Tinkerer

I got the 20awg PTFE stuff from naavships on ebay. It was the only multiKV silver plated stuff I could readily find. Unfortunately, only comes in one color so it's helpful to add some other markings to easily differentiate what goes where. Other stuff I just used the more standard silver plated teflon coated that's good to 600V or so. And I used two conductor silver plated shielded for the unamplified audio with the shield tied to ground. Been bringing an old IBM mainframe terminal and it's controllers back online lately so haven't got around to testing the new stuff yet. I did put in an order for replacement parts for my HV900 though. Hopefully, I'll get around to more testing later this month. Thanks for the troubleshooting stuff Soren, but there's so much energy in this thing, the 10M90S always gets toasted along with the 10V reference if anything else blows. The ones in the power supply that died were all brand new. I appreciate the slow power up info though. When I picked up my old Conrad variac last year, I wouldn't imagine I'd get so much use out of it on all sorts of projects, but it's become almost as indispensable as my multimeters.

Just loaded it with proper resistance the same way you did way back at the beginning of the thread. Looked like one of the CMF60's (the 100R next to the 1N4007 diode) had lost a chunk of insulation on the underside only and I didn't see it. Either that or the new SMD STN9360's were garbage. It's the only PSU that needed new ones since I had enough salvaged of the others. Also possibly the socket for the voltage reference as that seemed to have melted and soldered itself to the chip. Hard to tell what was what gave out and what got damaged afterward, especially when one of the resistors burned and covered a good chunk of things with soot. All the other three ran fine. This one got up to regulated voltage, lasted about ten seconds, then popped. All the solder joints were good and the board itself was squeaky clean before it got powered up.

Bumping this as I got some more multi KV rated silver plated wire in, enough to give all this a good test in a day or two. Had one of my reconstructed HV900 supplies blow under stress testing, bad resistor I missed I think, so will only be testing one channel. I'll let you guys know how it goes. It wouldn't be Fourth of July weekend without some fireworks after all. If anybody else has been working on this, now would be the time to pipe up if the latest current limiting mosfet additions worked for you or didn't.

I've been using a Radioshack one that I picked up on their going out of business sale for $30. It's similar to a Hakko 937 and uses the same tips but a little fancier in presets and stuff. I really like it a lot, especially the nice cheap variety of tips you can get from china for next to nothing. Best part is the quick heating and ability to keep a consistent temp on the tip even when soldering some larger stuff. It doesn't really bog down unless you're desoldering a big heatsink or something. Though I guess anything compared to my old plain twenty year old iron would be a pretty big improvement.

Turning an old capacitive terminal keyboard from 1978 into a modern USB keyboard with fully programmable keys. Got it working fine, but had to tear it back down for keycap polishing and a few minor tweaks. Currently reapplying dielectric lube to all the key stems. Need to finish up this project so I get back to testing Circlotron stuff.

Gonna test the current limiter on the HV 900's today. Just double checking that this is the right setup. If it works, I should have at least one channel up and running today. EDIT: I'm just trying to puzzle it out. Diode direction for an N channel mosfet should be Source -> Drain, right. Then the resistor is 60 ohms to get a voltage drop of -20VDC to activate the depletion mode gate at around forty something miliAmps to give some buffer room for the device to fully close before the 50mA limit on the output boards. I know this is simple stuff, but when it's a hundred bucks in exploded parts and worse, the time to pull everything apart, test it and fix it up again, I just want to be 100%. I had one other question though. The output stage bias trimmer on the driver board. We keep that shorted since the two pots on the Output V2 control the bias current directly now? And is the extra DC offset resistor still required if feedback is run from the output board like the silk says instead of from the driver board? EDIT 2: Nevermind, I get it now. I need to swap them around. The body diode is supposed to point back the opposite way to act as a suppressor.

I saw you had to send the PSU front plate back about three times. Magnificent patience all around to make everything just right. I barely had the patience to beat together my circlotron case out of a server rack and some extra parts when they mailed it with chips and scratches.

That's a shit ton of tubes and transformers. Looks like a mainframe module from 1956.

Gorgeous!

The toroid came from some other big amp. I bought it second hand awhile back. The setup for the primaries is weird. The 220 is a fully separate winding.

