Another K840CK15 Westinghouse restoration
 

I was recently contacted by an owner of a 15" Westy about restoring his set. Last week he drove from Canada to Milwaukee with the chassis.

The set has been in the hands of a "restorrer" in Canada for about a year and that tech was not able to get the set operational.

Upon inspection I found sub standard work which will have to be completely re-done and inspected for mistakes. All replacement caps and resistors were pigtailed to the old component leads. NOT up to my high standards.

The major issue is an un-obtanium 50 meg congergence pot which according to the former tech, is open. I am trying to get hold of the old part to see if anything can be done with the old one.

So now the big question and I know this is a long shot; DOES ANYONE OUT THERE KNOW OF A 15" WESTY PARTS DONOR CHASSIS THAT MIGHT HAVE A GOOD 50 MEG CONVERGENCE POT?

AND NOW FOR THE BEST PART.................................

The interesting aspect of this set is that it has a blonde cabinet in very fine condition, AND IT IS COMPLETE WITH A NOS 15GP22!!! The 15G has huge silver getters and when I tested it, and all 3 guns came up to 95 or better on the Beltron in less than 15 seconds. And whats more..... after a 10 minute warm up at 6.3V I did a life test and all 3 guns hung for 35 seconds or better, the strongest tube I have ever tested.

It appears that the original owner purchased a spare 15G for use and had it squirreled away for future use and it was never installed. CRT that was in the set had gone gassy.

Since nobody else will chime in I guess I will...

There are two possible workarounds for the 50 Meg pot that would work, though neither is particularly elegant:

First, you could always increase the values of the either the 100 Meg and 80 Meg fixed resistors, or both, and fit a new lower resistance pot to the set. Say add a 15 Meg resistor above and below the pot and buy a 20 Meg unit. You'd have to play with which side gets more fixed resistance than the other, and it would give a much smaller range to the control, but it would work.

The other, slightly better option, would be to grab a 6 to 12 position switch and build a sort of "stepped potentiometer" out of fixed resistors. A lot of audiophiles build these stepped attenuators and have great success with them in audio applications. Of course now you're limited to discrete, fixed steps, but given the fact that a 15GP22 based set rarely has anything approximating perfect convergence, I think that the stepped solution may well work. It would certainly be more reliable than any pot, and these are particularly failure prone parts.

Not at all familiar with the old 15" sets but.....
I assume its a voltage divider. What is the typical voltages on it ?
Lots of 70's to mid 80's SS sets used very high ohm focus dividers & bleeders.
Also focus pots that went to at least 15 meg. Should be able to put something
together that will work & not look messy.

73 Zeno:smoke:
LFOD !

Yes Ben, You have heard the old saying "brilliant minds think alike" so I had already thought of the plan to shift the range of a 15 meg High Voltage pot by using a 2 pole 3 position ceramic rotary switch. I would use 2, 15meg resistors on each side of the 15 meg pot in an arrangement whereby a rotary switch would move the the leads of the resistor element in the pot up and down to that I could have 30 megs above or 30 megs below the pot, or 15 megs at either side of the pot.

My only misgiving is that there is about 4250v across the original 50 meg pot so that puts about 1500v across each 15 meg element in the chain running through the ceramic rotary switch. Even isolating the switch frame and shaft, I am still concerned with the close proximity of the contacts within the switch and I am concerned with the possibility of arc-over from the high voltages involved.

EDIT....Although there is about 4250v across the 50 meg pot, there is a range of from 12.8KV down to 8.54KV at the wiper of the pot with reference to the 20KV anode voltage on the crt because the DC convergence voltage component is derived from the 20KV ultor voltage on the crt.

I guess the only thing to do is to buy the parts, and breadboard the whole thing outside of the chassis, connect it up and see if the components withstand the HV.

It may be a while before I get around to trying this but I will post the results when I have a chance to put it together.

On another front. I have located a company Metalux in Germany that makes HV resistors. They make a HV pot (POC-400) in 5m, 10m, and 15m (the 15m is what I will use in the scenario above.) The reason I mention this is that this item could be substituted in a CT100 for the focus (5m) and the convergence (15M). The item is encased in a small plastic housing about 2" square designed for mounting on a PC board. I think these may have been used in the high voltage sections of tv sets. The internal resistance element is placed on a ceramic substrate

The link to the data sheet is below
https://www.metallux-usa.com/uploads...-POC_400-s.pdf

There are some HV rated rotary switches out there, used mostly in old medical equipment, like 50s and 60s X-ray machines, particularly those used in dental offices. Some of them would be much too bulky for this application, but others might be small enough. I'll look around and see if I have anything rated at 1 kV to 2 kV. If not, a place like Surplus Sales of Nebraska or similar might have something in the ballpark...

