Bush TV22 returned for repair.

[FERNSEH]

The restoration of this Bush TV22 appeared in this forum in May 2014.
https://www.vintage-radio.net/forum/...d.php?t=105828

Last Friday the set was bought to the workshop for repair.
Yesterday, I set about finding what had caused the set to break down.
The mains fuse inside the set was found to be OC. To cut the story short the PZ30 HT rectifier and boost diode was the culprit. Fireworks display inside the valve. The faulty valve had taken out other components, a heater chain capacitor in the RF deck, yellow capacitor replaced by a polyester. See pic 1.
Switch on the set, only a tiny spark at the anode of the EY51 EHT rectifier.
Fearing the line output transformer was faulty I continued fault finding. Check the boost capacitor, dead short! After replacing the capacitor with a temporary component the results can be seen in pic 2, a badly focussed picture. EHT only 4,5KV.
Readjusting the line drive trimmer improved matters considerably, EHT 7KV and a bright picture with good focus.
Like all TV22s the interlace is not good. We learnt that was the result of too much integration in the frame sync filter circuit. a simple modification will improve interlace and reduce line pairing.

DFWB.

[peter_scott]

Excellent!

Peter

[stevehertz]

Very good. What is that mod to reduce line pairing?

[murphyv310]

Hi.
Reminds me of a TV22 I used to look after in Glasgow. I did the set originally in 2004, after a full restoration it ran until 2014 when the owner told me there was no picture. The fault turned out to be the ECL80 line osc anode load had gone O/C, a decent metal film type no end. A new resistor restored full operation and the picture was as good as it was 10 years previously, plus this was his only TV!
about 6 months later a rather rude call to say it was faulty again. This time total disaster, the set must have appeared to have gone dead and the owner probably thought it was off. When I got to the set the PZ30 had sucked in and the resultant heat cracked the CRT, not only that the LOPT had shed all its pitch. Eventually he came clean and admitted he had gone out to the pub but realised the set was on, he came back rather worse for wear and fell asleep on the sofa, by that time the damage was done, fortunately it didn't go on fire, the cabinet also had a crack on it.
The chassis from my Frankenstein TV22 is from the very set as is the cabinet. I was able to obtain another TV22 chassis and got a cabinet and CRT which turned out to be very good. I returned it to him a year later for far less than the cost to me and told him I cant be held responsible for his safety or further work. A difficult thing to do but there is a lesson to be learned that no matter how well you restore a set, in certain hands any vintage set could land any of us in deep trouble. The correctly rated fuses didn't blow, perhaps when the PZ30 sucked in and cracked that "may" have killed the HT current, who knows.

[FERNSEH]

stevehertz wrote: "Very good. What is that mod to reduce line pairing?"

In a previous topic about the TV22 Dr. Holden remarked that the frame sync had too much integration of the broad pulses.
He suggested reducing the value of R11 from the present value of 100Kohms to 10Kohms. See attachment.
I've tried this modification and the interlace is improved considerably.
In the unmodified state even the presence of equalising pulses didn't improve interlace.

DFWB.

[stevehertz]

Thanks! I'll keep a record of that for when I get round to restoring mine.

Out of interest, back in the 70s I restored an Ekco TSC193 and a TS188 that were essentially the same set, and one had line pairing and the other didn't. I checked every component on the 'faulty' one but could never get it to improve.

[Lloyd 1985]

I tried the mod for interlace on my latest TV22, and as far as I can see the interlace is still a bit naff! Maybe this set is just like Steve’s Ekco!!

I’m going to sound a bit dumb here, but could someone clarify the appearance of line pairing? Is it where the lines seem to sit in pairs with a slight gap between each pair, or is it when they seem to sit on top of each other? I’ve always looked at trying to adjust it so that the lines all look like they have even spacing, but without them sitting on top of each other. I should really try and get some photos of what I mean!

Regards
Lloyd

[Andy - G8MNM]

I did the mod on my TV22 and the frame wouldn't lock, I changed the resistor back to its original value and all was fine. I did leave the replacement capacitor in place though.

Also I added flyback suppression as in the TV32 schematic.

I hadn't powered the set up for months - It had been stored inside in a centrally heated bedroom for the duration.

On power up the frame scan was very distorted. Turns out the blocking transformer had gone high resistance on its windings.

Happily, Ed Dinning rewound the transformer for me and when fitted the TV was back to normal.

I still need to remove the pitch on the LOPT and reseal the windings afterwards.

Andy

[Ian - G4JQT]

Originally Posted by Andy - G8MNM

I did the mod on my TV22 and the frame wouldn't lock, I changed the resistor back to its original value and all was fine. I did leave the replacement capacitor in place though.

Also I added flyback suppression as in the TV32 schematic.

I did the interlace mod to my TV22 and I could see no difference at all.

How effective is the TV32 flyback mod on the TV22/24? If it works, I wonder why (presumably for only a few pence) it wasn't included in the first place?

Thanks

[stevehertz]

Ok, so as there seems to be thumbs down to the resistor mod, what is the TV32 mod?!

[FERNSEH]

Maybe the frame sync modification wasn't such a good idea after all.
First two waveforms were taken from the cathode of the WX6 interlace diode. It is clear the frame sync integration function is working.
Waveforms show the odd and even fields.
The third waveform was taken from the anode of the interlace diode, the line sync pulses have been removed.

DFWB.

[high_vacuum_house]

Originally Posted by murphyv310

Hi.
This time total disaster, the set must have appeared to have gone dead and the owner probably thought it was off. When I got to the set the PZ30 had sucked in and the resultant heat cracked the CRT, not only that the LOPT had shed all its pitch. Eventually he came clean and admitted he had gone out to the pub but realised the set was on, he came back rather worse for wear and fell asleep on the sofa, by that time the damage was done, fortunately it didn't go on fire, the cabinet also had a crack on it.
The correctly rated fuses didn't blow, perhaps when the PZ30 sucked in and cracked that "may" have killed the HT current, who knows.

I would have liked to see that!! The PZ30 must have really been glowing to crack the CRT. I presume the neck cracked and not the CRT cone which would have imploded with a bang!!
I presume the original fault that caused the thermal runaway was a shorting HT decoupler or a fault in the LOPT stage. Not much you can really do except fit a thermal fuse, say over the power stages, though that would then involve a modification of the existing set design.
Did you take a photo of the melted PZ30?

Christopher Capener