DuMont Inductuners

[bigaudioal]

Anyone know anything about the DuMont Inductuners? I recapped my RA-108 Mansfield a while back. Got a raster and it is very stable, but was not getting any signal. Set has a round dial inductuner, which I know was used in the 108 obviously and possible the RA-105 as well. It is a different tuner than the one used in the RA-103 (Doghouse) model. When the mode knob is in TV mode, I was getting no TV signal, and was getting one FM radio station over 60% of the dial.

I then decided to try inserting a signal into the grid of the first IF tube. When I did that using a VCR I got a good video signal on the raster. So figuring my issue must be with the tuner. Tubes in the tuner have been swapped with no change. Anyone know anything about these tuners or have any ideas how to troubleshoot further?

Thanks.
-Al

[reichsrundfu]

Sounds to me as though the front end mixer isn't working. You still need a local oscillator to oscillate and a mixer to create the intermediate frequency sent forward to the IF stages. Sounds to me as though the front end is not functioning as a superheterodyne. And the result is you're pulling in a local strong FM transmitter, perhaps on RF or maybe a harmonic. Your inductuner likely uses a 6J6 or similar in That capacity. Double check it is a good strong tube with good connectivity in the socket. You may find its a fast easy fix. If not, check the mica and ceramic caps within the inductuner to make shure they are not shorted.......

George

[Eric H]

Have you pulled the cover off the tuner yet?
Depending on which Inductuner it is, it's possible one of the three sliders that ride on the spirals has come off. It's not all that hard to clean up and reinstall.

[bigaudioal]

Thanks for these suggestions. What parts are the sliders you are talking about?

[Phil Nelson]

There are different styles of inductuner. After you open yours and operate the mechanism, it's not hard to see which parts make electrical contact (and should be carefully cleaned) and which moving parts need to be (carefully) lubricated. Here's the style of tuner used in my RA-113:



This article has more info & photos of that tuner, if it's the type you have:

http://antiqueradio.org/DuMontRA-113Television.htm

It's not uncommon for these to be frozen with old, petrified grease. If yours is hard to turn, be gentle until you have freed that up. The older type has a ceramic shaft that may break if forced.

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

[bigaudioal]

Originally Posted by Phil Nelson

There are different styles of inductuner. After you open yours and operate the mechanism, it's not hard to see which parts make electrical contact (and should be carefully cleaned) and which moving parts need to be (carefully) lubricated. Here's the style of tuner used in my RA-113:



This article has more info & photos of that tuner, if it's the type you have:

http://antiqueradio.org/DuMontRA-113Television.htm

It's not uncommon for these to be frozen with old, petrified grease. If yours is hard to turn, be gentle until you have freed that up. The older type has a ceramic shaft that may break if forced.

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

Mine looks like the one above that Eric H. posted a pic of. It is free as a bird. Was never frozen. I cleaned it up already and oiled the external gearing and such. Moves very smoothly.

[Electronic M]

Originally Posted by bigaudioal

Thanks for these suggestions. What parts are the sliders you are talking about?

They are little copper pieces that connect between the metal on the white ceramic and the coil wound on the brown phenolic. Their arms grab the coil and as the coil turns they slide along the ceramic. You may want to bend the arms closer together so they grab better....Also look for vertical motion in the shaft. If the shaft bearings are missing or miss-installed or the back pressure pivot screw for the shaft is too loose excessive vertical (or perhaps horizontal) slop may be making the sliders hop out. One coil is reverse wound so it's slider will move opposite the others. You will want to clean the coils and all metal surfaces with Goof-off there was some nasty dirty greasy residue on the tuner in my RA-103 that nothing else was getting (and I could not see it till it came off).

EDIT: Another thing you may want to do is sawp in as many different 6J6 tubes for the osc. as you can. Tube testers only tell you the emission/mutual conductance, and NOT how well it will oscillate in a given osc. circuit....A strong tube may suck at oscillating, but a weak one may do it like a champ.

[old_coot88]

Have you checked for plate voltage on the osc. tube?

[bigaudioal]

Originally Posted by electronic m

they are little copper pieces that connect between the metal on the white ceramic and the coil wound on the brown phenolic. Their arms grab the coil and as the coil turns they slide along the ceramic. You may want to bend the arms closer together so they grab better....also look for vertical motion in the shaft. If the shaft bearings are missing or miss-installed or the back pressure pivot screw for the shaft is too loose excessive vertical (or perhaps horizontal) slop may be making the sliders hop out. One coil is reverse wound so it's slider will move opposite the others. You will want to clean the coils and all metal surfaces with goof-off there was some nasty dirty greasy residue on the tuner in my ra-103 that nothing else was getting (and i could not see it till it came off).

Edit: Another thing you may want to do is sawp in as many different 6j6 tubes for the osc. As you can. Tube testers only tell you the emission/mutual conductance, and not how well it will oscillate in a given osc. Circuit....a strong tube may suck at oscillating, but a weak one may do it like a champ.

thx!!!!!

[old_coot88]

Just a thought- IIRC, some of those tuners used 6AB4 as the osc. Is there any chance it mighta got swapped for a 6J6 or vice versa?