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Well, the good news is that both the tube and LOPT are fine! At last a raster (of sorts) is obtained. To obtain this though, I first had to replace the DC-carrying brightness control, whose track had worn through at one point, leaving the tube biased hard 'off'. This is the view using a mirror, incidentally.

Attention now turns to the timebases. In particular, the frame timebase is currently very non-linear. I hope the new blocking oscillator transformer hasn't caused this. Plenty of scope for experimentation tomorrow...

However the next thing is to sort out the timebases..

Andy Beer's dropper is producing very close to the correct voltages at all the various points when the mains voltage is 235 volts.

Putting a signal generator through the set revealed that it is a 'London' model (ie. tuned to Channel 1) and all the tuning and IF circuitry seem to be fine.

Anyway, out with the vertical hold control, which measured 50K instead of the correct 30K. Careful crimping of the contacts at the end of the track and other minor attention put this right and gave a nice smooth action.

Unfortunately, these potentiometers are not of good quality and must have given lots of problems originally. However I'm stuck with using them, if I want to keep the appearance of the rear controls original.

On putting this back, the frame timebase behaved much more smoothly. However there was still far too little height.

After pursuing various blind alleys, I found that connecting the windings of the blocking transformer back-to-front produced more sawtooth drive - and thus sufficient height.

There's a lot of fiddling around involved with this sort of work. For one thing, the modern serviceman often has to adapt non-original parts (like the blocking oscillator transformer in this case), and also this frame timebase has no linearity control, so it's necessary to juggle around with component values to get it to optimum. All very time consuming.

However this set's firmly on the road to success now.

The timebases are now working reasonably well - the line and frame holds seem dead stable. However, the picture is still on the dim side, only really viewable in subdued light. This photo gives a rather flattering impression.

The EHT, at 7Kv, is fine - as are all the other rail voltages, so this must be due to either or both of the following:

1) The tube's emission is on the low side. Several hours more intensive use may wake it up a bit, or... some special treatment - see next picture.

2) The tube's heater is not being fed with enough current.

To date, I've set this up to work at the correct voltage (2v), but it is really the current which determines the heater's temperature.

Now, to gently recondition the tube's cathode, I'm feeding 5 microamps from cathode-to-grid using a 50 volt power supply and a 10 megohm resistor.

It's noticeable that the correct heater current of 1.3 amps is not being reached until the heater voltage reaches 2.5v. So upto now maybe I've been under-running it. This could be causing the low emission.

The outcome after several hours of this treatment did indeed seem favourable. I also experimented with raising the HT by bypassing some of the rectifier current with a 1N4007 and a power resistor. The picture then became still brighter and sharper. Although, purists might throw up their hands in horror at my doing this, it does have another advantage - it takes a lot of the heat load off of the potentially unreliable U801 valve.

About this time one of the 10F1's failed too - it was the oscillator valve - meaning no picture or sound. An unwelcome development that warns me that these miniature valves could prove as unreliable as those on many a post-war tv set.

However, I hit a big problem today which could cause significant delay to finishing the job. The frame collapsed - the replacement blocking oscillator transformer had failed - and I haven't got another spare. I think I shall have to 'borrow' the one from my own Baird to get this job through...


And here is the 'donor' replacement about to go in. It did the trick too, though then yet another 10F1 decided to go down - the frame oscillator valve this time - which gave me a nasty moment.

I've reverted to using the HT rectifier alone again now - as per original. The higher HT voltage with the other arrangement didn't prove worth it and seemed to upset the delicate interdependence of many of the circuit voltages. There was more width and height, sure, but at the cost of degraded linearity and focus adjustment.

One of the rear mains fuses failed today. I measured the current which was normal - the fuse was just 'tired'. Both fuses were removed and re-strung by soldering in wire taken from a 1¼ inch 1 amp fuse. All was then well.

We're now well into that period when the freshly restored set needs to be run intensively to encourage all sorts of little faults to surface so they can be dealt with. It's nice to believe that a set is 'cured' once the restoration is completed but in practice this is almost never the case.

The faults which have surfaced now are mostly the intermittent, annoying type I seem to specialise in getting: ie. variable sound volume, some hum on the frame scan, and a 'notch' on the contrast pot.

Slightly worrying is a comment in the Baird service manual... "If frame linearity appears to be defective, reversal of the mains plug will usually rectify this fault".

In the old days, when the frame frequency was locked to the mains, this method could have compensated for poor linearity by applying hum occurring naturally in the frame timebase to oppose the linearity error. The frame frequency is nowadays crystal locked, which means that any hum will often be visible as a stretchy region travelling up or down the picture. Let us hope this proves to be a real fault and is not a design deficiency which may have to be lived with.

