Black-level restoration mod, Bush TV125

One of the reasons for AC coupling from the output of the video amplifier to the CRT cathode is to reduce the heater to cathode voltage. An easy solution to maintaining the DC component is simply supply the CRT heater from a transformer and connect the cathode direct to the video amplifier.
The brightness control voltage will have to changed if it has not sufficient range.
Another solution is to fit a sync tip clamp circuit at the cathode of the CRT. Problem is that positive going sync tips are not so easy to clamp as negative going tips.

Till Eulenspiegel.

Ive managed to gain access to the Keth Cummins 'Adding Black-Level Clamp' article on That Forum.

Its quite a simple mod, and the article explains it well. However, it says the mod is ;Unsuitable for sets with Mean-Level AGC as opposed to Line-Gated.

The Bush appears to be Mean-Level delayed AGC according to the Trader sheet. I would just build it and try it, but currently don't have a transistor or diode suitable for it!

The article doesn't say Why the mod is unsuitable however--Any ideas why not?

A few replies and schematics came back that had been done to Thorn sets that should possibly transfer over to the Bush circuit.

Since then however Ive been banned from there for some unknown reason and cannot access the schematics posted...

Your wish is my command.....! I can only add JPG files here but I can email the others. Please PM me your email address. (won't be until tomorrow evening though....sorry)

Cheers.

Rich

The circuit shown on fig 3 is the answer, the emitter follower ((BF179 transistor) is a desirable circuit element. If you are after an all thermionic solution then the circuit employed in the schools receiver version of the Pye 368 could be considered. It's essentially a valve version of the circuit in fig 3 An ECC82 double triode is used, one section is diode connected and serves as the sync tip clamp. the other half of the ECC82 serves as a cathode follower.
The cathode of the clamp diode can be connected to the potential divider which is connected between the chassis and HT+ The purpose of the cathode or emitter follower is to isolate the sync tip clamp from the cathode of the CRT. It been found that variations in beam current effect the performance of the DC restorer.

Till Eulenspiegel

Here's a circuit of a black level clamp employing an ECC82. Note the cathode follower which isolates the clamp circuit from the CRT.
I back up what Murphy310 says about mean level vision AGC. it's not an ideal system, but it was cheap. The negative volts at the sync separator grid varied in proportion to the signal level and was applied to the IF and RF amplifier stages. All very fine but it did not take into consideration the picture content.
The correct method is to sample the signal level in the back porch after the sync pulse, this is what the Pye VT4 does.

Till Eulenspiegel.

Here's a clearer circuit diagram. The component values can be seen.

Till Eulenspiegel.

Doing proper AGC on +ve modulation meant the extra complexity of gating. On -ve modulation it's a simple matter to do peak level on the sync tips. Hardly surprising that most dual standard sets used simple mean level AGC, simply because of cost and complexity. Then having done mean level AGC (Always Grey Control) why bother with retaining the black level. Having done AC coupling why bother with decent EHT regulation.

It's those cheapskate circuits found in 1950s and 60s TV sets that are the reason why the 405 system was perceived by many as a junk transmission standard. Of course we know better.
However, I do remember in the 1970s when manufactures were beginning to do something about proper black level clamping and EHT regulation many viewers actually complained that during breaks and pauses in transmission the screen would go black. They liked it when the screen assumed a grey level between advert breaks and the like.

Till Eulenspiegel.

Two vision AGC circuits from 405 line receivers. The circuit from the Regentone TEN-17 is one of the simple mean level vision AGC systems mentioned earlier. The other is from the Ferguson 315T and is much better and really such a circuit wouldn't have cost too much more than the simple mean level AGC systems used in comparable receivers of the time.

Till Eulenspiegel.

Here's what Ive found so far.

The CRT Cathode in normal config is at 180V, the heater is in the last of the line to earth.

The Cathode of the cathode-follower from the video-amp is at 108V in normal config.

As a quick 'dodge' Ive shorted the 0.1uF cap in series with the tube cathode, placing it at the same potential as the video cathode-follower cathode, 108V
--Theoretically, there's less H/C voltage in this case.

