at unity gain, so there certainly needs to be a lot of gain on Q102, which I don’t see.either. I checked to see what the DCR was at the gate of Q102, and it was 220k, the resistance of R125, which before was sitting at 70 ohms. That had to have been the Reverb tank misconnected by the client, who replaced the tank. I had DCR of 220 ohms and 70 0hms, so the 70 ohms would be the input and 220 ohms the output. That would explain why I had seen 70 ohms before.

I have plenty of signal driving the tank, just have no gain or signal thru the return path. I do see the traditional low level signal coming thru the tank.

I swapped IC103 again, but still no signal passing thru it. Also, with the Reverb controls plugged in via ribbon cable, the signal level from Q102 MosFET is loaded down, so I still have to get to the bottom of that. First time I’ve encountered this problem on one of these Vox amps.

LND150-N3.pdf
NJM2120_E_sub for M5201 Marshall Dual.pdf
Vox AC30-CC2X Sch 1.pdf
Vox AC30-CC2X Sch 2.pdf

[Enzo]

Pull IC103, and tack a jumper from 5 to 7. Does that restore reverb sound?

And I would expect that pin 1 control voltage to shift between 15 and 7 rather than 15 and 12.

What happens if you ground pin 1?

SOunds like you also have trouble on the reverb pot at the other end of your ribbon.

[nevetslab]

I pulled IC103, connectd a jumper between pins 7 & 5, leaving the jumper in place bypassing the Reverb control, and that restored Reverb function. And, as I suspected, the client had mis-connected the reverb tank. It sounds right driving the low impedance side of the tank, with the 2200 ohm side being the output, though it does work the other way as well. And yes, restoring the ribbon cable to the reverb control kills the signal. Very odd failure here, having lost so many pieces of the Reverb Return circuit.

I havent' yet plugged a 2120D back into IC103. I too would have expected half the voltage at Pin 1 when switching between Reverb On and Off. So, On with pulling the chassis back out, pulling the main PCb out to get the top panel PCb out to see what wrong with the control wiring/ots.

[nevetslab]

Having Reverb signal get thru to the Summing Amp IC1B on the main PCB, I then pulled the chassis back out of the cabinet, then had to remove the main PCB away from the chassis in order to remove the top panel PCB and see why the pots are stopping the signal. I found a dead short across the Reverb Level Control pot, but no solder shorts or anything visible. I unsoldered the pot & removed it, then checked the PCB pattern again. It was now open circuit, and the pot was now measuring 500k, and behaving like a pot!???! Installed the pot back onto the PCB again, and checked again. Now normal. OK. I'd chalk it up to being Monday, if it wasn't already behaving that way since last week.

I put the Top control panel PCB back into place with it's hardware, re-mounted the main PCB with it's hardware and preamp tubes, and plugged in all the ribbon cables again to see if that is now working. Still had IC103 removed and a substitute for Q103 in place, jumper between IC103's pins 5 & 7. Now the Reverb controls are working. OK. I next removed the jumper from Pins 5 & 7, and installed one of the replacement 2120's I had tried on Friday. Powered back up, and I still have Reverb, now passing thru the 2120D IC. Checked it with the foot switch, and now that is working. This all MUST be due to it being a Monday., as I have no logical reason for this. I then removed the Reverb PCB again to remove the BF422 NPN I had installed in place of the 2SC2910, and restored that to it's position at Q103. All working fine.still.

And, just to Mock Enzo and I, the Control Pin voltage for IC103 (2120D) is still 12V & 15V...15V for NO Reverb/Ch A selected, 12V for Reverb /Ch B selected.

That left dealing with the replacement Reverb tank that was installed by the client. The RCA cable is color-coded RED and WHT. I opened the Reverb Pouch to find a Belton/Accutronics tank, whose color code was also RED and WHT...only WHT is the Output and RED is the Input, opposite that of the Vox color code. Korean Tank. I've had to deal with that color code confusion every time someone in our Guitar Dept swaps out a Reverb tank that's not working, and then it still isn't working as a result of the Korean color code.

