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405 line TV with black level clamp.

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abctelevision
(@abctelevision)
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I am wondering if any commercially built 405 line only TVs had black level clamp?

Did they give a better picture?

If someone can give details of make and model it will help me to keep a look out for one that I can probably do up. thanks

 
Posted : 23/03/2018 8:29 pm
crustytv
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PYE V4 is the first one that springs to mind, just one of the many innovations in its circuit, a very advanced set for its time. The cabinet also a very nice unusual design, all in all a great little set to add to your collection.

Picture #38 here

p.s.

I have the PYE manufacturers service manual, when I get a chance I'll scan and upload to the library.  ? 

 

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Posted : 23/03/2018 8:31 pm
abctelevision
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Thanks Chris. I had one of those in 1959, it was a 14inch. It had a vertical hold fault but I did not know anything about the insides of TV,s being only 9. I was constantly adjusting the hold. I will have to keep a look out, I think its the one with the sloping "washboard" front and the tuner at the back of the set?

 
Posted : 23/03/2018 8:44 pm
Nuvistor
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Hi Chris,

I am not sure about the black level clamp on the V4, I have never worked on one but a quick look at the circuit I couldn’t find one. The video signal is AC coupled from the white spot limiter to the CRT.

As you say a number of innovations including APC, Automatic Picture Control, this was gated AGC on probably the back porch of the line sync pulse, presume back porch due to it being longer the front porch.

If you get a chance to look at the circuit it would be nice to confirm or not. The TV had lots of innovation but I don’t think it was the most reliable of sets from what I have been told. That could be due to the circuit being too complicated for the components of the time or perhaps poor servicing.

 

Frank

 
Posted : 23/03/2018 8:53 pm
crustytv
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You're likely right Frank, I've not looked at the circuit. I vaguely remembered it had a number of features and thought B-L-C was one but now you mention it perhaps my fuddled memory mistook that for the APC.  Sorry Dave ? 

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Posted : 23/03/2018 8:57 pm
Cathovisor
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Of course, the other approach is to find a set that is DC-coupled from detector to tube.

 
Posted : 23/03/2018 9:35 pm
Till Eulenspiegel
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Pye V4.   No black level clamps employed in the video amplifier.   The DC component of the video waveform is maintained from the demodulator to the grid of the cathode follower.   The video developed across the cathode load resistor of the cathode follower V9A is coupled to the CRT cathode via a DC potential divider. A 2uF capacitor passes the AC component of the video waveform so in a sense the DC component is partially lost. 

However, things are different in the vision AGC circuits, there is no simple mean level vision AGC in the V4.  Back porch sampling of the video waveform is employed so that it is the true black level that is sampled and picture content has no influence on the performance of the vision AGC, or APC as Pye call it.

Till Eulenspiegel.

 
Posted : 23/03/2018 10:04 pm
Nuvistor
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Now that is interesting, the circuit I was looking at, the video was fed through the white spot limiter and via a 0.1 to the CRT.

Must have been some differences in models.

http://www.thevalvepage.com/resources/manuals/download.php?file=108&id=OmzB7srvR3

 

 

Frank

 
Posted : 23/03/2018 10:10 pm
Till Eulenspiegel
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Bush TV161, A640 chassis.    A clever black level correction circuit is employed, it's not a true black level clamp in the sense of having either a sync tip or back porch reference. Instead, the varying DC component supplied to the CRT cathode is tracked by a varying DC level which cancels out the changes of the video DC component.                      

The correction circuit works very well indeed.

Till Eulenspiegel.

 
Posted : 23/03/2018 10:18 pm
abctelevision
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Thanks Till for that.

 
Posted : 23/03/2018 10:21 pm
Till Eulenspiegel
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"Now that is interesting, the circuit I was looking at, the video was fed through the white spot limiter and via a 0.1 to the CRT. "    

Hi Frank,    It's a white spot inverter circuit.   Because the video DC component is maintained in the video amplifier stages and cathode follower, the white spot pulses are inverted and supplied to the CRT grid. White spot cancellation takes place in the CRT. 

Till Eulenspiegel.

 
Posted : 23/03/2018 10:25 pm
Nuvistor
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Ah yes missed the 2uf from the CF to the cathode.