I have seen extremely similar switches to these used at around a kV...

I have no idea what these are actually rated for, and they're a bit bulky, but it might help the switching issue should you decide to go that route...

https://www.surplussales.com/RF/rfcerrot_5.html

Some tube based Ham radio linear amplifiers have ~1-5KV on the plates and use ceramic rotary switches to change plate coils for band switching. Such a part could work here.

After reading Ben's post I was thinking what if that 'stepped potentiometer' had a pot as one of its resistors, and a switching arrangement such that the pot would change positions in the string? If you did something like a 6-12 position switch instead of a 3 position you could use lower value resistors and potentiometer and have less drop on each resistor and less chance of switch contact arcing.

The following probably is not mechanically practical, but it would be interesting to mount the body of the pot to the shaft of the rotary switch such that when the pot is rotated to it's stop some additional force would click the switch to the next range in the direction you're turning it...Thus keeping the control on a single knob and retaining access to its full original range.

Focus Controls
 

So I did an ebay search in consumer electronics for "focus control"

There are lots of focus control assemblies for 70's-80's tv sets. However I am not at all familiar with tv sets of that era.

Here is a url to the search I did.

https://www.ebay.com/sch/i.html?_fro...LH_TitleDesc=0

It came up with a lot of hits but you have to sort through about 170 items. A fair number are focus assemblies for tv sets, but there are also some "Ford Focus automobile controls". The problem is that I haven't any idea if any of these would have resistance values near what I am looking for in the area of 50 meg.

Do any of you younger techs have experience with the actual values of pots in these tv focus assemblies.

Another option, probably the worst, is to simply replace the pot with two fixed resistors and have a permanently "set" static convergence control.

It would appear that the static convergence control compensates for three dominant sources of error: manufacturing tolerance on the 15GP22 and other parts, which is static and fixed, B+ and B++ boost voltage drift due to line voltage drift, which is variable but can be fixed with a Variac or Sola, and drift in the fixed resistors that make up the divider from the HV rail to ground due to the temp coefficients of the resistors and simple aging, which can be minimized but not elimated.

I'm sure there are other minor sources at play, but those would be the three largest if my crude LTSpice mockup means anything.

Nailing down two out of three, minimizing the third, and using fixed resistors *should* work.

The 15 inch sets at the museum don't seem to need all that frequent adjustment of the static convergence control, and they are of course run straight off mains, with no regulation of the line voltage.

Something to consider if other options don't pan out. You should always run a vintage color set on a Sola or at least a Variac anyway just to avoid stressing components due to today's much higher line voltages, as the utilities desperately try to squeeze every last bit of capacity out of a broken and aging infrastructure.

How about this vintage Mallory 50M pot ? https://www.ebay.com/itm/VINTAGE-MAL...K9zv:rk:1:pf:0

Thanks for finding this for me Bob,

I bought it. I have been searching for several days and found nothing. I wonder if this auction was just posted???

You missed one. Looks like the Malory has a metal box & may
not work. See this one.

https://www.ebay.com/itm/Zenith-NOS-...pDL:rk:27:pf:0

That will take 6 KV & probably a LOT more. Maybe build it into an
old HV cup with resistor/s to make it 50Meg.

BTW I worked for a big Zenith dealer starting in 1971 til the "end".
I have never seen this part. The part ## would be late 60's. Its an odd
ball. Almost all sets then used the very common 63-6444. It may be from an
early hybrid. Seemed like every one you opened had a different HV
section.

73 Zeno:smoke:
LFOD !

The pot "50 meg ohm" pot arrived today. It was NOT 50 meg ohms, it was actually 50 K ohms. I thought it was too good to be true. I have received a full refund.

What about a Sony static convergence pot used in an earlier Trinitron set? One of those red jobs?

I am not familiar with Sony products. Do you have a photo, and how would this replace a conventional resistor in a voltage divider network?