DuMont RA-108 Mansfield inductuner - Videokarma.org TV - Video

bigaudioal · #1 01-03-2016, 10:21 PM

Hi everyone. Know I have not been on here in a while, took a break from the TV hobby. Working on my RA-108 finally. Got a raster finally. 3rd flyback was the charm.

However I have a lack of video signal and only one FM radio station coming in on audio. FM station is 91.9 FM and it comes in over a range of the dial. Same condition exists on the radio mode and the tv mode. Wondering if anyone who is familiar with DuMont inductuners has any ideas or guidance on how to best troubleshoot this condition. I know nothing about these inductuners. See below video which shows condition.

Thx.

http://youtu.be/cmLvkEaVhoE

old_coot88 · #2 01-04-2016, 12:10 AM

The LO (local oscillator) in the tuner is not running.

bigaudioal · #3 01-04-2016, 07:01 AM

My next step was going to be to inject a signal into the grid of the 1st video tube to check everything out from there. But I was leaning towards the problem being the tuner. The LO not running makes sense with the condition I have. Thx.

What might cause this? I have not inspected this tuner b/c it is difficult to access and must be removed to even inspect it. Guessing there are caps and resistors inside that if are bad might lead to the LO not running?

Dubis7 · #4 01-04-2016, 10:55 AM

Generally the resistors and caps in tuners don't go bad, though your results may vary. Replacing anything electronic in the tuner will wreck the alignment, and aligning a tuner is tricky from what I understand.

Could it be a mechanical issue? Something gummed up with old lubricant and not moving correctly?

old_coot88 · #5 01-04-2016, 11:01 AM

Have you tested/subbed the osc. tube? IIRC, these used a dedicated osc. tube rather than a combined mixer/osc. tube.

bigaudioal · #6 01-04-2016, 11:43 AM

I did open the top of the inductuner, which exposed the shaft. I lubricated the shaft at its ends and lubricated all the gears on the front of tuner. when the knob is turned, the gears and shafts rotate freely. I do not see any obvious mechanical issues with the tuner from the topside.

The tuner has three tubes. 6J6 - RF Amp., 6AK5 - Mixer, and 6AB4 - Osc. All tubes have been tested, swapped with no change. Next steps?

Gregb · #7 01-04-2016, 12:25 PM

Do you have a 7 pin tube test socket? If so I would install that under the osc tube and check some voltages.

Gregb

bigaudioal · #8 01-04-2016, 04:48 PM

I do have one of those test sockets. I will check the voltages and report back. Thanks.

bigaudioal · #9 01-04-2016, 09:28 PM

Ok gents. Here's what I found. Tested the voltages on the tuner osc. tube (6AB4)

Pin 1 (plate) 140 vdc
Should be 190 vdc

Pin 2 (NC) 0 vdc
Should be 0 vdc

Pin 3 (heater) - unable to get good reading
Should be 6.3 vac.

When I went to test pin 3 I moved my multi-meter to VAC. I noticed that with the ground to chassis connected only I was reading around 4 vac. Then touching the lead to pin 3 to try and get a reading I got smoke from under the chassis and the static in the volume faded at same time. So stopped. Something not right.

Pin 4 (heater) 0 vdc
Should be 0 vdc

Pin 5 (NC) 0 vdc
Should be 0 vdc

Pin 6 (grid) -1.5 vdc
Should be 0 vdc

Pin 7 (cathode) 0 vdc
Should be 0 vdc

I am all ears for suggestions. Thanks.

bigaudioal · #10 01-04-2016, 09:32 PM

Schematic for the osc tube in the tuner.

Gregb · #11 01-05-2016, 12:44 PM

Did you get that correct, the plate voltage is -140v? There is no way that tube will work with a negative voltage on the plate. Have to try and figure out why it is negative.

Gregb

bandersen · #12 01-05-2016, 02:01 PM

Well for starters you need to have around 6.3 VAC on pin 3 or the tubes not going to work properly. Pin #3 is going through a choke (L12) to the other tubes in the set. So I'd check that choke and also measure AC voltage in the heaters of other tubes in the set.

If they're all low, could be a problem with the power transformer. Or possibly something is shorted somewhere and really dragging down the filament supply.

bigaudioal · #13 01-05-2016, 02:48 PM

Quote:

Originally Posted by Gregb

Did you get that correct, the plate voltage is -140v? There is no way that tube will work with a negative voltage on the plate. Have to try and figure out why it is negative.

Gregb

Sorry, should not have used a "-" before listing voltages. That is 140 vdc. Not negative.

vts1134 · #14 01-05-2016, 03:07 PM

Did you check R23 (the grid leak resistor)?

bigaudioal · #15 01-05-2016, 03:07 PM

Quote:

Originally Posted by bandersen

Well for starters you need to have around 6.3 VAC on pin 3 or the tubes not going to work properly. Pin #3 is going through a choke (L12) to the other tubes in the set. So I'd check that choke and also measure AC voltage in the heaters of other tubes in the set.

If they're all low, could be a problem with the power transformer. Or possibly something is shorted somewhere and really dragging down the filament supply.

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?

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.

The set has been on for periods of half an hour with a good raster and good volume. Just get zero TV signal across the dial and only get one radio station 91.9 fm across a good portion of the dial.