Awakening Carver PT1800 amps from coma - Page 1
Forum section: electronics / repair

author: Audiorepair
author_rank: Frequent Contributor
timestamp: May 09, 2022, 06:07:59 pm
content: Ok, so I have a small PA to resurect, it has 5 Carver PT1800 amps for bass and mids and Carver PM's for tops.They have been in storage since lockdown in a supposedly dry outside building with no heat.Now, these are unusal amps in that they have regulated power supplies, but this is not strictly an SMPS.It uses a triac and PWM switching on the mains input to the mains transformer to regulate the DC rails.So, I need to power these things up without blowing them up.I really don't know if using a variac and a lightbulb is the right thing to do here, perhaps low voltage will cause high currents and blow stuff up that wouldn't otherwise have done so.Maybe it's less risky just turning the thing on at full mains voltage. (240v)They were all working fine before storage, and I had adjusted the PWM circuit and thus all the DC rails just a few gigs before that, so I know they were in reasonable condition.Its just I don't know what is best procedure to try and wake these things up without killing them, they are a very early and uncommon design.Diagrams here, unfortunately split in two halves.

author: bdunham7
author_rank: Super Contributor
timestamp: May 09, 2022, 06:17:35 pm
content: I've never known anyone to use a variac on these 'light dimmer' amps.I would bring them in and get them in a warm, dry location for a few days to drive off any humidity, remove the covers to inspect for spiders and such, but not use any lightbulb or variac for startup. Just make doubly sure that the proper fuse is installed for protection and fire them up. And maybe blow them out with compressed air if they weren't in sealed bags and seem dusty.

author: CaptDon
author_rank: Super Contributor
timestamp: May 10, 2022, 05:44:50 pm
content: You won't have much choice here but to plug-N-play. If you apply 60vac the triac is going to shoot for 100% duty cycle and the output of the transformer won't really be any lower than it would at 120vac. I know it is a bit scary but I would probably just go for it. I have multitudes of the CBA-1000 / PM-2.0T / PT-1200 units with the '14 capacitor trays' that sat for years with bad SMPS which died from bad carnival and county fair power. I bought the lot of them and fixed all but one which had a melted SMPS transformer. The only problem I ever encountered was one capacitor in one of the trays did actually explode. Amazingly, with aluminum foil and goo all over the guts of the amplifier it continued to operate with no additional problems for the rest of the day until I could pull it out that night and replace the blown cap! I never had a desire to own the heavy iron versions of the Carver amps. I do run the hell out of my C.B.A. conversion amps (All of mine a Clair Brothers spec'd regardless of model) and they have been flawless. I did lower the exact overcurrent fault circuit to a slightly lower value than the Clair spec's which pushed the darlington SMPS power transistors well beyond their design limit.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: May 10, 2022, 07:59:47 pm
content: These were all bought new in 1991.Out of the 6, 5.5 remain, one half having smoked its transformer a few years ago.I replaced a TIP driver on one, and a couple of Zeners melted on another, but apart from that they have been amazingly reliable.Apart from the hardware - these things just fall apart mechanically, like a snake shedding skin (bolts) whenever it likes.The PM 1.5's have also held up well, but they have only ever driven tweeters, so have had a very easy life.They need to do one more festival this summer, I'm really hoping they are all going to perform like they always have.I really don't want to have to repair any of them to make this happen, that would be brand new territory I don't need just now.

author: CaptDon
author_rank: Super Contributor
timestamp: May 10, 2022, 09:51:33 pm
content: It is funny that the PT (Professional Touring) models do seem to have had the most flimsy sheet metal and cheesy self tapping screws to hold them together!!!! They strip out and fall out like magic. A tiny dab of the blue Loctite semi-permanent seems to help unless the holes are already stripped. I have had to often go up one size in the diameter of the sheet metal screws being VERY careful of length!!! Some of those screws have very little clearance to live circuit parts!! I also have provided rear support brackets for every piece of gear in my road cases!! That really helps keep the gear in a secure and safe physical environment while the 'Samsonite Gorillas' throw my gear around and off the sides of ramps. Cheers!!

author: james_s
author_rank: Super Contributor
timestamp: May 11, 2022, 02:29:23 am
content: Just bring them indoors, let them warm up to room temperature and look over them for signs of rodent infestation or other damage. If everything looks good then plug them in and hit the switch. I've powered up equipment that was stored for many years without issues, there's no reason to expect any issues.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: May 11, 2022, 05:59:11 pm
content: Thanks to all, I kind of suspected I'd have to just flip the switch and hope it doesn't blow up.Not my preferred method of testing, particularly on power amps that are over 30 years old.Oh well, it is what it is, though I kind of suspect I'll have no failures, these seem to be pretty reliable amps so far.

author: james_s
author_rank: Super Contributor
timestamp: May 11, 2022, 06:15:15 pm
content: I don't see why you'd be worried about it. If they blow up then there was already something wrong with them or they were defective by design. Properly designed gear should be capable of being stored for years and then powered up without any special considerations, that's just normal use.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: May 11, 2022, 08:14:20 pm
content: Quote from: james_s on May 11, 2022, 06:15:15 pmWell in the past year or so I have had quite a few bits of gear brought to me that had been stored in a garage, shed, loft, container etc since lockdown.And when the owner got them out of storage and powered them up, they blew up immediately.I know this happens.I don't want that to happen to me, especially when it doesn't belong to me, and I am being paid to not blow stuff up.

author: BrokenYugo
author_rank: Super Contributor
timestamp: May 12, 2022, 06:28:17 pm
content: Then open them up and manually reform/check the power supply caps with a bench supply, insulation test the power transformers, check power semiconductors, etc. until satisfied there is low likelihood of an earth shattering kaboom having snuck it's way in there.I'm also more of a "flip the switch" guy.

author: SilverSolder
author_rank: Super Contributor
timestamp: May 12, 2022, 07:09:13 pm
content: Flip the switch with a suitably rated fast acting fuse in the line?With a DMM sniffing the PSU output. If not to spec, cut power manually immediately...

author: richnormand
author_rank: Unknown
timestamp: May 12, 2022, 07:11:38 pm
content: Usually in cases like that I usually pop the cover open and do a visual to make sure some critter did not make it home. Then a check the electrolytics for a proper response to the ohm meter.Then I fire it up while the cover is still off from a powerbar that I can flip off at the first sign of magic smoke.For a power amp I usually put a 10 ohm resistor at the output just in case the amp is unstable in a no load condition.Why pop the cover if stored for a while?I remember an open frame high power argon laser that refused to lase after storage. A spider had constructed a nest in the mirror cavity.Another was a SEM power supply that was swapped out as defective. A mouse was embedded in the primary line circuit. Must have been chewing the wires and zap. After 2 years of storage it was basically mummified.Cheers and good luck with it.

author: dietert1
author_rank: Super Contributor
timestamp: May 13, 2022, 11:58:52 am
content: In ancient times people used a 500W light bulb in series with the mains input as a resettable fuse. The lamp had a PTC characteristic. So if you can find one it may be a reasonable method.Regards, Dieter