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 More help on STC T212
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 Return to top of page · Post #: 1 · Written at 11:50:30 AM on 13 April 2020.
John116's Gravatar
 Location: Ryde, NSW
 Member since 29 October 2008
 Member #: 370
 Postcount: 92

All well except for a vertical linerarity problem I can't resolve. Height and linearity controls both work to a degree, but don't resolve an extremely 'stretched' top part of the screen.

The linearity pot does not have enough range to compensate.
Looking at the waveform at the output of the 6BM8, it seems the frame sweep starts at too high a voltage causing a very non-linear sweep rate.

The circuit around the linearity pot (P6) in my chassis is different from that shown on the circuit diagram:
• R108 is 4.3M
• R107 is 100k
• C106 is a fixed and variable cap in parallel
• R139 does not exist
• C108 does not exist
• There is a 0.27μF cap in parallel with P6 (to ground)


Relevant circuit section here

CRO waveform here


Any help/suggestions greatly appreciated.


 
 Return to top of page · Post #: 2 · Written at 3:22:17 PM on 13 April 2020.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1904

Looking hard at that circuit and substituting the parts you say are there, it doesn't make a lot of sense with those parts.

Put it back to the circuit is my suggestion. 100k vs 120k doesn't matter much, nor does the 4.3M resistor (Yellow Orange Green?) Not a preferred value but if it checks out it shouldn't make a lot of difference.

C106 needs to be a 1KV rated cap. P6 takes a hammering in this circuit, I'd be inclined to use a fixed resistor to take most of the current and use the pot for fine adjustment, once you get it working properly.

Don't be too concerned about the waveform. It's the current through the yoke that needs to be a linear sawtooth, The voltage will look different.
Remember with a TV the screen is your best CRO!

See how that goes. I'm not much enamored by the original circuit either. The existing P6 should function as an overall linearity control. R108 should mainly affect the top of the raster.

There are better ways of doing what the set designer was trying to do that work better and aren't so hard on the parts. But try that first.


 
 Return to top of page · Post #: 3 · Written at 5:03:48 PM on 13 April 2020.
MonochromeTV's avatar
 Location: Melbourne, VIC
 Member since 20 September 2011
 Member #: 1009
 Postcount: 1131

Here is a STC modification sheet that might be related to your query.

STC Tech Talk Vol 2 # 1


 
 Return to top of page · Post #: 4 · Written at 7:14:27 PM on 13 April 2020.
Brad's avatar
 Administrator
 Location: Greenwich, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 6348

Document uploaded to Post 3.


‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 5 · Written at 3:10:23 PM on 14 April 2020.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1904

That is relevant!

Only problem is, C138 is shown as 0.027PF! Should be 0.027UF of course.
That is, 27nF. Needs to be a 600v or higher cap.

You may need to make that up with two caps in parallel, 0.027μF is getting hard to find.


 
 Return to top of page · Post #: 6 · Written at 5:58:49 PM on 14 April 2020.
John116's Gravatar
 Location: Ryde, NSW
 Member since 29 October 2008
 Member #: 370
 Postcount: 92

Ian - thanks, noticed that already, and have made up a cap to that value. Still can't get the linearity right though.

Height control seems to lack enough range to lower the height, the linearity control seems to not do anything for much of it's range.

Picture of test pattern HERE


 
 Return to top of page · Post #: 7 · Written at 11:28:46 PM on 14 April 2020.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1904

Right! That looks like R108 is high resistance. Or possibly R142.

Looks like no, or very little, negative feedback is getting back to Pin 3 of V15

Or possibly that variable cap is shorting to ground??


 
 Return to top of page · Post #: 8 · Written at 9:29:49 PM on 15 April 2020.
John116's Gravatar
 Location: Ryde, NSW
 Member since 29 October 2008
 Member #: 370
 Postcount: 92

Spent some time on this tonight. While possibly a little barbaric, using a 1.5Meg resistor for R108 seemed to improve things greatly.

