Vox AC30-CC2X Maintenance...PITA

[nevetslab]

I moved a Vox AC30-CC2X into the shop from the shrinking repair pile, which had the remains of an aluminum electrolytic cap can taped to the top of the amp. Note stated 'blows fuses'. After removing the chassis assembly from the cabinet, then removing the wooden shelf from the chassis, I found all of the shrapnel of an exploded 22uF/450V Electrolytic cap everywhere. Most looked like confetti, while finding both axial component leads still solidly attached to the PCB. The rubber slug was among the shrapnel. I must say, this is the first time I've seen a main buss filter cap detonate with such force in an amp. I'm still picking up bits of the confetti.

One of the key 'features' of this model, is everything is tethered on a short leash, so to speak. To get to the bottom side of this main power supply board is VERY Tedious. I looked it over,and after giving firm tugs on the two remaining leads from the exploded cap, I cut them down, and with round-nose pliers, formed a curly-cue pig tails, tight enough to slip the leads of a replacement cap into the holes and tightened them up. Sits nice and solid on the PCB. Checked the AC Mains fuse, and the heater fuses, then plugged it in, ran it up n the variac, in Standby mode, monitoring the power analyzer for signs of failure. Seemed ok, holding steady at around 400mA/46W. While my attention was elsewhere for a moment, turning back, I saw there was now no current draw and the power LED was out. Popped the T4A fuse out, and it was blown. 5AR4 Rectifier tube, no doubt. I fetched another one and popped it in, then ran it up again and watched it for a while before switching out of Standby. Drawing the same 400mA/46W in Standby, then in Operate, now drawing 920mA/100W.

I connected a long speaker cable to reach to the check-out bench where the cabinet was sitting, and plugged it into the speaker jack. Fed input signal, but got nothing. After checking all the settings, I checked the supply voltages, and found 320VDC on the plates & screens, 11VDC on the cathodes of the power tubes, and around 280V on the supply side of the plates in the preamp section. Fetched the scope probe to see if I had signal anywhere. Found signal until I got to V3, finding nothing. Checked the output of the NJM2147 Hi Voltage Op Amp, and no signal on IC1A or B outputs.

Sigh. Getting that PCB out of the chassis is a major PITA. The preamp TUBE PCB has to be removed first, and it's tightly tethered in place with six wires from the other side of the chassis wall. I had to mark the thick BLU, BLK and RED wires so they'd be able to return from where I removed them from, unsoldered them and fished them out, now having everything else from that board disconnected, and extracted it from the chassis.

I removed the mtg hardware holding the bipolar supply board with it's Reverb circuits from the chassis, while it too is tightly tethered. And, just to make it all the more enjoyable, my right hand is now bleeding from encountering the sharp inside edges of the aluminum chassis that never gets deburred in the fabrication shop! I hadn't unsoldered all the wires to this supply board, but did find the 100 ohm series resistors from the half-wave rectifier feeding the 27V zener diodes were both open and burnt, while both of the 27V 1/2W zeners were shorted. The 15V zeners measured fine, as did their 470 ohm source resistors from the +/- 27V busses. I replaced those parts, then setting it back into place, fed what I had for supply voltage from my Tek PS503A (+/-20V is all I can get from it while my other supplies are back at the other shop. Something is still dragging the negative supply down, not getting +/- 15V.

So, more wires to remove, and marked the LED wires so I could restore order after figuring out what else failed. ONWARD.

I guess when that high voltage cap exploded, the shock to everything took out the bipolar supplies in the process.

[nevetslab]

I removed the power supply board, tried hooking it up again, and this time, nothing is dragging the supplies down. (??!?) Put it back into the chassis again, hooked it back up with just the external supply, and still working normal. So, finished installing the mtg hardware on that board. Then, had to contend with that tube PCB, of which there are those six stinking thick wires that get routed thru the board and back out to be soldered, and, no means of positioning anything to make it remotely serviceable. When it came to the two black wires, which I had been certain not to allow them to get mixed up, it seemed like I had too much length to be grouped with the three Blue wires. Installed it anyway, while soldering them in was touchy. Finally got that board mounted to the vertical panel, while dropping the screws/washers enough times to want to be elsewhere, as the sensation of claustrophobia was sneaking up on me working in such tight confinement. Finally got everything mounted, apart from the shelf panel.

