Dumont RA-103 Project Redux

[old_coot88]

...the bottom open circle and the arrow indicates that is the articulating contact in the relay switch correct? The top open circle is everything else and is the stationary contact?

No, in schematic arcana, the arrow is always the stationary contact and the moving contact is always what the coil "pulls" on.

[Chris K]

Originally Posted by old_coot88

No, in schematic arcana, the arrow is always the stationary contact and the moving contact is always what the coil "pulls" on.

GREAT!!! Thank you for that piece of knowledge! I think I have the relay hooked up correctly. The stationary contact has the tied together pin #8s from both 5U4s and the yellow lead from the small secondary coil soldered to it as well. I'm going to double check the wiring in the rectifier circuit then move on to checking the moving contact leads and how they trace out.

[Electronic M]

A couple of things:
The resistor that's smoking, if it tests fine then it's smoking because the short is on (or not far down stream of) the side of the resistor not connected to the high B+ rail....That resistors is the somewhat literal metaphorical smoking gun that points you to at least one killer short.

There were a few purposes to the relay. The main one is to delay the B+ until the tubes have warmed up to stable operation so when the set comes on you get a perfect picture and sound instantly without any concerning warmup instability. (Realistically you can delete the relay by shorting the contacts and the set will still work fine... At least for sets that work with the relay there) The other purpose is to protect most of the lytics from warmup surge.... Basically since the 5U4s are filament tubes they warmup almost instantly, but all the other tubes take ~11 sec.... During that time the B+ has no load from the signal/sweep tubes and skyrockets briefly this can be hard on the lytics and or require higher voltage lytics to cope.

[old_coot88]

Since the high B+ rail goes directly to the area of the focus control and H centering pot, I'd bet money that the short will be in that area (circuit-wise), most likely involving the focus control. The schematic is too fuzzy to read print, so if you could post a blow-up of the lower-right corner of the schematic, it should be pretty easy to nail the culprit.

[Chris K]

Here it is. Unfortunately, I'm at work so no troubleshooting until I get home!!!

[Chris K]

Originally Posted by Electronic M

A couple of things:
The resistor that's smoking, if it tests fine then it's smoking because the short is on (or not far down stream of) the side of the resistor not connected to the high B+ rail....That resistors is the somewhat literal metaphorical smoking gun that points you to at least one killer short.

There were a few purposes to the relay. The main one is to delay the B+ until the tubes have warmed up to stable operation so when the set comes on you get a perfect picture and sound instantly without any concerning warmup instability. (Realistically you can delete the relay by shorting the contacts and the set will still work fine... At least for sets that work with the relay there) The other purpose is to protect most of the lytics from warmup surge.... Basically since the 5U4s are filament tubes they warmup almost instantly, but all the other tubes take ~11 sec.... During that time the B+ has no load from the signal/sweep tubes and skyrockets briefly this can be hard on the lytics and or require higher voltage lytics to cope.

So I had it backwards in terms of what happens. The relay delay is for the rest of the TV to warm up before they get slammed with B+ from the 5U4s.

[Chris K]

…and maybe when u get time Tom, for my education, can you explain how you knew the short was after the CO resistor when it got screaming hot? Some things about DC voltage I just don’t get.

[old_coot88]

Originally Posted by Chris K

So I had it backwards in terms of what happens. The relay delay is for the rest of the TV to warm up before they get slammed with B+ from the 5U4s.

Exactly. That's its primary function. Less high-end designs didn't use it by designing-in enough 'headroom' that the surge doesn't overvolt the filter caps' rating.

[Chris K]

Horizontal position pot is shorted to ground. Based on the circuit, could it be a bad horizontal yoke coil in addition to a bad pot?

[old_coot88]

What point are you actually measuring from? Is it a dead short (zero ohms)? If not, how many ohms?

[Chris K]

Originally Posted by old_coot88

What point are you actually measuring from? Is it a dead short (zero ohms)? If not, how many ohms?

No not a dead short...48 ohms. From the two tied terminals on the pot. Am I measuring the horizontal yoke at 48 ohms?

[old_coot88]

Try lifting the lead that goes to the yoke coil.

[Chris K]

Originally Posted by old_coot88

Try lifting the lead that goes to the yoke coil.

I just did and forgive me now for the fact that I'm getting a little confused so I might give you information that doesn't make any sense or that makes no relevant sense to this troubleshooting. I lifted the lead off the positioning pot that goes to the horizontal yoke. There is no continuity to ground on the pot now..no tone on my DMM and the tab on the VP pot measures 98 ohms. The lead I removed reads a tone to ground at 48 ohms, the same reading I got when the lead was attached to the pot.

[Tim]

Is that the lead that goes to R286A 200 ohm candohm?

[Chris K]

Originally Posted by Tim

Is that the lead that goes to R286A 200 ohm candohm?

Yep