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Bush TV32 with frame timebase fault.

 
Till Eulenspiegel
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A Bush TV32 has come into the workshop with the report that the frame timebase has failed. Initial tests reveal that the blocking oscillator has an open circuit winding. The frame timebase circuit is very similar to the TV22. The exceptions are the omission of the interlace diode and the addition of a preset interlace control.
The oscillator transformer is of a different construction to the one in the TV22.

Attachments show the circuit of the TV32 frame timebase and the faulty oscillator transformer. Lots of Hi-Vis capacitors in this set.  

BushTV32 sync 2
BushTV32 FOT.

 

Till Eulenspiegel.

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Topic starter Posted : 11/10/2019 5:09 pm
Nuvistor
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Is it this series of sets that the blocking oscillator windings are a known problem? I could be thinking about other makes and models!

I don’t mind the modern capacitors used as replacements and left on show, I think what would have been use when the set was in service, if repaired in the 50’s it would be paper, 60’s polyester or mixed dielectric etc.

Still each to their own, the beauty of the hobby.

 

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Posted : 12/10/2019 1:36 pm
Till Eulenspiegel
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Hi Frank,

the owner of the set is happy with the Hi-Viz capacitors when such components are under the chassis and not seen. The blocking oscillator transformer is electrically identical to the one in the TV22 so with modification it will work in the TV32. However, I've found a NOS Radiospares FB7 transformer which was made as a replacement for the HMV 1824 series.  The substitute transformer will be tested in the set by long wires. If it works correctly then the  chassis will come out and the new part will be fitted in the position of the original transformer.

The blocking oscillator transformers do seem to be prone to failure but that's more  likely to be green spot corrosion rather than an inherent fault.

Till Eulenspiegel.

 

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Topic starter Posted : 12/10/2019 9:42 pm
Till Eulenspiegel
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Last night the substitute frame blocking transformer was tried out in the TV32.

Rather than take a chance of fitting the HMV transformer in the chassis long wires were used to test the alien part.  The good news is the frame oscillator is working again so the next stage of the repair will be to remove the chassis from the cabinet so that the new transformer can be permanently fitted.

Good interlace can be difficult to achieve in these sets and this one is no exception.  There is an interlace preset control, see the circuit diagram on a previous post, but the adjustment for perfect interlace is critical and not reliable. More work to do in this department.  

Till Eulenspiegel.

BushTV32 FOT 2
BushTV32 FOT 3
BushTV32 FOT 4
BushTV32 FOT 5
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Topic starter Posted : 13/10/2019 10:24 am
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Till Eulenspiegel
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The replacement blocking oscillator transformer is now permanently fitted on the chassis.  As the Radiospares  transformer is slightly larger than the original this necessitated  that two holes had to be drilled in the chassis. 

Most service tasks can be carried out with the chassis in the cabinet but on this occasion it was necessary to remove the chassis. 

Now it's time to test the set.  It has been reported to me that the set has sound distortion at low volume control settings.

Till Eulenspiegel.

BushTV32 FOT 6

 

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Topic starter Posted : 14/10/2019 10:03 pm
PYE625
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Posted by: @till

 

Now it's time to test the set.  It has been reported to me that the set has sound distortion at low volume control settings.

I wonder if the speaker cone is rubbing.

Well done for getting the running with a similar transformer.

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Posted : 14/10/2019 10:08 pm
Till Eulenspiegel
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Hi Andrew,

                        You could well be right. I've found that the loudspeaker suspension could be better, there might be particles of dirt between the speech coil former and the magnet. 

The chassis is still outside the cabinet so to maintain heater chain continuity tests are being carried out with tags 1 and 12 of the CRT socket connected together.  The 'scope confirms that the frame timebase is OK.  There is a -200V 1mS pulse available from the blocking transformer which can be used for frame flyback blanking.  Some TV32 sets already have this and it's something that might be worth considering for Mark 1 TV22 sets.

Till Eulenspiegel. 

 

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Topic starter Posted : 14/10/2019 11:36 pm
Till Eulenspiegel
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Posted by: @till

There is a -200V 1mS pulse available from the blocking transformer

Correction: the amplitude of the blocking oscillator flyback pulse is -50V.

Till Eulenspiegel.

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Topic starter Posted : 15/10/2019 10:02 am
Till Eulenspiegel
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Checking the main HT line for AC ripple we find that it is 2volts peak to peak.  Is that too high?   The TV32 has a 3 microfarad ripple cancelling capacitor which is connected in series with a 2.9Kohms resistor.  In normal operation the resistor runs very hot, hence it is a high wattage component.   The two components are connected between the "top" of the heater chain and the main HT line.

The capacitor has been found to be open circuit, no capacity in it whatsoever.  As a temporary measure I've tacked in a 4mfd motor run capacitor and the ripple on the HT rail is down to 1volt P - P.

It's likely the replacement capacitor will be smaller than the huge thing presently fitted in the set so will an adaptor plate be required?

Till Eulenspiegel.

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Topic starter Posted : 15/10/2019 4:28 pm
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Till Eulenspiegel
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Replacing the HT ripple cancelling capacitor.    

An RS Components 3uF 415AC motor run capacitor was chosen as the replacement for the old TCC "Visconol" capacitor.   The new component is much smaller than the original part so an adaptor plate and smaller capacitor clip was required.

BushTV32 FOT 7

The new parts were fitted yesterday and results are about the same as with the 4uF test capacitor, that is 1volt P - P ripple on the HT line. A new HT smoothing capacitor was tried but the ripple on the HT line was just about the same as with the original 1953 capacitor. 

Till Eulenspiegel.

 

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Topic starter Posted : 18/10/2019 10:32 am
Nuvistor
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Perhaps the HT current is slightly higher increasing the ripple, I don’t suppose we have any information about how much tolerance there was on HT ripple when the set was new. Fitting a tuned filter suggests the designers had some problems in that area.

 

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Posted : 18/10/2019 11:19 am
Till Eulenspiegel
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Hi Frank,  the ripple filter consists of a 3mfd capacitor and 2,900 ohm resistor.

There will be a phase shift function taking place in these components. The voltage across the capacitor will lag 90 degrees to that in the resistor.

Works in a similar manner as a capacitive main dropper in valve heater chains.

Till Eulenspiegel.

 

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Topic starter Posted : 18/10/2019 11:27 am
Till Eulenspiegel
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For the time being the original 60 + 250mfd HT smoothing capacitor has been refitted. As old as it is it's still usable, The capacitor was made March 1953. Replica Bush TV22 HT capacitors are available from various sources.

Ripple on the HT line is 1 volt P - P. I was hoping for a much smaller figure.

Till Eulenspiegel.

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Topic starter Posted : 19/10/2019 9:45 am