Dumont RA-103 Project Redux

[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

[Tim]

I think you may have found your problem.

[Chris K]

Originally Posted by Tim

I think you may have found your problem.

The terminal on R286A reads 48 ohms to chassis ground. And I lifted the leads from it and it measured 45 ohms to ground with a constant tone

[Tim]

yep, you found it. Replace the 200 and 250 ohm resistors and disconnect all terminals of the candohm. Use 15 watt for the 200 and 10 watt for the 250.

[Chris K]

Originally Posted by Tim

yep, you found it. Replace the 200 and 250 ohm resistors and disconnect all terminals of the candohm. Use 15 watt for the 200 and 10 watt for the 250.

Thanks Tim, Tom and "88"!! I have a parts chassis where all of the CO resistors were replaced with Ohmite ceramic wire wounds so I'm going to clip those out and just replace everything. I'll get new Ohmites but for further testing and resurrection, I'll go with the old resistors. I'm absolutely certain that's NOT the last issue of unknown origin I'll need help with running down but for now, I'll take a few hours tomorrow and replace the components. Thanks again all!

[old_coot88]

[Electronic M]

Looks fairly close to usable status. Good job.

[Chris K]

Originally Posted by Electronic M

Looks fairly close to usable status. Good job.

Thanks guys. There’s a lot of work I want to do on the underside including the resistor mounting plus a not so insignificant amount of wire replacement. Some of the insulation is cracked and crumbling from the chassis heat. I’m sure you guys noticed the ion burn on the screen. When I can turn this right side up and do all of the tweaking and chassis cleaning, I’m replacing the 12JP4 with a NOS 12QP4. I got it from a dealer in Florida who had an original ion trap as well. This is all very exciting!

[Tim]

Real nice to see good progress. Excellent job!

[Chris K]

What a dumb and fruitless time unless you count frustration and knowing your limitations as experience building.

The resistors came and I spent hours mounting and installing them. The area under the canohms seemed a perfect spot for bolting on these chassis mount resistors but I discovered the area was inaccessible from the top of the chassis due to the lower strap supporting the CRT. After making the appropriate changes to the layout plans, I finished the work and did a few other things most importantly, getting modern safety caps on the AC line. I buttoned everything up and began the testing. Turned it on...nothing. Panic. It was completely dead so I began investigating the AC hookup and the on/off switch and wiring. Long story short, I accidently clipped one of the wires going to the switch when I installed the caps. This chassis is a crammed full mess with hard, heat cured wiring twisted and hidden everywhere. I fixed it and began again. It powered on but my relief was short lived. When the relay closed there was no change in the amps being drawn. Obviously I had messed something up in the resistor install right? I spent several days tracing every circuit and checking the schematic over and over but I couldn't find any mistakes. I looked a square inch at a time over anything I might have moved or accidently disconnected or dropped solder on. Nothing. I even began putting together a detailed presentation for you guys with pictures of the work and the schematic detail etc. Maybe something in the relay got messed up again??? I started tracing voltages. In plane crash investigations there's something called "confirmation bias". You take an action and soon after, something goes wrong. Obviously it's something you did and you begin to see all subsequent events through the lens of your actions and focus just on that. The big picture is missed. The source of my problem with the RA-103? When I zip tied together some of the dozens of wires feeding up the side and across the chassis, I must have tied one too tightly. The wire from the choke to R285 snapped like uncooked spaghetti but couldn't be seen because it was hidden inside a wire bundle. Five minutes later it was fixed and the TV was working like it was before. Five minute fix after five days of fruitless troubleshooting over work that "obviously I must have messed up"! Well, technically I did mess it up but I lost an unbiased approach to the troubleshooting. Lesson learned...maybe. Can't make any promises on that one! More updates on progress, I hope, in the days to come now that I'm out of the self-induced quicksand.

[Chris K]

Are there 2 tuning dials for channel selection in various models of this tv? The brass gear on the back of my tuning dial stripped out at some point in its lifetime and I replaced it with the dial from my parts chassis and the 2 dials are completely different. The tv I’m servicing has the stripped gear and ascending channels when it’s turned clockwise. The parts chassis dial is the opposite…ascending channels when turned counterclockwise. I cleaned the tuner and the picture is so much better. Thought I was home free on this one