Progress on My Admiral 17T12

[Winky Dink]

My tubular HV caps arrived and I've spent the last two days finishing (I hope) the chassis work. I replaced or repaired everything questionable. Most of the previously replaced components would have worked, but the solder joints had to be redone and the leads had been cut too short for proper placement.



Several circuits had been modified. Since I don't have the expertise to assess the functionality of those modifications, I tried to put everything back to agree with the schematic. You might understand why the circuits had been changed, but I haven't got a clue. This shows the previous modifications in red and my reconstruction in blue:

https://picasaweb.google.com/lh/phot...eat=directlink

I don't have a Variac, so I hid behind a chair when I powered it up. Nothing exploded, no flames, no smoke, and after a minute of warm up, this image appeared.



Well, OK…that image didn't appear. I was delighted to get anything on the CRT, and with my test pattern videotape this is the best I could do:



There actually is an image in there. With a movie video I can see ghosts moving around. I feel some satisfaction in taking it this far. Nevertheless, I'd like to improve the picture.

Any suggestions about what I should next?

- Winky

[cbenham]

Looks like you should try adjusting the H & V hold controls to see if you can lock the picture in. Once you do that you can try the other controls on the back to get everything to look like it does in the test pattern.
Your work under the chassis looks good.
Good Luck,
Cliff

[Eric H]

It looks like your horizontal frequency is off, you have multiple pictures side by side.

I don't know if the 17T12 has a fine horiz frequency coil or not, I don't see one on the ETF schematic but if it does adjusting the slug may bring it into range.

If the only horiz frequency control is the horiz hold pot then you may have a component out of spec like a badly drifted resistor or you have put the wrong value cap in the horiz oscillator circuit, i.e a .01 where a .001 should be (done that more than once).

[Winky Dink]

Thanks. I did exhaustive adjustments of all nine pots to achieve this result. I'll look for wrong value cap first, reading and checking values against the schematic, then rechecking values with the multimeter. Then, regarding the horiz frequency, I see what I can figure out from the alignment instruction.

Appreciate the help.

- Winky

[Phil Nelson]

Yah, the horizontal frequency is way off, but you may be closer to a coherent picture than you think.

Your horizontal circuit is much like the 19A12 that I recently finished. The manuals say little or nothing about adjusting horizontal frequency because there is nothing to adjust, apart from horizontal hold and horizontal size. This simple circuit does not have a coarse frequency adjuster, horizontal drive adjuster, etc., such as you'll find in fancier TVs.

The best you can do is to make sure the components are correctly connected and have the right values. I replaced several resistors in that area, perhaps a couple more than absolutely necessary, but now the horizontal works well.

Interesting parts to check: R42, R43, R44, C36.

In addition to the Wallace Teleaide schematic from ETF, you can look at the Riders 19A1 manual there. It uses the same part numbers as Wallace.

Originally, the output of the sync amplifier was coupled to the horizontal oscillator with a single capacitor. Riders describes a sync filter circuit that was inserted in place of that cap for later versions (C61, C62, C63, R90, R91, R92). If your set has that circuit, those components are candidates for checking. If the signal coming from the sync amp is distorted, the horizontal oscillator can't work correctly.

Riders also says that C37, a .001/1KV mica cap, is there for horizontal "symmetry and wave form correction." Another one to check. Micas are reliable but they do break down sometimes.

On the positive side, a simple circuit has fewer components to check than a complex one. You may be very close . . . .

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

[Sandy G]

Houston, we have Lights...(grin)

[Winky Dink]

It seems pertinent that the one adjustment that gave me a barely discernible image was the horizontal hold--and it had to be close to full rotation in one direction or the other.

Coincidentally I was checking C37 this morning. Couldn't get a reading without taking it out of the circuit and was reluctant to desolder it. But I will tonight.

I also located all the coil screw thingies (you call them slugs) and noted which sections they worked with.

I'll look over the Rider 19A1 manual and see what I can figure out.

Thanks,
Winky

[Winky Dink]

I checked out everything Phil mentioned, including the sync filter circuit, and I took all the components out of the circuit for testing. Everything checked out pretty close to the specified values. I reassembled it all and, just for grins, replaced the horiz oscillator 6SNGT with a spare NOS tube. This is what I got:



You're looking at Lee J. Cobb. Or maybe it's Henry Fonda? Well, it's one of those 12 Angry Men.

This is better than the previous picture, BUT…then it started going out of focus, and randomly flickering into focus every few seconds. And now it's remaining unfocusable.

Is there any point in fooling with the alignment without equipment or knowledge?
Gee, I wish Mr. Grimes was here.

