TU56/TC11 restoration - VARIAC question

Plugging them into a variac won't address the primary issue of What's Wrong With This Widget? It can help you find something that's getting real hot if you can switch the unit on at about 60-70 volts and ramp it up slowly, if that doesn't blow the fuse. But I'd check the obvious stuff first, and then troubleshoot the power supply, before trying the above. Check the blatantly obvious first: 1. Right fuse? 1a. Right current? 1b. Right type (slow blow vs. fast blow?) 2. Shorted big filter caps or diodes in the power supply? 3. Lost screws or other metal parts loose inside? Don't laugh. I've seen it. Item two is easy enough to find, but only in a relatively simple power supply. If the PS is linear, not a switcher, I'd try disconnecting it from everything else and troubleshooting it in isolation. If it's a switcher, it's much to difficult (for me) to repair. Switchers have small transformers (proportionate to the current they produce, and employ a high frequency oscillator. You shouldn't run a switcher without a load. You may let all the smoke out. For linear power supplies, try to isolate the power supply circuitry from the rest of the unit. Find the transformer, see if the input and output voltages (AC) are reasonable. Should be, transformer failure mode in 99.999% of cases is no juice at all. Can you find diodes? Or a bridge rectifier? It is any good? A shorted diode or bridge will kill the fuse. You might have to unsolder one end of the diode to test it. Clip something metalic onto the wire lead near the diode so the heat from desoldering it doesn't do further damage. If you don't have any clamp-on heatsinks, loop a rubber band around the handles of a small needle nose pliers and use that. Classic linear power supples may have a grounded center tap on the transformer, or not, perhaps two diodes, more likely four or a bridge. Look for a capacitor or several on the output of the rectification stage. Depending on what sort of regulation circuitry follows, they may/may not have big ugly electrolytics on the output of the rectifiers. If you can unsolder or unscrew one lead on the capacitors, if present, try metering them for shorts or really low impedance that doesn't float as the ohms battery in your meter charges the cap. If you can find the outputs of the power supply, and you've been living right and they are all socketed, and this is a linear, and you can plug it in and run it unplugged from the rest of the unit, and have proper voltage on all the outputs of the PS, then obviously something is smoked further into the unit. Bad luck. Look for evidence of heat damage (fried components, burned printed circuit board, etc.). At 12:26 PM 3/17/2005 -0500, you wrote: [Computing] I have been told that _Introduction to Objectivist Epistemology_ was required reading at the Xerox PARC lab where OOP was invented, but this may be merely an urban legend. -- wilcoxb at cs.colorado.edu (Bryce Wilcox) --... ...-- -.. . -. ----. --.- --.- -... tpeters at nospam.mixcom.com (remove "nospam") N9QQB (amateur radio) "HEY YOU" (loud shouting) WEB ADDRESS http//www.mixweb.com/tpeters 43? 7' 17.2" N by 88? 6' 28.9" W, Elevation 815', Grid Square EN53wc WAN/LAN/Telcom Analyst, Tech Writer, MCP, Cisco Certified CCNA