Regular customer has brought this tidy Bush TV22 for repair and restoration. It's already on the workbench and the first action is to feed a small current through the EHT winding. Bench power supplier set to full 30volts. Coil resistance is 790 ohms so the current is just under 40mA. About 1.2watts dissipated in the windings.
Set is accompanied with a modulator unit. Circuit is the David Looser design and the complete unit has it's own power supply. The modulator printed circuit board looks very similar to the boards I made in the late eighties.
Anyone recognise this unit?
Till Eulenspiegel.
Hi, yes it looks similar to one I had specially made to order probably by the same chap you mentioned when his company was still making things and in the days before combined converters/modulators. Sold it eventually which I regretted. Fairly expensive but a very good product, mind you when I got mine there was no power supply, you had to supply your own.
The TV22 is now connected to the variac which is adjusted to supply the set with 165volts. The voltage will be increased in stages until the full 240V is achieved. The attachment shows a number of capacitors have been replaced.
Till Eulenspiegel.
Variac turned up to supply the set with 240volts. All's well for a few minutes and then the heaters of the PL38, PZ30 and V3 (ECL80) are glowing brightly. Switch off at once!
Almost certain one of the heater chain decoupling capacitors in the RF deck has gone short-circuit. Most likely culprit is C25, 0.001uF TCC waxie.
Till Eulenspiegel.
How about this?
Clipped short circuit capacitor C25 out of the heater chain and after a longish warm-up time and adjustments the set is displaying excellent pictures. For long term reliability all those waxies in the RF deck must be replaced.
Till Eulenspiegel.
That's pretty incredible! The TV22 I restored 10 years ago needed every wax capacitor replacing, they were all either open or very leaky. I suppose a lot is down to luck and how dry the storage conditions were.
That's great! Hopefully no major problems to sort out there. I've just realised this will be my oldest TV set, although perhaps only by a few months. The next oldest being the Decca 121.
That’s a nice clear picture! I notice it doesn’t have the usual Tv22 picture wiggle at the top! Mine has a slight fold over right at the top, which just affects the castellations, any idea what causes that?
Regards,
Lloyd
Hi Lloyd, the HT supply to the RF deck is decoupled by a 16mfd electrolytic capacitor which is situated on the upper deck. It is designated as C25 on the Bush timebase circuit diagram. Sometimes causes vision on sound. Also, there is an electrolytic on the RF deck which can cause similar problems. It is C4 and is 50mfd. Goes low capacity and causes impairment to the low frequency component in the TV waveform.
Till Eulenspiegel.
The frame sync in the Bush TV22 and TV24 does not perform very well if the standards converter is adjusted to include equalising pulses in the frame sync. It was discovered by Vintage-radio member Argus25 that the frame sync integrator time constant was too long. Resistor R11 (100Kohms) should be reduced to 10Kohms. The R11 is located on the reverse side of the component group board so to simplify matters just connect a 12K resistor across R11.
Till Eulenspiegel.
The Bush TV22 job can be signed off. Final results are impressive, the set is one the best TV22s I've ever seen. The MW22-16 CRT is excellent and thanks to Dr. Holden's modification interlace is perfect irrespective of frame hold and height control settings.
The RF deck. All the heater chain decoupling capacitors have been replaced along with the EF91 screen grid capacitors.
Coming up next. The modulator unit that accompanied the TV22 will be connected up to Derren's Pineapple field store standards converter. This converter PCB was assembled into a metal instrument box which also contains the power supplies and a modulator unit. The work on these units will be introduced in to separate topic.
The Pineapple converter.
Till Eulenspiegel.
I need to look into mine a bit more, I also tried changing R11, but the interlace was still a bit rubbish on mine.
Good result with the set you are working on!
Regards,
Lloyd
Showing the waxie capacitors which have been replaced on the RF deck.
Till Eulenspiegel.
A motley collection! Did you test any of them just to see how bad they are?
Have you ever noticed that some wax paper caps behave like batteries?
I once had a TV chassis full of such caps. Connecting them all in series I got 20 volts!
The TV had been out of service for at least 30 years and had survived a flooded garage.
All the the capacitors had leakage between 500Kohms and 1 megohm. It was not worth taking a chance to allow the suspect capacitors to remain in the circuit.
I have noticed that some suspect capacitors do hold hold a charge in the manner of a battery.
Till Eulenspiegel.
My 20v paper cap "battery" took a few hours to drop to 10 volts when connected to the DVM, so that doesn't sound like a charge (which is what I first thought), particularly since the TV hadn't been powered for 30 years!
More likely two dissimilar metals in an electrolyte! Paper contains acid and is hygroscopic. So hardly surprising, particularly in a warm, humid coastal climate.
What alerted me to it was when I read negative ohms on a leakage test!
That’s a nice clear picture! I notice it doesn’t have the usual Tv22 picture wiggle at the top! Mine has a slight fold over right at the top, which just affects the castellations, any idea what causes that?
Regards,
Lloyd
Hi Lloyd
From my experience, this image artifact arises from a wrong value of tuning cap (the 750V 470pF silver mica cap) and/or the wrong interwinding capacitance in the LOPT after a primary/secondary rewind. I have two LOPTs, one of them rewound and the second original. The rewound LOPT gives the wiggle/tilt at the castellations you mention when used directly but this can be corrected by adding more capacitance, which also lowers the value of the EHT to a more reasonable value and increases picture width. The other LOPT is original (although it has been stripped of pitch in a white spirit bath and was subsequently varnished) and does not produce the tilt when using the original 470pF cap. Exactly why the wrong capacitance produces the tilt is beyond my knowledge though.
Best
Helder
