Finally found an Ampeg SVT-CL with bad HT Pwr Xfmr!

[nevetslab]

One of the crew at AuntieM in Alhambra who I do work for regularly brought me an Ampeg SVT-CL and an SVT-VR to restore to working order. The SVT-CL had already been worked on by somebody else, who left the power amp chassis mtg hardware off, missing Output Xfmr mtg screws, loose hardware all over, broken input tube to the power amp, etc. After putting the two chassis' back into mechanical working order so I could proceed to begin powering the system up to see what other problems there are, such as more than one power tube.....it pulled like 1600W AC Mains with no power tubes installed, and blew the 10A AC mains fuse as I was reaching to turn it off. Replaced the fuse, after first replacing the CMA fuse cap to an AGC size cap.

Disconnected the two HT secondary windings from the power amp PCB, but it was still pulling high current...shut it off before blowing another fuse. Then, armed with the power xfmr wiring diagram for the two power xfmrs, verified the wiring was correct for the HT Primary. Then, unplugged those seven wires and applied power again, once restoring the AC mains PCB to the chassis. Now, power consumption remained at 24W. And, now have isolated the major high current fault source. Bad Power Xfmr # 94-602-40. I saw Flip Top had one listed for $230 + shpg/tax. Now waiting to hear from the chap at Auntie M, who's doing this for a buddy of his. Already 3-1/4 hrs into the project, so this one has the earmark of being VERY EXPENSIVE. The end goal on this was to make it ready for sale. Ouch!

[Leo_Gnardo]

The end goal on this was to make it ready for sale.

Might be best off being your SVT "parts car."

Transformer plus a set of tubes plus your troubleshooting time plus more parts besides, he'd be lucky to break even if he tried to sell it.

[The Dude]

FWIW, I think Full Compass has the transformer cheaper. They are currently out of stock, so it might take a little longer.

https://www.fullcompass.com/prod/512...mer-for-svt-cl

[nevetslab]

FWIW, I think Full Compass has the transformer cheaper. They are currently out of stock, so it might take a little longer.

https://www.fullcompass.com/prod/512...mer-for-svt-cl

Thanks for the heads-up on that. I first have to see what my client will say. I'll have to get his other SVT...an SVT-VR back up and running. I'll use that to test the sextet of 6550's that came in this SVT-CL. I was half-expecting to have one or more of the electrolytic caps in the power supply be the cause of the high current draw.

[nevetslab]

Might be best off being your SVT "parts car."

Transformer plus a set of tubes plus your troubleshooting time plus more parts besides, he'd be lucky to break even if he tried to sell it.

Interesting thought. I tend to doubt the owner would want to go that route. Seems like these sell used for close to a grand, don't they? I never looked, though they're definitely a nice amp to have. I'm more partial to the SVT2-Pro than the SVT-CL. As long as someone else is moving it. These old bones just ain't what they used to be.

[The Dude]

From what I've seen, yes- decent used ones are still over a grand. I'd think it'd be worth fixing even though it's not going to be cheap.

[Leo_Gnardo]

From what I've seen, yes- decent used ones are still over a grand. I'd think it'd be worth fixing even though it's not going to be cheap.

Well if you get upwards of a grand, then it's worth the trouble. Call me a dummy for not checking. I figured these days when you can get a 600W bass amp the size of a textbook for a couple hundred bucks, SVT's had gone the way of the dinosaur.

[nevetslab]

This morning, with the amp still sitting on it's end with the Power Xfmr's primary and secondary wires disconnected from the system, I wired up a spare AC mains cord with fully insulated Male Fast-ons so I could connect the two phased primaries to it, and check it directly thru my Variac/Power Analyzer. None of Ampeg's SVT-CL or SVT-AV documents show the actual HT power xfmr's schematic, nor that of the Heater Xfmr. So I still had to go thru and deduce those, at least having the BLK and BRN wires as a starting point, which were plugged into J27 & J28. For 120V wiring, one primary having BLk & WHT at the extreme ends also has BLU & BLU-WHT as taps, with the other primary having BRN & VIO as the exteme ends, with GRY being a tap. For Export wiring, there's enough to connect for 220V, 230V & 240V.

As I expected, nudging the variac up a touch, high current began to flow, and I backed it down to 0VAC & unplugged it. Confirmed failure. Now, from just the wiring, I found a short between the RED-RED and YEL-YEL Secondaries, as well as a short between the seconaries and BRN-GRY-VIO primary! Just what everybody needs for a good bass amp!

So, for now, I'll put this back into the cabinet, xfmr remaining disconnected. As luck would have it on this Preamp, one of the small format #10-32 cage nuts used on that chassis has snapped off the wings, leaving no way thread the fourth mounting screw thru the cabinet. I've still not found a resource for that small format cage nut, unless I purchase 1000 pcs! Nice move, Ampeg!

[drewl]

Weld a nut in it's place!

[nevetslab]

Weld a nut in it's place!

I don't have welding capability in my shop, and while our building maintenance chief does have arc welding provisions, I'll probably just resort to punching a hole adjacent to the square opening, and fashion a strip of metal & thread it for #10-32. Downside of fixed threaded holes is alignment thru the cabinet with that fixed hole. For the time being, I reached in with my hand, put that broken square nut in place and drove the screw on into it and tightened it up. I'm assuming I'll be the next one back into the cabinet. If not, there will be the sound of something dropping onto the top of the power amp chassis to instill dread.

[drewl]

I was able to solder four nuts on the bottom plate of an amp I built using my giant soldering iron.

Still holding thirty years later.

or how about the cage nuts like that are used on equipment racks, the ones that clip into the square hole?

[nevetslab]

I was able to solder four nuts on the bottom plate of an amp I built using my giant soldering iron.

Still holding thirty years later.

or how about the cage nuts like that are used on equipment racks, the ones that clip into the square hole?

Those cage nuts are the same size used in the power amp chassis. Those are the 'standard' size everybody carries. The smaller ones that Ampeg elected to use in the preamp chassis...........very little call for them, so nobody carries them. Default is order 1000 pcs and a distributor will sell them to you gladly. And, of course what Ampeg has been using now for some time are the cheap Chinese crap whose cage is stamped from non-spring steel. The wings snap off with the greatest of ease, as do the larger ones in the power amp chassis. On those....nothing more irritating than having previously removed the M6 chassis mtg screws, but try to get them to thread back into them during reassembly? Won't thread, then on the third cage nut, having fought these first two, that cage nut pops out of its' square hole, now with a broken wing! I have at least stocked up on properly made M6 & 1/4-20 cage nuts. And pinched fingers too many times to count during installation of them from my bent-nose pliers!

Meanwhile, I'm trying to track down an Ampeg SVT-CL, same later vintage amp as this one with the shorted power xfmr....this one I'm seeking was damaged by one of our clients, and I was told he had to pay for the replacement cost of it, with that amp no longer being in our inventory. I never saw it nor got any report of the extent of the damage.....but, hoping it's total loss, while being a likely candidate for salvaging a power xfmr. Waiting to hear back on that.