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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--- In TekScopes@..., "whippletreefarms" wrote:
Hi all -
I recently turned on my Tek RM503 scope after giving it a couple year break. No trace appeared. Since I hadn't used it in a while, my first thought was to get rid of it on Craigslist. I don't use the scope too much, but it is handy to have to work on the occasional radio, theremin, synthesizer or organ (my son collects musical instruments).
However, after opening it up, and seeing how beautifully it is designed and built what with the ceramic terminal strips and glowing tubes, and realizing how cool it looks in a retro sort of way, I figure I'll at least try to fix it myself first before selling to someone for parts.
Trouble is, I have no experience with this kind of equipment (I'm not a EE although I had to take one Circuits class to get my engineering degree. I mostly work in software, and am just barely young enough to not have worked with tubes before). I don't have any other test equipment except a basic multimeter (). And I do have a soldering iron and some silver solder. So, I need some basic tips from those of you willing to assist me in keeping this machine alive if possible. Based on some of the posts here, it seems the hard-to-find-or-repair HV transformer is one of the likely culprits, but I don't know how to verify that.
Here is the background info:
Tektronix Type RM503 Oscilloscope
Serial: 000708 Mod 113N
What I have checked so far:
1. Power lamp lights up; Graticule lamps light up and are adjustable.
2. Phosphor glows lightly after having been on for a while, but maybe that is just from my basement fluorescent lights?
3. All tubes that I can see light up. There are three tubes in cans that I can't see light up, but one feels warm to the touch. Two feel only very slightly warm, but that may be caused by the nearby tubes. They are V334 and V434 - the horizontal and vertical amplifiers.
4. Line in from outlet and at T601 pins 1-4 measures 124VAC (recommended 117VAC per schematic)
5. Power transformer T601 pins 6-7 (CRT heat) = 7.03VAC (should be 6.3VAC)
6. Power transformer T601 pins 10-11 (heaters) = 6.94VAC (should be 6.3VAC)
7. Power transformer T601 pins 8-9 = 208VAC (should be 190V RMS, but my meter is not RMS)
8. +12.6V rail at end of R605 = +5.34V
9. +250v rail at end of R659 = +153V
10. +85V rail at V659 pins 1&5 = +100.5V
11. +100V rail at end of R634 = +60.0V
12. -100V rail at end of R642 = -58.8V
13. -3000V rail (no way to measure, meter only goes to 1000V DC)
14. +500V input to T620 HV Transformer at end of R626 = SPARK and blown fuse on RM503 front panel!
15. Pin 4 of V620 = +105.5V (should be +128V)
16. Pin 5 of V620 = -36.7V (should be -87V)
17. Junction of R630/R631/R632 = -12.2V (should be -6V)
18. Pin 3 of V634B = +17V (should be +60V)
19. Pin 6 of V634B = +0.6V (should be +22V)
Obviously lots of voltages out of whack....
Questions:
Q1. There are "Early" and "Late" versions of the manual. I presume with the low serial number this would be early, correct? It seems from the schematics that 'early' is correct, since I have 1/2 6DJ8 tubes instead of 8393 tubes in the horizontal and vertical amplifiers. Also neither the early nor late manuals show a separate scale illumination adjustment and power switch, with separate power light like on mine. I presume that is because I have the rack mount version.
Q2. What would have been changed in the mod? This mod is not listed in the back of the 'early' manual. Does the mod mean it was changed in the factory prior to original sale, or after it was sold? Seems like at least one of the listed mods was performed - changing the Power transformer circuit to the new style.
Q3. How can I measure the -3000V rail? Can I build a resistor bridge to bring it down to 1000V for my meter to measure, or are there some sort of consequences to doing that like damaging parts or equipment?
Q4. What caused the big spark and blown fuse when I tried to measure the +500V HV input?
Q5. I remember my high school electronics teacher showing us how to de-energize a TV tube by grounding the HV wire with a screwdriver and lead. Are there any similar safety precautions I need to take to work on this type of tube equipment?
Q6. Any obvious reason for my problem, based on the measurements I've made?
Q7. What are my next steps?
Thanks everyone!
Ken