Although not exactly fully DC coupled from the detector, (Det has 0.1uF cap coupling into grid of the vid amp) it appears a somewhat better displayed pic. The Video-amp is DC coupled to the cathode-follower that (now directly) drives the CRT.

It still has the tendency to grey-up on dark scenes, although the irritating 'feature' of giving streaks of black either side of bright captions against a dark (grey) background are very much lessened.

On scene changes from a dark or low contrasted scene to a bright high contrast scene, the picture will 'fade down' slightly, time-constant about 1/4 second--but it tended to do this anyway.

I'm pretty happy with it at the moment--Time will tell if I find any issues with this 'Botch'!

Ive played the set now for a while, tried various sources, and films etc.

It works very nicely with all media, particularly old 40's B/W movies. These are displayed much better than before.
--Where before the average background brightness was all over the place and that irritating streaking on dark scenes with bright captions, now the the background average brightness is much more stable. High contrast scenes also appear clearer and no loss of definition of the bright parts...

I would like to thank all above who added info and diagrams for the DC Restoration. I may try a few, although to be honest, I dont think its needed.

AC coupling between the vision detectors and the grid of the video amplifier in dual standard TV receivers can be a good idea. The attached diagram shows why. On 405 the video developed across the diode load is positive going and on 625 it is negative, athough the active video and sync pulses are in right sense.
So when the video is AC coupled to the video amplifier, the DC component is lost and the waveform assumes an average value. What was found that when employing a valve such as the PCF80 as the video amplifier, crushing of the sync pulses is possible on 625 as the grid goes more negative during the sync period.
The PCF80 has a short grid base. I discovered this problem in the sixties after converting Thorn 800 series TVs to dual standard operation. The conversion involved adding a separate 625 IF amplifier unit. The video amplifier used in the unit was a PCF80. Sync crushing occured on high contrast levels. The problem was solved by introducing AC coupling between the 625 video detector and the grid of the PCF80 video amplifier.
The choice of the PCF80 was bad one when one considers proper video amplifier valves had been around for a number of years before the introduction of 625 in the UK. The PCL84 dates back to 1958 and was first used in the Ferguson model 406T. The PFL200 came out in 1964 and there was the old PL83. Philips used that valve in their 19TG155 dual standard receiver of 1963.

Till Eulenspiegel.

The Bush does use the PCF80 as Video amp--Great explanation BTW!

I was rather surprised to see it doing that job myself, and notice the set is rather 'fussy' about the quality of that valve in that position.
--Either that, or I have a few PCF80 that are poo!

I was concerned about the negative nature of the video at the detector upsetting the bias of the video amp if DC coupled and this was confirmed when tried. A total loss of sync and very low signal resulted as well due to the AGC signal taken after the video amp stage, I guess a little messing round with a cathode-follower/emitter follower could well sort the DC level into the G1, but is it really worth it--I doubt it!.

At the moment and for the forseeable future, the set will be used exclusively 625 UHF, where with just the CRT coupling-cap shorted, gives good results.

We were told at Philips that using a PCF80 as a video amp was 'pushing it' but in the interests of economy, manufacturers went ahead. Poor thing was really run at it's limits and in a way, TV gave valves a bad name as being unreliable. I hear that the humble EF80 was also used in some circuits. Again an economy measure considering that decent video valves were about.

Rich

Here's an idea. In the the Bush TV125 the video amplifier consists of the pentode section of a PCF80 and the triode section serves as a cathode follower. It should be possible to insert the DC restorer between the anode of the pentode and grid of the triode. The circuit will resemble Pye circuit shown earlier in this topic. A silicon diode can be used for the sync tip clamp. The potential divider supplying the cathode of the diode can consist of two 100Kohm resistors thus setting the cathode of the picture tube to about 100 volts.

Till Eulenspiegel.

Here is my suggested circuit for a black level clamp for the Bush TV125, in fact it could be incorporated into other Bush receivers which employ the PCF80 as the video amplifier and cathode follower. Models TV105 and TV115 come to mind.

Till Eulenspiegel.