I'm back revisiting the Dumont. It's taken a bit of time and motivation to get it out and on the "bench". I finally got around to testing the CRT and it pegged on the grid emmisions. Now I have an idea of why the brightness was so bright even when the control was at its minimum.

Theres a few 1000pf caps that didn't look right on my cap tester, the eye on the tester didn't open as fully as a new cap does. So that got me going down the rabbit hole of looking at some other areas. I remember last time I was testing the set I had excessivly high b+. Its highly likely that the grid emmisions on the CRT might have cause that.. But shouldnt that load down the B+ instead of raising it?

A big issue I stumbled apon was v6(3rd video IF). The grid(pin 1) connects to a 3900K resistor and two coils. The resistor goes to ground. The schematic shows the resistor going to a 6800mmf cap and then to ground as well as through the 100 ohm cathode resistor.

Why would my set be wired this way? The service notes don't mention a production change...

Thanks!
Matt Davala

I looked at the RA-103D schematic and how my set was wired matches. I must have a late production model that incorportates changes in the RA-103D model but do not have the 6x4 bias rectifiers. .

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Matt Davala

The CRT is definately the issue. I get about 15 seconds after the set kicks on of normal brightness control and then the CRT goes super bright. Its going to be a bear sourcing a good 12JP4.

This one really wasn't a winner tube anyway as it has a nice big ion burn in the center of the screen.

Is there any harm in running the set like this with high grid emmisions? Its not like I'm going to make the CRT worse....

Thanks!
Matt Davala

Not sure what you mean by grid emissions. Is there a G1 short? Have you tried to clear it?

You can sub a 12LP4, 12KP4, 12QP4 or 12RP4 but you'll have to change the HV connector.

I'm using my B&K 466 and on the G1 leakage test the needle pegs fully to the right.

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Matt Davala

I ended up trying to remove shorts at least 20 times. The G1 test would indicate good then randomly peg again. So I went between rejuvinate 1 and remove shorts over and over. I figured the tube is tired anyway.

Its been about 10 minutes now with the Dumont running and I now can actually go fully dark for the first time ever. Now I'm dreading moving the chassis back into the set in fear of jarring the CRT.

Thanks Bob for telling me to remove the G1 short. I completly misunderstood that test.
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Matt Davala

Back fitment also changes as the 12LP4 at least is a bit longer and bulges the back out on the doghouse (mine has an LP4 so I know first hand).... It'll still fit but provides no protection for the CRT neck. One good slam against the wall and it's done.