Pilot tv brightness problem
 

What would cause the loss of brightness in the pilot tv the hv is up ok but the brightness is 90% gone and the crt is fine.

Hi Timmy,

Main question:
Do you have both schematics (Sams and Riders)?
I should not start any work without them!

Re-cap-job done?

There is some safety work to safe the life of the CRT to do, too.

I have collected many reports about this little Pilot TV.

I like to send you this literature by email if wanted.:yes:
To anybody else who likes that stuff (print-friendly), too.......

It will give you a perfect overview about issues and how to fix them!

Regards,
TV-collector:stupid:

Hi Timmy,

If you dig into the material I just sent to you, you will be traced to that video-diode 1N34!
This will be your first job to check.:yes:

We have to realize, that because of aging these old diodes becoming more and more the status of troublemakers, like the old ugly caps and resistors, which are often out of tolerance.:thumbsdn:

Good luck and happy weekend,
TV-collector:stupid:

Picture is ok hv is good all tubes check ok b+ is also good. If the 1n34 was bad I would think the picture quality would suffer not the brightness. The brightness problem came out of nowhere. There is no mention in the rider’s about the 1n34 and the loss of brightness only the loss of picture and probably sound.

Sure would luv to see the circuitry by which brightness would be affected by the diode.

:saywhat::headscrat

What is the heater voltage on the CRT?

jr

low!
If i recall, he is using some odd rare tube with a very low heater v.

Yes I’m using a d100 crt but it’s not that I unplugged that crt and plugged the 3kp1 in and same thing heater voltage is good.

Which CRT was in the set when you first noticed the brightness drop?

jr

D5-100 crt

It’s odd pin 1 of v9 if I use the meter on resistance from ground to pin 1 I get full brightness and when I take it off the brightness slowly fades back to dim almost like depleting a cap. No bad resistors that I found yet.

So what is V9? Is it the CRT or something else? Any chance of posting a (hopefully legible) print of the video and CRT region?

V9 is dc restore and sync tube

Sams here:

http://www.earlytelevision.org/pdf/p...sams_62-16.pdf

jr

Yes that’s the one and I also checked the 1n34 for laughs and it’s good. And also checked all the power supply electrolytics all good. I’m lost at this point. The resistors at the bright pot are spot on.

Hey Tim, just to cut to the chase, what are the voltages (measuring from ground) to the CRT cathode (pin 3) and G1 (pin 2)?

Since the brt. control varys the cathode voltage, what is the voltage swing lock-to lock?

Hope you're running the set on an isolation xfmr since it's a hot chassis.

Should I swing the brightness control up and down while checking these voltages or just check the voltages. And no don’t have an isolation xfmr. Lock to lock ?

Yeah, with the meter connected, just turn the control lock to lock and jot down the reading at both ends. The range can then be compared to the G1 (control grid) voltage. What you're in effect doing is varying G1 with respect to cathode, and this varys beam current (brightness) in the CRT

BTW, your meter is a digital 10 meg impedance (not an old analog VOM), right?

No old analog meter I’ll check those voltages.

Using 3kp1 crt pin 3 -50v brightness up and 67v brightness down and pin 2 -120v brightness up and no change in voltage brightness down. No antenna used.

Hm.. lessee. G1 fixed at 120V. Cathode (pin 3) has negative 50V, positive 67V. That's a total voltage swing of 117V. That seems like a gawdawfully large swing. Are you sure the neg50V is correct?

Yes the voltages are correct. What should the -50v be ?

Is -39v better or more normal?

Well, I'm gonna hafta bail on this one and leave it to the experts.:no: With 106V positive on the grid relative to cathode, and still be showing an in-focus (albeit dim) raster is beyond credulity. Maybe a quirk of electrostatic jugs? I may learn sumpthln new here.:deal:

Can you check some more to see if one of the connections is maybe open? I'm thinking possibly an electrode floating freely to an unusual voltage.

The voltage chart in the Sams gives this:

Measure from pin 5 of V19 (35W4) which is raw negative 135V to:

CRT pin 2 (grid)..13V
pin 3 (cathode)...185V

Timmy's pin 2 from ground... 120V
Pin 3 from ground varies from negative 50V to +67V w/brt. control setting

So... calling all xperts:bigok:

The DC restorer is not a restorer per se, but it is supposed to develop a varying DC on the grid based on sync tip to average ratio that tracks the same ratio that occurs naturally in the AC coupled cathode drive.

As such, it's possible that it will not work normally if you don't have a signal with picture coming in.

Can you retake the voltage readings with a normal signal please?

You missed the minus sign

Pin 2, G1 is -120, pin 3, K is -50 to +67

So G1 to K is -70 to -187, Sams says -172, so that's within range of the SAMS.

I think the next thing I would do is check the resistance chart readings all around the CRT and V9. Also check C41, C42, C5 for leakage.

Thanks. Geriatric brain fart.:bash::yuck::o

These voltages don’t seem correct... according to the 3kp4 data sheet, typical grid cut off is at most -90 volts not -172 :scratch2:

https://frank.pocnet.net/sheets/168/3/3KP1.pdf

jr

Thanks, 172 seemed high, but I had no knowledge of the tube spec, so kept my mouth shut.
Still need to hear the results of resistance readings and voltage readings with a good signal to try to figure this out.