Newbie to Televisions working on GEC BT320

[NickG0HIK]

Televisions have not really got me excited in the past, I started with transistor radio's, particularly ones with S.W.
I have also worked on a few Domestic valved radio's, but it is WWII technology that has interested me in recent years.
As I side line I intend to build a Green screen television using the old Type 62A GEE display in the future.
But walking through a salvage yard recently I came upon a 405 line set, it interested me, I went home and did a little bit of homework and the idea of watching the old Dr Who's really struck me.
We returned to the yard a week or two later and the set was still there. I got them to take the back off and all looked in reasonable condition.
The fact that the case is in so good condition was the clincher.

So it's been in the living room for a couple of weeks and this week I made a start even though plenty of other projects have priority really

I have given it a clean and a once over, tested all the valves except the two EHT one's
Resolded a loose electrolytic and replaced a Black Hunts that was in bits.

I've powered it up on a Variac, at about 70% supplies, all seems ok. The heaters are on and the heater voltage seems reasonable.
The H.T. line seems reasonable and I have small hum of when touching the volume wiper terminal.

There is no raster or audio hiss.

I'm just wonder what to do next.

Replace all those black Hunts ? as they always seem to have failed by my previous experience.
I need to dig out my H.T. probe and see the timebase is running, I'll not hear it now a days.
The potting on the LOPT is very cracked.

BTW, are there any X-rays from the rear of the screen or the rectifiers.

Any tips would be great.

Nick

[AC/HL]

A little history, if you haven't seen it already: https://www.vintage-radio.net/forum/...06&postcount=4

[NickG0HIK]

Originally Posted by AC/HL

A little history, if you haven't seen it already: https://www.vintage-radio.net/forum/...06&postcount=4

Thanks for that, I had spotted it and looks to be the exact model I have.

Nick

[dazzlevision]

Before you replace many of those black Hunts Moldseal capacitors, I'd see if you can get the line output/EHT stage working first. That way you can see if the line output transformer is OK and maybe get an idea of the state of the CRT.

Is there a negative voltage on the control grid of the line output valve ? It's usually around -50V, or so. Try adjusting the line hold control whilst measuring. If yes, that suggests the line blocking oscillator is working. If not, then the line oscillator stage needs investigating.

If you do have line drive, are either of the two line output valves overheating (glowing red/orange)? If yes, I'd check the boost HT reservoir capacitor for short-circuit.

Let us know how you get on with these initial checks.

The EHT on these sets isn't high enough to generate any X-ray risk, unlike early colour TVs, with a GY501 valve EHT rectifier / PD500 valve shunt stabiliser triode.

[NickG0HIK]

Thanks for the info,

I've been in to the loft to get my probe and can confirm there is no EHT.

I've measured G1 of the Line Output valve, it is around -15v

I put a scope on and it's 10Khz, varying about 1khz with the Horizontal Hold control.

After a few minutes there was a pop and a very unpleasant smell, sort of a cross between raw meat and PVC, but I cant see any evidence to what caused it.

The valves are not hot and no warmth in the LOPT.

I've just checked the Line output valve and it's showing 100%

Nick

[dazzlevision]

I'd check the two large axial leaded capacitors on the left hand chassis upright tag strip, above the LOPT.

One is the boost HT capacitor (0.1uF) and the other is the CRT first anode boost HT decoupling capacitor (0.5uF) - both are highly suspect. Best replaced straightaway with high voltage polypropylene film types.

I'd also replace the 0.01uF capacitor that couples the line drive to the LOP valve. It's between the line oscillator valve and the line blocking oscillator transformer (and almost certainly a black Hunts moldseal).

[Lloyd 1985]

The pop and funny smell; does this set use a selenium rectifier at all?

Regards
Lloyd

[dazzlevision]

Originally Posted by Lloyd 1985

The pop and funny smell; does this set use a selenium rectifier at all?

Regards
Lloyd

It would have used a finned metal rectifier when made, but that has probably been replaced by a silicon rectifier and series surge limiter resistor. It is located below the front of the CRT.

[NickG0HIK]

Originally Posted by dazzlevision

I'd check the two large axial leaded capacitors on the left hand chassis upright tag strip, above the LOPT.

