Ampeg SVT-VR Protection Circuit

[nevetslab]

Most of the Ampeg SVT's I service are the SVT-CL's or SVT-AVs. We also have a eight SVT-VR's in our rental inventory at CenterStaging here in Burbank, CA, and I have one that's not coming out of Protect. While I haven't yet opened up the chassis to check the AC Mains PCB, I do know that the control circuit to the Time Delay relay comes into that board by way of J36 via a short twisted cable off the nearby edge of the main PCB.

But, none of my schematics show ANY of the Protection Circuits that will finally feed the relay circuit. I haven't yet spent the time to look at the main PCB to see what Ampeg is doing. At least in the SVT-CL, they show the protection circuits.

This particular SVT-VR remains in Protect with the power tubes removed. While it could be as simple as the mating connector to that 2-pin header has fallen off, it bothers me not having the circuit diagram. There are other incomplete circuits in the standard SVT-VR Schematic documents from Loud Technologies.

Does anyone have the rest of the -VR Protection Circuits schematics? Love to get a copy of that.

Steven

[Enzo]

Did you call LOUD TECH? Just because it isn't online doesn't mean they don't have it.

[Jazz P Bass]

Hey, Steve.

Long time no see.

[nevetslab]

Yeah, it's been a good chunk of time since I"ve been on, and have a lot of posting to do from the travels chasing electrons.

I did see a statement on a different forum regarding the differences between the SVT-VR and SVT-CL. basically stated there ISN'T ANY circuit protection in the SVT-VR as there is in the SVT-CL. I haven't lifted up the VR main board to see just what's attached to the mating connector that feeds J36 on the relay board. +FIL (3VDC) and -FIL (-3VDC) is shown on the schematic for that relay board. No doubt a similar circuit as seen in the SVT-CL for sourcing those voltages to start up the time-delay relay circuit. Both wires to that mating connector are the same color. I had one 'VR whose connector had come off when I lifted the relay board out. The locking ramp had already broken off the PCB header, replaced with a mound of RTV that didn't help. Mis-connecting the mating plug does prevent the relay from turning on, leaving you with a RED lamp status on the preamp. Reversing it made the relay work again, so in that instance, there wasn't any issues on the mother board/power tubes.

I'll get a closer look at the SVT-VR motherboard next week, as I have one that is pulling fault current. One of the 'NEW' AMPEG production units where they don't bother tightening hardware inside or outside the cabinet. There was a big pink 'X' gaffer-taped on the cabinet top. When I picked it up off the inventory shelf, I felt the chassis sliding around, as well as the transformers moving. When I got it to the shop and had the chassis ready to come out of the case, grabbing hold of the two large transformers to aid in lifting to pull it out, the Output Xfmr lifted right up off the chassis!! The Pwr Xfmr lifted up partway, held by two remaining screws ready to come out. Wonderous quality control at Ampeg!! Nothing was tight inside, not to mention missing a number of screws. The sleezy M6 cage nuts being used these days break their hook wings off, leaving sizable metallic hardware inside the chassis to find high voltages if not caught in time. Somehow this amp didn't get back into the road case, as that O/T was ready to mow down the cluster of power tubes like bowling pins! But, all that will wait for another thread.

Good to be back.

[Enzo]

The FIL circuit at the two pin connector is just the DC heater supply for the preamp tubes. It is the power source for the relay, but not its control. If the heaters are not powered, then the relay won't be either. You can see wher the control signal comes from on the CL drawing. Don't know about the VR.

[Leo_Gnardo]

Good to be back.

Yes and it's good to see you back. Last we heard you were moving out of your old apartment . . . then nothing for a year or so. Couple of times I wanted to refer some people with questions about BGW's and other whatnot to you. Well it's good to hear from you and that you're still holding down the fort at CenterStaging.

[nevetslab]

Having finally restored the SVT-VR that was missing all the output & power transformer mtg hardware, chassis mtg hardware, with cage nuts lost inside the chassis, broken wing flap metal from the cage nuts loose inside, flimsy lock washers that are sometimes used under the transformer mtg screws. I was able to restore order after tightening all the power tube socket standoff hardware. Those have recently had the nasty habit of locking the hex-drive truss head M3.5 in place (screws that hold down the clamps), and cause the hex driver to slip inside the screw. I had only gotten as far as powering the amp back up in two pieces on the bench, to find besides all the mechanical issues, there was fault current to find, it immediately blowing the T10A mains fuse. I had several other SVT's lined up waiting for service and set this aside until yesterday. I did find a source for decent spring-steel M6 cage nuts, having had it with Ampeg's cheap faulty hardware.

All it took was one of the 1N4007 diodes in the -150V Bias circuit to be shorted to take it off line. I still had lifted out the main PCB PCB to dig out all the parts totally covered in RTV. Always something to slow down maintenance efforts. Anyway, contrary to what's called out on the SVT-VR power supply schematic, what feeds the Relay control port on the AC Mains PCB assy comes from the 5V IC Regulator, and not that other circuit fed from a 15V supply. And, as I had read elsewhere, THERE IS NO PROTECTION CIRCUIT in the SVT-VR, other then a T10A Mains Fuse (US wiring). +5VDC and Gnd are what's on the two-wire cable that plugs into the AC PCB's relay control jack J36. I forgot to trace out where the PCB fuse installed in clips was connected. It's not shown on the schematic either.

Last month, finally getting around to adding 1/4" split lock washers under the chassis mgt screws on our collection of SVT amps, re-installing the M6 chassis screws were no longer engaging any cage nuts that had been in place on the bottom flanges of the power amp chassis. Great! more loose hardware to float around inside with 650VDC present. So, I ended up with a room full of SVT-CL's and -VR's to pull apart to deal with the displaced cage nuts. Found decent M6 Cage nuts from Rackfinity on line P/N GM6-SNP-Z25 for $17.47 w/shpg, 25 pcs. Good for Marshall amps as well.

At least in the process, I discovered new arrivals to our inventory that was never sent over to the shop to be set up. These days, with what's coming out of Ampeg, if YOU take an SVT-CL out of the box, load it into a road case and go traveling with it, IT WILL FAIL. At least the SVT-Cl's and -AV's have a fault protection circuit that works.