nevetslab 7/31/2018 10:03 PM
Ampeg svt-cl ht xfmr cycling on and off
The last of the Ampeg SVT-CL’s in our rental inventory (going thru preventative maintenance) finally returned from being on the road with a client. It failed to operate, not blowing mains fuse, or going into Protect Mode (flashing RED Status LED)…it would attempt to power up, but then the AC mains relay would drop out, and go thru attempts to engage, but shut back down repeatedly.

The last time I had a similar problem like this, it was from the main heater wires’ right angle ” fast-on connectors that got crusty, as did the wires crimped into them, overheating the connections.

Those who don’t know the SVT-CL system, the heavy heater wires come onto the Power Tube PCB from the bottom side of the tube sockets, inaccessible until you remove the power tubes, hold-down clamps, power tube PCB cover plate, and finally the Power Tube PCB. The preamp chassis assembly gets it’s power off that power tube board by the 2-pin right angle connector, with a pair of white wire mating connector attached to it. It plugs onto J5 in the preamp, then gets rectified and converted into roughly +/- 3.5VDC, and goes out on J6 and connects to the AC mains PCB J36 to operate the HT relay. The heaters for the two driver tubes and preamp tube ahead of them in the power amp circuit are powered by a separate heater winding.

On other SVT-CL’s, I’ve had to chop off those right-angle connectors that plugged onto the blade terminals on the bottom side of that power tube PCB, and directly solder those heavy leads to restore order, when warranted.

That wasn’t the case this time, though on the AC Mains PCB, the relay power terminals had solder fractures, and the right-angle connector on the power tube PCB that feds the Preamp heater supply, those terminals also had solder fractures. I repaired those connections, but, the cycling of the relay continued.

Sometimes the preamp’s status LED would go GRN, even after the relay would drop back out…though eventually it would go RED and remain RED.

I monitored the control voltage that comes in on J35 of the AC mains PCB assy, which connects to the base of Q1 MPS A13 Darlington by way of R3 220k. And, monitored the relay voltage connecting across D1. What I was seeing on the initial power up, was J35 immediately went to about +10V, and the relay voltage across D1 went to around 6.5VDC. Then, the relay voltage would slowly drop, finally the relay drops out, and you could hear the power transformer shut off, then hear it engage again, only to drop out again. I was watching the AC mains current change from around 200mA to around 650mA. I had already removed all the power tubes, and just working with the control circuits for the system. Just to say I did, I replaced Q1 and D1, and as expected, to no avail.

I also was seeing the control voltage on J35 had dropped to around 950mV, where it had been a bit above 10V.

That led me to the system’s low voltage bipolar supplies. Looking to see what the voltages were across the +/- 20V supplies at C24 and C25 (using the parts placement guide as a map, since the chassis was upside down, showing me just the solder side of the main power amp PCB). I was only seeing about +16V, -17V on those caps, and the voltages across the ‘+/- 15V supplies’ they were around +11V, -13V at first, but then kept dropping. After shutting off the amp, discharging all the HT caps, I was curious about the two thermistors CB1 and CB2, which couple the AC CT Xfmr from the Heater Xfmr to the low voltage bipolar supplies’ discrete diode bridge. One was reading 47 ohms, the other 230ohms.

Are these even needed in this system? I knew I didn’t have any replacements, and ended up removing them, replacing them with 10 ohm 2W MO, laid horizontally, since the tube PCB sits into that cavity above the main power amp PCB. I also pulled the two pair of caps C24, C25, C26 & C27, all of which were heavily gooped with RTV and firmly holding them onto the PCB, defying all attempts to remove them! Installed 330uF/35V Nichicon PW series for C24/C25, and 220uF/35V for C26/C27.

I had also installed a 220uF/35V cap for C12 and the diode D1 (1N914) across R43 68k, where R43 & C12 formed a 15 Sec Turn-on Delay to the base of Q1 on the AC mains Relay board. Output from this turn-on voltage is by way of J12 on the main power amp PCB. Neither part was present on the PCB, though not sure they were really required.

The two thermistors, CB1 & CB2 looked like the size of a 470pF candy-coated ceramic cap, around 10mm dia. I’d never looked at that part of the circuit on the SVT-CL, not having any problems with it there until this amp. The schematic simply refers to them as 0.4A thermistors. No idea what there no-load resistance is or their 0.4A resistance is. The 10 ohm substitutes was an educated guess. I only found 1V AC across them when I powered the system back up, and now had stable +/- 19VDC across C24/C25, and +/- 15VDC across C26/C27, with the relay now pulled in, 6.7VDC across the relay coil, and 10.5VDC on J35.

I had also swapped out IC2 on the main power amp board, which controls the Fault circuit, and IC1 on the preamp PCB, which drives the LED Status Light, just to be thorough. That got the amp stable again with reliable turn-on/stay on, once out of Stand-by.

I haven’t yet measured the caps C24-C27 that I removed. I think that one thermistor that measured 220 ohms was certainly one of the faulty parts, and no doubt one or more of the caps. The solder joint fractures in the path didn’t help either.

But, you had to go all over the place in the amp to get at everything. I was told by our Guitar Dept mgr that the band who had rented it and a back-up SVT-CL, they had taken it to some shop, who replaced all the preamp/driver tubes, and told the band the amp was NOT repairable. YUP. Certainly not by that method!

I went to attach the SVT-CL Relay Control/Fault Circuit schematic, the LV power supply schematic and the AC Mains / Relay PCB schematic, but for some reason, the pdf files came out as GIF and blank. Sorry about that.

g1 8/1/2018 11:39 AM
Quote Originally Posted by nevetslab View Post
I went to attach the SVT-CL Relay Control/Fault Circuit schematic, the LV power supply schematic and the AC Mains / Relay PCB schematic, but for some reason, the pdf files came out as GIF and blank. Sorry about that.
If you zip the file and post it as a zip, it will work.
nevetslab 8/1/2018 1:27 PM
Thanks, g1....that did indeed work.

I was mistaken regarding how the two 12AU7's and the 12AX7 get their heater voltage supplied on the main PCB. It comes down from the Power Tube PCB by way of the 20-pin ribbon cable, pins 6-8-10-12-14 & 16. So all of the heater voltages come in by way of those two heavy leads on the back of the power tube PCB.