Pye 51BL7215 (Philips L7) B/W TV from Welsh Anorak

[hamid_1]

I received this TV from forum member Welsh Anorak (Glyn) at this year's NVCF. He told me it had 2 problems, firstly the picture went dark but returned when he tapped it (sounded like a dry joint) and secondly the line output transformer had been arcing at some point.
I tried it and the set worked brilliantly for about 40 minutes, then the picture suddenly went dark as he had said. There was a bright vertical stripe down the left hand side of the screen; the rest of the picture was dark but just visible with the brightness and contrast turned fully up.
I looked for dry joints and thought I found one, but re-soldering it made no difference. No amount of tapping the chassis with a screwdriver handle could bring the picture back. In fact it was getting worse. The bright white stripe on the screen faded to a very dim grey and almost completely disappeared. In a darkened room, I could see a blue glow coming out of the line output transformer, and could also hear a hissing sound. I assumed the line output transformer had started arcing again and this was causing the no picture fault.
I really wanted to get this old TV working again. Amazingly I managed to track down a new (old stock) replacement LOPT for this set in a shop in Portugal. I placed an order. Problem, they couldn't accept credit cards unless you went in the shop with the card and typed your PIN into the reader. Eventually I got a friend who was in Portugal to visit the shop and pay for the transformer on my behalf.
Finally I received the new LOPT yesterday. Unfortunately I fitted it and there's still no picture! I'm really gutted, especially because I sent someone else out of their way to get me the part for it. I'm even more determined to fix it now.
With the new LOPT there appears to be no more arcing. There seems to be some EHT present - I can feel static on the screen, and the CRT heater is alight. If I twiddle the contrast control, occasionally a few white streaks appear on the screen, but only rarely. The old LOPT did the same. The brightness and contrast pots measure OK on a multimeter.
What else should I check? The Pye 51BL7215 apparently uses the Philips L7 chassis from 1982. Does anyone remember this, or have a manual?

[Electrical]

Hello Hamid 1
I don’t think I can help much with the fault on this set, but I have found the service information in the Radio and TV Servicing Books. I have scanned the information and converted it to a PDF fife, please find attached.
I hope the information is of some use and it may help other members diagnose the symptoms.
Regards Stan.

[Welsh Anorak]

Hi Hamid
Sorry you're having these problems - as I said I had the set on test for a while and this fault happened, but the picture returned with a tap, so I assumed a dry joint. Life's not that easy, though, is it?
I'd start by checking the CRT base voltages. I think we have a blanking fault here - that odd line I saw suggests that to me. I had replaced the tube a while back so I think that's unlikely to be the cause - the fact that the picture faded gradually backs this up.
Valveman's done a great job of finding the diagram - especially as it doesn't have any valves in it!
Let's start with the +200v line and see how that is looking - check C2148 by replacement. The EHT overwind is connected to earth so it's unlikely any damage was done by the arcing LOPT. Next have a look at the CRT's aquadag connection - that can cause odd faults.
Now I'd concentrate on the CRT's grid as the brightness control works on it - there's a capacitor and high value resistor there along with a zener that look a bit suspect (C2291, R3291, D6291). Check the voltage swing is 0 - 60v.
If all's OK here then we must have a cathode drive fault - voltage checks on 7360 should be revealing, and if a scope's to hand then life will become easier.
It's a circuit that's been designed to a price, and there's no clever touches or exotic circuitry here which will help you in fault finding. Do remember it's been in a damp environment (sorry!) for some years so maybe some corrosion or similar has taken hold - so expect the unexpected!
We'll have it going in no time - don't worry.
Glyn

[hamid_1]

Thankyou both for your help so far. The circuit diagram was very useful.
The replacement line transformer seems to be working. The voltages derived from it are present but slightly high (200v reads 228v, 30v reads 33v and 14v reads 14.1v). EHT is present.
Looking at the tube base, the voltages are:
Pin 7 (cathode) 199v (should be 130v)
Pins 2 and 6 (grid) voltage varies from 0-130v with brightness at minimum - maximum. This seems a bit too high but there is no brightness on the screen at any setting. Thinking there might be an open circuit, I unplugged the tube base, squirted some Servisol onto it and re-inserted it a few times - it did look a bit tarnished.
Moving on to the video output transistor 7360: Base=2.2v, Collector 199v, Emitter 2v. The base voltage varies slightly when the contrast is adjusted.
I think you could have a point about the CRT aquadag. About half of it seems to have peeled off, probably due to damp. The CRT rimband is rusty too. Despite that, it did work initially. The metal strap across the back of the CRT seems to be touching most of the aquadag that's left.
I haven't replaced any parts yet. Do you think I should replace C2291 in the brightness circuit or C2148 (200v HT)?

[Maarten]

The cathode voltage is too high, so the tube is in cutoff. The video output transistor doesn't seem to be driven.