Powered it up, connected the speaker and signal cable, and now I had a working amp. Finished putting it all back together. If I never have to gut one of these that far in the next ten years, should I live so long, it will be too soon! AC30-6/TB's are FAR easier to service then this thing!

[nevetslab]

So, I moved the amp from the bench to a dolly, and wheeled it across the aisle to the Guitar Dept. 20 minutes later, they call me over to tell me the Vibrato ain't working. Odd. It was working fine here in the shop. Now, I didn't have a pedal for the amp here. They had one over there, so I unplugged the pedal, and still no vibrato. It's as though somebody read my comments about having to tear back into one of those again.

I already have an Ashdown EVO III 900 whose output on a sustained LF note begins distorting and fuzzing as the note trails out. I heard it do that on the one I just serviced, (mounting the steel Pwr Xfmr hold-down plate on), while an identical 900 in the stack didnt' have that problem. So, brought that over to the shop. I just had it on the bench, and.....NO trailing fuzzy distortion on the LF notes here in the shop. Misbehaves in the Guitar dept, works fine in the shop. Simple answer. Use the amps only in the shop.

[nhbassguitar]

EDITED

That fragmented cap reminds me of stunts we used to pull in tech school. We'd connect a low-voltage electrolytic backwards to the output of a high-voltage DC power supply that could be remotely triggered, and then step back a few feet and wait for the right person to walk by the bench. That was before fun was outlawed by today's armies of annoying unemployed uplifters. The VOX's electrolytic blew up because of what was happening upstream, yes?

Does that amp have a reverb tank connected by thin wires with RCA (phono) plugs? If so, I'd wiggle those a bit and rotate the connectors a bit & see what happens. Those things are the bane of nearly every HiFi/stereo setup, unless the cable is quality and the connectors are gold plated.

[nevetslab]

EDITED

That fragmented cap reminds me of stunts we used to pull in tech school. We'd connect a low-voltage electrolytic backwards to the output of a high-voltage DC power supply that could be remotely triggered, and then step back a few feet and wait for the right person to walk by the bench. That was before fun was outlawed by today's armies of annoying unemployed uplifters. The VOX's electrolytic blew up because of what was happening upstream, yes?

Does that amp have a reverb tank connected by thin wires with RCA (phono) plugs? If so, I'd wiggle those a bit and rotate the connectors a bit & see what happens. Those things are the bane of nearly every HiFi/stereo setup, unless the cable is quality and the connectors are gold plated.

No idea what was occurring when the amp blew the cap. I assume it was on-stage with a client (rental gear). Yes it does have reverb tank, with quality cables connected. Tank works fine. Now I'll have to deal with the Vibrato circuit that was working until I rolled it back to the Guitar Dept, There are T0-92 size MosFET's involved in the circuit, which, regrettably is located on the chassis floor's Bipolar supply/Reverb circuit PCB. Nightmare to extract due to the progressive unbuilding required.

[nevetslab]

I finally got back to this Vox AC30-CC2X Amp's Vibrato Circuit. The preamp tube PCB, which is mounted to the chassis wall, and has ribbon cables connecting to the control panel PCB and the Reverb/bipolar supply PCB down below it, plus it has six thick-insulation wires coming from the HV Power Supply PCB on the other side of the chassis wall. I had to remove that PCB in order to lift up the Reverb/Bipolar supply PCB below it to do those repairs, then, after it was remounted to the floor of the chassis, I had to fish those thick insulated wires from the HV PCB thru the anchor holes, and finally slip the ends back thru to solder them into place.....Resistor R39 connected to one of those blue wires @ W1. I managed to disrupt the solder joint of R39 that feeds the TO92 MosFET MF3 by way of R37. When I wasn't finding any voltage on the Drain of the MosFET, I looked down that string of resistors, and found I could wiggle R39 to make it conduct, as long as I held it just right. Jumpering the solder joint restored the Vibrato, so had to again lift that PCB out just enough to resolder that resistor to the W1 Blue wire joint. Cured the problem. While it was on the bench during the initial surgery, Vibrato WAS working....I just managed to miss that mistake, and moving the amp was enough to jostle the resistors' connection to stop the Vibrato from working. Sigh.........

At least I didn't have to fully disassemble it as I did to extract the Reverb/Bipolar supply PCB. I should have taken pictures of this whole operation from the start, but failed to do so. Next time.