Need some help on an 8TS30....

[kramden66]

It needs to be installed in a airconditioner to be kept cool loll. If you have two 10 watt resistors of equal value you could put them together to get the 20watts but they must be equal and add to the ohms you want. Or put two parallel to decrease the ohms to the value you want.

[old_coot88]

Just a coupla thots for when you tackle the beastie again.
You mentioned R127A running "pretty damn toasty". That's interesting since it shares the same current loop with hot-running R127B.

According to Sams there should be 117V DC on the low end of R127A, at its junction with R128A.
See simplified diagram attached.

What voltage do you get at the junction? Is it signifigantly lower than 117V?

Filter cap C5B connects from that junction to ground. Sams lists it as 80 mf.@350v.

Is there any chance that cap coulda been replaced with one having too low a voltage rating? If so, it could be breaking down and pulling excess current.

But if the 117 V is OK, then there isn't a problem.

[Kamakiri]

Nah. All of the bypass caps I stock are 630V and all of the electrolytics are always 450 V. But I'll poke around in there after Christmas

[Kamakiri]

Originally Posted by Kevin Kuehn

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.

Okay....so here we are with the holidays behind us. I've got a 25 watt NTE ceramic resistor ready to go in. Just install it and let it run....?

[old_coot88]

I just got one outstanding question.. Does that large voltage drop still occur across that resistor with those four tubes removed, or not?
(H.out, damper, audio out, and vert out.)

[Kamakiri]

No change at all when I pulled those 4 tubes.....

[Kevin Kuehn]

Originally Posted by Kamakiri

No change at all when I pulled those 4 tubes.....

That does seem almost impossible, that the voltage across that resistor wouldn't budge with those 4 tubes removed. After all, it's the load on the B+ that is pulling current through the high voltage winding center tap, and that same current has to pass through that resistor on it's return path to ground.

[Kevin Kuehn]

Originally Posted by Kamakiri

Okay....so here we are with the holidays behind us. I've got a 25 watt NTE ceramic resistor ready to go in. Just install it and let it run....?

Yeah, that's basically the plan. You might keep your volt meter clipped across it and monitor that your previous voltage reading doesn't increase dramatically over time . Wouldn't be a bad idea to pick up something like a Simpson 260, so you could take a average current measurement in series with a load. Considering the volume of set's your working on, it would be a good long term investment.

[old_coot88]

Referring back to post# 122, what voltage do you get at the R127A/R128A junction? It should be 117V according to Sams. (That's extrapolated from the 117V called for on pin 6 of V9 and pin 4 of V10, which are connected to that junction.)

[Kamakiri]

Sometimes having a TV friend staring at something helps you to bring clarity...

Ed: "Okay, but you didn't replace one section of the candohm"

Me: "Right, but it ohms out perfectly"

Ed: "But did you ohm it to ground?"

Me: "Well, no. I usually replace all the sections, but this one tested fine. Wouldn't the value be off if it was shorted to ground?"

Ed: "Don't EVER trust a candohm. Replace that section and power it up and see where you are"

So I did, and now it's got a stable linear picture, and nothing's sizzling. Takes a few minutes of running to get decent AGC and there's some occasional snapping static in the audio (could be a tube socket that needs cleaning), but we'll see if that levels off with repeated 5 minute stints of usage. Have to tweak the adjustments too, of course.

The moral of the story: like Ed said, never trust a candohm......

[old_coot88]

So which section of what was bad? Was it open or shorted or...? (I'm guessing R128A.)

[Kamakiri]

R128C. I replaced all but that one section which tested fine, and figured with it left in circuit that would be a good mounting point for the other stuff.

It bothered me the more I thought about it, so I tested it. It's not shorted to ground, and it's not open....in fact it's right on the money. Must have been doing something when the set was under load. Who knows, all I know is that it's approaching operational now....

[Kevin Kuehn]

I think I'd have to split that candohm open and see what was going on.

[Eric H]

I'd be careful doing that, I think they were insulated with Asbestos.
The insulation can get damp and break down, especially after nearly 70 years.

[Kamakiri]

Originally Posted by Kevin Kuehn

I think I'd have to split that candohm open and see what was going on.

After the hell that this sucker's put me through I have lost any semblance of curiosity as to why it hasn't worked