Early Color Television
Restoration of the DuMont Industrial Color Monitor
November 6, 2012
I took apart the monitor today. It is complete, and in excellent condition, other than the cabinet, which needs to be refinished. The construction is commercial quality. The color wheel needs to be rebuilt (Cliff?). It is a sandwich design, with thin filter material in between two plastic pieces. There is no magnifier, and uses a 12 inch CRT. The DuMont brochure describes it as a 12 1/2 inch monitor. I wonder how they came up with that? This should be easy to restore. I'll start by documenting everything. Once I find out if there is a field detection system and if so how it works I'll work with Darryl to get a converter.
November 8, 2012
Some bad news on the DuMont. It is missing the motor control chassis. I can build one like the one in the Gray Research monitor, which should work fine if the saturable reactor can handle the big motor on the DuMont. There is not a coaxial input to the motor control chassis, so I doubt that composite sync was fed to it (cables would be 5 or 6 feet long), which might rule out a sync pulse for the red field. Some other interesting things. The CRT has the pin type anode connector that DuMont used in the late 40s. That means the tube was a 12J or Q probably. There are two flybacks - one for horizontal sweep and one for HV. It will be interesting to see how high the HV is. The video amp uses parallel 6BG6s as ouputs. I guess they needed it to handle 18 mHz? The power supply is a monster, with two regulated B+ outputs. I will be posting schematics as I get them done, starting with the video and vertical sweep chassis.
November 12, 2012
Today I finished documenting the 3 chassis and everything fell into place. I am still trying to figure out what kind of horizontal oscillator transformer was in the set. It plugged into an octal socket and I think it was wired like this. I think there were 3 windings on it, and since it will be difficult to find a 3 winding transformer, I will try and modified circut, also shown on that link. That schematic is very similar to the oscillator used in the HV section of the DuMont Royal Sovereign, which was from the same year as this monitor.
January 28, 2013
I have replaced all the paper and electrolytic capacitors. Everything seems to work - HV is 15 KV. I used a standard horizontal blocking oscillator transformer, and it worked fine at 46 kHz. Since I don't have a signal source yet I'm not sure how the sync will work.
The power supply has a 2 minute timer that delays applying power to the B+ transformer until the filaments have warmed up.
Cliff Benham has the wheel, and is building a replica. He has also volunteered to build a motor control circuit, which will be locked to the power line. Here is the schematic of the CBS slave receiver motor control, which he intends to duplicate:
Darryl Hock is figuring out how he can build a standards converter. The problem is the high bandwidth (18 mHz).
I hope that all of this will be ready for the convention in May.
The original CRT was a 12JP4 or 12QP4. I don't have a good one of either, so I installed a 12LP4. It seems to work fine.
With the rebuilt chassis and CRT installed.
A nice, bright raster
February 11, 2013
Cliff Benham is making progress on the new wheel and the motor control chassis. Here are the dimensions for the wheel.
He is almost finished with the motor control chassis:
I have ordered a replica saturable reactor transformer, which will be shipped directly to Cliff to put on the chassis.
Cliff has completed the motor control chassis. He has tested it with an oscillator to simulate the generator and a 300 watt light bulb for a motor.
I looked at how the motor control would work if the sync was from the power line. Since the generator is single pole, the motor would always sync to the same filter segment. So the pushbutton wouldn't select the filter segment. In that case, what did the pushbutton do? Why didn't they just use a synchronous motor and eliminate the motor control stuff? That makes me think that they didn't use 60 Hz sync. There are only two connections to the video-vertical sweep chassis. One is a 8 pin terminal strip, with all pins in use. The other is a 3 pin Cinch Jones connector that mates with the vertical section of the yoke. I think that the yoke actually plugs into the motor control chassis, where the vertical pulse is obtained, and that there was a cable and 3 pin plug from the motor control chassis to the video-vertical sweep chassis.
Darryl Hock has ordered the parts to make a converter for this standard. His job is much easier now that the vertical doesn't need to be locked to the power line.
Cliff now has the plastic pieces for the new wheel and is in the process of making it.
May 15, 2013
Cliff arrived at the museum before the convention to install his motor control chassis and new wheel. Darryl delivered the converter and we were able to get the monitor to work. However, there were still problems with it, and Cliff will rebuild his chassis and bring it back later. Here are pictures from the Convention.
May 24, 2014
Cliff made another trip to Ohio to do more work on the monitor. He was able to get it working well, but the CRT failed. When he returned for the 2014 convention, he installed a good 12KP4 CRT. We had determined that this is the best tube, since it is aluminized and would produce the brightest picture.
An intermittent flashing started to happen, which we diagnosed as being from the high voltage supply. John Folsom worked on the monitor before going home from the convention. He replaced some capacitors that had been missed in the original rebuild, and the problem disappeared. He adjusted the color balance, using settings on Darryl Hock's converter.
The monitor now displays an excellent, bright picture, with much more resolution than the Gray Research monitor. There is still a vertical jitter which occurs occasionally.