several months have passed and the set has now been returned by
the customer for two reasons:
1) It has gone wrong
- it's ceased operation altogether apparently.
2) A re-gunned,
re-screened and aluminised CRM123 tube is to be fitted.
On this page you
can follow a blow-by-blow account of what happens next. We'll
discover what has gone wrong with the set and also see what difference
a 'brand new' CRM123 will make.
Here's the new tube
in its box, having just arrived from RACS
on the continent. It's a valuable service this French company
very well packed it was too. I'm expecting this tube will give
a brighter and more sparkling 'modern' looking picture than the
original un-aluminised CRM121.
The reason the set
had ceased to work? This 20P1 line output valve had blown its
heater, meaning one of the two valve heater chains was open-circuit.
Fitting a new 20P1
immediately restored full operation.
Not much I could
have done about this. Unfortunately I'm dependent on the quality
(or lack of it) of the original components.
The next thing to
do is to try the new aluminised CRM123 tube. This replaces the
non-aluminised CRM121, also previously re-gunned by RACS.
Once the new tube
is installed a picture is immediately obtained. It's definitely
brighter but is blurred. Presumably there are minor differences
in internal electrode positioning between the two tubes that will
account for this.
To obtain focus,
it's necessary to remove one of the two yellow sponge rubber spacers
in front of the black focusing magnet and move the whole magnet
about ½ inch further forward. This is mounted on three
studs so it's easy to do.
This photo doesn't
give a good impression of the actual quality of the picture -
but shows it's definitely brighter !
This is a better
view though it doesn't really capture the 'new tube' sparkle of
the picture. The picture is sharp and video bandwidth is very
good with plenty of gratings clearly visible.
The set is now being
left on to soak for long periods - with a clear conscience - this
new tube is loafing along on a mere 7Kv of eht. With its aluminised
protection it should last for years of regular use.
There are two 'sort-of'
faults remaining. Sort-of, because they're not usually present.
The first is occasional mild hum on the frame and line scans (but
on nothing else) and this is proving very hard to trace. The second
is variability in the sound. I've been trying swapping valves,
but in both cases I feel it might be worth carefully checking
the valve bases.
Think I've gripped
the sound problem... First I found that pin 2 on the base of 10P13
sound output valve (the anode connection) was showing signs of
charring. This was scraped away. The base appears to be otherwise
undamaged. I then checked the valve wasn't running too hot and
that the cathode bias was OK.
A connection to
the sound detector valve base also seemed noisy and (for now)
has been bent to a position where it remains silent.
The hum problem
has been largely solved. It was a layout issue.
This was probably
my fault, since I had had to mount an extra resistor (with the
thermistor) directly behind the U801 HT rectifier. This was necessary
because the replacement dropper didn't have all the sections needed.
AC hum was getting
into the sync separator valve nearby.
I've now added a
screening panel (arrowed), resulting in a significant improvement.
It is probably impossible to remove this hum entirely since it
may well have been a deficiency of this design. After all, Baird
mentioned using it for linearity correction! As it is, it is only
visible on Test Card, and only then on close inspection.
Here's the sound
IF section. I've now re-soldered all the connections. So far,
so good - the sound is stable though I won't have faith until
a lot more long term testing.
Then the picture
went to a lower contrast... this was correctable by the contrast
control but shouldn't happen. It will be have been caused by one
of the wretchedly unreliable 10F1 valves going low gain. This
seems to be a regular feature of these valves. Mazda didn't only
make poor cathode ray tubes. They made poor valves too...
The New Year arrives
and with it a complete set of brand-new 10F1s ! All are
tested and then plugged into the set, which is now to undergo
soak testing for days on end...
The bar on the picture
is caused by the camera shutter, since it's now operating in daylight
conditions. The actual picture is really quite good. I've also
changed the value of a capacitor in the line timebase to ensure
the picture width will be ample in future.
And so the set was
returned to the customer, 110 miles distant, and re-installed
in its cabinet still in his drawing room. Then the next day I
received a phone call from him. Normally when the phone rings,
it means a problem has cropped up, but this time it was different.
The customer proclaimed himself 'thrilled' with this television
and has suggested I quote him here. With this Baird, he's regained
something he remembers in his home as a small boy. To give pleasure
like this is one of the perks of my job !