Bryan's Philco TV-123

[bgadow]

1) The doorknobs. I had an early b/w set with a bad doorknob once; easy enough to repair. I have an assortment of "pulls". From what was on the webpage that Dave had for his TV123, he had to replace them due to a rectifier that was red-plating. I had a no-HV condition earlier in the resurrection process that was cured by changing them...I'm not sure which was bad. The 3 higher value caps were replaced with new production Murata units ($$$) while the one in the focus circuit is the same rating typically seen in early b/w stuff so I swapped in another I had on hand. Tonight I disconnected or swapped around the replacements with no change. While I was at it I also replaced the 4.7 ohm resistor in the base of the diode coupler.

2) I never thought about restringing the filament windings. I can't see anything obvious but its hard to tell what might be hidden where it wraps around. I have plenty of replacement HV wire so maybe I'll give it a try. This raises a question: is there any polarity that has to be observed? (I don't see how it could matter)

3) The damper-Nick included a NOS tube when I got it, plus I bought a spare. Swapping brought no change.

I need to reexamine the stack of 68mmf/5kv caps in the width coil/yoke circuit. I previously loosened one end and subbed something halfway close to see what would happen but nothing changed. Now that I have the HV cage all torn apart, my access is as good as it will ever be. I should just order some and be done, whether they're good or bad. I'm not sure how failure prone they are-I guess they're ceramic.

[miniman82]

No polarity, it's heaters.

The wiring doesn't typically go 'bad' per se, but rather begins to leak HV the older it gets. Thus you won't typically have a direct short in the filament windings (which usually results in an audible hiss and the smell of ozone), but you will sometimes have quite a lot of leakage which tends to drag down the power supply wholesale. Remember that any current lost in the HV circuit is also current which is unavailable to drive the CRT, which is one reason I pay so much attention to the horizontal section. The other being unobtanium flybacks.

This problem is exacerbated by the attractive properties of HV, leading to the accumulation of potentially conductive dust and in turn more leakage. The cure for this is of course cleaning, but if you're gonna go that far you may as well just restring it anyway. I keep a spool of Belden 40kv stuff around for precisely this purpose. Don't forget to look for tin whiskers on the HV tube sockets either, those took my Hoffman out of service for a little bit and the length of them can be really surprising. If you see any, clean with fine grit sandpaper. A rag will just move them around, and they really need to be broken up to be rid of them. Protect bare metal from corrosion with WD-40, or other light water displacing oil. Do not paint!

Your problem happened all of the sudden though, which makes me think a doorknob failed on you.

Let's get systematic about this: Set the chassis up so that you can read horizontal cathode current, and take a reading with the whole circuit in play. Next see what happens if you pull V24 and V25, if cathode current is in a more or less normal range and you have good focus voltage, reinstall V24 and see what happens. Isolate the problem by process of elimination. You should expect to see a slight bump in cathode current with each tube that is reinstalled because of the heater, but if it jumps significantly it may point to a problem.

Don't forget to rule out the CRT either, if it suddenly went gassy it would cause these problems. I hope not, but something to look for. Your shunt tube grid circuit is another area to investigate, the 6BK4 will drag HV down real fast if something went wonky with it. Could be tin whiskers in the HV pot as well, just gotta get to eleiminating each part in turn.

[Phil Nelson]

Originally Posted by bgadow

1) The doorknobs. I had an early b/w set with a bad doorknob once

Yeah, not a high-failure item, but I ran across a bad doorknob cap while restoring my Dumont RA-102:

https://antiqueradio.org/DuMontRA-10...n.htm#doorknob

At first, the doorknob created subtle horizontal blips -- maybe leaking a little -- and then it failed dramatically.

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

[miniman82]

Hot coil implies dissipation- power should not be dissipated in the linearity coil. Switch from reading HOT cathode current to reading total input power to the horizontal circuit- put your ammeter where the fuse normally is and see what you get. Power is going somewhere it’s not supposed to go, either you lost drive or something is shorted somewhere. Either way you have to track it down before you burn something out, I don’t think you’ll find a LIN coil for this thing if it goes bad...