Frank

 
Posted : 23/03/2018 11:55 pm
Till Eulenspiegel
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Pye 368.  A special version of the Pye 368 chassis was made to serve in special schools sets. An additional valve, an ECC82 double-triode was employed to serve as a cathode follower between the video amplifier and CRT cathode, the other  section of the valve was connected as a diode to function as a black level clamp.  The DC component of the video signal supplied to the CRT cathode was restored and the DC reference level was reduced to much lower potential than the anode voltage of the video amplifier.  The reason why a DC  potential divider was used between the video amplifier and CRT was in order to reduce the heater to cathode voltage supplied to the CRT.  A capacitor is required to pass the video signal to the CRT which results in the loss of the DC component.

Till Eulenspiegel.

Polite Mod Note: Not sure why but many of your posts have lots of spaces included which results in the post appearing badly formatted. This happens quite regularly and predominantly just yourself. The mods tidy them up and they don't mind doing so, however we bring it to your attention that you have the ability to edit your own post for up to 20 mins to make the corrections yourself.

Thank you for your anticipated cooperation in helping to keeps posts looking tidy.

 
Posted : 24/03/2018 10:42 am
Focus Diode
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The Thorn 950 series gives excellent results but does not employ a black level clamp circuit. I remember an article in Television magazine about adapting these sets with this facility. I wonder if anyone did this?

Regarding the polite mod note, at least when submitting posts using the phone I'veoften encountered problems with extra spaces and jumping back a paragraph when making corrections and the likes. Submitting this post is no exception. Don't understand why this is.

I've personally observed David typing in posts, he takes the upmost care at all times.

 

 
Posted : 24/03/2018 6:34 pm
Till Eulenspiegel
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I've personally observed David typing in posts, he takes the upmost care at all times."

Hi Brian,   thanks for the vote of confidence.   It seems when the "align left" function is implemented the messy layout problem manifests itself.    Using the edit facility I attempt tidy things up but the results are still a messy layout again.  Best not to use the align left function. 

The topic about video DC restoration has appeared in the UKVRR&R Forum.

Till Eulenspiegel.

 
Posted : 25/03/2018 2:06 pm
Till Eulenspiegel
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I've been using my Pye  V600A on a regular basis and without doubt there's a TV set that really needs a black level clamp.   The clamp circuit  employed in the Pye 368 schools set would be the ideal solution to the problem.  

The V600A was one the first 23" CRT TV sets to be marketed in the UK, uses an imported 23SP4 tube.  

From an earlier post about the subject:

 Pye 368. A special version of the Pye 368 chassis was made to serve in special schools sets. An additional valve, an ECC82 double-triode was employed to serve as a cathode follower between the video amplifier and CRT cathode, the other section of the valve was connected as a diode to function as a black level clamp. The DC component of the video signal supplied to the CRT cathode was restored and the DC reference level was reduced to much lower potential than the anode voltage of the video amplifier. The reason why a DC potential divider was used between the video amplifier and CRT was in order to reduce the heater to cathode voltage supplied to the CRT. A capacitor is required to pass the video signal to the CRT which results in the loss of the DC component.

Till Eulenspiegel

 
Posted : 27/11/2018 9:58 pm
Nuvistor
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Do you have a circuit for the special Pye 368 video stage?

 

Frank

 
Posted : 27/11/2018 10:17 pm
Till Eulenspiegel
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Hi Frank,                                                                                                                                         

BL clamp modification to an Ultra V17-63.  The attachment shows the circuit of the black level clamp that was used to  solve the total loss of the  DC component of the video signal supplied to the CRT cathode.   In  the Ultra V17-63 the contrast control is  the load resistor of the video amplifier.  A similar arrangement can be found in the Pye model 11. My circuit is similar to the BL clamp in the schools version of the Pye 368. The semi-conductor diode and capacitor form a beam current limiter. copied from the Rank A816 chassis.

Till Eulenspiegel.

BL Clamp 01
 
Posted : 28/11/2018 12:30 am
PYE625, PYE625 and PYE625 reacted
Nuvistor
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Thank you David, I will look at it later today, I always like to look at circuits, sometimes don’t get how they work correctly though, keeps my mind active.

 

Frank

 
Posted : 28/11/2018 7:32 am
MurphyV310
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Interesting circuit David. The Pye 11U was one of the sets I particularly disliked, it was grim on dark scenes and for a company to make a set like this after the advanced V4 beats me.

Cheers,
Trevor.
MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member

 
Posted : 28/11/2018 8:34 am
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