Now, I just have to try and catch the client before they toss out a perfectly good Reverb tank. Now, what caused all this weirdness in the first place? Perhaps having the amp fall over while on and the Reverb output take out the Return circuit MosFET. I can see that, but a short that was found and went away after removing the boards and the pot to get at it, then the Switching IC not working until Monday after the pot was restored? Glad it's over!

[Enzo]

Function is the test. Just because I EXPECT a certain control voltage doesn't mean that is what will be there.

[g1]

Here's the datasheet, pg.9 talks about the switching voltage but the language barrier is throwing me off.

[nevetslab]

Amp came back with no reverb!

I hope I havent' inherited an amp that only works in Burbank....after spending way too much time to find nothing and have it start working again. Sure enough, this amp arrived in it's road case, one of 5 like it in the client's inventory. My heart sank when I saw the asset number, and the complaint NO REVERB. Put it up on the bench, and sure enough, no Reverb.

With my scope probe attached to Q102, being the MosFET return amp, and saw no signal there. Pulled the Reverb return cable, and injected signal into the return jack, and saw still no signal on the Drain. I pulled the ribbon cable that sends the buffered return signal up to the controls, and looked again. Now I had signal. Before, I had found a short appearing across that Reverb Return pot. Not this time. Pot measured fine. But, wen measuring on the cable to see the top and bottom of the pot, adjusting the wiper changed the reading down to 0. Disassembly time again.

But, leaving that control cable unplugged, I jumpered the send/return pins of the header, to see what appears at the 2120 Switching IC. I now got signal at Pin 7, but nothing at Pin 5. 2120 is not passing signal. Pulled the IC, placed a jumper in the socket Pins 7 and 5, and then moved the scope probe up to the Mixing resistor at IC1B. No signal there either. What in the land of Montrose is going on (client's home base)

Pulled the chassis out, pulled the boards free to get the control panel PCB out, and found nothing wrong, and now the pot is reading correct. Just so I didn't do this for nothing, I scraped a clean path between the traces of the pot and the ribbon cable header with the Xacto knife, checked again...still measuring right, and re-mounted the top panel PCB. Put the main PCB back into place, re-connected all the ribbons, and powered back up.

Injected signal again, and now the reverb return is working as I did when I last had it on the bench last week. Weird!

[g1]

When you had the jumper from pin 7 to 5, and no signal at 5, did you look back at 7 to see that there was no signal now when it had been there with the IC in?
Just wondering if any socket weirdness could cause your problem.

[nevetslab]

I did have signal at Q102 Drain, had signal at the jumper plugged into Pins 7 & 5, but no signal at the mixing resistor. That was a new finding. But, after pulling the control PCB out, finding the control ok, and still ok when I put that and the main Preamp PCB back into place, I had looked to see if I got signal from Pin 5 over to the mixing resistor (don't have the schematic number for it in front of me, but it's IC1B where it mixes with the main signal from the preamp)....I had continuity.

I wasn't finding any issues with ribbons when flexing them...though you can't very far as they're short and multi-wire....like 8 conductors each.

To find this working again, with the latest new 2120 Switching IC installed, which an hgur ago wasn't passing signal.....this may be another amp that only works correctly in Burbank. I left it running for the remainder of the day, and will resume in the morning. The IC socket is a nice machined-pin pair of 4-pin in-line sockets with tight fit on the IC leads. Ive never had this sort of oddness with them before.

[nevetslab]

My client came by this morning to pick up this AC30, along with some of their other repaired gear. The rain had finally let up. allowing me to take it down the driveway and roll it around on the pavement, up and down the driveway to bump it around in the road case. Rolled it back to the shop, pulled the case cover off, plugged it in and the amps' Reverb was still working.

The client's truck driver told me they have one equipment truck that has a fuel line issue, that has yet to be cured....it conks out on them while out doing cartage, with it having multiple trips to the repair shop. So, perhaps it's them what's got the voodoo and not me. As good of an answer to the mystery of why the reverb return path keeps failing.