View Full Version : CTC4 chassis variants

01-22-2016, 02:39 AM
I am in the process of preparing for the restoration of a chassis for my Cheltenham set.

As you may already know, this Cheltenham came as a gutted cabinet without anything (less chassis, crt and crt bezel and no safety glass or trim and no back) Just an empty cabinet with the 3 speakers.

So I need to prepare a chassis. To provide a correct chassis it needs to be a CTC4-B variant. So I have been researching the 3 varieties of the CTC4 chassis.

Early production consisted of only 2 models, the 21CT661U (Seville) and 21CT662U (Director). These were the only 2 models available in the early production. The early production 21CT661U (Seville) used the CTC4 chassis. The early production 21CT662U (Director), used the early production CTC4-A chassis which only differed from the CTC4 chassis by the change of 2 capacitor values in the audio section to accommodate 2 loudspeakers in the director.

One of the ways you can tell if you have an early or late production chassis is to look in the area of the back side of the convergence control chassis. If you find what looks like a small choke about the size of a small audio output transformer with 2 wires coming out of it (this is the convergence choke); then you have a late production chassis. None of the early production chassis have this convergence choke located in the back of the convergence control panel.

At the point when the 3 new models were added to the lineup, all chassis, in all 5 models, were getting all of the circuit changes, and the chassis were referred to as "late production". The 3 new models were the 21CT660U (Haviland) table model, the 21CT663U (Cheltenham) and the 21CT664U (Gainsborough).

At this point all late production chassis had many circuit modifications from the original early production CTC4 and CTC4-A chassis. The late production chassis still used the CTC4 and CTC4-A designation. The CTC4 chassis was used with the table models sets 21CT660U (Haviland) and 21CT661U (Seville) and the CTC4-A chassis which only differed by the 2 capacitor value changes in the audio circuit was used in the 21CT662U (Director)

The 21CT663U (Cheltenham) and the 21CT664U (Gainsborough) were given a chassis designation all their own; CTC4-B. The CTC4-B variant was essentially a late production CTC4 chassis with all the many circuit changes, however because the 21CT663U (Cheltenham) and the 21CT664U (Gainsborough) had a 3 speaker system with tweeters, RCA gave a special "B" variant suffix to the chassis designation. The CTC4, CTC4-A and CTC4-B late production variants only differ from each other in the audio section where various capacitors are added and values are changed to accommodate the cabinets with 1, 2, or 3 speakers.

To be more specific; "some or all of the late production 21CT661U (Seville) and 21CT662U (Director) chassis have some or all of the circuit modifications made to them" Chassis starting with serial numbers B-8075000 and above, and all chassis starting with an "A" prefix have ALL of the late production circuit modifications made to them. All of the 21CT660U (Haviland), 21CT663 (Cheltenham) and 21CT664U (Gainsborough) chassis will have a serial number of B-8075000 or higher or begin with a letter "A"

Fortunately the chassis I will be restoring for the Cheltenham came from a Haviland model. By virtue of it being a Haviland chassis, by definition, it is a late production chassis. Therefore you would expect the chassis to have a late production serial number. The serial number begins with the letter "A" so it is definitely a chassis that has all of the late production modifications. Therefore I only need to make some small changes to modify some capacitor values in the audio circuit to turn this chassis into a CTC4-B to make it correct for the 3 speakers in the Cheltenham

So, to sum things up if you have a Seville or a Director, you could have EITHER an early production chassis or a late production chassis. If you have a Haviland, Cheltenham, or a Gainsborough, you can only have a late production chassis. (that is if your set has the original chassis it was manufactured with) But remember, if you have an early production serial number Seville or Director, it could still have some of the circuit modifications of the late production serial number chassis. So it looks as if RCA was in a state of improving the early production chassis, and when they had fine tuned it; they finalized the design, and then introduced the Haviland, Cheltenham and Gainsborough, and all 5 models, from that point on, got late production chassis with all the circuit improvements.

I hope this may be of help to someone trying to determine which variant of CTC-4 chassis they are working on. All of the CTC4 Chassis modifications are detailed in the manual posted on the ETF site at this link Refer to pages starting at 77 through the end of the manual. If you are working on a CTC4, I highly recommend that you use the above referenced manual or another RCA 21CT660 series service manual. The Sams folder is helpful from a standpoint of locating specific components, but I find that the RCA schematics are far more accurate when it comes to the fine details (at least on the RCA CTC4 series).

01-22-2016, 07:01 PM

Thank you for the interesting and detailed notes about the CTC-4 versions. My Gainsborough is #B8082343. The tubes that are likely originals have white RCA labels and date codes of 5-43, 5-48, 5-52, and 6-04; more than half of its tubes were replaced over its service life with red-label RCA tubes with two-letter date codes in the 1959 to 1965 range, plus a couple of GE tubes with 1965-66 date codes.

I would be very interested to know how many of each of the five models still exist, out of the total number of surviving CTC-4s.