One is the boost HT capacitor (0.1uF) and the other is the CRT first anode boost HT decoupling capacitor (0.5uF) - both are highly suspect. Best replaced straightaway with high voltage polypropylene film types.

I'd also replace the 0.01uF capacitor that couples the line drive to the LOP valve. It's between the line oscillator valve and the line blocking oscillator transformer (and almost certainly a black Hunts moldseal).

The 0.5uF was .25M Ohms
The 0.01uF was 0.75M Ohms
The 0.1 was short.

I still dont have much more than -18v on G1 though.

But yes, it still has the selenium rectifier, It appears this set has never been in for repair before.
I measured the H.T. and it is about 155v the ERT sheet says 210v, so I presume it's failed.
Replace with a IN5408 ? and what value of limiting resistor ?

BTW, now getting a narrow raster and some Audio and I'm getting excited.

Nick

[dazzlevision]

Have you replaced the three leaky capacitors? You should do before running the set for other than a minute or two, or damage to the LOPT might occur - it’s already probably going to have dampness related issues.
The series resistor for the HT rectifier should be around 20 Ohms and rated at least 7 Watts. Keep the resistor away from the CRT glass and anything else the heat might upset.

[NickG0HIK]

Yes, I changed those caps.

I've found a few dry joints and sorted those.

I was glad to get shot of that selenium rectifier, it had a bit of paxolin between it and the CRT, but it still looked, like it would not take much to punch through.

The raster is not even, the spacing between the lines is greater at the top than the bottom, is this called Linearity ?

Looks like I'm going to need something to generate a signal before I go too much further though.

I'll look into this, I presume a Hedgehog ?

Still got the case to spruce up in the mean time.

Thanks so much for your help, it would take so much longer on my own.

Nick

[dazzlevision]

If you’ve fitted a silicon HT rectifier snd 20 Ohm resistor, I expect the HT is back to 200+ Volts now...?

The field linearity is poor primarily due to leaky black Hunts moldseals on the timebase panel.

You will need a 625 to 405 standards converter, such as the Hedgehog, fed with baseband 625 video from a DVD player or Freeview box (examples). I can’t remember whether the Hedgehog has an RF modulator built in. You will need one if not.

[NickG0HIK]

H.T. is 205v

I'll spend some time checking out those Hunts next.

A friend has just let me know he has a 405 line pattern geny.

I'll report back when I've got some more progress.

Nick

[dazzlevision]

You’ll probably also need to replace the frame output pentode’s cathode bias resistor and parallel decoupling electrolytic capacitor.

There will also be several/many carbon composition fixed resistors that have drifted out of tolerance, but that can wait for now.

[Lloyd 1985]

Hedghog does have built in RF modulator (if you fitted the parts while building it!) as does the Aurora standards converter. Both excellent devices!

Regards
Lloyd

[mark pirate]

I'll spend some time checking out those Hunts next.

I am sure this will sort the majority of your problems, these are a replace on sight item on any set!

Mark

[FERNSEH]

Link to work done to improve picture quality on the very similar BT302.

https://vintage-radio.net/forum/showthread.php?t=123587

DFWB.

[NickG0HIK]

It looks like the set is capable of great results then.

I'm going to spend the evening checking the caps.

The Hedghog is on order, so hopefully it will be displaying some sort of picture soon, I'm sure I'll be asking more questions before long though.

Nick

[Ed_Dinning]

Hi Nick, when you get round to it PM me as I have a "compendium " of 6" green screen TV designs and some of the guys over her have also built them (as I did more than 50 years ago.)

Ed

[NickG0HIK]

An update,

The Hedghog arrived today, I now have a test card.

Its not perfect, but not too bad, any tips on alignment?

There is a black line about one 1/3 of the way up the screen that comes and goes.
The picture also appears tilted.

I cant hear any audio from the Hedgehog either.

I still have four Hunts capacitors to change on the Timebase board and all of them to change on the I.F. board, they were all around 0.25 Mohm.

I've not checked any of the resistors yet.

It's just blown the 1.5 amp mains fuse, H.T. fuse is OK, the only thing I can think doing that is the filter cap across the mains, I'll change it for an "X" class cap as I've known these failing quite often.

Nick