1948 Bakelite Admiral Question

[old_coot88]

Try disconnecting R35 from the top lug of the brt.control and see if it's open or gone 'waaay high in value. If R35 is OK, measure from the top of the brt.control to ground and see if it's shorted or very low resistance. Both R35 and the control should read 100K according to the schematic.

[Unimatic1140]

OK guys I suspect the brightness control is the culprit. So I disconnected R35 and it read nearly a perfect 100K ohms. Then I measured from the input to the brightness control to the output of the wiper and it ranges from 25 ohms to 108K ohms from stop to stop. BUT when I measure from the input of the control to the chassis ground it shows only 135 ohms. Am I correct in assuming 135 ohms to ground is a problem?

If so is it safe for me to experiment by hooking up different resister ranging from 100 ohms to 50k between the output of R35 and the input of Pin 2 just to be sure this really is the problem, bypassing the brightness control? Also the brightness control is a part of a combination, brightness/contrast control is this going to be next to impossible to find?

Thanks again everyone for such great assistance.

[old_coot88]

Originally Posted by Unimatic1140

OK guys I suspect the brightness control is the culprit. So I disconnected R35 and it read nearly a perfect 100K ohms. Then I measured from the input to the brightness control to the output of the wiper and it ranges from 25 ohms to 108K ohms from stop to stop. BUT when I measure from the input of the control to the chassis ground it shows only 135 ohms. Am I correct in assuming 135 ohms to ground is a problem?.

Yes indeed. At the impedances involved, that would functionally be a dead short.

If so is it safe for me to experiment by hooking up different resister ranging from 100 ohms to 50k between the output of R35 and the input of Pin 2 just to be sure this really is the problem, bypassing the brightness control?.

What i would do first is this- disconnect the CRT grid (pin2) wire from the brt.control, and connect a 47K resistor to ground (in place of the brt.control element). Then connect R35 and the CRT grid wire to the junction. Then fire the set up. You probably won't get a raster at first. So replace the 47K with a 56K. If still no raster, keep bumping up the value till you get a raster. Optimize the ion trap adjustment and select the final resistor value for the brightness level you want. This'll serve temporarily till you figger out what's shorting out the control.

Also the brightness control is a part of a combination, brightness/contrast control is this going to be next to impossible to find?

Very probably it's fixable. But you may have to physically remove the control and disassemble it to find the short.

[kvflyer]

I skimmed the posts in this thread and didn't see if you had tried to clean the outside of the control. If not, you might want to take the control out if possible and thoroughly clean it with at tooth brush and some cleaner. I have an Admiral 26R12 and the height suddenly dropped and the control had little or no control over the voltage. It appeared that the control had developed "tin whiskers" on the phenolic material shorting the lugs. May be a long shot...

[Unimatic1140]

Thanks guys, here is an image of the control, the brightness control is on the top against the chassis, the bottom control is the contrast control. You can see where I've disconnected R35 from the brightness control. I assume that once I take out the control the I should be able to somehow separate the two controls. I also assume these controls are delicate and taking them apart could make matters worse. Hopefully I'll be able to fix it, I will keep you all posted.

[kvflyer]

You may want to test the control after you get it out. Look for short circuits between the lugs and short circuits from a lug to the metal outer cover. You can do that without trying to take the control apart. Taking a control apart is a little tricky and if you find a "tin whiskers", you may be able to clean it externally. I would certainly try that first.

[old_coot88]

If you have any of those itty bitty dental thingys that look like a micro-size bottle brush (for cleaning between teeth), it might be a good tool to try before removing/dismantling the control. Just a thought.

[Tom Albrecht]

Actually potentiometers can be disassembled and reassembled quite easily. They are not particularly delicate and you will be successful as long as you are reasonably mechanically inclined.

In general, replacement potentiometers are hard to find, so it's quite worthwhile to develop the skill to mix and match components from various potentiometers to build replacements when needed.

In this particular case, the first thing I would try (same as kvflyer recommends above) is cleaning with a toothbrush between the control and the chassis, without even removing the control. The failure I see most often is growth of whiskers between the chassis and the terminals of the control. If that doesn't work, remove the control and disassemble to clean.

[Unimatic1140]

Just wanted to say thanks to everyone here for some great assistance. I removed the brightness control, gave it a good cleaning without having to take it apart, put it back together and the set works absolutely beautifully again!

[old_coot88]

Say hey! Good job dude. Got my all-time favorite show on too.
http://www.youtube.com/watch?v=kkRkG...eature=related

[Tom Albrecht]

Nice work. That's a good looking picture!

[vts1134]

Great work
I've been following this thread from the beginning with great anticipation. Every time Unimatic1140 would post something I'd try to work through the problem and potential solution to see if my idea would match the suggestions of the vastly more knowledgeable posters who were sure to follow. This was another great mystery solved by the VK Imperial Council.
Enjoy the nice new picture on that set.