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  #16  
Old 12-06-2014, 01:41 AM
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Tubejunke Tubejunke is offline
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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.
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  #17  
Old 12-06-2014, 01:41 PM
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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.
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Last edited by ohohyodafarted; 12-06-2014 at 01:54 PM.
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  #18  
Old 10-06-2016, 11:36 AM
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A Hoffman Hiatus

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:

Last edited by benman94; 10-06-2016 at 01:14 PM.
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  #19  
Old 10-06-2016, 01:04 PM
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Electronic M Electronic M is offline
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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.
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  #20  
Old 10-06-2016, 02:48 PM
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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.
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  #21  
Old 10-06-2016, 08:09 PM
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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-50...3D232072262896
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Last edited by ohohyodafarted; 10-06-2016 at 08:25 PM.
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  #22  
Old 10-06-2016, 08:27 PM
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Ben,
I see the pot just sold. Did you buy it?
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  #23  
Old 10-06-2016, 10:05 PM
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That was infact me Bob.
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  #24  
Old 10-06-2016, 10:34 PM
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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.
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  #25  
Old 10-07-2016, 01:28 AM
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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.

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  #26  
Old 10-07-2016, 02:14 AM
Tom9589 Tom9589 is offline
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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.
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  #27  
Old 10-07-2016, 05:36 AM
walterbeers walterbeers is offline
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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.
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  #28  
Old 10-07-2016, 10:59 AM
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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.
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  #29  
Old 10-07-2016, 12:37 PM
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Tom of course is correct. The 12 volt rail comes from the power supply. Sleep deprivation strikes again!

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.
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  #30  
Old 10-07-2016, 06:05 PM
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Quote:
Originally Posted by benman94 View Post
Tom of course is correct. The 12 volt rail comes from the power supply. Sleep deprivation strikes again!

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.
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