TU56/TC11 restoration - VARIAC question

[Tom Peters]

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:
> > 20amps, wow. Someone give me this for christmas!
>
>
>I now have Two nice machines (Tek scope, VT100) that blow
>fuses and have been dancing aound the issue to get one for a
>few weeks now.  I'm not really an electronics guy but I've been
>led to believe that a variac is the single best thing to get to help
>remedy fuse blows.  Any strong yes or no votes towards this
>decision?
>
>John A.


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