Picture here

A little concerned that I haven't found the actual cause of the problem, but for the moment, the end result is good, and certainly watchable.


 
 Return to top of page · Post #: 9 · Written at 9:59:55 PM on 15 April 2020.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1904

Yes I wonder about that circuit.

I assume you checked the 470k in the grid circuit, that it is 470k and not 47k or something silly like that.


 
 Return to top of page · Post #: 10 · Written at 6:36:19 AM on 16 April 2020.
John116's Gravatar
 Location: Ryde, NSW
 Member since 29 October 2008
 Member #: 370
 Postcount: 92

Yes, checked all those values and the variable capacitor.


 
 Return to top of page · Post #: 11 · Written at 9:32:55 AM on 16 April 2020.
Johnny's avatar
 Location: Hobart, TAS
 Member since 31 July 2016
 Member #: 1959
 Postcount: 409

Can only assume that you have tried a new 6BM8, and actually replaced all caps, some of which are critical in value.
It’s very similar to an audio amp displaying some distortion.
Also check/replace the 100mfd cathode bypass cap.
The cap across plate/screen .033 were very troublesome according to some of my very old notes.
The preset pots are also very troublesome, leakage to deck etc.
JJ


 
 Return to top of page · Post #: 12 · Written at 10:36:35 AM on 16 April 2020.
Irext's avatar
 Location: Werribee South, VIC
 Member since 30 September 2016
 Member #: 1981
 Postcount: 399

When restoring my W101 Pye I had severe vertical crushing at the top of frame.
I checked everything but could find no crook components.
In the end I swapped the Vertical O/P tube and the audio tube (both 6BM8) and the fault was cured.
The original Vert O/P tube was a Trigon brand.
I learned a long time ago not to trust Trigon tubes but never had this particular fault with them.
Interestingly the Trigon 6BM8 worked fine as an Audio O/P tube.


 
 Return to top of page · Post #: 13 · Written at 11:14:42 AM on 16 April 2020.
John116's Gravatar
 Location: Ryde, NSW
 Member since 29 October 2008
 Member #: 370
 Postcount: 92

Thanks for that. I have also swapped in replacement valves (and a number of different brands), which unfortunately made no difference in this case.

I guess for the moment, am happy that's 'fixed' even though the actual fault has escaped (my admittedly limited) circuit design knowledge and analysis.

John


 
 Return to top of page · Post #: 14 · Written at 12:22:20 PM on 16 April 2020.
John116's Gravatar
 Location: Ryde, NSW
 Member since 29 October 2008
 Member #: 370
 Postcount: 92

Also, could anyone provide an explanation of how the height/linearity works in this circuit? eg, what does the negative feedback on V15 do? And the function of the resistors/capacitors? Is it a timing circuit?


 
 Return to top of page · Post #: 15 · Written at 2:23:25 PM on 16 April 2020.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1904

The height in that circuit is a bit dumb, it's just like a volume control.
Normal practice is to vary the resistor from B+boost to the anode of the oscillator. Reducing the resistor charges the cap up more betwen cycles. Doing it this way keeps the sawtooth as linear as possible by approximating a current source.

The other components provide negative feedback around the pentode. The feedback is shaped so as to make the pentode work like a triode. The yoke needs a linear CURRENT sawtooth. So the voltage waveform is deliberately distorted by the R and C network to get the linearity correct.

Of course you also need to ensure that the anode of the output stage doesn't jump up too high on the flyback of the waveform and cause the valve, the socket or the transformer to break down. This is why you sometimes see snubbers (R and C in series) as in this case across the transformer primary. Later designs used VDRs. Ceramic sockets are also seen.

I can give you a more in-depth analysis if you like.

HMVs used a more elegant approach. There is a small current to voltage transformer that samples the yoke current and feeds an error voltage back into the bottom of the sawtooth cap, hence, in series with the drive waveform. By doing it this way they can avoid the need for a linearity control.


 
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