Tube experts! - I need your assistance.

Upon the date 10:42 PM 12/3/99 -0500, John B said something like: Well John B., I *like* tubes :-) As you may have noticed I'm an electronic historian and old radio collector. Tubes pervade my very being it seems. I grew up learning about and fiddling with tube gear :-) Love it! Wish I could afford getting (or even *finding* an old IBM tube machine like a 704 or such.) A few others here enjoy the same background. Anyway, I went up to my library and dug out my Tektronix type 503 manual. The RM503 is simply a repackaged 503 which fits into a 19" rack. Below, I'll speak as if you've had very little exposure to tube circuits, 'scopes of this vintage, etc. as I really don't know your old technology background as of yet. At least others here who are not tube savvy and will someday have to fiddle with an old scope may get something out of this anyway. Not good of course. Limit the on-time while testing please. I sense you have a manual too as you give a good basic layout of the power supply and indicate expected voltages. So, by removing the -3KV line, things settle down. First thing in mind is that either of the two pots in the voltage divider resistor string may be arcing over to ground. They are the FOCUS and INTENSITY controls. Second thing and at least this is easy to check, does the CRT heater light up? With the -3KV left disconnected from the CRT do you measure 6.3 to 6.5 volts AC across the heater connections (pins 1 and 14)? Third thing, and most undesireable, is the CRT envelope got broken and the tube went to air. This will *definitelly* cause arcing inside the CRT's electron gun. You know the shiny metallic spot you often see inside vacuum tubes? That's the gettering which basically had taken up most of the leftover oxygen after the manufacturer had drawn a vacuum on the tube and tipped it off. Never saw a 503 tube so can't say where to find it but it usually would be on the inner surface of the neck somewhere maybe 5 or 10 CM in from the base. If you see a milky white spot on the inner surface of the envelope in that region then the tube's gone to air :-( You mention that instead of 10 volts you measured 500. Thank Heaven this is not a solid state scope!!! Smoke City!! A shorted 5642 HV rectifier tube would present a high frequency AC voltage of some level on the -3KV line. Maybe 5-7 KVAC peak to peak. The freq would be 25KHz as generated by the 6DQ6A oscillator tube. Hard to see the little filament in the 5642 to verify whether its glowing and you really cannot do any measurements with a standard voltmeter on this part of the circuit (your meter would not tolerate the high voltage unless you use one designed for, say, 5KV or more.) I'll assume you may not have such a meter and cannot measure even the -3KVDC. Check to see if the tube envelope is broken. This tube has gettering also and see if it's milky. The filament could fail and flop down onto the plate thus making a short circuit. Been there, done that. I haven't hunted for 5642's for a long time and they may be hard to find now. But I know one of the ham radio community folks could come thru if they have a junker Tek 'scope on hand for parts. Let me know. Turn off all room lights this evening, close shades if the city lights are bright too. Turn on unit and look for faint arcing around the circuit. Keep your hands in your pockets! ;-) You may smell ozone from the arcing. The FOCUS and INTENSITY pots are in the resistor divider circuit and are mounted on the front panel. Listen carefully to help zero-in on the noise. Keep your earlobe in your pocket too! ;-) {ZZZapp!} Yes. Do the lights out trick to verify . . . Hope this helps John. Sorry to be late with this but family stuff and my schoolwork got in the way all day. Let me know if you need other info/help. Regards, Chris -- -- Christian Fandt, Electronic/Electrical Historian Jamestown, NY USA cfandt(a)netsync.net Member of Antique Wireless Association URL: http://www.antiquewireless.org/