I've replaced all the caps...

in my DuMont RA112, and the chassis is on the bench in the basement and the rest of it is upstairs. I'm going to re-check all of my work against the schematic for continuity and shorts as much as possible, and I've already done that as I've gone along without finding anything out of order. I've got a table of resistance readings to check the tube pin sockets against. I've got a 2' by 4' blowup of the schematic, and the rest of the Sams folder on this set.

Question 1. When checking resistance, do I do this with the tubes in or out? When I built my pre-amp, the instructions were with the tubes out, but there are a lot more tubes in this TV.

Question 2. Assuming 1 above checks out, do I plug it and bring it up slowly with a variac inside the cabinet with the picture tube and everything else connected, or do I bring it up on the bench, being careful with the connections that they are not touching anything else, to make sure I'm not going to smoke it first, then put it all back together.

Question 3, big safety question. Once I've applied power to the chassis, I know enough to discharge the electrolytics before working on it again, but I've also been reading on the picture tube capacitance. As I understand it, you want to discharge the picture tube HV+ to ground on the tube itself, not the chassis. What size/wattage resistor should I use there, will it make a noise when I do this? Should I attach a wired resistor to a big stick and stand back when I do this. (I'm only half kidding here.)

Question 4. I haven't replaced any resistors, should I? I know that the high ohm tend to drift. Will the resistance check show this? Maybe I should order a bunch from Mouser.

Question 5. If the picture tube works (still untested), but I still have problems, what next? I know how to replace things, but beyond replacing tubes that might be bad I know nothing about using a scope to diagnose or do alignments. Anybody in the NYC area?

I know this is a lot of questions, but I don't have anywhere else to turn. I wish I had a digital camera so I could post pictures of what I have, and the repair job I'm doing. Other than 50+ years of dust, this thing is clean, and all the wiring still checks out.

Thanks in advance for all your help.

[Frank]

Did you replace the high voltage filter caps? These are the paper 500pf at 10,000 volt caps inside the high voltage compartment. You will find three of these and they are usually bad. These caps, if bad, could kill the flyback! I am in the process of rebuilding a RA-112 and when I get home, I could post some pictures if you like. When you replaced the filter caps in the black cardboard tube, you need to reuse the tube to keep the cap above chassis ground. You do need to check resistors to see if they are in tolarence. I found several in my RA-112 that needed replacement. Yes, you do need to bring the set up on a Variac, you may have made a wiring error! The set can be run on the bench, but you should leave the horizontal output tube and the damper tube out. You should connect a speaker and hear sound with an antenna.

More info. when I get home if needed.

Frank

Thanks, I did replace the 500pf caps. With the cardboard caps, I used the ground posts as an anchor point and connected the negative of the new caps there, leaving the original connection in place. I connected the lead going to positive to the new cap positive, cutting the old cap out. In any case, I checked continuity between cap negative and ground and there isn't any.

I'll keep the two tubes out, this is something I wouldn't have known to do. Does this disconnect the HV? I have speakers there to test with from my speaker building projects.

The resistors that you replaced, were they the high value ones? And what did you use to replace them, carbon comp, metal film? Or does it matter as long as they are the proper wattage.

[Tubejunke]

Don't make the mistake of thinking that all - symbols on capacitors mean that side goes to ground.

No, that's why I used the original negative post from the original cap, it was already connected to the right points. I didn't open them up and put the new caps inside, but I kept the new caps' leads oriented pretty much as the old ones were. There was plenty of room to work.

A few electorlytic caps were definitely not to ground. There was one multisection cardbord cap in the PS whose negative became the -60vdc source.

From the outside of the chassis, you can't see that its been rebuilt, but I made no attempt to hide the new caps, way too many to do.

[Frank]

Sounds like that you are well on your way. Don't forget to provide an output transformer if you use one of your audio project speakers. The speaker lead from the RA-112 provides B plus to tthe output tube and the output transformer is mounted on the speaker. I use carbon comp. resistors for replacement in older sets. Metal film does not hold up well for higher voltage drops and they are inductive. Removing the two tubes removes the high voltage and the B plus from the vertical stages as well.

