Need some help on an 8TS30....

[Kamakiri]

Originally Posted by old_coot88

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?

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

[Kamakiri]

Originally Posted by Kevin Kuehn

Any progress on this?

I'm curious how much your B+ increased when you pulled those 4 tubes?

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

[Kevin Kuehn]

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.

[Kamakiri]

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.

[Kevin Kuehn]

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?

[Kevin Kuehn]

Originally Posted by old_coot88

FWIW, I would first suspect a hard B+ short-to-ground (possibly from the H.centering control or V.centering control to ground).
A hard short on the B+ line would throw a huge current draw on the 5U4 and its centertapped HV winding, and cause a large overcurrent through the resistor chain off the centertap, which includes R127B (it's the highest resistance in the chain, so it will run the hottest).

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

[old_coot88]

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.)

[Kevin Kuehn]

Originally Posted by old_coot88

(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+.

[old_coot88]

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.

[Kevin Kuehn]

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.

[old_coot88]

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)

[Kamakiri]

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

[Electronic M]

Originally Posted by Kevin Kuehn

A good rule of thumb is to have something at least as physically large as the original.

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.

[Kevin Kuehn]

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.

[Kevin Kuehn]

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.