Vertical Blocking Oscillator Transformer
In my experiences with restoring valve
television sets, particularly ones from the 1950's, one of the most common
faults is the vertical blocking oscillator transformer failure.
Usually, the transformer shows signs of
impending failure before it actually stops oscillation. Such signs include
poor field linearity which cannot be adjusted out, strange looking interlace,
and poor synchronisation.
The worst transformers seem to be the
ones which are used in the plate circuit of the vertical oscillator (often
a 6BM8 triode). This type of oscillator design is common in the European
type of design, used by such brands as Philips and Kriesler. The other
type of oscillator circuit employing a blocking oscillator transformer
has the transformer in the cathode circuit of the oscillator triode. This
type is common in sets of US type design, such as AWA. It could be that
having the transformer in the cathode circuit means less turns on the windings
(lower impedance) thus resulting in better reliability. In fact, I don't
think I've ever had a failure of this type.
So, what to do? At first I would replace
the transformer with another one, but eventually it would fail again, and
I didn't always have the right transformer. Besides, the vertical locking
never seemed to be much good anyway. So, I decided to design an electronic
solution and eliminate the transformer altogether. Why this type
of circuit found favour I can't understand. Plenty of sets used a multivibrator
instead, eliminating the costly transformer.
While a few sets used a separate twin
triode for the vertical oscillator (often a 12AU7), most actually used
the output triode or pentode as the other half.
My design uses the separate oscillator
approach. I did this to allow flexibility of design; i.e. using it with
other sets than the ones I'm describing here. Also, using the output valve
in the oscillator circuit is more critical in that linearity and height
settings can change oscillator performance.
This first circuit
was implemented in my 1958 Philips 21CT335. A 6C4 triode was added to form
the multivibrator in conjunction with the existing 6BM8 triode. You can
of course use other valves; a 12AU7 triode for example. I simply mounted
the extra socket on a piece of aluminium and screwed it to the wooden side
panel of the chassis. The improvement in performance was amazing. Lock
was so strong that the hold control had to be turned right to the extremes
to cause loss of synchronisation.
Note that the output
side of the circuit is no longer original. That has nothing to do with
the oscillator modification and was actually done with the original oscillator
circuit in place. I had to rewind the output transformer as the primary
eventually went o/c. It's 3000 turns of 39 gauge wire if you're interested.
As it happened, after rewinding there wasn't enough room for the feedback
winding to go back on, so I ignored it and modified the circuit to a more
conventional design. Note also the blanking level has been increased. This
is due to the teletext problem.
This next circuit
was designed for an 1957 HMV F1, but is obviously applicable to the other
F series chassis. These HMV F series sets are notorious for transformer
failure. Despite that, they are one of the best performing sets in Australia.
If you wanted a set designed to text book principles with no cost cutting,
this is it. As far as the vertical section goes, linearity is so good that
a user control is not provided. As you can see, current feedback is used
to optimise the waveform. The yoke current is sensed and fed back to the
6BM8 output pentode grid via a small transformer.
Again, the substitute
circuit uses a cathode coupled multivibrator. The transformer was removed
and the 6BX6 mounted on an aluminium bracket in its place above the chassis.
In view of how the sync pulses are fed in to the oscillator, I decided
to use a pentode with the sync pulses fed into the screen grid. This worked
exceptionally well. Incidentally, an interesting feature of the HMV is
that the vertical oscillator plate supply is taken from the normal B+ rather
than the B+Boost. No doubt the feedback network in the output stage compensates
for the less than linear waveform.
The third circuit
was for a Kriesler 79-1. Not suprisingly, the design is similar to
Initially, out of
curiosity I did try using the 6BM8 pentode as one half of the oscillator.
While it worked, it was unreliable and critical, so went back to the tried
and trusted method. I have a number of 12AU7's with one triode faulty,
so I used one of those. Note that R138 is reduced to 1.2M in order to get
sufficient height. To see how good
the linearity is, have a look here.
It should be obvious
that the circuit can be adapted to other sets, but clearly some experimenting
will be required to optimise performance.