ITS WORKING!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Special thanks to Tony's idea of hooking up an isolation transformer (6.3V but rated to 3000V).. Okay, I did some of Christians tests and I found that the Transformer indeed was arching into the main supply. (I won two of these scopes on EBay). Here is the funny part.. I rarely go into surplus stores but tonight I made an exception. I told the guy at the counter what my problem was and he had a box full of 6.3Volt isolation transformers rated to 3000KV HAHAHAHHAHAHAHAH.. I could not believe it. I bought a few and tried one out. The scope now has a perfect trace. On XY the dot is crisp and very stable. I have to align the scope.. but will do so after I get all brand new tubes in it. Second funny part, As I was leaving the store I went over to his lightbulb section (get the hint yet).. Yes, brand new bulbs [equivs] for my 8/S and my 8-I still in the package! What a day... I can't wait to get spacewars up and running. Thanks for the info Chris and Tony, I have one more scope to restore and have placed orders for another 8 of them... so I have some serious tube work over the next couple of weeks. (comments below) -----Original Message----- From: Christian Fandt <cfandt(a)netsync.netTo: Discussion re-collecting of classic computers <classiccmp(a)u.washington.eduDate: Saturday, December 04, 1999 5:34 PM Subject: Re: Tube experts! - I need your assistance. a The only reason I don't like tubes is because they are very flakey in old mini computers.. From what I have heard from people who use to support them every power cycles was a nightmare. I am trying to stick to minis that can run off of 110/200Volt. I have a lot of design experience in transistors and IC/analog stuff... little to no tube knowledge. anyway. Very limited.. but I have to use these old tube scopes as I am putting the original tek scopes back into the minis. >>Okay, I bought the Tek RM503 for my PDP-8/S. When I turned it on I heard >>some terrible noises and found the power supply voltages were all over the >>place (way off , like 10V was 500 etc..). >Not good of course. Limit the on-time while testing please. > I did.. to about 30 seconds... Got lucky though.. the 4 transistors did not die. (12V >>to -3000). >I sense you have a manual too as you give a good basic layout of the power >supply and indicate expected voltages. Yes, I got it with the scopes.. I do have to buy a bunch of RM560s. Do youhave any manuals for those? >>>Picture this... the -3000 volts is fed right into the CRT heater (which >>happens to be directly coupled with the primary transformer). I have been >>able to locate the problem somewhat. If I remove the -3000 volt line between >>the HV rectifier tube and the CRT itself then the scope works fine! All >>waveforms are proper and the power supply works great (no picture of >>course). >So, by removing the -3KV line, things settle down. Yes >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. The focus *kind* of worked... nothing on intensity. (they both work now) >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)? Yes >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 :-( Thanks.. I wil check out the other scope I have to work on next. >You mention that instead of 10 volts you measured 500. Thank Heaven this is >not a solid state scope!!! Smoke City!! I know.. What scares the hell out of me is I am hooking this thing up to my 8/S.. I am going to put some highvoltage diodes between the 8/S,8I and the scope to make sure if the scope goes bananas I don't blow a few hundred transistors in the minis. >>>It can be a few things... I am hoping someone here who use to work on tube >>units might be able to tell me which problem below it most likely is: >>>#1) A bad HV rectifier tube causing the HV to come back to the second >>transformer which would put a few thousand volts back into the secondary >>winding taps causing high voltage everywhere. >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 The 5642 was glowing and arching inside (looked really bad).. It is happy now. I do. I worked in a TV repair shop in my early teens for weekend money.. I picked up the high voltage meter when the business went south. >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. I found a canadian retailer that has them in stock: http://www.sphere.bc.ca/test/tekparts.html also, http://www.vacuumtubes.com/ has all the tubes in stock (all new in the box).. It is costing me over $300 for three sets of the tubes though :-( 6DJ8 Amperex tubes are over $30 each :-( >>>#2) Bad insulation on the primary transformer secondary "crt heater" winding >>which jumps over to the other winding that happens to be the main >>powersupply winding (125V X 2) >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!} >>>#3) The -3000 volt wire is closely tied to the other low voltage wires. Is >>insulation breakdown possible due to a crack? >Yes. Do the lights out trick to verify . . . >>>My next step will be to take a reading on the primary transformer (first >>transformer, secondary winding [doubled winding]) and see if thousands of >>volts are there... That might help determine if there is an insulation >>breakdown but from what I can tell when the -3000V is hooked up every >>voltage goes crazy. >>>I am going to bed.. Hopefully I wake up to a great answer ;-) >Hope this helps John. Sorry to be late with this but family stuff and my >schoolwork got in the way all day. Thanks for the info. It is much appreciated. I have a lot of these scopes to quickly restore.... john http://www.pdp8.com/ >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/