Today, the power supply for the Texas Grinder or Sideband Squisher or anyway the 3CX3000 amp that followed me home, it got checked out and repaired. As wisely warned by the previous orner, the electrolytics in the PSU might need to be reformed since it has been sitting for a decade or more. So my apprentice helped me put it on the work bench
(this is the example for Burt of the power supply size to be used for an AM linear http://amfone.net/Amforum/index.php?action=post;board=28.0), and I go to use the 20A variac. No output from the variac. This is very annoying, so I remove the rack panel to check it out. It is going to need some more detailed repairs. It turns out there are 2 sets of brushes, One is in the coil, and the other set is two small round contacts near the hub of the wiper plate that press by springs against a metal disk attached to a bar that then goes to the output terminal. The ones in the hub were not making good contact.
Well now this is all taken apart, and I says to my apprentice, here take this away, and I can fix it later. So then we go looking for another big variac. The ones I spotted were the 120/240V 56A ones. So ok that will do, we get one and lug the 100 LB thing to a cart and set it there, a PITA to move it, but there it is now and there it will stay, and I am hooking up a power cord to it. It has a large handwheel so that is an improvement.
Anyway this done, I jack the output to the transformer primary on the PSU and start cranking it up, and get to 2400VDC with the variac at 100%, reforming he caps as I go.. All is well. So, need 240V to apply the full alternation to the thing and reform the caps all the way. So we go hunting this 120/240/480@1KVA "control transformer" that I know is somewhere in the lab. In doing so, I happen to notice a dual 15A variac I had passed right by and somehow missed before. Now remarking, well we could have used this one, it's alot lighter. Wise apprentice says; 'well, you won't be able to blow up the big one as easy'. Ok. he is right on that account.
So we found the control transformer and set it on the workbench, and got out the big clip leads and hooked it up. Nice. Start cranking the variac, watching the clamp on ammeter, AC voltmeter on the variac, and the HV DC meter with its HV probe and the clipleads and all on the work bench, telling the apprentice don't stand too close to that, and don't stand on the side where the caps are in case one decides to blow up . So we get to 4800V, and the variac is still not at 100%, but more like 60%. So. Hmm. Ok, what is the rating of the electrolytic stack? because this is getting weird. It's a pile of 800uF 450V caps. I am getting nervous about increasing the variac any more. (apprentice cheats and uses cellphone to multiply 450VDC*14) - so 6300V is the limit.
So, I am slowly spinning the handwheel, but on the other hand something is not right, in the back of my head I know that thing is not supposed to be putting out 5500V at 70% of 240VAC. Ok, well I did take it to 5900v, but I stop, because something is just not right, and the variac is not at 100%. The transformer in the PSU is humming only slightly. Nobody is smoking or getting hot, but I shut this down anyway, and after looking at the control transformer, I saw that it had been jumpered for 480V instead of 240V. Duh.
So at least the caps got reformed properly. Rewired the control transformer for the correct 240V and let the supply sit at 4800V for a while. That is alot of volts and I am sure it will be more like 4200 with a load. The bleed was the stack of equalizing resistors, a total of 350K Ohms (13mA). The caps are pretty good, not leaking. It took a long time to bleed that voltage off the 57uF stack. So next we will get to work on the amplifier itself.
I mention my apprentice Jacob, because he is really helpful around the lab, especially when something is heavy. He has been sorting parts for me too. Many boxes of mixed parts. I help him out as well; fixed his OS-8B and his 2215 tektronix scopes this weekend. He wanted to build a plasma gun this wekend. He said that if we draw an arc of high voltage DC of 2-3KV at 1/2A between two carbons (using a choke to keep the arc stable), then place a 1/2 turn of thick wire near the arc so it is between the carbons but not arcing or touching, and then discharge some 1000uF/450V cap into the 1/2 turn, the magnetic field will blow the plasma arc off the carbons and out into the room. I do not know if this is true but I really wanted to fix the power supply instead.
