Sounds like too many caps ago, and too little time. :)

You're not kiddin'.....

There's got to be a 6.8K ohm resistor around here somewhere. Time to go dig through my wirewounds.

Okay, something's still very wrong. Here's where I'm at.....

I found a 7K ohm 10 watt in my stock. Close enough to give it a try. I'm getting 39.8 V across R127B, so we're right there in the correct range. Now, the horizontal linearity is WAY off, compressed on the left side (as I recall).

But everything's getting hot still. Really hot...to the point where something was starting to give a wisp of smoke. When I lifted the leads off R127B after the set was shut down for a couple minutes they were still extremely warm. The contrast/AGC control does almost nothing now, and the picture has slightly less width than it had.

Here's my work as it is now. The 12 ohm resistor to chassis ground checks out fine.

OK, now try the same power-up except with the 6BG6, damper, vert.output, and audio output tube removed, while monitoring voltage across R127B. It should drop 'waay down with those heavy-draw tubes removed.
If it doesn't, try the ohmmeter test suggested in post# 75.

I'll pick it up here tomorrow. Gotta go work the night shift on the railroad tonight :)

Good ol' Lionel "O" scale model RR...Used to set up my father or grandfather's set 'round the tree most Christmases....The tracks liked to push apart when the train went over it and speed dropped a lot on the far side from the power connection to the tracks.....Until I tried tying the track sections together, then they did not come apart and power loss along the tracks dropped significantly.

Voltage test didn't change with all 4 tubes removed, so I went on to this. The meter stopped at almost exactly 6.8 K Ohms.

Well, 6.8K is reasonable enough, considering that the B+ bleeder string to ground (R127A and R128A) add up to 8.1K. Current will only be around 45 ma., nowhere near the 175 or so ma. that's going thru poor R127B.

It almost looks like a B+ filter cap is breaking down under voltage, like maybe a cap has been replaced with one having too low a voltage rating. The most likely suspect would be C4A, which should be 40 mf and rated at 450V. Does the C4 can show evidence of heating?

Any progress on this?

I'm curious how much your B+ increased when you pulled those 4 tubes? Or if it didn't, I'd continue to pull tubes, one at a time, except leave the audio output tube in. Continue to check voltage across R127B and B+. Make sure B+ doesn't exceed your 450v cap ratings while doing this.

Good timing, I just pulled the chassis back out a couple minutes ago. Christmas stuff's delayed my work a bit.....stand by.

C4's been replaced with all new Nichicons. C4A is now 47 uF @ 450v. Don't see any signs of anything under there.....

No change. With all those tubes out and still that much of a current draw, are there any likely suspects at this point?

How bizarre. So with those tubes removed, what voltages do you measure across C1A,C3A and C1C? Measure directly across their + and - terminals, NOT from + to chassis.

Gonna have to do it with the picture tube pulled and the speaker disconnected because of the way everything's mounted in the cabinet on this set, or do I need to jig anything up? I assume that it would be fine, but I'm asking anyway, since I've got so many things pulled as it is.

Hmm, that sounds like a major pain, but I think it's come down to getting that chassis up on it's side for voltage measurements. I'm not sure why we'd need the picture tube installed at this point, anyone else have a better idea?

Did you ever verify that these controls, or possibly one of the yoke windings are not shorted to chassis, or to each other?

One question. With those four tubes removed, is R127B still running severely hot like it was? If so, something else (besides the focus coil) somewhere in the chassis oughta be running overly hot also.

Or else... what kind of meter are you using, analog or digital? The fact is, I would not trust a digital meter on anything other than pure DC or pure sinewave AC (when measuring voltage or current). If there's any ripple component to voltage or current as there is here, a digital meter could be giving screwy readings.

If you were to disconnect one end of R127B and put an analog milliameter in series, it would give a bulletproof reading of the actual current. Do you have a Simpson 260 or anything of the like?

(Those main filter caps C1A, C1B, C1C, C3A and C2B would not be suspect as they are not in the current loop with R127B.)

No, but the voltage across them could be revealing . And I think that chassis should come out of the cabinet to get any meaningful measurements, as we're beyond pulling tubes. As you mentioned way back, something besides a tube must be drawing major current on the B+.

At this point it might be well to consider that the replacement for R127B, although rated at 10 watts, is intended to be heat sinked by mounting to sheet metal. Its unmounted rating is gonna be considerably lower than 10W, and it will get overly hot even at normal current.

And if voltage and current readings are being taken with a digital meter, all bets are off. At the very least an analog milliameter should be used to verify that the current thru R127B is actually too high.

Well I'd have to agree. It would be a real shame to keep chasing ones tail, all over a underrated resistor that's overheating. A good rule of thumb is to have something at least as physically large as the original.

Yeah, the point was made earlier that a temporary sub for R127B could be strung together, something like a couple of 100 ohm 10 watters and a 33 ohm 2W in series, as a fill in during troubleshooting.

