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CTV The Shunt Stabiliser in a 1st Generation Colour Receiver...

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Captain Peacock
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Hi,

In the first generation of UK colour TV sets, in order to maintain constant picture size, a shunt stabiliser valve (usually a PD500 or PD510) was used to keep the EHT current constant independent of picture/scene brightness, with the PD500 shunting the EHT current the CRT was not using to chassis. On a dark or cut off CRT, the PD500 would therefore shunt around 1mA i.e. 25 watts or thereabouts.

My question is, where did this 25 watts go as, according to the dual standard Philips G6 Spanish Waiter (Manuel, Manual, geddit? Oh well, please yourselves!) the cathode current on the PD500 in the dual standard G6 is around 1.2mA at 1.2V via a 1K resistor, meaning the power dissipation in the 1K cathode bias resistor is very small indeed, with very little if any grid current flowing and only through the potential divider network R5052, R5053 and R5035.

So have I forgotten something in my old age, or where does the roughly 24.9 watts go? It can't be via the cathode as there is around 1.2mA max at 1.2V which is a tiny fraction of a watt: so does it flow to chassis via the screen electrode (pin 2 of the PD500) to chassis via the CRT external aquadag, or was it dissipated in heat and X-rays from the PD500 anode?

Thanks for any help as this problem just won't leave me alone. I blame old age and senility!

 

 
Posted : 02/03/2024 12:33 pm
colourmaster reacted
Nuvistor
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@captain-peacock I was under the impression that it was dissipated in the internal structure of the valve, anode mainly.

See : http://www.r-type.org/exhib/aah0027.htm

 

 

Frank

 
Posted : 02/03/2024 1:31 pm
colourmaster reacted
dtvmcdonald
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Like any triode its all dissipated in the anode, both as heat, X-rays and a bit as secondary electrodes.

At least in American tubes (6BK4) the cathode and grid are essentially those of a CRT. The anode is long and narrow to spread out heat, and to shield most of the X-rays.

 
Posted : 04/03/2024 1:12 am
ntscuser reacted
ctc15
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Hi the anode just gets red hot!

Keith

 
Posted : 04/03/2024 10:51 am
Captain Peacock
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Thanks all: I did wonder if that may be the case: no wonder that valve got so hot in normal operation! It's a wonder the PD500 didn't develop a suck hole in the glass envelope as say a PY500 or PL509 used to when its anode glowed red hot for any length of time, eventually softening the glass.

G6DS LOP Stage

 

 
Posted : 04/03/2024 12:16 pm
irob2345
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I suspect you've got the B+boost control turned up too high. That's a recipe for killing the LOPT.

 
Posted : 05/03/2024 8:48 am
Nuvistor
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@irob2345 I suspect there won’t be a B+ control, setting the EHT to 25Kv should give the correct Boost voltage. I don’t remember any Boost control only EHT. 

But then again I have been wrong so many times, must be a song in that comment. 

Frank

 
Posted : 05/03/2024 2:52 pm
irob2345
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The EHT control only adjusts the shunt regulator. It's like using the brakes to slow your car down, without taking your foot off the accelerator!

It's probably off the circuit fragment to the left, connected to G1 of the Line Output.

It would be unlike Philips not to have a stabilised line output!

There should be a procedure in the manual for setting it up.

 
Posted : 06/03/2024 6:47 am
Nuvistor
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@irob2345 Attached circuit snippets from the Pye CT70 Dual standard CTV. The set EHT control is in the control grid of the PL509 along with the stability VDR. The Set EHT control will affect boost voltage but the Pye set up measures the EHT, the shunt set up alters the current through the valve at no beam current. The two controls interact but are easy to set up. Also shown is adjustment for 3rd Harmonic tuning if required. 

Screenshot 2024 03 06 at 12.23.52
Screenshot 2024 03 06 at 12.28.08

Frank

 
Posted : 06/03/2024 12:41 pm
ctc15
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The shunt regulator takes the current that the C.R.T. is not using so the load stays the same and the E.H.T. stays the same voltage (25kv)

Keith

 
Posted : 06/03/2024 6:53 pm
Nuvistor reacted
irob2345
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Yes I know all that.

But what is in G1 of the hor OP valve? I can't see that.

 
Posted : 07/03/2024 8:11 am
Nuvistor
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Posted by: @irob2345

Yes I know all that.

But what is in G1 of the hor OP valve? I can't see that.

 Are you referencing the Philips circuit in an earlier post?

 

Frank

 
Posted : 07/03/2024 8:49 am
irob2345
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Yes. Now I see the relevant part of the circuit.

It seems to me that the strategy for setting these controls for maximum efficiency (and component life) should be to first disable the shunt stabiliser - probably by adjusting it for minimum effect.

Next, use the Set EHT (both standards?) to do just that, without the shunt stabiliser shunting the EHT, and using near maximum beam current.

Once that is OK, adjust the shunt stabiliser so that the EHT does not rise excessively (and the picture shrink) on minimum beam current. Use the absolute minimum of shunt that you can get away with, let the primary side control SET EHT circuit do most of the work. You should never see the shunt tube red-plate, ever.

