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.
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
next thing is to sort out the timebases..
dropper is producing very close to the correct voltages
at all the various points when the mains voltage is 235
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.
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.
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
this back, the frame timebase behaved much more smoothly.
However there was still far too little height.
various blind alleys, I found that connecting the windings
of the blocking transformer back-to-front produced more
sawtooth drive - and thus sufficient height.
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.
set's firmly on the road to success now.
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
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.
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.
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.
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.
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
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
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.
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.
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
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.
the tube heater current on my Avo, and sure enough
it was near spot-on. No point in doing anything here...
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
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!
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.
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
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.
viewing will have to be done in subdued light. Under such
conditions viewing is really very pleasant but ideally this
tube should be replaced.
'improvement' I've carried out is to increase the capacity
of one of the audio coupling capacitors - for that little
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.
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.
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.
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
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
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
will be able to enjoy a two-hour programme of vintage entertainment,
very similar to what would have been seen some time around
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.
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.