DuMont RA-108 Mansfield inductuner

[bigaudioal]

Originally Posted by vts1134

Did you check R23 (the grid leak resistor)?

I did check R23. It was almost dead on 12K, as it should be. All the resistors inside the tuner were good.

[bandersen]

Originally Posted by bigaudioal

Yep, L12 is inside the tuner. 0.1 ohm fil. choke. Do you just check the resistance on it like a resistor? If it is bad, is it replaceable?

Yes, it should read nearly 0 ohms. Replacemenets are available. You could also just short it out with a jumper while testing to eliminate it as a problem. I had a Motorola where several of the filament chokes had corroded and gone open.

Testing pin three was very strange. With my multi-meter on the 20 VAC setting, I connected the ground wire to the chassis. Saw the meter register around 4 VAC. Then when I touched the other lead to pin 3 the volume on the set faded and then I saw smoke rising from a hole in chassis near where tuner is mounted. As soon as I removed the lead from pin three volume was restored. Reading was jumpy at best but did not get into double digits. I will test some of the other tube filaments to see that they are 6.3 vac.
...

I'm not sure I understand when you say you connected the ground lead and saw 4 VAC. What was the other lead connected to ? Was it not connected to anything ? What type of meter are you using - is it battery or AC powered ?

Touching a meter lead shouldn't cause something to smoke unless you were on the current or resistance scale by accident.

[old_coot88]

'Scuse the dumb question, but is there a healthy orange glow to the 6AB4 cathode?

And another very dumb question. Is there any chance the 6AB4 and 6J6 coulda got interchanged; that is, the 6AB4 in the 6J6 socket?

[Gregb]

Originally Posted by old_coot88

'Scuse the dumb question, but is there a healthy orange glow to the 6AB4 cathode?

And another very dumb question. Is there any chance the 6AB4 and 6J6 coulda got interchanged; that is, the 6AB4 in the 6J6 socket?

Not dumb questions, spent endless hours on a Philco radio with the ocs and the 1st rf tube swapped.

Gregb

[bigaudioal]

Originally Posted by bandersen

Yes, it should read nearly 0 ohms. Replacemenets are available. You could also just short it out with a jumper while testing to eliminate it as a problem. I had a Motorola where several of the filament chokes had corroded and gone open.



I'm not sure I understand when you say you connected the ground lead and saw 4 VAC. What was the other lead connected to ? Was it not connected to anything ? What type of meter are you using - is it battery or AC powered ?

Touching a meter lead shouldn't cause something to smoke unless you were on the current or resistance scale by accident.

Battery powered multi-meter. Set it to measure VAC on the 20 setting. Connected the ground lead of the meter to chassis and with the other lead still in my hand, not touching anything, meter was reading 4 VAC. Then touched it to pin 3 and that's when normal static in audio faded out and then saw smoke. Tried it twice with same result just to be sure.

[bigaudioal]

Originally Posted by old_coot88

'Scuse the dumb question, but is there a healthy orange glow to the 6AB4 cathode?

And another very dumb question. Is there any chance the 6AB4 and 6J6 coulda got interchanged; that is, the 6AB4 in the 6J6 socket?

I thought of this and doubled checked to make sure the 6J6 and 6AB4 were in the correct tube sockets. According to the SAMS and original DuMont paperwork they appear to be in the correct sockets.

[Electronic M]

Originally Posted by bigaudioal

Battery powered multi-meter. Set it to measure VAC on the 20 setting. Connected the ground lead of the meter to chassis and with the other lead still in my hand, not touching anything, meter was reading 4 VAC. Then touched it to pin 3 and that's when normal static in audio faded out and then saw smoke. Tried it twice with same result just to be sure.

Your meter, does it measure current and voltage, and have 3 terminals to attach test leads to? It should not be possible for a meter to cause smoke...Unless you had the 2 leads plugged into the high current measurement terminals of the meter (those terminals are shorted together with a short length of thick current sense wire).

Always measure voltage with the leads plugged into the voltage terminals on the meter....I made the mistake of measuring line voltage with the leads in the meter's high current measurement socket once and fried my meter!