I'd check the following things first:

+11 rail low
7269 leaky
7276 leaky or not driven correctly (blanking)
o/c resistors in the drive of the video output

[hamid_1]

I've fixed it
It turned out to be a blanking fault after all. Transistor 7276 (BC548B) was leaky. This was causing the video output transistor 7360 to be turned off all the time, hence the cathode voltage was too high.
I proved it by disconnecting one side of R3277, disabling the blanking. A nice bright picture appeared, with teletext lines on it due to lack of blanking. The cathode voltage returned to normal. Today I popped into Maplin and bought a replacement BC548B transistor (29p!), replaced 7276 and all is well.
Thankyou again to everyone who helped. It's great that service manuals and spare parts are still available for vintage black and white TVs. Compare that with modern cheap TVs sold by supermarkets ...

[Tazman1966]

Excellent news! How about a couple of pics?

[Welsh Anorak]

Hi Hamid
Glad it's going again - it did have a good picture. Odd how the fault was intermittent.
Glyn

[hamid_1]

As requested by Tas, some pictures
The first shows the excellent picture - the tube was replaced and is nearly new.
The second picture allows you to see the cabinet. Sadly it's not in great condition; the plastic wood effect covering has peeled off in places due to exposure to damp. The paint around the first four channel buttons has worn off after years of use.
The third picture shows snow with a black vertical line down the screen. This is not a fault, it's an unusual feature. The black line is a tuning aid. It appears when there is no signal. When tuned to the lowest UHF channel (21), the black bar is at the far left of the screen. As you tune up through the UHF band, the bar moves to the right and disappears when you pick up a station.

[Tazman1966]

A really nice set. A final flourish from the designers of monochrome sets before the end. I like the tuning aid too

[slidertogrid]

Hi Hamid1
Well worth the effort to repair ! I would think this is Probably quite a rare set as most people would have colour sets by the time it was made.
I would not worry about the wear to the controls etc, it shows that the set provided someone with many hours of entertainment , all part of it's history.

Rich.

[Hybrid tellies]

Nice picture that, well worth the effort. I would imagine this may well be the last generation of large screen black and white sets. So well worth saving.

[Welsh Anorak]

Hi
Yes - the renter of this set was a lovely old lady who is now in the great TV lounge in the sky. I inherited the rental from a colleague and was paid a pound a week! Eventually she decided to go for colour but I didn't get rid of the set as the picture was so good, though i must admit it didn't get the best room in the house....
Glad it's back to life again - well done Hamid!
Glyn

[yampy187]

About the black tuning bar. I have had a couple of PYE/Phillips portables in the past and they have the same. the colour sets have a green bar and iirc their was a tuning scale underneath the screen that went from 21-68

Hope that is of some help.

Jan

[Sideband]

Ages since I've seen one of these let alone worked on one. Excellent pictures and one of the best of the last monchrome sets.

Well done for fixing it.


SB

[AidanLunn]

Originally Posted by hamid_1

As requested by Tas, some pictures
The first shows the excellent picture - the tube was replaced and is nearly new.
The second picture allows you to see the cabinet. Sadly it's not in great condition; the plastic wood effect covering has peeled off in places due to exposure to damp. The paint around the first four channel buttons has worn off after years of use.

Brill sets, I'm considering collecting a black and white set at the moment (portable of course).

If it's not the camera's settings producing this effect, I'd give it a little tweak on contrast and brightness, the sea of the weathermap should look a little brighter than that on black and white.

Originally Posted by yampy187

About the black tuning bar. I have had a couple of PYE/Phillips portables in the past and they have the same. the colour sets have a green bar and iirc their was a tuning scale underneath the screen that went from 21-68

I've noticed this on some Philips/Pye sets as well, although others of a similar size didn't have it. I had a Pye (Philips) CF1 chassis sets that had this - and yet also have a Philips CT2006 (14" CTX chassis - mechanical button tuner) that doesn't have it. I believe they both date from between the years 1983 and 1987, the Pye being the latter model.

As your set suggests late 70s/early 80s to me it'ss obviously not a case of this feature being dropped and one set being made after they dropped it, so I wonder what made Philips decide which of their sets would have this feature? IIRC, the CF1 chassis and the CTX chassis both came from Singapore so it wasn't a case of which factory they were built in.

Anyway, brilliant set, coming from a Philips lover

[Sideband]

Originally Posted by AidanLunn

I've noticed this on some Philips/Pye sets as well, although others of a similar size didn't have it. and yet also have a Philips CT2006 (14" CTX chassis - mechanical button tuner) that doesn't have it.

...but the CT2206 remote control version does have it


SB

[ben]

Just read this interesting thread. Great to see that set working! Here's a thread of a Philips monochrome one of similar vintage I picked up a few years ago. I have a soft spot for Philips stuff of this era.