A little while ago it was mentioned on this site that the RCA schematic had errors. Yep. Here's what I can share on the subject...
There are about six versions of the CT-100's CTC2-chassis schematic in use. Most common today would be the SAMS, then the RCA versions, and the Telaide version. SAMS and RCA tend to show early CTC2-design circuitry configurations. Telaide however in its "RCA Victor TV volume two" CTC2 schematic shows late prduction changes.
[1] The RCA CTC2 schematic (
http://www.earlytelevision.org/Deksnis/images/CTC2.tif) is Figure 24-37 of Grob, "Basic Television Principles and Servicing" 2nd edition, 1954:
This is a usable schematic for restorers except for two areas: the (1) horizontal drive control circuitry and (2) the vertical centering potentiometer (there are also two hard-wired fuses in the rectifier cage whose schematic annotation is sorely lacking). Telaide however does show prduction changes for the Vertical Centering pot (there are two; a single-pot and a dual-pot configuration), which are not shown on earlier RCA releases. So, to update the Grob-based RCA schematic, you can refer to the Telaide schematic for vertical centering circuitry and the SAMS schematic for horizontal drive circuitry (this pint-sized upgrade runs between pin D of the horizontal sweep oscillator transformer and the 120-ohm resistor in series with the 6CD6 control grid at pin 5). See also on Phil's excellent site a partial schematic that includes the production horizontal drive circuitry:
http://www.antiqueradio.org/art/RCAC...CAManual9B.jpg Incidentally, why would you even use the Grob-RCA schematic? I find it a much better troubleshooting tool than the others because RCA engineers annotated this schematic with bold lines to indicate primary signal flow.
[2] The SAMS CTC2 schematic found in Fotofact folder set 252:
This is the schematic I use for restoration work and therefore have spent more time comparing the CTC2 chassis to this schematic than any other version. It is an early production schematic. Exactly WHEN production-line chassis' were blessed with upgrades is still steeped somewhere in color television history. But important to CTC2 restoration is an accurate-as-possible schematic. To that end, I share with you the SAMS schematic inaccuracries I found in 1999 using early-production CTC2 chassis B8000173 as the reference standard.
(2.1) Eight-pin 'M16' BALLAST TUBE socket shows pin 4 blank. It is not. Pin 4 of M16 is a tie point for the 375V SOURCE.
(2.2) AGC KEYING 6AN8 V11. Pin 7 goes to Pin 4 of M16 (not 380V).
(2.3) AGC KEYING 6AN8 V11. Junction of R93 (47 meg) and R95 (820K) goes to Pin 4 of M16 (not 380V).
(2.4) One end of the HORIZONTAL HOLD CONTROL pot goes to R170; the other end of R170 goes to the 375V bus (not 380V).
(2.5) Vertical convergence transformer T12 BLK lead goes to the 380V bus (not 375).
(2.6) Pin D of horizontal sweep oscillator transformer L38 goes to R174; the other end of R174 goes to the 375V bus (not 380V).
(2.7) The lead from HV REGULATOR 6BD4, pin 1 (incorrectly labled 1 - 8) is fitted with a bare wire jumper. The jumper, between two lugs of a terminal strip, is located inside the chassis under the H.V. cage area. The jumper can be added to the schematic in the vertical part of the lead from the 6BD4 pin 1 (incorrectly labled 1 - 8) but after (below) the C140 (0.47 uF) junction.
(2.8) SIXTH VIDEO IF 6CL6 V8 and FIRST VIDEO AMP 6CL6 V9 have pin 4 marked as pin 5 and, therefore, pin 5 is incorrectly marked as pin 4. This occures on every schematic of the CTC2 chassis I have seen. (But it doesn't amount to much until you try to build a plug-n-play Composite Interface Adapter that plugs into V9!)
[3] For general info:
(3.1) The boost B+ was upgraded in later production to improve vertical linearity (specifically, boost B+ now powers the vertical sweep oscillator). These later sets can be identified by checking pin 1 of the flyback transformer (part number 107355-1). Without the upgrade there is only one connection on pin 1. The later sets have a second connection on pin 1. See also on Phil's excellent site a partial schematic that includes the vertical oscillator B+ production upgrade:
http://www.antiqueradio.org/art/RCAC...CAManual9C.jpg http://www.earlytelevision.org/Deksnis/changes.htm
(3.2) As the story goes, in order to meet UL requirements, the vertical centering pot had to be removed from the 380-volt bus and connected basically to ground (the -30V bus). It is believed no production CT-100's had 380 on the pots. Both this pre-production change and the production switch from a ganged- to a single-pot circuit is shown in the Telaide document. There is also an RCA Service Co. document covering the pot change (RCA Service Tips publication Volume V, Issue 5, April 15, 1954). See also on Phil's excellent site a partial schematic that includes the single-pot circuit:
http://www.antiqueradio.org/art/RCAC...CAManual9A.jpg
(4) A subtle reminder about the 6BD4 High-Voltage regulator:
(4.1) Most CTC2 schematics are incorrect at the H-V regulator. They improperly show the 6BD4 cathode as pins 1 and 8, which would be correct only for a 6BK4. Only pin 1 is connected to the cathode in a 6BD4.
(4.2) Remember, as shown in the link below, a CTC2 uses pin 8 as a tie point, which negates the use of a 6BK4 in a CT-100. If you have a 6BK4, ring out pins 1, 3, 4, 6, and 8. You'll see they are common. Do the same with a 6BD4; pins 3, 4, 6, and 8 will be open.
(4.3) However, if a 6BK4 is used in place of the specified 6BD4, the circuit alteration is not severe enough to cause H-V regulation failure. The smart thing to do, however, if you must use a 6BK4 in place of a 6BD4, is to remove the pin 8 tie-point connection at the octal socket.
(4.4) See also:
http://www.earlytelevision.org/Deksn..._6BD4-6BK4.htm
Pete Deksnis
8 Jan 2017