Fixing Tektronix RM503 power supply

[Henri]

Hello, I have a Tektronix type RM 503, the power supply section stopped working after some use, and the Armstrong oscillator does not start oscillating on power up. I have already replaced the 6DQ6A tube, 5651 tube, C624, and R626 which was burned. On power on, the anode of the new 6DQ6A starts glowing cherry red after a minute and gets way too hot in my opinion so I switched back to the tube old tube which does not glow so hot. I have checked almost every part in the power supply section, I also have measured all of the connections on T620 all of them have a reasonable resistance. still can't find out why it does not work.

[Henri]

Any ideas on what is wrong? Here is the schematic:

[Henri]

Here is the schematic:

[CaptDon]

When the circuit is not oscillating the control circuit allows for high screen voltage causing heavy plate current draw. The new tube probably has red plate glow probably because it has excellent cathode emission. The old tube is probably about 3/4 dead with low emission, so low that the plate doesn't cherry. Even the slightest thing like a diode on the secondary with high reverse leakage will kill the oscillations. An excessive load on any of the supply voltages on the output of the transformer would also kill the oscillations as would a shorted turn within the transformer. It is really a self excited flyback circuit similar to that of an old C.R.T. television. As the output voltage rises the feedback circuit will pull the screen voltage down and become a stable regulation circuit. With main power DISCONNECTED you might try pushing a 10KHz audio sinewave into the plate lead of the 6DQ6 and run the audio voltage slowly up to maybe 50vrms and have a look at the rectified output on the secondary side of the flyback. In normal operation the P-P voltage on the 6DQ6 plate lead probably approaches 1000vpp. I have a nearly indestructable 100 watt RMS audio amplifier that I use for this type of signal injection testing. When I need larger P-P voltage I use the amplifier to drive a 120 / 24 volt transformer backwards easily getting over 200vpp from the transformer. Even though the flyback is a pulse transformer it will respond well to a sinewave. Cheers!

[BlownUpCapacitor]

Huh, I didn't know tek scopes as early as that used an inverter! Very interesting! 

[Henri]

I injected a 1v 60 hz signal to the primary of the transformer and could see that all of the outputs were working. I measured the primary side to be about 2 ohms seems very low for 500V. The secondary 100v and -100v taps were 0.8 ohms. I don't understand how the feedback is supposed to start. The tube V620 just slowly warms up and starts drawing DC current through the primary causing no feedback in the secondary to shut it down. How is the oscillation supposed to start?

[Arts]

Check C621.

[Henri]

I isolated and checked C621 which measures 5.26Nf so it seems good, I have no way of checking for leakage. When I put the 1v 60HZ on the primary a small voltage does make it past the cap so it is working? I am pretty lost.

[Henri]

 I don't understand how the DC voltage is supposed to excite the transformer to start the oscillation. The tube slowly warms up and draws current through the primary causing no excitement in the secondary because the current increase is so slow. How is the feedback supposed to start?

[med6753]

Did you check all the electrolytic capacitors on the secondary side of T620? If one of them is shorted it can kill the oscillator. Same with the diodes (rectifiers) on the secondary side. Also, if you red plated the new V620 it's probably damaged and I suggest replacing it. But not until you insure the circuit is oscillating.

[Henri]

I have lifted one side of all the diodes so they are no longer connected and removed V634A. So the circuit is pretty much isolated. The primary of the transformer is only 2Ω which seems very low for 500 volts. still no oscillations.

[BlownUpCapacitor]

2 isn't unreasonably small as the oscillations should be at 30kHz. Inductive reactance should bring up the impedance of the primary.

[Henri]

Do you know if the oscillations are the full 500 volts? Odd it seems that the oscillations should start but no feedback is getting through the secondary of the transformer there so it does not start.

[BlownUpCapacitor]

Remember that vacuum tubes offer fairly high impedance when they are on. They can't quite be close to a full short like a regular transistor. So in order to get the required current flowing, they use a very high voltage of say 500v. This should apply to the 503 as well. In my tube transceiver, I use 250v B+ voltage. My 6AQ5 tube drives the antenna coupling transformer with the coil connected directly to 250v, and the cathode of the 6AQ5 is connected directly to GND. The coil has less than an ohm of resistance and offers only 50 ohms of impedance at 3.57MHz; yet the circuit draws only a few milliamperes from the 250v.

If you can, hook up a function generator to the transformer primary and set it to 30kHz. See if the voltage ratios match up with the input RMS voltage of the transformer. Be careful, if the transformer is good, there can be lethal voltages on the secondary!

[George Edmonds]

How that is a blast from the past, I last worked on a RM503 when I was about eighteen, and I am now eighty.

Having had a very quick glance at the service manual I am not surprised that you are having problems as this is an early valve/tube switch mode supply.

The oscillator AC feedback path is via C621 and the DC regulation control path is via V624A.

In my experience by now most of the resistors will have gone high in value and the capacitors failing.

Please be aware that this power supply can be  lethal,  Strange way of drawing the valves/tubes

G Edmonds

[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.