- Winky

[Phil Nelson]

Don't mess with alignment. Without proper equipment (and expertise), that would likely make things worse.

Have you double-checked every one of your replacements against the schematic? It is distressingly easy to make a wiring mistake. Everyone reading this thread has done it at one time or another. A wiring error can sometimes create a weird symptom that's hard to debug. Instead of the classic repair situation where you're trying to track down a failed component, you may have a case where every component is good but you accidentally connected the neckbone to the kneebone.

Incidentally, if you are checking capacitors, they need to be checked at their normal working voltage. A modern digital multimeter can't apply enough voltage for a meaningful test. My cap checker only works up to about 500V, so for anything beyond that -- say, the 1KV rated cap mentioned earlier -- the only way I can test it is by substitution: installing a new one.

Phil Nelson

[Winky Dink]

I recognize that my multimeter capacitor testing isn't good for much other than confirming a nominal value. I thought about trying to substitute for the 20pF caps, but the best I could do with materials on hand was 50 .001mfd caps in series. I ordered an assortment of 10pF-500pF caps today.

I think the focus problem is fixed. A focus pot lug was nearly touching a lug on the adjacent pot. I pushed them apart and ran the set for 30 minutes with no focus problem.

I rechecked my circuits multiple times, but if I've done something that I thought was right the first time, then it's hard to recognize it as an error. I'll try tracing the circuits backwards from the way I would usually look at them. I may even experiment with reversing some of my "corrections."

Thanks for addressing my many issues.

- Winky

[Winky Dink]

Last night while puzzling over what could do bad things to the horizontal frequency, I made a fortuitous accidental discovery. While checking some voltages I inadvertently touched the probe to the case of the horizontal centering pot and heard a "Zap!" Yup, the metal cases of both the horizontal and vertical centering pots were energized with big voltage.



I disconnected and removed both pots, on the horizontal pot found a short between a lug (not the wiper) and the case. Couldn't see anything wrong on the outside. Disassembled the pot expecting to find a stray big of solder or wire fragment. Didn't find anything.



Reassembled the pot, and there's no short, and the resistance range seems to be a normal 100 ohms to 1.9 Meg. So I'm gonna put it back together and see how it works.

BUT…the wiper lug is about to break off. It would be difficult to fix it in its normal position, but could I just solder it to the metal case? The metal tabs on the case are continuous with the wiper terminal, so if I soldered one of the tabs to ensure a good connection, then I could use any part of the case for the wiper connection. Would that work?

- Winky

[Winky Dink]

Put a new wiper lug on the horizontal centering pot, then reassembled and reinstalled both the horizontal and the vertical pots. Checked that there were no shorts between the wipers and the other terminals. Hoped that this would somehow cure the horizontal frequency problem, but no-o-o-o! And then I checked the pot cases and found that they're still hot. Bummer! I assume the metal cases should shoot sparks when they're grounded, but I can't find anything wrong with the circuitry.



Other than retracing all the circuits for the fortieth time, I'll try switching out one or more of the presumed-good mica caps. I don't have the parts for that yet, but I expect the dog sled from Just Radios to arrive any day now.

- Winky

[Phil Nelson]

Perhaps those pot cases are hot by design. That would explain why they're mounted on an insulating phenolic board rather than on the metal chassis. The nylon shafts also insulate you from Bzzzt.

Mica caps can and do fail, especially in high voltage and/or high frequency situations.

Phil Nelson

[Winky Dink]

I'm constantly amazed at my own ability to overlook the obvious. And I just rechecked the focus pot which I thought wasn't hot--but it is hot, too.

Thanks,
Winky

[Winky Dink]

The more I fix it, the broker it gets. When I fried the horizontal size pot I noticed that I'd lost some horizontal dimension--couldn't get it to fill the screen. I thought the horizontal size would go back to normal when I replaced the pot. But after the replacement the size continued to diminish. The next time I powered up the set, the horizontal size was even narrower. I started rechecking voltages and found that the low voltage rectifier filament was getting no voltage (previously it was 275 VDC as specified).




So I guess I have a problem with the transformer. I hadn't noticed before, but the transformer is missing four out of six bolts, so maybe someone has taken it apart before.



So, I'm requesting advice on these questions: Why did the transformer fail? Is it worthwhile to disassemble the transformer? Is there anything fixable inside? Is there a work-around for a single dead secondary coil (keeping the other two functional)? Finally, should I just go back to working on radios? I have a nice Atwater Kent that I haven't broken yet.

For background, this is a link to the full schematic which shows the wiring as I found it in red and my rewiring to conform with the original schematic in blue.

https://picasaweb.google.com/lh/phot...eat=directlink

Always appreciative of your guidance,
Winky