Obscure problem cured with a Bush TV22

[Boom]

I thought others might be interested in a problem that I had recently with my TV22.

I often get remarks on what good condition the set is in. It has been restored to what I think is a very high level with every nut bolt and washer that should be in the set present. The tube is exception with no Ion burn and 100% emission.

Lately the set has been coming on apparently fine but after 30 minutes or so the width would come in slightly from the left, the picture would go smeary as though something was up in the video output stages and the focus control would drift off requiring setting to an extreme end.

I had assumed that the only common factor for all three faults would be the EHT dropping down but replacing the PZ30 and PL38 made no different whatsoever to the HT rail which seemed fine anyway and I never had an EHT meter so was starting to doubt the LOPT insulation. Even worse, at the back of my head something was saying 'maybe tube H-K problem.'

The boost rail is very difficult to measure even with a digital meter as it is so high impedance but measured 150VDC at the metrosil. A bit low but I'd hardly have thought it was responsible for all three symptoms and I was fairly confident that I was looking for a common fault rather than three separate faults.

After measuring just about every voltage in the set I found that there were no problems whatsoever with all of the measurements that I took proving fine. so what was causing it?

I'd boxed the set up and was wondering what I was going to do next when for some reason I thought I'd take voltage readings at the tube to make sure the heaters were not partially shorting intermittently. The picture came on so perfectly though that I still doubted this really.

While waving the meter about I put it on the tubes A1 connection (pin 10) and lo and behold there was no reading! There must be volts there I thought as the metrosil has 150VDC on it but no amount of scraping the meter probe against the tubes pin brought forth anything.

At that I fairly flew at the A1/metrosil circuit and found that a wire which goes from the metrosil to the tube base and which I had replaced when rebuilding the set had never been soldered The A1 was effectively floating!

Resoldering the lead brought the picture back to perfect with all of three of the faults gone but I was left wondering how the A1 volts going missing due to an o/c lead could have caused lack of width and drifting focus. I'd have thought that the A1 voltage going missing would have reduced rather than increased the load on the LOP stage. Obviousness not so.

Once the set was running properly I also noted the effect on the picture of taking a voltage reading from the A1 pin 10 to deck and there was a noted decrease in brightness due to loading so the 150VDC probably is far higher than what I am actually measuring. It would be interesting to know what an Avo gives on the A1 but an Avo is something that I have not had for years.

[KeithsTV]

The service manual says the A1 voltage should be measured with an electrostatic meter. Presumably to eliminate the loading effect of the meter. An AVO8 on the 1000v range represents a load of 20M which may be too much load for tha A1 supply.

Keith

[Boom]

Originally Posted by KeithsTV

The service manual says the A1 voltage should be measured with an electrostatic meter.

Thanks Keith, I suspected that might be the case.