Horizontally Segmented Picture
 

My recently resurrected experimental Delco car TV, just developed a new problem. The picture is torn into horizontal segments that don't quite line up correctly. (See picture.) The number of segments continuously changes. This problem just suddenly started. The picture was fine previously. (The picture tube is an old 5AXP4 test CRT for now. The TV is B&W early 50s.)

I'm hoping that someone can identify the problem from these symptoms. I'm not a TV guru.

http://antiqueradios.com/gallery/mai...serialNumber=1

Bob D.

Possibly an open connection in the CRT socket. One of the lugs not making connection with the pin, and a little arc jumping the gap. Just a WAG.

I'd look for loose connections first, as stated. Not only the CRT, but check out the tube sockets in the vertical section. Wiggle tubes, tap gently in various places, and so forth. Also try wiggling vertical hold, height, vertical linearity controls.

This looks to me like a vertical sweep problem, not a CRT one, although a loose and arcing connection almost anywhere could cause it.

The horizontal shift is probably from the collapsing vertical output tube changing the load on the flyback, assuming the vertical output tube uses B-boost from the damper tube.

It is hard to judge vertical linearity from this picture, however, with that said, it looks compressed at the top and stretched at the bottom.

Try substituting both vertical sweep tubes. If you don't have alternate vertical tubes, try tapping them gently and see if this changes the problem or you see internal sparking on the tube.

Visually inspect the vertical circuits for loose connections. If none are found try gently probing them with an insulated rod. Listen for the sound of arcing in the yoke assembly. You also can do this with an insulated tube for the rest of the set.

Note: The vertical output transformer may make a sound that matches this, if the plates are loose. To determine if this is a problem or not, tighten the screws slightly. If this reduces the sound, this is normal. If not, suspect an arcing transformer. (Smaller vertical output transformers don't have screws, just metal crimping.)

Have you replaced the grid coupling condenser between the vertical oscillator and the vertical output. If not, it could be breaking down.

A resistor (cathode, plate, grid bias) in the vertical output circuit could be breaking down. Check for changes in value.

If you are using the yoke that came with the Delco set, do a test with the one that came with the test tube. These are usually general purpose ones.

James

Stupid question, but are the various parts of the image somewhat overlapping each other? If so, the problem may be spurious oscillations in the vertical section or maybe the horizontal pulses are coupled somewhere in an unwanted location in the TV set, rendering the vertical scan so "strange". If this is the case, I'm afraid, it's usually oscilloscope time

Cheers

Giulio

Edit: not exactly the same problem, but the image attached should clarify what I meant in my post

Exactly!

What I think is happening here is electrically the same thing that motor-boating is to a vacuum tube audio power amplifier, and the same checks will usually work. In both cases, for pre-1960's equipment, it is most likely in the output stage, but can be anywhere in a related circuit.

The problem here is this Delco Auto TV is probably a proof of conception prototype, most likely made to determine if the public would buy one if GM offered a car TV for sale. This prototype only had to work well enough for display and there is no known documentation on the set, including what voltages the missing power supply provided.

Since this spurious signal is probably happening in the vertical output area, the scope will be of little use here. If Bob cannot localize the problem, he may have to find a TV restorer locally who has a scope and is familiar with the older vertical circuits, but that is the last resort.

Bob, I forgot to mention earlier, if there is an electrolytic across the cathode resister of the vertical output tube or in the plate circuit and/or screen grid of the vertical output, make sure they are good. This test can be made by simply jumping them with a second electrolytic, since over capacity here should not do any harm.

James

Edited to delete incorrect assumption. I incorrectly remembered that GM completely switched from subcontractors to made-in-house auto radios as 1953 or 1954.

The General always built their own radio's since 1946.
All GM cars, with factory installed radio's, were Delco built.

Interesting. The unknown TV in my Scott 800B radio/TV/phono set recently developed exactly one horizontal fold similar to the ones you are seeing. My first thought (fear) was that something may be going wonky in its vertical output transformer. This discussion gives me some other ideas. It will be quite a while until I have a chance to wrestle this 200+ pound monster out for an exam, however, since I have other projects in the queue.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

Problem found :banana:

The picture tube anode cable was broken internally. A quick resistance check showed no continuity. I made up a new cable with some HT wire I had on hand and now the picture is fine. The old cable must have been arcing across the break inside the wire.

I'd like to credit my trouble shooting technique, but the only reason I checked the cable was it looked really beat-up. Sometimes you get lucky :thmbsp:

Thanks for all the good advice.

Bob D.

Phil, did you think "vertical" and write "horizontal"?

Bob's picture shows vertical folding.
Giulio's picture shows horizontal folding.

For a single horizontal fold, check the horizontal drive or do a complete wave shaping in the horizontal circuit.


Congratulations!

Just wondering, did fixing the HV lead cure the bad vertical linearity problem too?

Usually arcing across an open connection appears differently, plus seeing the vertical linearity issue is why I incorrectly suspected the vertical output stage.

James

Arcing across an open connection was what it looked like right off the bat. But i was guessing it to be in the CRT socket rather than the anode lead (per post# 2).

+1, James! When I had some bad EHT arcing on my 27" Allocchio Bacchini, it lost sync, as I expected, but it never broke down the picture as shown in this thread. Well, another precious lesson learned

Cheers

Giulio

I meant a fold that stretches horizontally across the screen, which might be caused by some problem in the vertical sweep circuitry.

I am going to check the anode lead connection. This set has a metal coned CRT and a previous owner had ripped off the lead and basically reattached it with duct tape. I moved the set recently and maybe something jiggled loose.

Phil Nelson

Hi James, no the vertical linearity is still poor. I've read warnings not to do alignment with the 5AXP4 test CRT in place, so I'm not too concerned at this point. I have an 8AP4 that, hopefully, will work and have better linearity.

I guess I will eventually need a test pattern signal. Is this normally generated by special equipment? I still have an old VCR. Would a VHS recording of a test signal work? (I don't think a DVD recording would work. I've never seen a DVD player with an RF output.) This subject might make a good new thread.

Bob D.

Yes, you might want to start a new thread on that topic. Or search the archives for past discussions of same:

https://www.google.com/?gws_rd=ssl#q...videokarma.org

Phil Nelson