View Full Version : FINALLY! DuMont RA-108 Mansfield lives


bigaudioal
08-25-2016, 12:02 PM
After months of frustration and not being able to track down why I could not get a signal through on my RA-108, I did some intense reading about the inductuners. Thanks to those folks who have posted online about servicing these tuners, I discovered that the rear slider was misaligned. Only thing I could think of was it came out and someone in the past just shoved it back underneath and it was in the wrong spot. Instead of being at the extreme rear of the tuning barrel it comes in contact with when the tuner is bottomed out, it was in the center of the barrel and not sliding at all when the tuner was turned. Lucky it did not break. So I gently popped it out and re-aligned it where it should be and viola!!!

Set looks ok for its first image. Have to go through a proper setup now, but cannot wait for my Mom to see this set working, as it was the first model TV she remembers watching as a little girl.

vts1134
08-25-2016, 01:11 PM
You're on a roll lately! That set looks great...you've got to love those Dumont sets.

Radiotronman
08-25-2016, 03:36 PM
Cool looking set, great job on the restoration!

hi_volt
08-26-2016, 08:45 AM
Very nice. Great picture!

DavGoodlin
08-26-2016, 11:18 AM
Great work Al, that is the Packard of TV sets there.
Ask anyone who owns one.

reichsrundfu
08-26-2016, 04:40 PM
Great work Al, that is the Packard of TV sets there.
Ask anyone who owns one.

Absolutely! I LOVE my RA-103 Chatham!

George

Phil Nelson
08-26-2016, 08:43 PM
Looking good! And congratulations on finding that obscure problem.

Regards,

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

bigaudioal
08-30-2016, 02:59 PM
Set has a few issues that I need to work on. Was totally recapped and all resistors tested and replaced if more than 20% out of tolerance.

Width will not fill out fully. Gets very close.

There is a white, squiggly vertical line in the picture on the left center. Cannot get it to go away. Tried all the horizontal controls (which move it some) and swapped tubes in vertical, damper and Horiz.

Vertical linearity is not great. Top squashed. Height and vert lin pots are maxed out. I notice a slight clicking sound which seems to be associated with vertical pot adjustments. Could that be the vertical output transformer? Might it have an issue? Clicking seems to get more rapid as I try and fill out the height using the vert. height pot in conjunction with vert. linearity pot.

Top left corner of pic seems to pull to left slightly as well.

AGC pot does not seem to do anything. Wondering if some of these issues might be related to a problem in the AGC circuit? Those tubes are lit and I swapped them as well. 6AL5 and 6AU6 IIRC.

So I will keep plugging away. Anyone have any guidance for me? Thx.

kvflyer
08-30-2016, 03:18 PM
Make sure you don't have "tin whiskers" on any of the pots. I had similar issues with an Admiral set and aggressive cleaning of the exterior of the controls fixed the linearly issue. That's the easy thing to do. Still may be an issue with the yoke or flyback. Let's see if it is an easy fix.

bigaudioal
08-30-2016, 03:25 PM
Make sure you don't have "tin whiskers" on any of the pots. I had similar issues with an Admiral set and aggressive cleaning of the exterior of the controls fixed the linearly issue. That's the easy thing to do. Still may be an issue with the yoke or flyback. Let's see if it is an easy fix.

Will try cleaning pots again. Original fly was shot. On the advice of Nick Williams I pulled the flyback out of a spare 630 clone set I had and threw it in. Putting out plenty of HV for the set and I can adjust it with Horiz drive control from 9kV to upwards of 14kV. I have it set on about 10.5kV, the mid range. Stays cool and raster is stable overall. Now the yoke might be a different story. Have not tested or messed with it at all.

earlyfilm
08-30-2016, 05:52 PM
Big Al,

That picture is starting to look like a Du Mont !!!

On your Barkhausen, check the antenna dress between the tuner and the plug on the back of the set. I cannot remember which RA's had this very unusual cause for Barkhausen, so check your service bulletins on the ETF website. It had to do with how the lead was grounded and where they were clipped.

On insufficient width, resolve the vertical linearity problem first.
The reason:
Vertical linearity being off can cause the vertical output tube to conduct too much and that drops the B Boost and drops your width.

On any pre-about 1952 set that uses 6AL5 tubes:
If the set uses a center terminal post for those sockets, test each 6AL5 by removing any shield and with your finger on the tip of the tube, attempt to gently "bend" the tube moving your finger in a 1/2 inch circle for this test to see if the tube is seated properly in the socket.
The reason:
Early 6AL5 tubes had a recessed area in the center between the pins. Most manufactures switched to the standard flat pin base for their 6AL5's about 1952. If this problem is found, try removing the center pin tip extending from the top of the tube socket with wire cutters or a small grinder.

James

bigaudioal
08-30-2016, 07:22 PM
Big Al,

That picture is starting to look like a Du Mont !!!

On your Barkhausen, check the antenna dress between the tuner and the plug on the back of the set. I cannot remember which RA's had this very unusual cause for Barkhausen, so check your service bulletins on the ETF website. It had to do with how the lead was grounded and where they were clipped.

On insufficient width, resolve the vertical linearity problem first.
The reason:
Vertical linearity being off can cause the vertical output tube to conduct too much and that drops the B Boost and drops your width.

On any pre-about 1952 set that uses 6AL5 tubes:
If the set uses a center terminal post for those sockets, test each 6AL5 by removing any shield and with your finger on the tip of the tube, attempt to gently "bend" the tube moving your finger in a 1/2 inch circle for this test to see if the tube is seated properly in the socket.
The reason:
Early 6AL5 tubes had a recessed area in the center between the pins. Most manufactures switched to the standard flat pin base for their 6AL5's about 1952. If this problem is found, try removing the center pin tip extending from the top of the tube socket with wire cutters or a small grinder.

James

Thanks so much for the tips!!! Will try some of these over the next few days. Really appreciate it!!!