Hi everyone
I’ve been working on an STC Commander combination radiogram / TV
The TV is working but suffers from unstable horizontal and vertical sync.
The chassis is not easy to identify and I would guess is not made by STC but another company.
I’ll email a pic of the chassis to Brad for uploading in the hope someone can help identify it and also help with a schematic for it.
The vertical output transformer has been replaced and apart from being the wrong physical size, I’m wondering if it’s even the correct type.
Any advice appreciated
Cheers
Scott

The old tele's are very much like an oscilloscope but in most cases took their synchronisation from the 50Hz mains. As it lead & lagged often pictures started to roll. This was always exacerbated by poor quality valves going out of spec. & a major contribution was made by wax paper caps leaking. This problem will be likely improved substantially by getting rid of all wax paper caps & old electrolytic caps and overhauling the synchronisation circuits.

I have had similar issues in a Heathkit Oscilloscope which was cured by replacing everything on the circuit board, as most of it was out of spec. and caps were leaking. There were signs of previous work, so parts were failing.

The physical size of a part, often does not relate to its performance or suitability.

Marc

Photos uploaded.

With that valve line-up, I'm guessing later 1960s - even though there's a printed circuit board. Newer sets would more likely use valves such as 6EH7, 6GV8, 6DQ6A etc.

I can see that the EHT rectifier is a 1B3GT rather than the usual 1S2A. There's no HT rectifier listed at all, not the usual 5AS4 or pair of 6N3's. Maybe it has solid-state diodes.

That transformer at lower left definitely looks dodgy.

Unfortunately I cannot identify the set.

The chassis has a serial number. That may be useful & often transformers and speakers had a date on them. As that lower left transformer shows no likely hood of belonging on the top of the chassis; Its possibly a sub.

Marc.

My JR field service manual (1971) stops at model T236 (730 SU-11).
The chassis is very similar with a similar IF strip but more PCB's, one at the bottom.
But it does have the vert opt at the bottom left and a pair of trannys mounted on the the top edge of the chassis.
Also has SS rectifiers no bottle rectifier.

Ian would probably have a better idea.
Fred.

Thanks Gents, I’ll be interested in what Ian says too
Cheers
Scott

That is an STC T236, the rare fringe chassis. Dates from 1961.

I have the schematic in a JR manual, but not scanned. Can scan it if you need.

The vertical output transformer is unique to that model.

It was probably the best performing chassis STC ever made.

No TV in Australia EVER took synchronization from the mains.

The idea was tried in the 1930s and quickly rejected.

Some very early security cameras derived vertical sync from the mains.

Weak sync in that circuit was usually caused by resistors around the 6CS6 going high.

Agree with Ian!
Scotty, when I look at the T236 circuit the valve types fit neatly to yours.
Your chassis build has less PCB's than the one in my JR manual.
Over the length of the build they have worked their way by batches with PCB updates.
The design must have been successful to warrant updating like that.
Re the jittering, agree with Ian, the components around the 6CS6 have critical time constants and any resistors will have changed value and those Ducon caps weil be leaky. I see they have marked the circuit up with the expected waveshapes.
Be nice to check the waveshapes before and then test and replace all the crap r's and c's and see what difference happens.
Fred.

The full service manual for the STC T236 is here:

https://www.kevinchant.com/uploads/7/1/0/8/7108231/stc_t236_envoy.pdf

Thanks Gents. Kevin sent me the service manual yesterday too.
Another anomaly I’ve noticed is the Aquadag on the back of the tube is flaking off, I haven’t seen this before. The tube itself is fine as the picture is both clear and bright. Is there a fix for flaking Aquadag?
Cheers
Scott

They were always interesting the mains did affect them & the synchronization was always unstable. as soon as meal times came around and the voltage changed off it would wander, lots were the same. no sooner had you sorted it, off would come the peak load and the Horizontal would start rolling,

You must have had a faulty set Marc.
The sync pulses are transmitted as part of the whole 6 megs of picture, sound, horizontal and vertical sync.
The whole television signal from memory was called vestigial sideband modulation. (Can’t think of the correct spelling at present).
Sync was nothing to do with mains. Portable battery sets worked fine.
JJ

Another thought, poor reception etc could cause the sync separator circuit to try and latch on to anything, including mains hum.
JJ