Thread: Ctc 16ex sos
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Old 08-23-2018, 12:13 AM
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Electronic M Electronic M is offline
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Join Date: Jan 2011
Location: Pewaukee/Delafield Wi
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Not having a variac here is what I'd do.
Resistance tests:
With the set unplugged (wait at least 15 min for lytics to discharge), and the positive end of the bridge rectifiers disconnected from the B+ system try measuring the resistance between the chassis and the 5 highest (by listing on the schematic) voltage B+ rails (do them in order from highest to lowest). The resistance of the 400V rail should be higher than 250 ohms (number obtained by applying ohms law with the breaker current and high B+ rail voltage as inputs). It should be MUCH higher than 250 ohms (at least once your meter charges the caps) since the tubes aren't acting as loads since the heaters are out.
If the resistance is lower than 250 ohms start unhooking wires from that point and check the resistance of each separate wire...Follow the path of least resistance to find the short.

Another troubleshooting option is to do 'unloading' tests (keep the H and V output tubes removed during these tests). For example (using sams 818 for reference) Disconnect L45 from where it connects to C1A/the rectifiers (disabling power to the choke and all but the 400V rail) then power the set up. If the breaker does not trip the C1A and everything on the 400V rail are working okay. Now power down, reconnect the choke to 400V and disconnect Resistors R112, R213, and R214 from the choke. If you power it up like this and no trip then 390V is fine too. You can reconnect a resistor, power up then reconnect another resistor and power up, and so on.
The first rail that when reconnected causes a trip is the source of the problem*.
*If there are lower voltage rails derived from the culprit rail disconnect those from it and check for a trip again to incriminate or clear the derived rails of suspicion.

There are many possibilities what could be wrong. A lytic installed with reverse polarity could do it (double check your work, I've made that mistake more times than I care to admit), a winding to frame short in the 400V rail choke (L45 sams 818) could do it, a wire with damaged insulation rubbing on something could do it, a conductive spec fallen in exactly the wrong place could do it...The trick is finding it.
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