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Re: RM503 Troubleshooting tips needed for beginner
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Hey Ken, ? I probably missed your earlier posts. ?I am on vacation on ?a little itty bitty laptop so can only be so helpful this week... First, a bit of background, because I don't think we've addressed this. ?From my experience with this scope, and other 5xx series Tek scopes I've had, and other
discussions posted in this group, there would seem to be three common failure modes in the RM503/504 scope. ? ?The first is probably the most common. ?The filter capacitors (C611, C612) get dried up and start to fail. ?This is why the first priority should be to get the 500V rail in operation. ?That voltage should be relatively independent of the state of anything else in the circuit. ?If you pull V620 out and you do not have close to 500VDC on the 500V line, we have a problem in the first power supply and its likely those filter caps. ?You only have 260V. ?Is that with the input
switched to European 220V? ?If so, great since at 110V you should then have the full 500V. ? I would concentrate on getting this rail up to par if it is not, though it seems there may be other problems. ? The second most common problem is a breakdown in the coils of the second transformer, T620. ?This one oscillates at several tens of KHz driven by V620 and generates all the other voltages the circuits need. ?The problem is the insulation of the transformer windings break down with time (and possibly long-term absorption of atmospheric water vapor) and the coils short out or open up or short to
ground. ?The circuit relies on the inductances of the coils and their capacitances to oscillate properly, so if any one coil (e.g. the HV coil) shorts out, the rest of the voltages go out too. ? The third failure mode is the HV shorting out through a breakdown in the insulation of ?T601. ?This is because the CRT filament is driven by T601 *raised* to -3KV by the HV circuit of T620. ?We've seen evidence of this failure mode in some scopes sold on e-bay. ?One solution that has been found is to get a cheap 6.3 V filament transformer, hopefully with >3KV isolation, and replace the T601 filament
coils with that. ? ? ? What I would concentrate on is getting the +500V up and running. ?But from what you indicate below, T620 may be no good. ?That;'s harder to deal with. Question 1: ...a tough one. ?Here's where the wisdom of this group will come in very handy. ?Could we replace V692 with a series of silicon diodes? ?Would this work? My first thought would be to run a chain of 1N4007 diodes in series. ?You will need at least 4 of them since they have a peak reverse voltage of 1KV. ?Wired in series, they should be able to rectify the output of T620. ?Two questions for the group: ?will they be low enough capacitance to function in place of V692, and do we need some high-value resistors in parallel
with the diodes to ensure a uniform voltage across each? ? Ken, if I were you, I would leave V692 soldered in place until you can determine if you have 3KV ac on the ouput of the T620 HV coil. ?Those HV rectifier tubes are very delicate and the leads can break off very easily. ? Question 2: Probably not. ? Get the RCA handbook mentioned below. ?Gas regulators are basically fancy neon lamps. ?They maintain a constant voltage within a certain current range, but they have a startup and a dropout voltage. ?The tube and its series dropping resistor must have at least the startup voltage to fire, and at least the dropout voltage to operate. ?When operating, there is a current through the series resistor ?(R659, 100K). ?In this case and the voltage supplied to that resistor must be at least 85v plus I*100K where I is the operating current of the tube (see handbook). ?These are generally pretty robust tubes - if it is glowing it is probably working. ?It should have a purplish-orange glow in operation. ?Check R659 to
make sure it's still 100K. ? If it gets warm, check it warm to make sure it is not opening up or becoming much higher resistance as it warms up. ?If it's glowing and it's at 97V it's not good. ?But I still suspect that the 85V rail will fall in to line when the +250 works. ?You could try temporarily jumpin another 100K resistor in parallel with R659. ?That will pull V659 up a little harder, and give it some more voltage. ?It should then glow and run at 85V. ? Question 4: Oscillator: ?V620. ?It oscillates at ?~30KHz or so (could be different - I'm going from vague memory of the measurement on my working scope) to generate the AC which drives the high frequency transformer. ?The tube is a beam power pentode, so it dissipates a lot of power and should get hot. ?What you describe sounds normal and is the filament glow. ?What I was seeing in a defective circuit was the outside envelope of the guts of the tube, normally grey, get glowing hot. ?I am not sure if this element is the plate (anode) of the tube or an external shield, and it might glow a little in normal operation, but not much. ?The glowing spot starts out as an oval and grows to encompass the plate and the envelope gets very hot as well. ?Eventaully it will weaken the metal structures, possibly warp them, and drive out gasses that ruin the vacuum and make the problem