I was still worried about the picture brilliance - or lack of it - and the condition of the tube. When the fuse went earlier, this produced an excellent image of the cathode momentarily on the screen as the set expired, and I could see about 50% of it was dead.

I measured the tube heater current on my Avo, and sure enough it was near spot-on. No point in doing anything here...

Time then, to consider the last resort - re-activation. There was really nothing else left to try. In my careful hands, this process proved well worthwhile. Within a few minutes the emission had been doubled, and I switched back on to a much better picture. The emission is still nothing to write home about, but it is no longer dire, and I hope a week of soak testing will make it better still.


The next thing was to reduce the hum on the power rail to the frame timebase as much as possible. I found this old-style capacitor in stock was still in excellent condition and I bolted it in, to increase the smoothing on this rail.

The hum on the rail before (above) and after (below) the new capacitor was fitted. Doing this has indeed reduced the hum on the frame scan. It does seem that Baird's originally counted on using hum to counter non-linearity on the then mains-locked timebase. But as discussed above, this isn't good enough today.

In case you're wondering I had also tried other remedies, like decoupling the heaters, checking layout on the high-impedance parts of the circuit, and changing the valves - but to no avail. Any other suggestions welcome!

Anyway doing this has greatly improved the situation on the screen - and along with some attention to the sound detector - this means the set now is fault free at last...

And what of the actual results? Well this gives a good impression of how the picture looks under direct fluorescent light. It's not a bright picture - I would sum it up as 'good pre-war' for brightness, which it is really is in a way, since the CRM121 tube is basically a pre-war pattern triode.

Before getting this far, I re-visited the tube rejuvenator one last time and determined I really could get no more out of this tube. I've also given a 10% boost to the operating voltage of the previously slightly under-run tube heater, with good results.

I've now got it running through the Aurora and DVD player which will be included in the package supplied to the customer - and the delightful Sylvia Peters is now seen.

In practice viewing will have to be done in subdued light. Under such conditions viewing is really very pleasant but ideally this tube should be replaced.

Another minor 'improvement' I've carried out is to increase the capacity of one of the audio coupling capacitors - for that little more bass...

An interlude comes on. Turn the lights off and the picture leaves little to be desired.

But then I notice the picture is a little dimmer than it was. This has been caused by the mains voltage, which has just dropped from 240 to 230. Restoring it to 240 on the variac immediately brings back the brighter picture. A mains supply of steady high voltage is important for this set.

I've decided to matt black the active sections of the new dropper to assist radiation and so provide cooler running.

No this is not as original to be sure, but better-than-original - for increased safety and reliability.

It's now time to finally assemble the set. All the various fasteners and small parts are set out for cleaning up.

Here's the Baird badge, which sits at the front of the cabinet low down, being cleaned with some wire wool. This is very much like cleaning a coin.

At this time the grille guards which cover the small cabinet apertures are re-secured with large tacks. The tacks each have a spot of glue added on top, to ensure they cannot shake free in future. The knobs too are cleaned and polished.

Whilst being about to re-mount one of the grille guards I espy a manufacturing chalk mark on the inside of the cabinet. I don't know what it signifies but it's a piece of history, so it stays.

The back cover retainers are screwed back on. Each one has a 6BA washer added either side to make for smoother motion and less cabinet wear in future. Again, not strictly as original, but better than original.

Meanwhile, the grille spars have failed quality check. It's possible to see some unevenness at the edges of their upper (side) surfaces. The rest of the day now has to be spent re-finishing these so they will be fitted tomorrow, along with the other loudspeaker grille parts.

The next morning things are starting to come together...

And so the set is finished. Just as I was about to take a picture the sun came out (after several weeks!) making a good photo difficult.

It's hard to decide whether the tube should be sent away for re-gunning or not. Whereas it is obviously worn, and it flares on the whites with an excessive brightness or contrast setting, with proper adjustment of the controls it does give a pleasantly 'vintage' viewing experience and a quite acceptable picture. There are already signs it is improving under soak testing too.

The customer will be able to enjoy a two-hour programme of vintage entertainment, very similar to what would have been seen some time around 1950.

Like any vintage television set, it really wants to be enjoyed in a domestic setting. Here is my own set, which has been substantially modified to take a Mullard tube.

Owners of vintage television sets are members of a very exclusive club. They can tune in to a very special viewing experience, one which brings back the magic atmosphere of television - as it was at the very beginning.

The story continues - click here...

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