View Full Version : RCA CT-100 Chassis Intrigue...


Pete Deksnis
01-08-2017, 09:29 PM
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/RCACT-100RCAManual9B.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/RCACT-100RCAManual9C.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/RCACT-100RCAManual9A.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/Deksnis/CAUTION_6BD4-6BK4.htm

Pete Deksnis
8 Jan 2017

ChrisW6ATV
01-15-2017, 11:23 AM
Thank you for posting your notes, Pete. I will keep a copy for when I get back into my CT-100, which is working but needs some refinements.

dtvmcdonald
01-15-2017, 12:18 PM
I am making up a schematic with these changes indicated in red. It will
also include the B-boost mods. I've got to get back home for that as Deksnis's
little hand-drawn schematic is not clear and I have my chassis conveniently
on my bench so I'll check reality. I'll make it available when its done.

dtvmcdonald
01-21-2017, 11:53 AM
I am making up a schematic with these changes indicated in red. It will
also include the B-boost mods. ...

This is now available at the ETF.

Pete Deksnis
01-21-2017, 05:25 PM
...Deksnis' little hand-drawn schematic....

Here it is. http://www.earlytelevision.org/Deksnis/changes.htm I added it to my 1-8-2017 post above. It was meant to be sort of a 'pictorial/schematic' presentation aid for anyone attempting to locate the components on a late-production CTC2 chassis.

Thanks for the complete updated schematic Doug.

dtvmcdonald
01-21-2017, 05:52 PM
I knew it! I knew I had missed removing a bogus decimal point.
Its in front of the 100 ohm resistor from the -30 volt line to ground.
This is 100 ohms, not 0.1 ohms. In a set its rather obvious, however.

Steve McVoy
01-27-2017, 02:44 PM
There is another problem with the Sams. One of the waveform drawings is inverted. Can't remember which one, maybe someone else has noticed this and can tell us which one.

dtvmcdonald
01-28-2017, 11:28 AM
It may be W5, which should closely resemble W2, and does on the 21CT55 set
of waveforms.

Pete Deksnis
01-28-2017, 04:34 PM
There is another problem with the Sams. One of the waveform drawings is inverted. Can't remember which one, maybe someone else has noticed this and can tell us which one. According to my notes, although it's been years since I've checked it out, it's on page 3 of the SAMS. Transpose the images only of W1 and W2. Not the callouts.

Waveform W29 is probably/certainly not correct. It is supposed to be a Q signal just as W31 is an I signal.

Pete

dtvmcdonald
01-28-2017, 09:33 PM
W1 and W2 look correct. The sync pulses are negative going on W1,
and therefore on CRT grids. So note, of course, that only W1 is triggered
on sync pulses.

I carefully looked at the schematic for all color waveforms, W29 through
W44. They looked correct and I have looked at them on
a real TV in the last week, and recall no inconsistencies.

Again, you gave to look very very carefully to see what's signal and what's
blanking pulse, and the time they sync is various. Its much much clearer
when turning knobs. Also ... few people have the actual type
color bars specified, which I do.

Pete Deksnis
01-29-2017, 01:22 PM
W1 and W2 look correct. The sync pulses are negative going on W1,
and therefore on CRT grids. So note, of course, that only W1 is triggered
on sync pulses.

Two Points:

Back when I started the CT-100 site in June 1999, the first person restoring a CT-100 to respond to the site emailed that the first two waveforms were reversed. I noted that on page 3 of my folder. That was 18-years-ago this July. Back in 2005, when I got a CTC2 up and running, I recall checking the waveforms and not disagreeing with the info. But, Doug's post puts the question on the table again, so let's see what happens...

And the second point.

I don't have a working CTC2 chassis just now, so I can't check anything out, but on March 15, 2015, I added "Really ?" to W29 on page 3 of SAMS Set 252 Folder 11. Here's my reasoning which tentatively concluded that W29 was 'probably/certainly' not correct.

Checking the I-channel Inverter waveforms, it shows that W31 (grid), W32 (cathode), and W33 (plate) all display I-signal waveforms.

The equivalent Q-channel waveforms show that, of the three -- W29 (grid), W35 (cathode), and W34 (plate) -- only W35 and W34 display Q-signal waveforms. W29 shows a pulse. Somehow SAMS turned a pulse into a Q waveform. History: I knew the guy who set up the SAMS 'production line', as he called it, the one that produced data for the Photofact Folders. He was a publisher, not a technical guy, but I know he'd have been on top of apparent mistakes like this -- keeping such things from being repeated.

dtvmcdonald
01-29-2017, 02:27 PM
Two Points:


I don't have a working CTC2 chassis just now, so I can't check anything out, but on March 15, 2015, I added "Really ?" to W29 on page 3 of SAMS Set 252 Folder 11. Here's my reasoning which tentatively concluded that W29 was 'probably/certainly' not correct.

Checking the I-channel Inverter waveforms, it shows that W31 (grid), W32 (cathode), and W33 (plate) all display I-signal waveforms.

The equivalent Q-channel waveforms show that, of the three -- W29 (grid), W35 (cathode), and W34 (plate) -- only W35 and W34 display Q-signal waveforms. W29 shows a pulse. Somehow SAMS turned a pulse into a Q waveform. History: I knew the guy who set up the SAMS 'production line', as he called it, the one that produced data for the Photofact Folders. He was a publisher, not a technical guy, but I know he'd have been on top of apparent mistakes like this -- keeping such things from being repeated.

Ah, but W29 and W30 don't go with W31 through W35. W31 through W44
are color bars, in order green yellow red magenta blue. W29 and W30 don't
display those bars. They display I and Q bars, in unknown order, see Note 4.
I checked polarities with the official matrix and they look right. They look
right compared to my color +-I +-Q test slides ... (plus is first in two triples
+I-I+I Y=50% +Q-Q+Q)

Red bars are positive for both I and Q
Green bars are negative for both I and Q
Blue bars are different for I and Q

Salmon (pure I) bars are +I
Pure Q bars, +Q, are sort of violet-magenta

Pure -I and -Q are both sort of greenish, -I being a bit bluish.

Doug

dtvmcdonald
01-30-2017, 12:36 PM
For those of you with CT-100s, what fraction of the way up do you
usually set your brightness control?