1948 Bakelite Admiral Question
 

Hi Everyone, about seven years ago I recapped a 1948 Admiral Bakelite television, the model with the 10BP4 picture tube. During my recapping process I found a rebuilt 10BP4 picture tube (which had been never used since re-building) and the set ran beautifully with an amazingly bright and clear picture.

I used the set a few times a year and its always worked perfectly. I powered the set up the other day and it was working great for a few minutes, I left the room for a minute and came back and the screen was dark, but sound was fine.

I pulled the chassis and measured the high voltage at the picture tube anode connector cap, its reading between 10,000 and 11,000 volts. I also measured the voltages coming off of the cap that connects to the neck of the picture tube and they were all close to what the Sams Photofact says they should be. What is odd is when I turn the set off I get a mildly bright dot or two near the center of the screen for about 60 seconds. This makes me think that maybe its not the picture tube that has gone bad.

I'm curious how you guys think I might troubleshoot this issue? What should I be checking?

Thanks in advance everyone!

If it has an ion trap magnet, have you tried adjusting that? It's on the neck of the picture tube and it can be rotated or slid forward and back.

You can't hurt the TV by experimenting. Note the initial position of the magnet (in case it's already in the right spot). Move it around until you find a "sweet spot" where the screen is brightest and you don't have any shadows on the edges.

There may be other causes of a dark screen, but that's an easy & obvious thing to check.

Phil Nelson

I usualy get the dot with the brightness turned down.. :D

If you're getting a dot then the picture tube is most likely good and the ion-trap magnet position is OK. It seems to me that you might not be getting a video signal through to the base cap. Can you get a scope on it ? Might be as simple as a bad video IF, detector or output tube.

Also possible that the bias voltages that the tube is receiving at the socket are causing the electron gun to be biased to a strong cutoff. I'd do a voltage check there, and if you can't get the readings close with the brightness and contrast controls then start checking the components that connect to the miss biased element.

It is not your picture tube, as CRT's don't typically fail like that and it is not your ion trap (if so equipped) unless someone moved it. Try cleaning all of the frontal controls such as the brightness, contrast etc. You may also check for dirty tube sockets in the video circuit. I have an early Philco that did the same thing that was a dirty tube socket and an early Sparton that had a dirty brightness control..

admiral
 

Maybe cap under the 1B3 shorted

I had a similar problem with my Admiral, think I solved it by cleaning the CRT pins and the CRT tube socket. I say "think" as the problem didn't come back so far...

In addition to cleaning the CRT pins and socket, you might want to try re-soldering the CRT pins.
But be prepared for the worst...I had a similar problem in a 14'" DuMont portable that was caused by a bad weld to the cathode sleeve inside the CRT. The picture was very dim and smeared, in fact that I did not see it at all until I darkened the room. I tapped on the neck of the tube, and the image appeared at full brightness, but was quite intermittent. I tried to "spark weld" the connection, but it did not fix the problem. :(

jr

Thanks everyone for your suggestions, I will definately try cleaning the CRT tube pins and socket.

It's for sure not the Ion Trap, that was the first thing I tried.

I was under the impression that if something in the video circuit signal went out I would still see a white raster without any picture, but I guess that isn't the case.

I'll keep you guys posted to what I find. By the way how does one clean the front controls? I assume that they shouldn't be opened in any way.

If the video goes out such that the signal to the CRT is down in the black or lower voltage region, your screen would be dark. Although, I would think you could still see a raster by cranking up the brightness and decreasing the contrast.

If your set is like my Admirals, there's a gap between the metal control housing and the phenolic wafer the pins are mounted on. You can squirt a little DeOxIt up in there and rotate the control to clean the contacts.

Also hit the terminals of the control with a tooth brush to remove any tin whiscars that might have grown.

You might try subbing a different ion trap. Just because you get a dot when you shut the set off doesn't mean the ion trap is working properly. When the set is shut off, anode voltage may be down quite a bit, and the function of an ion trap is affected by the acceleration voltage of the electron gun. A field that will pass electrons at low anode voltage may not do the same at proper operating anode voltage.

Otherwise, try shorting cathode and grid together at the CRT socket. If you have proper screen voltage and anode voltage, and a working ion trap, this should give you a full brightness raster. If you don't get it, it would suggest something came disconnected in the CRT socket or gun. It's not unheard of to develop an open circuit in the electron gun.

Thanks everyone, Tom I like your test of "shorting cathode and grid together at the CRT socket". I just want to be sure I understand how to do this without creating sparks and explosions lol.