Got my spare parts GRLV PSU all finished up for the mini stax amps today. Wiring is a little ugly since I left the transformer leads long in case I ever want to repurpose the transformer again and left the other primary intact for 220V. Used an inline fuse because there wasn't really a good spot on the back panel that wouldn't look weird or collide with the board. The Kemet caps were nice and short enough for me to put in the case but since they're fat required adjusting some of the other bits before soldering to make sure they fit. Had some decent little heatsinks on the pass transistors originally, but even though the amp only pulls about 300-400mA, they got way too hot too quick. The bottom plate worked fantastic though and once assembled, the whole chassis spreads heat to all parts pretty well. And I just noticed in the picture that I forgot to scrub all the flux off those when I remounted them. Well, better go fix that. Hope you guys enjoy this particular combination of spare parts pile and overkill. Only cost me about $70 to put together with what I had laying around and half that was the case.

Just a heads up that 7v reference and 1.07kOhm RN60 worked fined for 12V output. I powered it up and +11.98 and -12.00 dead steady on the board. Only need the +12 half for my purposes but I figured if I ever repurpose it later, it's easier to swap around a couple resistors than hunt down some transistors that might be OOP years from now.

Does anybody know if the new US STAX distributor is up yet? I need a couple parts to refurbish an old Nova, and they're the cheapest for what I need.

The nice thing about that case type if you look at the back is that it already has a few milimeters of isolation around the hole, unlike a TO-220. So the screw is all you need.

I just used the same ones I use for all my TO-247 stuff. http://www.mouser.com/Search/ProductDetail.aspx?R=4180G

Bumping this because I got my HV900 supplies all back up and running and need to make my last parts order to do some testing soon. I need to know the general voltage and current rating for the 60ish ohm resistor on the new current limiting adapter. Now that I looked at it closer, I think I get it. I didn't see the normally open part of the mosfet before. Also, if this works, I would assume the big resistors are not needed on the output boards. Though since I already have them, I'll probably install them anyway as long as I can find the sink space.

Paid

Won't that need to be a 1kv'ish rated resistor though? Or were you guys thinking more of putting a couple smaller ones in series?

Just a couple small questions. What type of case is best for the device for how much heat it's going to be shedding? TO-220 or TO-247? What value for the trimmer pot? Can we get a quick diagram of the circuit modifications with all alterations on the HV900? I just want to make sure I'm doing this right the first time. 50mA should be .6V across the 12ohm resistor. And about what's the minimum AC voltage on a variac you can set the current for this and the output boards before powering them up all the way?

Yeah, didn't finish pulling, testing and cleaning everything until now. New 12V diodes are 5W and along with the new current resistors should beef that output board stuff up considerably. Plus being able to turn that on second after giving the driver boards a bit to power up should help too. I got them from Arrow. With the shipping difference it's about the same for small quantities. But thanks for the tip on the direct line. Might need that for a 7V ref for a different project. Those things can be a pain to find in stock at places for cheaper prices.

Just a heads up that Mouser just got some of these in a couple days ago. http://www.mouser.com/search/ProductDetail.aspx?r=588-AP10162RJ Bit cheaper than DigiKey at the moment. FInally got all my stuff sorted and a big parts order to get the PSU's and output boards up and running. As far as I could tell, the things that blew were consistently. 12V diodes on the output boards all the time (some fused open is why I didn't realize this earlier) The diode side of the 4N25 half the time Big Cree devices on the output boards. Survived on the PSU's. Both 120 ohm current resistors on the Output boards always blew, the right less obviously as the damage was usually underneath. On 900V PSU's, 10M90S behind the DN254 every time. Other 10M90S and DN254 survived every time. SMD stuff, STN0214 always died, STN9360 survived rarely Voltage references always died without exception. Not fun on the wallet. BC557's died every time without exception Might save someone a bit of time if something goes wrong. Like you don't have to go hunting in the rectifier bridges or anything.

So, I was just fooling with an SRM-Xh I got in. Everything turns on fine, all LED's on the boards light up etc. The balance on the right channel is within a couple volts. Offset is fine on both channels. But the left channel balance is almost -20VDC. There's no pots or anything on the inside to adjust. What's the likely deal? Carbon film resistors aging or did these things ship this way?

So to reiterate on this. I should just need a lower voltage reference (7V) and 300? ohm resistors in place of the top 1.5K in each pair for 12VDC operation? Just want to make sure I have the resistor value right since I gotta make a part order soon for some other stuff and I'd like to get it all together. Also, is there any particular reason the STAX people put the positive on the barrel sleeve of the plug for the small amps? If not, it seems smarter to just swap the socket wires around so it can use any old common 12VDC wall wart.

The problem with the single bridge on my boards (blue ones from the last GB) was the holes were smaller than the side ones so you had to drill it out to fit. I left the bridge lifted up a bit from the board so I could solder that top board trace from the top side to be sure it was connected right.

Probably the trace on the top of the board for the bridge, that last leg closest to the AC input.