Need some help on an 8TS30....

[Kamakiri]

R127B I have a 10 watt, R127A is a 20 watt. R127B is a chassis mount, but since the arcing issue a couple pages back, it's just floating loose in space at the moment.

[old_coot88]

So what's the actual DC voltage you have across R127B (at normal line voltage or whatever voltage you normally run the variac at)?

[Kamakiri]

Originally Posted by Kevin Kuehn

Based on Riders voltages I'm coming up with 39.4v across R127B, which would be about 6.77 watts.

I'm at 45.8v across R127B, running the set at 117 VAC.

[old_coot88]

Originally Posted by Kamakiri

I'm at 45.8v across R127B, running the set at 117 VAC.

Kinda high. The current will be 196 ma. @8.96 watts. A 10W resistor will run sizzling hot.
196 ma. is the total current of the entire B supply going thru that resistor
What does the voltage across it drop to when you pull the 6BG6?

(Edit)
In a darkened room, does the 6BG6 show any sign of red plating?

[Kamakiri]

With the 6BG6 pulled, I'm at 48.9 VDC. Can't see any hot plating.....

[old_coot88]

Originally Posted by Kamakiri

With the 6BG6 pulled, I'm at 48.9 VDC. Can't see any hot plating.....

Holy makrel! That's over 10 watts. Try pulling the damper, and then the rest of the tubes one at a time, starting with the audio output, while watching the voltage across R127B. If the voltage stays up, it'll prove there's not a tube overconducting.

To save brutalizing R127B while troubleshooting, you could sub it with a couple of 100 ohm 10 watt resistors in series if you happen to have such.

[Kamakiri]

Just so that I have this right, I'm gonna next pull the damper, power it up, then start powering up the chassis, each time yanking a tube and watching the meter, looking for a change in voltage?

This is something I've never done before. I was always under the impression that I'd end up damaging something doing that if I yanked more than one out.

[old_coot88]

Originally Posted by Kamakiri

This is something I've never done before. I was always under the impression that I'd end up damaging something doing that if I yanked more than one out.

Ordinarily*, pulling tubes might allow B+ to go too high, stressing the filter caps. But in this case, there's already a huge draw on the B supply. And you have a variac to keep it under control.

Just so that I have this right, I'm gonna next pull the damper, power it up, then start powering up the chassis, each time yanking a tube and watching the meter, looking for a change in voltage?

Basically yes, using your variac to bring it up each time. Actually, just do the damper> AF out> vert.out for now.
-------------
*One big no no would be pulling the H.osc tube while the H.out tube is still in place.

[Kevin Kuehn]

Wow, I don't understand how removing load from B+ can cause the voltage across R127B to go higher.

[earlyfilm]

First verify that your rectifiers are 5U4's and the damper is a 5V4, which is a rather unusual choice for a damper.

Yes, pull both at the same time. A shorted damper can kill a new horizontal output tube and I normally yank both together. The multiple tube pull method won't work on stacked power supplies, but the stacking is limited in this set.

If the voltage across the resistor is as high as before, immediately kill the power and feel all electrolytic caps.

Don't worry about the HV, as it wont be working, but don't touch the chassis and an electrolytic at the same time as some might have lingering B+. If one is heating, check for incorrect wiring, ratings and polarity on the hot one(s).

If no electrolytic is heating, print out the RCA Factory Schematic and Wiring diagrams from the RCA 8TS30 PDF on ETF and give them to your son and offer him a reward if he can find your wiring error starting from the Power Supply and Reaction Scanning (damper) tubes. The wiring error could be something as simple as a tube in the wrong socket . . . . . or a grounded floating ground . . . . or an electrolytic with polarity inverted.

Jas.

[Kamakiri]

With both the 5V4 and the 6BG6 removed, it dropped to 44.9 VDC. Continue pulling tubes and checking, or stop here?

[Kevin Kuehn]

Any chance you could show us a picture of the inside chimney wiring? Another very strange thing, is how you mentioned earlier that R127B was arcing to ground. There shouldn't be any voltage potential on that resistor high enough to arc to ground, unless you meant one of it's terminals was touching ground. Is R127B an aluminum cased or ceramic coated?

What's the voltage reading from either side of R127B to ground?

[Kamakiri]

Bear in mind that this isn't the "finished product", as I'm moving things around, testing, etc. My finished work is much neater.

R127B is on the right, that's the gold chassis mount resistor. R127A is the enameled wirewound. You can see how the previous R127B just let loose. It wasn't touching ground, it had an arc jump TO ground on the right side. It was close to ground, but not touching. Probably 1/8" or so.

[Kevin Kuehn]

It's hard for me to see clearly what's connected to where, but is that a 10 ohm cement coated Dale, with the left end connected to chassis ground? And where is the other end of that same resistor going? Edit- I guess that 10 ohm Dale is in series with the 12 ohm(128B)?

[Kamakiri]

Left end is to chassis ground, right end is connected to the 3rd terminal of the candohm. That section of the candohm was dead.