Howdy folks,
I don't often post here, but have been a Videokarma member for some time now, and read the postings religiously. In any case, I was the fellow that acquired the Hoffman Colorcaster in the Bay Area that was on eBay recently. I apologize in advance for the large number of photos, but I'm hoping this thread will serve as an excellent set of "notes" when I get going on the restoration, and (hopefully!) successfully complete it.

The story of how exactly I came to acquire the set is a bit peculiar. My grandfather is a bit of an eccentric, and has a penchant for random gift-giving. A few weeks ago, he decided that all of his grandchildren ought to have new hunting rifles in time for the opening day of rifle season here in Michigan. I don't hunt however, and I only occasionally shoot trap and skeet, and he knew this. He happened to be at my home one afternoon in late October, and noticed that I was admiring the Hoffman on eBay. I briefly explained what the set was, and he muttered something under his breath, saying no more about it. The morning of Halloween, I received a call from my grandfather saying that he had been in touch with the seller, had purchased the set for $500, and that he was working with a shipping company to have it shipped back to the Metro-Detroit area. I then posted the set on uShip, and paid to have it shipped back to my home in Michigan, having felt that his incredibly generous gift was already much too great.

It arrived the morning of Monday, November 10th, 2014 in a pickup truck driven by one Robert Giacometti of Buzz's Transport. He took the utmost care with the set, and even went to the trouble of purchasing a large metal bowl to place over the plastic bump out on the rear of the set. It arrived in Michigan precisely the way it was when it left Saratoga, California. Needless to say, I was thrilled with Mr. Giacometti's service, and would highly recommend him to anyone here. As soon as it was moved into the basement I began poking around. It's by no means perfect, but with little work it should be pretty presentable.

http://i.imgur.com/bjT1uUrl.jpg

Here's the set with the back off:

http://i.imgur.com/7CZnrBfl.jpg
http://i.imgur.com/5AEmm6xl.jpg

The top of the chassis:

http://i.imgur.com/Mbc6p7Cl.jpg

Rear apron of the chassis:

http://i.imgur.com/R9QDwqFl.jpg

I quickly figured out how to remove the side panel that hides the underside of the chassis:

http://i.imgur.com/HasubEnl.jpg

And then the panel covering the controls up front:

http://i.imgur.com/mzhEHjnl.jpg

And then finally I removed the front safety glass and CRT bezel. It would look that the CRT is one of the later "grey screen" 21AXP22s:

http://i.imgur.com/8A0ds62l.jpg

It was at this point that I noticed the serial number sticker on the front of the CRT. The serial number on the CRT, and the serial number on the chassis match, thus indicating that the CRT is likely original to the set.

http://i.imgur.com/4R734sjl.jpg
http://i.imgur.com/v3vS8lEl.jpg

It was at this point that I decided to test the CRT. I noticed that the CRT base is Hoffman branded, but the tube is RCA built. It has a date code of 6 -13, which I believe means the CRT was built during the 13th week of 1956, between March 25th and March 31st.

http://i.imgur.com/FhoJAVVl.jpg
http://i.imgur.com/ohAgAf2l.jpg

I nervously connected my Beltron to the CRT, and was greeted by normal looking glowing heaters!

http://i.imgur.com/xM2okRAl.jpg

With 6 volts on the heaters, the CRT looked like a dud for a few mintues, when the meter movements suddenly sprang to life, pushing up toward the top of the scale. I allowed it to sit for a minute, and then looked at the life test. It took a full 15-20 seconds before the red gun (the weakest) began to fall!

http://i.imgur.com/yNjYPe0l.jpg

Curious, I looked to find the lowest heater voltage the CRT would still "pass" at. I let the CRT cool for a number of hours, and then tested again inching the heater voltage gradually. At 3 volts I found pretty decent looking emissions on all three guns and an 8 second life test on the red gun.

http://i.imgur.com/ylH92G8l.jpg

I then turned out the lights and looked for any abnormal glow in the neck or near any of the elements. I found none, so I'm guessing (and hoping) that the tube is still under a hard enough vacuum to operate correctly.

