Dumont RA-113 (B3) restorations

[bandersen]

Making good progress with the cleaning but noticed that a pulley appears to be missing. The other tuner has it and without it, the dial cord rubs at some dial positions.
Not sure what might have happened to the old one or how to replace it The shaft is riveted on and I really don't want to remove it if possible. Maybe a nylon pully could be mushed over the end ?

[Phil Nelson]

Maybe you could grind the head off that shaft with a Dremel tool, slip on a new pulley, and epoxy a little round piece of metal or plastic back on to replace the head. Tape the string farther down the shaft and out of the way to avoid cutting it.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

[Electronic M]

Rather than try and glue the metal back on I'd try to drill and tap it so I could screw a washer down to replace the head.

[M3-SRT8]

I love dem DuMonts.

[bandersen]

While I ponder the pully, I started on the second chassis. It's definitely earlier and a crustier.



First I pulled the 5U4 and did a power up test. All the tubes lit and I have about 860 VAC on the 5U4 plates.


I'm going to start in the HV box this time around. This one has three 470K resistors in series while the other had a single 2M HV resistor.

[holmesuser01]

So, the one with the 3 470 caps are the early version? I wasn't sure where my RA-113 came in the model run. I got those 3 caps from Just Radios in Canada.

The replacement of these 3 brought the HV up from 1500v to 9000v.

I'm enjoying reading about the restorations here.

EDIT: I must be losing it. I started out thinking about those 3 resistors, and somehow, while typing, I switched to the capacitors and what happened when I replaced them. It's been a long day, so far...

[bandersen]

They all have three 500 pF HV caps as far as I know. I was referring to the three 470K resistors.

[holmesuser01]

Originally Posted by bandersen

They all have three 500 pF HV caps as far as I know. I was referring to the three 470K resistors.

OOPS! So was I. I started thinking about the 3 caps and... well.... I'll go back and fix the boo boo.

[bandersen]

I finished up inside the HV box including a new HV anode lead. Making good progress underneath the chassis now.


This one is much dirtier and crustier than the other. The wiring insulation is rather brittle so I'm having to be more careful. There is also a difference in component type, placement and tube wiring. For example, this 0.01uF cap was a conventional looking paper cap in the other set. I'm pretty sure this is also a paper cap housed in a mica looking package.

The biggest headache so far is the vertical oscillator. The 6SN7 dual triode wiring is flip-flopped from what's shown in the schematic and the circuit is a little different

[M3-SRT8]

Those paper "micas" can be deceptive.

When in doubt, I check the parts lists for descriptions and see if any of the replacement parts have an "M" in them or not.

I tend to shotgun ANY caps in the horizontal circuits now.

[bandersen]

I finished recapping the second chassis and fired it up with an 8XP4 test CRT and substitute yoke again



Noticed a few problems right away.
* No dial lamps and eye tube barely glowing.
* Good FM radio reception, but no sound for TV stations
* No sync
* Power resistor subbing for focus coil cold
* Horizontal periodically "freaks out"

[bandersen]

I quickly solved a few issues. One dial lamp was burned out and the other two had dirty contacts.

The focus control has a large "dead" area. Setting it in a good area gets the sync working better and the brightness/contrast improves. However, the power resistor I'm subbing for the focus coil is now roasting.

Eye tube is totally dark.

Horizontal still freaks out periodically and no sound for TV reception. I'm thinking the sound alignment may be way off.

Also discovered the tuner mounting screws very all very loose and reception cuts out as the fine tuning is adjusted. I bet it's very dirty inside. it will be unmounted,cleaned and lubed.

I'll clean all the controls and tube sockets and swap in some fresh tubes.

Not the worst first power up, but still many issues to resolve.

Need to swap the horizontal yoke leads too.

[bandersen]

Sure enough the focus rheostat is bad It has a Dumont poart # and is likely the original. Should be 1250 not 7.8M !


The break is somewhere near the scratch mark..


So with the tap on one side of the break, all the power goes through the 1450 ohm power resistor, on the other side the focus coil which explains why it was sometimes cold and others blazing hot. Either way, the set is being starved of power for the tapped bleeder resistors.


I'll have to track down a replacement. I'll see if I can dig up a couple 680 ohm power resistors in the meantime as a substitute.
Also tested all the tubes and found a bad 6AB4 and 6J6 in the tuner and two bad 6AU6 in the IF.

The set must have been in a heavy smokers home. All that goo protected the chassis nicely.


Also discovered this dangling coil near the tuner. That doesn't look good

[IsthmusTV]

Bob,

Surplus Sales of Nebraska has a 1K 25 Watt wirewound pot that's a good substitute for the focus control, and it's only $8:

http://www.surplussales.com/Potentio...sRheost-4.html

Good luck,

Clark

[bandersen]

Thanks for the tip. Now I'm looking for more goodies there to fill up my order