You see what I mean about the accelerator and brake analogy.

But of course, I'm only coming from control system theory, I have no experience with EHT shunt stabilisers. Probably why I have kids!

In Oz we only saw stabilised line op circuits in B&W valve TVs, where they were common practice. No valve colour sets here. But even then, the stabilised line stage was the most misunderstood and abused circuit amongst TV service techs.

 
Posted : 07/03/2024 11:52 am
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Till Eulenspiegel
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Circuit diagrams of the line output stage in the 1955 RCA CTC2B. This set was a development of the famous CT100.

The shunt stabiliser valve is a 6BK4. This valve was also employed in early 405 line colour receivers.  Pye 405 colour set had a 6BD4.

RCA CTC2B Line osc
RCA CTC2B Line output

Till Eulenspiegel.

 
Posted : 07/03/2024 6:33 pm
colourmaster reacted
irob2345
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Just read those manufacturer's setup instructions.

Incredible! They are just plain WRONG! I bet they sold a lot of replacement LOPTxs!!

Can you see the problem?

If the shunt stabiliser is active when the "Set EHT" pots are set, you will be setting the TV up for failure by getting the line stage to drive an unnecessarily high load, it ends up fighting the shunt stabiliser.

As I indicated earlier, no-one seemed to understand how that VDR stabilised circuit worked, or its purpose. In the original US designed TVs, where as often as not they didn't have primary stabilised line OP stages, you only had a brake, i.e. the shunt regulator. No accelerator!

The shunt regulator should only be active on low brightness settings where the EHT would climb on light loads, despite the primary stabilisation, due to losses in the EHT rectifier.

Use the instructions I posted earlier!

What used to happen all too often here on Oz with B&W TVs was, a service call was made for low width, caused by a shorted S correction paper capacitor.

The (so-called) tech would turn the B+Boost control to max and pronounce the TV fixed.

A few days or weeks later the LOPTx EHT overwind would fail shorted turns. Or the EHT rectifier socket would start arcing over.

 
Posted : 08/03/2024 8:57 am
Nuvistor
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@irob2345 Only ever replaced one and that was in a 19 inch set that had the same electronics as the 25 inch set but in a much smaller cabinet, bit of a hot box. The end result of those instructions would be 25kv and 1.1ma through the shunt at no brightness, it does say there would be some interaction. In practice I didn’t find it a problem.

Most of the technicians in the UK had done the the RTEB/City and Guilds exam, 5 year day release course for Radio and mono tv and an extra year for Colour. Later exams were full time I think for two or three years for the later exams. All the techs I knew had completed the education and despite pressure of work did a decent job. I left the trade in 1980, don’t know what went on after that.
No doubt there were others who didn’t.

Frank

 
Posted : 08/03/2024 9:48 am
Till Eulenspiegel
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Ten years after the RCA CTC2B was on sale RCA in the CTC20 continued to employ the 6BK4 shunt stabiliser tube.

RCA CTC2B Line output

Till Eulenspiegel.

 
Posted : 08/03/2024 12:50 pm
Nuvistor
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@till V123 is interesting, it’s as though it’s using the Focus voltage 6,700 volts to even out the EHT pulses that provide the 25Kv EHT. There would always be a positive voltage on the EHT rectifier anode.

Does that make sense?

Frank

 
Posted : 08/03/2024 1:07 pm
Captain Peacock
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According to the Philips G6 SS Spanish Waiter, this is how one set up the EHT and shunt stabiliser bias (see attached schematics).

1) Set R5040 for minimum boost voltage. Connect a minus 30V DC feed to the grid (pin 8) of the PD500. Disconnect the luminance feed (brown wire IIRC) to blackout the CRT. Connect an oscillioscope to pin 5 of socket 8 (lower LOPTX socket IIRC). Adjust L5515 (3rd harmonic tuning coil) for equal peaks on the waveform on pin 5 of socket 8. This will change to the first peak being higher than the second once the -30V has been removed from pin 8 of the PD500 and R5040 has been set for 590V (see below).

2) On a normal picture, set the Line Stabilisation Control (R5040) for 590V across C5013 (pins 11 & 12 of LOPTX or pins 2 & 4 of Socket 8).

3) Connect a high impedance voltmeter across R5054 (cathode bias resistor of the PD500): unplug the luminance feed to the CRT to blackout the CRT and adjust R5053 for 1.2V across R5054. Reconnect the luminance lead.

Hope that helps.

Philips G6 SS LOP Stage 2
Philips G6 SS LOP Stage 1

 

 

 
Posted : 09/03/2024 11:21 am
irob2345
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Same issue.

I reckon the PD500 should be disabled when the boost voltage is set. It does not say that.

The instructions make no assumption about where R5040 is set when the line stabilisation pot is adjusted to set the boost voltage. What if it's set incorrectly and the line stage has to make up the difference?

At the very least, the boost voltage should be re-checked after setting the no-beam-current PD500 bias with R5040.

There are two control loops at work here, care is needed to ensure one doesn't impinge on the other.

 
Posted : 15/03/2024 10:25 am
Nuvistor reacted
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