From here I turned my attention to the schematic found here. (http://i.imgur.com/Azb4Zj3.jpg)

And the tube location chart found here. (http://i.imgur.com/K3C5esR.jpg)

After examining a number of other schematics from other early 21 inch color sets, it looks to me to have the "RCA Licensee" circuit, but with the high voltage section more closely resembling that of the 21-CT-55, and a few tube types changed. Perhaps Hoffman received an excellent deal on the 6K6 used as the audio output, and opted to use that instead of the 6AQ5 used by other manufacturers. In any case, if anyone has a copy of RCA Licensee Bulletin No. 962 (dated September 16th of 1954), I'd love to see it. I'm curious as to what exactly RCA specified in that circuit, and where Hoffman deviated from that.

For comparison sake, the schematic of the Sentinel IU-816 (http://www.earlytelevision.org/images/Sentinal_color_Schematic.jpg) bears some very striking similarities, especially in the IF strip and color circuits.

Any info on the RCA licensee circuit would be greatly appreciated. :)

I guess this is it for the time being. I have an ongoing, perplexing medical issue that will very likely leave me hospitalized for some time. I hope to be back home and digging into the set by Christmas, but I'm not holding my breath...

- Ben

What model number is it? I have a NOS Hoffman remote kit for color TV's on eBay right now. You can see the models listed there.
http://www.ebay.com/itm/301383354181?ssPageName=STRK:MESELX:IT&_trksid=p3984.m1555.l2649

Model number SP4021A, built between late 1955 and early 1956 as near as I can tell.

Looks like just 7000 series sets. Sorry

Wow, a Colorcaster with a good 21AXP22 -- Santa came early this year!

I really like that side service panel. What a thoughtful feature.

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

What a fantastic set -- I'm looking forward to seeing your progress on it.

Wow! Me likey a lot! :drool::drool::drool:

Great story and what a find! congrats :thmbsp:

Ahh HATE Yew...-Yosemite Sam, Esq. Seriously, EXCELLENT score ! Your Papaw might be a bit Odd, but Odd is GOOD...(grin)

WOW! Another jaw-dropping rare, beautiful early color set with a good 21AXP22 has surfaced within the last month or so! Incredible find!

jr

I nervously connected my Beltron to the CRT, and was greeted by normal looking glowing heaters!

http://i.imgur.com/xM2okRAl.jpg

I am sure that you are correct about the manufacturer of the CRT, 274 is RCA. Congratulations on taking possession of quite a nice set.

Great scroe! I would have bid myself, but in truth I'm tired of shipping sets all over the place. Guess the only way I'd get involved is if one popped up in my backyard.

Nice gift! Sure makes the Philco cathedral I got for my birthday pale in comparison...

That can't be a Hoffman (QU)"EASY-VISION" CRT because it is not a GREEN face 21AXP22 :tongue:...Yup, definitely an RCA fake. :D

That should be an excellent project. I hope your health issues do not keep you away too long.

With a set like that being found in the "Bay" area of California, one must wonder what its history is. Who did it belong to? Uncle Miltie perhaps, or Marilyn Monroe perhaps. Could be anyone really, but the bottom line is that they weren't working in the factory that built it.

The pictures simply excite me. I have seen and own what I call working originals, meaning working unrestored and by the looks of this set it may be one. Unless it has already been thoroughly cleaned, you can see that it was well kept and used very little which was often the case in those days as people didn't watch TV all day or night when given a chance. It may have been used once a week, or hardly at all.

I have learned that extremely low use can be good and bad. The caps would be more likely to still be good if the set was powered at least say once every few years as opposed to complete dormancy. I would definitely, and very carefully see what the set will do as is. The access panels will make that all the more easy. Maybe get a laser thermometer and keep a close eye on everything for excess heat. Of course things local to tubes are going to seem to be getting hot, but it's a good way to carefully observe the set in action.

