Bush TV 12 TV12 TV12a TV
12a TV11 TV22
Back to home.
Bit of a challenge this one. After 50 odd years gathering dust, I was asked if this old TV12a could work again. I have steered clear of TV so far, as the line output transformers were so unreliable, and getting a 405 signal to test a set will be difficult. It would be heartbreaking to do hours of work only, to be thwarted by a dodgy transformer. Spare ones just don't exist. After drying out the set thoroughly, I had a fault finding session on it. looking first at the line output tranny, it looked pretty horrid, the pitch covering being cracked all over. The owner of the set Peter Carlton from North Devon had done loads of excellent work on the chassis already,with the Main smoothing, and all the paper capacitors changed. The AC/DC design of these early sets are pretty lethal to work on, as the chassis can be live if the 2 way mains connector is plugged in the wrong way, or the on/off switch goes faulty. I use an in line mains R.C.D. This gadget will cut off the supply if there is a mains leak to earth.
Anyway think
positive, check the heater chain, (All the valve heaters are in series, except
the EY51 which gets its heater supply from a small winding on the LOP tranny.)
and connect to the mains supply via a variac, to bring up the mains supply
very slowly. All the heaters and the H.T. come up O.K. but no healthy line
whistle. With no line oscillator, and hence no drive to the PL38 line out
put valve control grid, the the PL38 will draw way too much HT current the
overstressed anode glowing cherry red. Switch off quick. A quick look at the
circuit shows a self oscillating output stage with the triode section of the
ECL80 and the PL38 and feedback from the line output transformer. To make
hot checks to the line stage, the best way is to disconnect the HT feed to
the line output valve screen grid, this will stop everything getting overheated.
I suspected the line out put Transformer with a shorted turn, as it did look
in a bad way, however it turns out to be falsely accused. After a few DC checks.
The line drive adjuster C28 a 800 pf variable mica capacitor turned out to
be the culprit. With a fixed 500 pf mica cap fitted and the feed to the PL38
screen reconnected. Switch on and check. Relief a line whistle, now loud and
clear. But still no raster on the screen. A glow can be seen from the EY51
indicating the line output stage now working. More DC checks reveals no anode
1 voltage on the Picture tube. C31 was dead short a 0.1 uf paper capacitor
missed by Peter, as it does hide away underneath the tag board. With this
fixed, a raster of sorts was obtained.
See the great layout for servicing. The sound and vision signals
panel slides out for acceptability The line output stage on the top right.
After obtaining a raster, The next problem was to get a suitable signal to
test the set with. I have a philips digital signal generator PM5326, The set
is a band 1 TRF, All the EF50 sockets and valve pins had to be serviced and
run through the Mullard High speed tester, a couple
of low ones were replaced, and with the internal modulation set to on, the
vision and sound was now being received. (Dark horizontal bars on the screen).
The Philips generator has an external modulation facility, but according to
the book only up to 30Khtz, it's worth a try, so, with Peter's recording of
some old 405 stuff on the standard VHS video machine, It was all lashed together.
There was something shadowy and jumping on the screen, but nothing resembleing
a picture. I Thought it was a waste of time, then remembered video is Positively
modulated on 405. Changing the input around to the Sig gen worked the Miracle.
It's going !
A pretty poor picture, but great to see the old 405 again after all these years.