Hi folks, would anyone have a vertical output transformer for the STC T236. The one in it is not original and is too big to fit on the chassis properly. According to the schematic the part number is SP54349E
Hopefully someone can assist.
Cheers
Scott

Don't like your chances. That one has extra windings and was unique to that limited-run chassis.

Ahh crap, is there a suitable substitute

There is one in it but I have no idea if it’s correct or not. The vertical output stage is not working properly

Hi Scotty, I would eliminate all other possibilitys before thinking of changing the transformer!
The tranny you have is probably a later better version used on later chassis.
You have to assume it is connected correctly is ok and your problems are crap 50 year old caps and res.
The circuit has to generate specific waveforms and run at certain voltages as shown on the circuit.
I would measure and scope all the key points and record for reference.
Then measure all resistors, replace if out of spec, replace all capacitors they will all be leaky.
Substitute valves of course.
Some where in the process an out of spec or faulty part will be found that corrects a waveshape and removes the problem.

Hours of fun for all the family!

Fred.

Hi Fred,
Thanks. I have recapped the whole thing replacing all the old wax caps and majority of electrolytic capacitors. The Vertical problem still remains with the picture being stretched in the centre and compressed at the top and bottom of the screen. I cannot adjust the height enough to get rid of the black frame line at the top of the screen. Adjusting the linearity doesn’t change much other the resulting in a fold over effect at the bottom of the screen. I’m suspicious of the resistors in that circuit.
Unfortunately due to the design of the set, the chassis can’t be lowered to test when the set is on as the cables are all too short. To lower the chassis, it has to be slid back, crt unplugged along with the tuner and vertical scanning coils…
Cheers
Scott

I have a funny feeling I saw a fault like that about fifty years ago. There were some sets that had magnet problems with the CRT and had to be degaussed & electrolytic's drying out also caused issues.

The shape of the yoke deflection coils is of course married to the CRT & not just anything fits.

Marc

Is that what they call vertical cramping? I'm not sure. It could be caused by the components in the cathode circuit of the vertical output valve (or the valve itself). But what would I know....

One can but speculate; It really comes down to originality. We know that things change & fail, that is somewhat predictable.

However, when non original alien parts to the device are added and perhaps the original circuit monkeyed with; Outcomes do tend to become a tad unpredictable in most cases.

Like it takes a finite time for a line scan to travel a distance, anything that changes the timing, or magnetic pattern has to cause a problem, which may not be able to be addressed.

Hi Scott, what a pest where the chassis is hard to get at.
The symptoms you describe could be range of items all the way from the valve down in emission, to the yoke or transformer having a turns fault (or the nearest substitute). The vertical output amplifier seems to be running out of its linear deflection area. (run out of steam).

For the valve to have a chance, it must have good heater voltage and fed with the correct HT.
Its great to work on a stage live as you can check voltages and waveshapes on the run.
However you might like to work on the chassis cold and do a parts replacement approach.

Assuming the coils and valve are ok, even though the basic waveshapes are controlled by the THREE inductors involved ( osct T2, opt T3, yoke) there are a pile of crappy carbon resistors of high values and leaky little wax caps around the triode and pentode sections of the vertical oscillator and amplifier that can spoil the party. ALL the resistors will be guaranteed to be way off centre value and the caps leaky like 100K resistors. I know you have replaced a lot of caps but it only needs one hidden little cap to cruel the oscillator wave shape or drive.

A sensible approach (after a HT check and a good valve substitute check proves negative) would be to work on the dead chassis and REPLACE ALL the resistors and capacitors in the VOP section. While you are doing that I am sure any 470K resistor will read 1.5 Megohm (R234) and any 1.5 megohm (R 227) will read 2.3 megohm! And C217 will have a 50Kohm leakage!!

You can spend a happy couple of hours doing that and if the gods are smiling you will power it up and find the previous setting are now producing wild overscan and you have back it all off and then the 6BM8 will be cruising happily and a linear picture on the screen.

If not you are free to curse me and the fates!

Fred.

Hi Fred,
Well I replaced to 2 electrolytics in the VO stage and that improved things significantly. Now working on the sync circuit as the picture becomes quite unstable with changes in brightness or scene changes
Cheers
Scott

Stability problems with scene or brightness changes indicate poor HT regulation.
Suggest TV has numerous but fixable problems due to age.
Will need to go through all components, and change all wax/paper caps and all electros to start with.
Big job and time consuming, but it’s the only way.
JJ

Hi Scott, so far so good.
Just imagine how good it will be with ALL the caps and r's replaced!

If the sync pulses get interfered with by bad components causing cramping or deletion or, as Johny points out HT rail coupling then the pix will never be steady.

The sync pulse train is the ONLY thing that properly triggers the running of the H and V oscillators and exact timing of the interlacing.
Anything that interferes with that pulse train and the seperation of the H, V and interlace pulses will give a "jittery" picture.
Any HT rail that has EXCESS ripple or COUPLING sections that need isolating will give a jittery pix.
Very rarely the problems may go back to the IF section, but most likely is just high resistors and leaky caps around the sync seperator and poor decoupling.

Keep it up!
Fred.

Thanks gents
All caps have been changed out. I have ordered the 2 remaining electrolytic caps and a new 1B3GT valve. Checking the resistors around the Vertical and Horizontal section proved they were pretty much at their original values give or take 10%.
I suspect replacement of the last 2 electrolytics will help significantly. One is in the 200v supply line and the other in the Vertical stage 250v supply line. Someone has also added a resistor to the Top linearity pot which has reduced the 330 ohm (as per the schematic) to 220ohm. I will remove it once I have the 2 new caps and see what it does.
Cheers
Scott

The vertical output cathode bypass electro is usually the first suspect, a very common bad linearity fault back in the day.

G2 bypass if it has one, ditto.

If the vertical output stage is decoupled form the main B+ with a resistor and an electro, if the electro dries out all the scan energy goes into the resistor instead of making its way to the yoke via the transformer.

All the things that can cause weak, distorted audio in an audio amplifier apply to a vertical output stage, which is really just an audio amplifier.