Ampeg SVT classic Loud technologoies

[nosaj]

Got an SVT classic powers on bias lights show green after a few seconds relay clicks lights go out. Checked LV on the preamp board +-15 are good. the 14v line stays solid. Pin 5 voltage when bias lights are on voltage goes up to 350vdc then relay clicks and HV trickes down to 0v after a few minutes maybe it bias comes back then disappears.

Since there are 6 tubes I've tested them in a tester for shorts all good.(no shorts leakage good) Can I halve this some how to determine if it is a tube causing this issue? I only have one bias probe.

also I do not know how many different schematics there are for this amp.. But on back it says Loud Technologies 2012.
Thanks,
nosaj

When Tubes cool I will pull them all to see if HV rises and falls.

[Enzo]

SVT-CL

Just a thought, more than anything else, on these amps, especially when bias lights are awry, the trouble is a bad 12AU7 or a 12AU7 in the wrong socket, or 12AX7s installed instead of 12AU7. Worth checking.

[nosaj]

SVT-CL

Just a thought, more than anything else, on these amps, especially when bias lights are awry, the trouble is a bad 12AU7 or a 12AU7 in the wrong socket, or 12AX7s installed instead of 12AU7. Worth checking.

All of them are correct. Pulled all the 6550's powered on while monitoring HV at the preamp board HV still rises and falls can hear a relay clicking when it happens . So we know it's not tube related.
nosaj

[The Dude]

If B+ drops even with tubes removed, I'd be looking at snubber diodes.

[nevetslab]

I've come upon this problem and numerous others, and finally started a project to eventually publish a thorough service book on the Ampeg SVT-CL, SVT-VR and SVT2-Pro. While the SVT-VR is more like the original SVT, the SVT-CL and SVT2-Pro are in the same family with the protection circuit that controls the HV power Xfmr. The problem you're describing is in the control circuitry, which is scattered among the Relay PCB, the Main PCB and the Preamp PCB, just to make life interesting. One of the continual problems with these amps, from their sheer weight, is the imposed vibration inflicted upon all of the PCB's connectors. The power & output transformer mtg hardware gets loose, so those transformers start functioning like a shaker table in transient.

I suspect the same will occur (relay clicks open) even with the power tubes removed.

I've written this following service note, which will end up in my book when I return to that project, but adding it here.

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 ¼” 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. 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!

With the amp’s control circuits working properly, I went thru and checked the power tubes for plate current balance. I found I had to swap V1 with V4 to get better plate current matching of the two sets. Biased up, remained stable.

Treble control knob had internal D-shaft mechanics partially break. Installed shim stock to restore, knob now fits tight again.

I'll follow this post, and no doubt get back to you, as I have years of service notes accumulated. I also have drawn up the power transformer and output xfmrs schematic diagrams for both the SVT-CL and the SVT-VR, and did publish them here on MEF last year some time. Ampeg never bothered with that, making you have to piece together the details from multiple schematics.

[nosaj]

If B+ drops even with tubes removed, I'd be looking at snubber diodes.

So D1 is a snubber diode across the relay? Sorry my electronics terminolgy is still amatuer at best. schem for ac board linked.
nosaj
http://www.tangible-technology.com/s...C-Term_Brd.pdf

collection of docs
http://www.tangible-technology.com/s...g/SVT/Classic/

[The Dude]

I was looking at the schematic I have- maybe wrong one? If not, it would be diodes D4,5,6,10,11,12. If dead shorted, they usually blow a fuse, but I have had them leaky and cause the symptoms you are describing.

Ampeg_SVT-CL.pdf

[nosaj]

i can make it happen on demand now. J36 cable connects to preamp board at J5 wiggle it HV stays on move it again it goes off. Got to dig up the tool to release the pins from the molex and expand it out. before pulling preamp board to check for solder fractures. So far it's been up for 10 minutes solid without touching it.
nosaj

[nosaj]

ok dumb move here Accidently reversed J5 and J6 and now relay chatters before engaging Relay chatter.m4a

measuring 14vdc on each wire to ground. both positive. with cable disconnected from preamp. Relay does not chatter on power on with j5 unpluigged.

nosaj

[nevetslab]

i can make it happen on demand now. J36 cable connects to preamp board at J5 wiggle it HV stays on move it again it goes off. Got to dig up the tool to release the pins from the molex and expand it out. before pulling preamp board to check for solder fractures. So far it's been up for 10 minutes solid without touching it.
nosaj

I've had issues with those 2-pin connectors on the preamp board....and Ampeg's PCB quality on that board sucks big-time. I've had to repair foil breaks on that one over time in our rental fleet here at CenterStaging. When you open it up, look close at all of the pot terminal solder joints, as well as the input jacks. Solder joint fractures on that board is VERY common. Also on the 5-pin power connector at the back of the preamp pcb. Also look close at the soldering of the 2 x 10 pin header...same issue, as well as the rest of the joints thruout the board.

[nosaj]

ok dumb move here Accidently reversed J5 and J6 and now relay chatters before engaging Relay chatter.m4a

measuring 14vdc on each wire to ground. both positive. with cable disconnected from preamp. Relay does not chatter on power on with j5 unpluigged.

nosaj

After replacing the the bridge rectifier circuit. Relay chatter is gone and voltage normal. Went through and resoldered all pots, tube sockets, jacks, cable connections. Seems rock solid on a dummy load. Will bring it out to the shed tomorrow to bring some misery to the neighbors Shame I don't have the 8x10 cabinet. just a 15in cab.
Thanks all for the help
nosaj

[nevetslab]

Good to hear, and happy to hear you've done the preventative/corrective maintenance. These amps ARE a challenge for sure. I do have to put myself into a certain mind-set in order to move forward, as they demand a lot of attention to get them to shine and behave. Well worth the effort, if you have a strong back (or roadies to carry them for ya).