Where are the purity magnets??

Does anyone see a purity magnet assembly around the front of the 12AX ?

Can't tell from the photos.

EDIT: OK Forget it. Looking at the schematic, I see a "field neutralizing coil" supplied from a couple of 100 ohm pots from the power supply. Must be something like was done on the 15GP22 sets.

I think I like the Permanent magnet ring as used on the 21CT55, CTC4 and CTC5 and other manufactures sets, because it gives you 6 to 8 quadrants of variable adjustment. You can fine tune purity quite well with the magnets.

All in all a very cool set. I am specially fond of sets with a separate powers supply chassis.

Shortly after acquiring this set, I traded it away to another collector. Recently, this other collector contacted me and offered the set back. Having regretted making the deal ever since I moved the Hoffman in 2014, I jumped at the opportunity. I'm now both the 2nd and 4th owner of this set.

Anyway, the fellow that had it in the interim did a recap on it and got the set about 80% restored. It has poor sync. Part of the issue was the 6V6GT vertical output tube. It was gassy. I think the other major issue is the contrast control. It's a 500 ohm potentiometer tapped at 75 ohms. The one in the set is shot. The contrast control is in the cathode circuit of the 12BY7 video amplifier. The signal going to the sync separator (the triode section of a 6U8) is picked off from the plate of the video amp. If I can't get clean enough video to the sync separator, it doesn't matter what's going on down stream of it. In addition, the -12 volt supply for the 6AV6 audio amp is picked off a resistor network tied to the contrast control. Basically, without a new contrast pot, I have a very expensive paper-weight. Does anyone here know where I could find a replacement pot, or how I might modify the circuit to use a normal 500 ohm untapped pot?

Here's the schematic of the video amplifier:
https://s11.postimg.org/dumhnxpjn/Azb4_Zj3.jpg

I'd try a 1K pot with a 1K voltage divider for the tap in parallel with the pot's end lugs. Make one of the divider resistors (the one in the tap section) 75 to 80 Ohm.
With 75 the parallel/series network will turn a 75 into a 72 which probably won't matter, but some are perfectionists.

Found yet another problem: someone at some point replaced components (I can't tell if this was recently or back during the set's service life) in the HV cage and did a piss poor job soldering. If I even look at the HV adjustment pot wrong I get bad arcing across a terminal strip in the bottom of the cage. The fly, thank God, wasn't damaged in all of this. I guess I'll sort out the mess in the HV cage first and move on from there.

Hi Ben,

On a couple of occasions I have, added a tap to a regular control. In both cases I used a 4 watt clarostat wire wound control. The procedure is a bit of a pain but it can be done.

First remove the wiper and control shaft from the body. Then I grind off the 2 rivets that hold the wire element and end terminals into the bakelite body. Now you can remove the entire resistance element. Then using a Dremel I make a slot in the bakelite body where the new center tap terminal is to be located.. I make a new terminal of brass shim stock about 1/4" wide and 1" long and slide it through the slot I made in the bakelite body. Then I re-install the wire wound element into the body in such a way that the brass shim stock is wedged tightly between the outside of the wirewound element and the inside wall of the bakelite body, thus making a tight mechanical/eelectrical connection. Then re-assemble the original terminals at either end of the wire wound element replacing the old rivets with tiny brass screws, nuts and washers. Re-install thee wiper and shaft, and you're in business.

You will need a pot like this one to do the conversion.

http://www.ebay.com/itm/CLAROSTAT-500-OHM-4-WATT-WW-POTENTIOMETER-RA30NASK501A-LONG-SHAFT-NOS-1-PC/232100987424?_trksid=p2047675.c100011.m1850&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%2 6asc%3D39240%26meid%3D5a263ea21d614595a716c13db747 fd70%26pid%3D100011%26rk%3D6%26rkt%3D14%26sd%3D232 072262896

Ben,
I see the pot just sold. Did you buy it?

That was infact me Bob.