worse. ?Just be aware of the potential for this problem and keep an eye on that tube while you are troubleshooting. Question 5: By the way, get a copy of the RCA Vacuum tube handbook from the mid 1950's. ?It's fun reading and very helpful as well. ?I did a brief check and it looks like you might even be able to read it on google books. ?Also I would not be surprised if Archive.org has a copy. ?But it's better in my opinion to have a paper
copy in hand. The encouraging news, at least to me, is that the secondary voltages of that transformer are all in the correct proportion if low. ?Note the +250 is at +160, which is 2.5x the +100, which is at +60. ?I think the bad news is you have an open in the HV circuit. ?4.7 MOhm to ground sounds like too much to me for the HV coil of T620. ?? I hope this measurement is wrong because T620 is "Unobtanium" without a donor scope, or re-winding the thing (an ordeal). ?After verifying the +500V, I
would try to verify voltage on the HV side of this transformer, maybe by substituting some diodes for V692. Ok, being dragged to the beach now. ?Good luck and keep me posted. ?Also: ?danube@... if I don't respond here. ?Sometimes I lose messages in the hundreds of posts. ? Dan From: whippletreefarms To: TekScopes@... Sent: Thursday, January 17, 2013 1:56 AM Subject: [TekScopes] Re: RM503 Troubleshooting tips needed for beginner
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So the V620 and V634 tested OK. My buddy wasn't able to test the V659, but it shouldn't be the main problem at this point anyway.
My plan is to next replace the V692 diode tube. I still would like to know about my questions 1, 2, 4, and 5 below before I search for the diode tube though. Thanks, Ken --- In TekScopes@..., "whippletreefarms" wrote: > > > > Thanks for all the input everyone. I got another DMM and an HV probe. My old meter did indeed start giving erroneous results. So here is where we stand now: > > Normal situation (nothing removed and no wires lifted, measured with new meter) > -3kV rail = 0V > +500V rail = +260V to +270V > +250V rail = +150.4V > +85V rail = +97.8V > +100V rail = +60.1V > +12.6V rail = +4.97V > -100V rail = -57.2V > > I know the HV probe is working, because I measured the +500V rail with it both before and after measuring the -3kV, and it corresponded to the direct measurement. > > I'm guessing that my old scope somehow shorted the +500V to ground and that was the spark I initially saw. It hasn't repeated since, so this must have been a red herring. > > I removed the -3KV rail from the end of the diode tube, and the voltage at the end of the tube still measured 0V. > > Per Albert's comments: > * When removing both the V620 and V634, the +500V rail increases to +476V. > * With R628/C628 shorted to ground, the +500V rail reads +315V. > * The voltage decays to about +120V about 20 seconds after power off. > * The V659 is a 87V voltage regulator, so I would be surprised that it hasn't ignited by +97.8V. > > Per David's comment: > * C611/C612 probably isn't an issue now that I am getting consistent (although incorrect) +500V readings. > * On a related note, however, they held voltage a LONG time after shutoff, so long in fact, that I had to actually remove the white/red T620 wire to measure the resistance! > > Per Dan's comments: > * The V620 tube has an orange glow on a vertical element. I'm not sure if this is the same as the plate glow you mentioned. It does get hot - too hot to touch. > * See comments above on removing V620, and also measuring decay time > * -3kV to ground measures 4.7 MOhms to ground > * I removed the CRT filament (white/yellow wire, pin 14) from everything, and measured that it was open to ground. > * I measured the other CRT lines to ground, and they all seemed reasonable based on the circuit diagram. > > I suppose it is possible I added to my woes by bunging up the V692 lead when lifting wires and thus causing 0V on the -3kV rail. Or maybe it has always been 0V, but since I couldn't measure before, I don't know. > > So my new round of questions: > 1. Is there a way to measure the input to V692? The power-off T620 measurements all seemed correct. > 2. Does the +97V on the +85V rail indicate V659 is bad? > 3. I plan to take V620, V634 and V659 to the hardware tech at work. He says he has two tube testers in his attic and can measure most tubes. Thank goodness for his "things were so much better in the old days" curmudgeonly attitude! > 4. (tube circuit newbie question) A couple of you mentioned the "oscillator". Which tube is that? The V620 or the V634? And what frequency does it oscillate at? > 5. What to try next? I will say I am learning an awful lot, and am not ready to give up yet! > > Thanks again, > Ken > > |
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