Golborne Vintage Radio

I have just scanned the service manual for Prowest PM14/1A and PM19/1A professional dual standard video monitors and uploaded it to the archive: https://golbornevintageradio.co.uk/forum...p?tid=8059

My scan is from a photocopy I made many years ago, probably when I was working at Thorn-EMI CRL c1986. I don't have an original copy. I don't know why the photos have come out so small compared to the text. I did all the scans as 300dpi TIF. Then I cropped them, reduced to 2 bit monochrome and saved as GIF. Apart from the photos which I cropped and saved as JPG. I then used Irfanview to assemble all the pages into a PDF. I suspect this wasn't the ideal tool for the job but it was the most convenient. If anyone wants to have a go at doing a better job I can send the set of files.

These are probably the last 405 capable monitors ever manufactured. They also do 625 with automatic sensing of line standard. Fully soild state except that some versions had a valve EHT rectifier. Capable of giving superb quality pictures. They have separate EHT, fully stabilised.

Prowest also did lower quality 625 monitors, the 3 series and 5 series. The 3 series were actually very good, though the 11" version usually has a low emission CRT. I've rarely seen a 5 series. The PM22/7 was an early solid state colour monitor with 22" delta gun shadowmask CRT. Very nice pictures when they were working but reliability was poor (I'll admit they were quite old when I got some) and servicing a nightmare due to interlocks. I never got the knack of fixing them and if ever I see another one on the bench it will be too soon.

Out of interest Jeffrey, do you know what year approximately that these last 405 monitors were manufactured, please?

Best regards,

Francis

c1967

Thanks Jeffrey - so a couple of years before 405 became a 'legacy format' (if that's counted as November 1969).

Best regards,

Francis

They might have been in production for a couple more years after that. I'm pretty sure they were not made by 1970. The only requirement for 405 monitors after 405 line programme production stopped would have been to check the output of standards converters at transmitter sites.

Not sure when programme production in 405 stopped. Probably not long after BBC2 started in 1964.

I wonder if the ITV companies switched to 625 for programme production early on or waited till nearer duplication?
Certainly STV had the edge over BBC1 on picture quality even after duplication. Many customers in the early 70s didn't want to ditch their 405 line sets. Sets like the Ekco T368/377, Bush TV115 and Murphy V759 gave brilliant 405 line pictures and they weren't interested in high brow stuff on BBC2 so didn't want the expense of a dual standard TV and new aerial either.

Turned on the PM14/1A this morning for the first time in ages. Guess what..... It didn't work. I could hear line whistle and it autoswitched to 405 when given a 405 line input. No EHT rustle. Remember this has separate EHT and line scan so you can hear line whistle even without EHT.

Drag it out of the rack on to the bench and probe around. About 2kV EHT. Normal-ish waveform on the EHT output transistor collector but something is clearly not happy. There's a 1R resistor where it's convenient to see how much current the EHT output is taking. I don't know what it should be but 2A is much too high.

Disconnect EHT smoother and EHT comes up with much discharge around the disconected point. One of the photos is of the EHT transformer with the cap just visible bottom right. This was taken in 2013 when I had much fun fixing the EHT generator. The output transistor had blown and I had fitted a BU208A. This was fine on 625 but overheated on 405. Took a while to find that one. The BU208A has a much lower current gain than the original BU102 and was coming out of saturation at the end of the forward stroke. Somebody gave me a BU102 and that fixed it. I also rebuilt the EHT rectifier assembly which was very prone to corona. There should probably be an anti-corona metal cup on the EHT cap terminal but I didn't have one and it would have been awkward to fit. I think the original mechanical design of the EHT stuff was a bit iffy and I got it to work OK without corona. EHT is about 16kV.

The EHT smoother cap (1000p, 20kV) was near enough short circuit. Look in spares drawers and by a miracle there's a similar looking cap. See both in other photo. Try to fit replacement and find it's got a 2BA screw hole instead of 4BA. Not hard to fix but I'll have to take out the EHT board which is a hassle. Also ream out the tag without damaging anything. Or replace it.

In theory the monitor ought to be able to work without an EHT smoother, just relying on the tube's graphite. If I can get it to stop corona'ing like made without the cap I'll give it a go.

Seems to work OK without the EHT smoother so I'll fit an insulated spacer in its place. With luck I'll have something tapped at 4BA. Still need to dress the EHT wiring (from diode, to CRT and long yellow 250M sense resistor) very carefully to avoid corona.

It looks quite compact in there Jeffrey.
It can help to lightly cover the EHT cable with silicon grease or better still specific EHT resistant grease, also around the CRT anode cap.

It's more spacious than it looks in the photo. The CRT anode cap has never caused any problems. I've backed off the EHT slightly, from 16.4kV as stated in the manual to about 15kV. Makes no visible difference to the picture quality and will reduce any tendency to corona.

I've fitted a plastic spacer instead of the capacitor and all seems well.