Postwar American Television
Hoffman Colorcaster Restoration
We have started restoration of our Hoffman 19 inch Colorcaster. The set uses a 19VP22 CRT and appears to be a prototype, since it has typed paper labels for all the controls and adjustments. The CRT was bad, but we have a good tube we can replace it with.
When I removed the CRT I found that the plastic wraparound insulator around the ultor was missing, along with the HV cable and clip. Fortunately, these are easy parts to find. Another interesting discovery was that the yoke probably came from a Philco set, since it has a connector on it of the type used by Philco. There is an adapter cable that mates it to the octal socket on the chassis.
The set had been recapped by the previous owner.
March 23, 2012
Because the chassis had been recapped, I decided to plug it in and see what happened. Nothing smoked, and the high voltage came up nicely. I connected a scope to the video output and found that I had noise but no video. My spectrum analyzer showed me that the local oscillator (6U8) was not working. Changing the tube didn't help. I measured the voltage on the plate and had nothing. I removed the turret from the tuner and noticed that the connections on the inserts were corroded. Cleaning them solved the problem.
Next I found a spare surround insulating plastic piece (from a 21AXP22), attached a spare high voltage wire and clip, and put the CRT in. After a few experiments with alignment, I was able to get it to fit nicely.
I installed the chassis in the cabinet and fired it up and had a nice bright vertical line, all three colors. This verified that the CRT is good, but I couldn't understand how I could have good high voltage and no horizontal sweep. I finally figured out that the yoke was in wrong, tilted 90 degrees, and that actually I had horizontal sweep but not vertical. The mounting arrangement for the yoke is different than on other early color sets.
I tried a new vertical output tube, but no luck. Next I'll take the chassis back out and use the scope to troubleshoot the verical sweep.
March 24, 2012
The problem with the vertical sweep was a bad pot on the convergence chassis. The B+ lead from the vertical output transformer goes through three pots before being connected to the +400v line. The line is connected to a center tap on a 100 ohm pot, and that connection is bad. Otherwise the pot is OK. Since it is unlikely that I'll ever find a replacement (it is a dual pot), I connected the line that went to the center tap through two 120 ohm resistors to either end of the pot, to simulate a center tap. I'll find out how well this worked when I attempt a convergence.
The convergence chassis
I put the chassis back in the cabinet and now I have good sweep, but the high voltage is very low - only about 12 kv compared to the 27 kv that is is supposed to be. The set uses a voltage combiner and 3 - 3A2 rectifiers (7 kv from the focus rectifier plus 20 kv from the flyback). Unfortunately, I don't have any spares, so I will have to put the set aside until I can find some.
March 25, 2012
Today I tried new 3A2s (had some in aother set) and all three doorknob caps. I also checked the filament series resistors in the 3A2s. No luck. I now have 7 kv on the output of the focus rectifier, but only 8 kv from the flyback HV winding. The problem has to be the flyback.
Since I have good horizontal sweep, I will try a solid state tripler from the 8 kv I have coming out of the flyback. Bob Dobush has a box full of triplers and is coming through Columbus in a few days.
If that doesn't work I'll find a flyback from a set using a 21 inch CRT. Since the Hoffman flyback doesn't have any convergence or AGC windings it should be easy to replace it with one from a 21 inch set. If the yoke doesn't work I'll use the yoke from the 21 inch set, which will be physically very similar.
April 2, 2012
I have been working with a tripler that Bob donated recently. It does produce the proper voltages, but the width is inadequate. Using the trick of putting a small high voltage capacitor across the damper I was able to get adequate width, though the high voltage is not quite high enough (22 kv). I can get 25 kv by increasing the horizontal output tube screen voltage, but the flyback gets too hot.
Even without increasing the screen voltage, the flyback gets hot and begins to smell a little after an hour or so. I'll buy a small computer fan from Radio Shack and install it near the flyback, and hope that keeps the temperature low enough, and I'll only operate the set for a few minutes at a time. 22 kv produces a nice bright picture.
Next I started working on the video. Though I had a good black and white picture the vertical sync was very unstable. Replacing the 12AT7 vertical sync separator fixed that, but still no color.
I put the scope on the output of the 3.58 mHz reference oscillator, which had no output. When I took the tube shield off I noticed that the tube's filament wasn't working. Wiggling it brough the filament up, so I need to clean the socket and tube pins. As the filament came on, so did perfect color.
Now I will connect the convergence yoke and do a setup. I don't have any documentation for this, so I will have to experiment.
Today I added a 120 vac fan to keep the flyback cooler. I ran the set for an hour with no bad smells. I connected the convergence chassis and did a quick setup and crude convergence. The picture below doesn't accurately show the colors - there is green and yellow.
The HV tripler
The cooling fan