Electronics > Repair
Fixing Tektronix RM503 power supply (Fixed)
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floobydust:
I think it's easier to first test the usual suspects - C624 10uF, rectifier diodes, the other electrolytic caps C611, C612, those big can caps C652, C654 etc. C620 tunes it I believe.
I would not run it with all loads disconnected so you don't make a Tesla coil here.

The 6BL8, I think it lowers the 6DQ6A screen voltage to lower its gain when rail voltages are where they should be.
I have to wonder how that section works- if V634A needs to be turned off (negative bias to give the 6DQ6A some screen voltage) Hmmm. It must use grid-leak, with the high values for R630, R631? Check them and C630 for high leakage.
Does that make sense? If the regulator V634 hold down the 6DQ6 it could not start up.

Something else I would try is with power off, use a muscular sig gen and inject say 30kHz several VAC at the secondary winding for filament power (xfmr at D672 and GND). You can pull the 4 tubes to off-load that winding. Just to see if there is an obvious heavy load on one output or the primary, and you can poke around with a scope or voltmeter. Not to take it to full voltage but look at at even 1/10.

If you are suspecting the primary has shorted turns, then I'd use a Ring Tester.
George Edmonds:
Hi

Having now had time to look at the schematic and consider what is happening it looks very much like the classical problem with CRT oscilloscopes in that the EHT secondary on T620 has become damaged/fried.   This will cause the oscillator to stall and V620 to draw excessive current.

There is only one way of validly checking if this is the case and that is to do a “Ringing Test” on T620 with it removed from the oscilloscope.  A “Ringing Test” basically involves applying a pulse to one of the transformer windings and looking at the decay wave form which should be a decaying sine waves .  With a good transformer you should get up to six cycles of decay, and possibly more, depending on the Q of the transformer, whereas with a bad transformer you may not even get one cycle.

There are commercial ringing testers available, if you can locate one of them in your area a conversation with anyone who repairs CRT gaming machines would be the way to go as they should have such an item of test kit.

Regrettably if  T620 has failed it is almost certainly the end of the road for the RM503 as such transformers are difficult if not impossible to rewind.

G Edmonds
Henri:
I just got another RM503 with a power supply that is oscillating, I plan to compare the two and find out what components are faulty.
Henri:
I want to thank everyone for helping me get the scope working. I injected a 30khz signal into the 12.6v tap, which proved that the transformer was good. I had tried many 6DQ6B tubes but finally got a 6DQ6A tube I put it in and I had power!
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