Hi everyone,

I am repairing a GE 'Stereophonic' TV/radio/turntable unit. The TV currently has no EHT HV. Grid drive into the output tube is correctly biased within a few volts and the waveform is as per the schematic.

There is only 7 volts on the plate and the HV diode filament is not lit. Plate waveform shows 30kHz ringing from the 15kHz drive pulses, at around 20vpp.

I have looked thoroughly at the schematic but I cannot find the B+ source to the primary. The only supply is fed via a 33nf and 47nf paper cap.

Coming from a solid state background I'm familiar with a direct B+ to the primary, often via the horisontal def coils. I understand that the high impedance of tube electronics is different but I still can't see how you can feed a single ended flyback transformer with only a cap.

I've got new caps on the way which may restore the HV but I'm curious as to the topology / design of the flyback stage.

Can provide schematics if there is any interest.

Thanks guys!

Ben

The 6AL3 damper diode would be suspect.
Have you got decent voltage on the plate.
We used to just momentarily short out the plate to deck with a long screw driver, should produce a nice blue arc.
JJ

Wow you're good! I had the set powered up with the lights off and saw a nice blue glow from the damper diode. Swapped it out and powered it up to hear the flyback running with some eht crackle.

The image was about 3" thick vertically so I swapped out the 6BM8 vertical driver and got a raster almost the full screen minus about 3 inches down the bottom. The raster was distorted (stretched) towards the top so I'll measure grid bias and drive on that tube next. A quick look showed only ceramics to the previous stage but might be worth swapping them out anyway.

No snow either so I'll need to build some RF oscillators for alignment and keep working back.

Thanks for the reply Johnny. I don't suppose you know where I could find a service manual for a TE108F chassis?

The cathode bypass electrolytic capacitor on the vertical output valve is probably O/C.
Which will cause bottom of picture to be cramped up.
No idea on a manual, just the circuit should be enough.
JJ

I have a GE TD1 presently in a state of restoration. This uses a TE108F chassis.

These sets were originally Crosley-Kirby VB24 "Concerto" when released in 1961 and were later rebadged General Electric in 1962.

There is some service data for the TE series chassis that I'll post here.

General Electric TE107F, TE108F & RA1 Service Data

Doesn't look to be any bypass caps or resistors on either half of the output tube. The cathode goes direct to ground.

I'll swap out the grid caps and double check the resistors are approx the correct value and go from there.

Was hoping for some alignment info. Not even too sure on the IF freqs.

If you could find that service manual for the original set that would be great!

Thanks again for your help!

Ben

I have not got a circuit in front of me but I doubt the vertical output cathode goes directly to deck.
From memory pin 2 on the 6BM8, and usually something like a 330/470 ohm resistor with something like a100 mfd across it.
JJ

http://m.imgur.com/3y0lZH9

Image link above. If I'm not supposed to link images I'll remove it.

It is important to note that its not just filter electrolytic caps that deteriorate & dry out in power supplies. It's old electrolytic caps anywhere and dried out ones and those that loose "form" (polarity) & present as a short, can and will cause voltage and coupling problems.

Hi Marcc,

There are surprisingly few electrolytics in this set. The power supply stage are all fine with the handfull of others getting replaced today. I'll stop by jaycar and see what they have.

I'll try get some grid readings on that 6BM8 after work today.

Does anyone have a suggestion of an RF oscillator/transmitter to broadcast video? Or should I feed composite right after the final IF detector?

Ben

Ben,

Linking to images is fine. The only rule is that any linked material needs to be relevant to the topic and be able to be considered G rated.

I'm at work and don't have the schematic with me but many GEs of that vintage used to derive bias for the vertical output stage from the negative voltage on the 6CM5 grid, suitably bypassed. That would explain why the 6BM8 pentode cathode goes to deck.

Sounds like either resistors going high or leaky paper coupling cap to grid of 6CM5.

OK, found a schematic and now have egg on my face.
The cathode in the vertical output does indeed go to deck.
The bias is obtained via a 560K resistor from the horizontal output grid circuit.(6CM5).
Check the 560K resistor, it should be located on one end of the linearity pot.
Also check/replace off pin 3 6BM8(grid) an 0.01 and an 0.047 cap.
Should be approximately 15 volts negative on pin 3.
My only excuse is that they did not sell many GE's in Tasmania.
JJ

File uploaded to Post 5.

If you've got a raster with no snow, a common cause was a faulty video detector diode. Often found in an IF can with a removable top. Hope this helps.