Quote:
Originally Posted by Penthode
I just acquired a Du Mont RA112-A3. It is a late run chassis according to the Rider's service manual which puts it manufacture probably at the end on 1950.
I noticed on the back cover "Video Out to Color Converter". The video output is via an RCA connector and this connection is not shown in the diagram from Riders Volume 6. Nor in Riders is the DPDT switch next to the connector which is not marked. (A standards switch?) I have yet to follow where the video is extracted but I expect it is somehere from the video amplifier stage. And as for the mystery switch, I will have to trace this out as well.
This is interesting since the late 1950 coincides with the FCC acceptance of the CBS color system. My guess is that perhaps the switch alters the horizontal and vertical time bases and the video out may be to provide sync for an external sequential filter device? If a separate color wheel was considered for this set, it would have to be a monster diameter: the tube is a 19AP4 and therefore the disc would have to be about 40" in diameter! 
(It make me chuckle to think how hazardous television servicing could be in those days: HV on metal cone CRTs and huge spinning discs could lead to horrendous accidents!) 
Has anyone else seen such connectors and switches on 1950-51 vintage TVs? |
Yes, and there are some on other mfgr's chassis as well.
CBS showed an add-on "color converter" consisting of a video monitor set with a color wheel that took a video feed from an existing B&W set, so I'd think
this was the intent of the jack on the RA 112.
Another DuMont Set of the era, the 30 inch Royal Sovereign had this output connector marked the same way. It is a cathode follower output off the first video amp tube, a 6BA6, through a 0.1 uf cap to the output jack. This can be seen in Sams 156 for the RA-119-A on the ETF TV schematics pages. The output is about 2.0 volts P-P video with negative going sync.
I would think the color output on the RA 112 would be from the same point, pin 7,the cathode of the 6AH6 video amp tube through a 0.1 uf cap to the jack. You'd have to trace out the switch wiring, but it probably disconnected the video from the video amp.
This output could have also been used to drive a motor amplifier circuit for an external color wheel if such became available as an upgrade to this DuMont set. Modifications to the vertical and horizontal sweep sections would have been necessary as well.
Cliff