If you need any additional guidance making the mod, I will be glad to assist with what ever information I can provide. Good Luck. I know this will work, because it worked very well for me. A lot will depend on the skill of your craftsmanship. But with the right tools and materials, it is easily done.

One tip, if the brass shim stock fits to loose between the bakelite body and the resistance element, make the brass strip longer and double the thickness where it makes contact with the resistance element. Or possibly use thicker shim stock. The idea is to get a nice snug/tight fit so the shim stock makes good contact.

Hey, if you try the add-a-tap trick, how about taking a couple of photos? I have a set with a tapped 10-ohm WW pot that needs help, and a pic is worth 1,000 worlds.

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

Ben, the -12 volt supply does not originate off of the tap of the contrast control. The -12 volts originates from an unusual arrangement in the power supply where the two selenium rectifiers are connected to different points on the power transformer instead of being connected together to the same point on the transformer. Pretty neat way to generate negative bias voltage.

A very nice unique and rare set indeed. Nice and clean, and looks to be well taken care of with low hours use. Glad the CRT came to life, so it should produce a really good picture when your finished with it. I didn't even know Hoffman made any color sets back then. Seems as if the HV section is the first thing people messed up back then with poor solder joints, bad lead dress, etc. The 21CT55 that I restored, someone had plastered the area with HV dope, instead of replacing the big doorknob capacitor, making quite a mess of the HV cage. Hope your contrast control works out ok for you. Hope to see a picture of it working when you get it up and running.

Given the nature of the circuit at the contrast control, which is just a bypass cap
(and there is also a 0.0015uF in parallel, more or less!) I'd just use a 500 ohm
non-wirewound pot with the cap simply across it.

The over-meticulous person would run a computer simulation to see what
difference it would make. I did ... very little.

At low contrast this circuit a 6 dB boost at 3 MHz. The center of the
curve rise is about 100 kHz.

Tom of course is correct. The 12 volt rail comes from the power supply. Sleep deprivation strikes again! :screwy:

Cleaned up the HV cage today. Still arcing somewhere; I can hear it, but I can't actually find it. Bob G. has graciously offered to tap a new pot for me at a reasonable cost. I'll take him up on that offer. I may try it with a standard linear pot however and see if it makes any visual difference. I had suspected based on the changes from the 703 chassis that such a modification would cause only a slight change. Given Doug's simulation, I'm inclined to believe I probably won't notice the difference at all.

Tom of course is correct. The 12 volt rail comes from the power supply. Sleep deprivation strikes again! :screwy:

Cleaned up the HV cage today. Still arcing somewhere; I can hear it, but I can't actually find it. .

Ben, When I cant find a HV leak I try these two tricks. First and easiest is to run the set in total darkness to see if you can see the arcing in darkness.

If that fails I get a few feet of heavy wall 3/8" ID vinyl tubing and stick one end in my ear and probe the suspected area where the HV leak is, with the other end sort of like a stethoscope. The sound of the HV leak will get louder as the end of the tubing approaches the location of the leak.

Hey, if you try the add-a-tap trick, how about taking a couple of photos? I have a set with a tapped 10-ohm WW pot that needs help, and a pic is worth 1,000 worlds.

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

When I do the mod for Ben, I will document the procedure with a How To page on my website and place a link on this thread.

WARNING!!!! DO NOT use automotive heater hose for your "listener"... radiator/heater hoses are impregnated with carbon to mitigate electrolysis in the cooling system... be careful!

Jim

Very nice so far. I hope you can effect a complete repair.

Hey, if you try the add-a-tap trick, how about taking a couple of photos? I have a set with a tapped 10-ohm WW pot that needs help, and a pic is worth 1,000 worlds.

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

I just finished doing this center tap mod on a 10 ohm 4 watt Clarostat pot for Phil Nelson. I have documented the procedure and posted instructions and photos at my web site under the "My Methods" section. It's the bottom entry "Pot Center Tap Mod"

Link to my web site is at the bottom of this post.

Excellent! It'll be a labor of love to restore.

A really nice find! Congrats