Frank

[Charlie]

You can't bring your Horzontal circuits up on a Variac because your horz osc won't reach the correct operating frequency. As a result, the osc won't provide the horz output tube enough drive... then that would make the output tube overheat and ruin the tube. Removing the output tube keeps you from putting the strain on the tube and HV circuits that follow.

Once you've ran the chassis for a while with no apparent problems, then try firing it up with those tubes in place. If you've replace all your caps, chances are you'll be alright. The only thing to keep a lookout for is any HV leakage at the flyback and surrounding area.

Good luck!

Hey Charlie, if you're soft starting a TV with the light bulb trick can you still leave the horiz output tube in place?

Anthony

[Charlie]

No, same rule applies to the lightbulb trick. Pull it out. You're basically doing the same thing. Using the lightbulbs inline with the power cord still gives the chassis less voltage, which still reduces B+, and still makes the oscillator run at a lower frequency. Only difference is your method... using the bulb instead of a variable transformer.

There will be a quiz on Friday!

I think I better study for this quiz.

Thanks to everyone for all the input. I'll let you all know how this comes out. And pictures too. Maybe I'll take some film shots and scan them.

Thanks for the info Charlie, that's what I suspected. Was up all night studying for a urine test once.....still missed the cup!

Anthony

[bgadow]

Anthony, you have insulted me deeply! I live across the street from a cup factory & how dare you talk like that!!! You are a terrible, terrible person, heathkit tv!

Then my work is done here. Ehhh, go piss up a rope!

Anthony

I finally applied power to this thing, slowly with a variac, with the horizontal and damper tubes pulled as suggested, and NO SMOKE or blown fuses. Every tube had the heaters going too, which I know doesn't mean they're all good, but at least they all still had vacuum. There was a slight smell of something hot, but everything is covered in dust, and this stuff is hot. It wasn't a burning smell and I actually had expected it to smell even more.

One thing I did find though. The Horizontal tube's base was slightly loose. Since I didn't put it in I don't know if it even will take the heater power. Short of disconnecting all but the heater leads, is there any way to test it? Both Angela and my local guy have the 6BG6G so maybe I should pick one up although it would be helpful if I could find out what other tubes I may need.

I wish the speaker transformer wasn't mounted on the speaker so I could test that out on my bench. I suppose I could bring the chassis upstairs to the cabinet since I'm close to needing to do that anyway. I was going to test some voltages first.

Also, I checked resistance and some of them were right on the mark, while others were off by some, and still others were so far off I checked the schematic, only to find that the connections did not match. Since these were the original wiring should I assume that my chassis has some revisions and just go with it? The tubes were different too, but this was documented as a later version. My guess is that they were constantly changing the circuit to take care of problems. (If every version of every computer program I ever worked on was documented I'd still be working on an IBM 360)

[RetroHacker]

No smoke! This is a good thing. You are, of course, familiar with how electronics work, I assume. All electronic components, (especially modern integrated circuits, but it still applies to capacitors, resistors and power transformers) function by way of electrical current interacting with magic smoke. That's right, magic smoke. It's sealed into the components at the factory, and it is the basis of all electronic technology. If you let the magic smoke out of the components, however, they will stop functioning. So be careful.

That "hot dust" smell is pretty normal, and most tube electronics will carry with them a pleasent "hot" smell when operational. I would, however, reccomend cleaning everything as best you can. Too much dust and crud in there can be a problem, and it's a lot easier (or at least more pleasant) working on a clean chassis. Brush the dust off the tubes the best you can, and I'd suggest pulling out all the tubes and brushing off the chassis too. Personally, I use an air compressor to clean the chassis before I start working on it. Just be careful when cleaning tubes not to clean the numbers off too...

A loose base on an octal tube is pretty common. The glue used to hold the base onto the tube has deteriorated with time and heat. It's normally not that much of a problem, provided that the little wires inside the tube base aren't broken. Always pull octal tubes out by their bases, to avoid putting stress on that connection.

Speaking of loose tube bases - anyone ever tried regluing these? You'd need some sort of a glue that would bond with both the base and the glass, and something that would withstand the heat of the tube, and preferably not give off noxious fumes when it gets hot.

Good luck in your restoration, and keep us posted with the progress!

-Ian