1948 Bakelite Admiral Question

[miniman82]

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.

[7"estatdef]

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

[kvflyer]

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.

[jstout66]

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.

[Tom Albrecht]

Originally Posted by old_coot88

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

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.

[old_coot88]

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).

[Unimatic1140]

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!

[old_coot88]

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?

[wa2ise]

Originally Posted by kvflyer

A 6AU6 for a video amp?

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

[old_coot88]

Originally Posted by kvflyer

A 6AU6 for a video amp? I didn't think that little tube could be up to that challenge.

Heck why not? 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.

[dieseljeep]

Originally Posted by old_coot88

Heck why not? 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.

[Unimatic1140]

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?

[bandersen]

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

[old_coot88]

Originally Posted by Unimatic1140

So as for the CRT socket:
Pin 2 +0.6

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).

Pin 10 437 (sams says should be 400)
Pin 11 116 (sams says should be 115)

[Unimatic1140]

Originally Posted by old_coot88

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.