View Full Version : Problem With My DuMont RA103 Chatham

09-06-2017, 12:06 PM
Post subject: Trouble With DuMont RA103 ChathamPostPosted: Sep Wed 06, 2017 8:51 am

Joined: Sep Tue 15, 2015 4:25 pm
Posts: 217
Hi Guys,

Ran into a problem with my DuMont RA-103 today: I'm noting two things:

1-vertical roll. Picture rolls upward. I can slow the roll with the vertical hold control but can't stop it.

2-When I shut off the TV I'm noticing that the round "spot" in the center of the CRT is not as bright as it normally is, and it fades away in about half the time as previously (maybe 1 second instead of 3)

Any ideas or thoughts???


Electronic M
09-06-2017, 12:39 PM
Your vertical osc. is running off frequency: primary causes bad and or drifted value vertical stage resistors, caps, tubes. Check and replace as necessary.

Also since spot persistence varies you may want to check all B+ voltages in the vertical stage, especially any shared with the CRT electron gun and horiz/HV system (since the change in spot time may indicate a change in PS loading/operation).

Don't worry about spot time too much. The spot was something later sets designed out of the system.

Big Dave
09-06-2017, 02:34 PM
Check the primary winding of the vertical oscillator blocking transformer. Open primary is common on these.

09-06-2017, 04:53 PM
Eesh! Hope it isn't the vertical oscillator blocking transformer. I don't have one and damn knows where I'd find one!


09-06-2017, 07:23 PM
If you end up needing a transformer, be sure to post the number of it here. Some of us might have one laying around...

09-06-2017, 07:42 PM
Can always rewind it if that's the cause. :D

Electronic M
09-06-2017, 08:04 PM
IIRC I have a spare 103 chassis, so I may be able to spare that part if it becomes needed.

Phil Nelson
09-07-2017, 03:44 PM
I had to replace the vertical oscillator blocking transformer in my DuMont RA-102, and the folks here supplied the right cross-reference to order one from Play Things of Past. This thread has more details:

The RA-103 part may be a little different, but these transformers are out there, if you need one.


Phil Nelson
Phil's Old Radios

Big Dave
09-07-2017, 05:46 PM
There is a fix to get around replacement. With the primary open, the sync tubes aren't getting B+. I will need to look at a print since it's been a while since I did this.

09-07-2017, 09:58 PM
Hi all,

Well here's the scoop to date; it's been a very rainy and extremely damp few days here. My significant other haxhad the AC off and a window fan blowing. The fan is very literally a foot from the TV and has been blowing damp air on it for three days. Well, I shut off the fan, turned the AC back on and let it run all day to dry out the house. Now with nice dry air I went and jiggled the vertical tubes and sync separator tube to make sure they were fully seated in their sockets and switched the set back on. I immediately found the picture was rolling, but now downward, not upward. Ah ha. So I adjust the vertical hold and the pictured locked right in, and has been plying about two hours perfectly stable.

So it seems the damp air was a factor and equally likely a poor tube connection was involved. Hopefully that's all it is. But I've been watching it all evening and it's perfect.


09-10-2017, 07:20 PM
More on above: been playing the tv since and all has been mainly well. Only thing is I wish there was an easy adjustment for horizontal hold which there seems not to be. This is unrelated to my beloved's desire to blow damp air on the tv, but rather my cutting the cable cord and going with streaming. Since hooking to a Roku box via an hdmi to composite downconverter box I've had three occasions where a slight voltage spike such as the air conditioner compressor coming on, causes horizontal to go out of home and go to diagonal lines. Switching the tv on and on again r switching no off the video section corrects the problem. I'm believing the hdmi to composite down converter is slightly off with its sync signals, but tweaking a tvs horizontal hold would compensate for it. Any ideas???

Electronic M
09-11-2017, 08:21 AM
IIRC one of the cans at the back of the 103 is the horiz osc. can. It is slug tuned, the top adjustment has a big nut threaded on (so you can adjust it by hand) and on some sets there is a good sized hole in the factory back cover to access that nut with the words 'horizontal hold' embossed along the edge... See if adjusting that helps.

09-11-2017, 04:47 PM
That's awesome info guys. Many thanks! Sadly my Chatham no longer has the rear cover or I'd have seen the stamped marking. Next time it jumps out of sync I'll give if a try!