But at least get an analog reading of the actual current ('stead of computing it from a possibly-questionable voltage reading)

Welp.....I've been using a digital non auto ranging meter. I do have an old analog I can use to take new readings. At any rate though, the hot resistor was the problem before but now that the resistor network is straightened out I now have raster issues.

At any rate....I'll reassemble it, dig up the analog meter and verify the readings

A better rule of thumb is to use a replacement at least double the power that the circuit is designed to dissipate in the resistor (using: Power in Watts=IV=(v^2)/R=(I^2)R ) for a cool operating part.

Do you happen to have a test CRT hanging around, like a 5AXP4 or an 8XP4? Life would be a lot easier if you could keep that chassis away from it's cabinet and up on it's side while troubleshooting.

Yeah, Tom, that works fine, but for the sake of simplicity, we know the original was designed conservatively. Also modern components may be able to withstand more heat, but there's still nothing like good old surface area to dissipate the heat away.

Shucks, for some reason I thought the chassis was already out of the cabinet. If you haven't pulled it yet, and while those tubes are still removed, try to get an analog current reading on R127B.

Another thought - Those resistors may actually run hotter if you're leaving the back cover off the chimney. Something about convection not being able to pull cool air up through the bottom opening, and the heat just sort of hangs around.

Chassis is out of the cabinet at the moment. Putting it back in is a 10 minute job....no big deal there. If the HV lead doesn't need to be connected I do have an adapter lead to hook up the socket so I can set the tube next to the chassis. I don't have a test tube here. And the cover is definitely off the chimney :)

Just tell me what's next. This set has a ride back home next weekend if we can get it figured out. If not no big deal.....it would just have to wait till the next time.

I've kinda lost track of what kuszactly the current state of affairs is?
Maybe there should be a live web camera aimed at the Kamakiri workbench. :D

With the chassis out, you don't need the CRT hooked up for troubleshooting this particular problem. It's not going to make HV anyway with those tubes removed.
If you've got an analog milliameter with a 0-500 ma (or 0-250 ma) range, just stick it in series with R127B, variac the chassis up to normal line voltage, and see what the current thru R127B is.

Well, if a visual aid helps.... ;)

FYI, here's the meter I've been using.

I don't have an analog milliameter. I have two analog meters, a basic Micronta voltmeter, and a prehistoric Weston model 697 that works, but doesn't have the range.

Decided to power it up to get a reading on my digital meter. Started it off on the variac at 75v, and all of a sudden heard and saw a SNAP. Looked like the spark came from the area of the chassis ground lead below the chimney, but inside the chassis. The *only* thing in that area, but further down below, is the horizontal centering control.

:tears:

Y'know, I'm about ready to give up on this stupid thing......

Now let's back it up a sec. When I had missed the ground lead on R128A, everything was acceptable except the heat going to R127B. I'll say this out of frustration, but can't I just leave that ground lead off, and stick like a 50 watt wirewound in R127B?

Yeah, didn't think so.....

Wait, wait, wait wait. Careful study of the chassis just revealed something to me.

It seems that someone back in the day replaced C3 before I recapped the set, but didn't anchor it properly to the chassis. With the jostling of moving the chassis around, C3 must have knocked loose and the floating ground was very lightly touching chassis ground.

Odd thing is that I don't recall it being loose during the restoration, but there it is loose, with only one rivet hole drilled all the way through and one missing rivet.

Gonna fix that and try it again.

Got that all fixed, and powered it up. Now I've got no HV. Dammit.

So now with that cap not shorting to ground, are things running any cooler? Did you re check the DC voltage across R127B?

Friggity frack, this thing is haunted.

I did NOTHING to it since the last power up, and now I have a picture.

:screwy: :wtf:

Here's what is on the screen, bear in mind I've made no adjustments because I didn't want to run it out too long. Contrast control does nothing, brightness past halfway causes the picture to bloom and lose focus.

Resistors R127A and B are pretty damn toasty, but measuring 40.3 V across R127B.

Well 40.3 volts DC across R127B is really close to where it's suppose to be, provided your voltage reading is accurate. I'd replace R127B with like a 20-25 watt unit, install the chimney back cover and let it run a while.

Well sunnava gun. Now, with that Micronta meter, you can get a reliable current reading using the analog voltage across R127B, per I=E/R (current=voltsXohms). Then for wattage, P=EI (watts=voltsXamps).
Be interesting to see if the voltage reading on the Micronta matches the digital's.

This has GOT to be the world's crappiest analog meter. The DC V scale only goes up to 15, and then a VERY coarse 0-1000 V. Ugh. Model 22-027U. I'll be honest in saying that I hate analog meters....had a couple Simpson 360s along the way that I really wanted to love.....but just couldn't. Guess it's all in what you're used to.

Time to take a Holiday break and order up some big honkin resistors for this sucker. I don't expect they'd be in any time before next week, so unless anyone has any other thoughts (which are always appeciated), we'll pick this one up after Christmas.

Meanwhile, I just might toss this Zenith roundie chassis on the bench and get going on it :)

You will get this set working, I am sure. Enjoy your holidays, and get back to this set when you have both time and desire. Lots of us here are ready and willing to offer help.