Here is an image of the schematic, is the cathode Pin 11 and if so which pin is the grid pin 2 or 10 or both 2 and 10?

If I understand you correctly I should put the cap on the picture tube, but not all the way down. Then take a jumper cable and short pin 11 to pin 2 or 10?

Thanks again for you help.
http://www.automaticwasher.org/TEMP/Admiral.jpg

He means jumper pin 2 (control grid) to pin 11 (cathode).

My guess is bad video amplifier tube, I have an RCA set that started out having no picture till I replaced that tube. Sounds to me like the CRT is being cut off, read voltages on all the CRT pins and report back.

I had a Zenith set that the coupling cap from the video output to the crt was leaky and caused a black screen. Took me a while to figure it out.
Terry

A 6AU6 for a video amp? I didn't think that little tube could be up to that challenge. You may want to change the tube and see what happens. 6AU6 tubes are plentiful.

Hi Robert, My first guess would be the video output tube.
Granted.. in a set this old, it could be anything...but...that is the most common/likely problem.
I'd try that before I tore into anything.

Yes, that's right. Another way is to put a fine piece of wire into the holes of the socket from pin 2 to pin 11. When you push it on, the wire shorts those two pins. But be careful not to short to either 1 or 12. Your idea of partially inserting the socket should also work just fine, and may be easier.

You could also place the jumper from the brightness control wiper to the jct. of R36/R37 (that's assuming wiring continuity through the CRT socket to the pins is intact).

Well guys, shorting pin 2 to pin 11 brings full bright raster to the screen. When I remove the jumper wire, the screen goes completely dark again. I'll record the voltage values at the socket and report back tomorrow. Thanks again everyone, I sure do appreciate the help!

These are voltages you want to check in particular:

CRT grid, pin 2 (should vary with brt. control setting from zero to some positive value).

CRT cathode, pin 11

V7 (6AU6) pins 5, 6, and 7

BTW, the 6AU6's heater is lit, right?

An old friend and video expert suggested using a 6AH6 in place of that 6AU6 in my Bakelite Admiral, Seems to work just fine,

Heck why not?:D And even dinkier tube, the 6C4 (a triode) was used as the oscillator of the RF high voltage supply in some of the old electrostatic-deflection sets. It had the job of lighting the screen itself.

My entry-level model RCA 17" table model uses a 6C4 as a video output. A 6C4 is one half of a 12AU7.

Hi everyone, sorry for my delay in getting the voltages out, we've been particularly busy lately, ugh!

So as for the socket:
Pin 2 +0.6
Pin 10 437 (sams says should be 400)
Pin 11 116 (sams says should be 115)

I did replace all the tubes originally, but just to check I replaced the 6AU6 and still no raster, so that's not the issue. All heaters on all tubes are lit.

As for the voltages on the 6AU6 / V7:
Pin 5: 200volts (sams says should be 120)
Pin 6: 180volts (sams says should be 180)
Pin 7: 0.80 (sams says should be 28)

So it seems to me that something around pin 7 is bad, I wonder if the contrast control went out?

How about the focus wirewound control ? It was burned out in two Admiral 20X11/12 sets I worked on recently.

That's the control grid (G1). It should rise to around 100-115V as you advance brightness to maximum. Does it?
In other words, its bias relationship to the cathode (pin 11) should be at or near 0V at max brightness.

What is the voltage directly at the brt.control wiper as you turn the control stop-to-stop? What is the voltage at the top (hot) terminal lug of the control? Just for the heck of it, check for continuity in the wire going from the wiper to pin 2 (just on the chance there might be a break in that wire).

Thanks again everyone here is what I found...

on the far end of the 100K resister (R35) it reads 330 volts, after it crosses R35 the voltage drops to 0.69 volts which is the input to the Brightness Control. The wiper of the brightness control seems to vary the voltage from stop-to-stop at 0.04 volts at min to 0.67 volts with the dial turned to max and this is what is output to Pin 2 of the picture tube cap. The continuity of the wire is just fine, but always good to check! Old_Coot88 are you saying with the brightness turned up Pin 2 the output of the wiper should be 100 to 115volts? If so 0.69 volts is obviously way off.

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.

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.

Yes indeed. At the impedances involved, that would functionally be a dead short.
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.
Very probably it's fixable. But you may have to physically remove the control and disassemble it to find the short.

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...

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.

http://www.automaticwasher.org/TEMP/admiral_control.jpg

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.

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.

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.

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!:thmbsp:

http://www.automaticwasher.org/TEMP/...trol_fixed.JPG

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