Pye B18t Lopt

[radio1932]

I am currently rennovating a PYE B18T, which has not been switched on for many years - I always run through the components before applying power, but I think I have come unstuck at the first hurdle !

It looks like I have an O/C LOPT - I am not familiar with the 'pin out', since there is no terminal information, but I think I have deduced it - can anyone confirm ?

The Pye B18T service manual indicates that the top pin of V7A ( EY51 EHT rectifier ), should be connected via a center tap of the primary / EHT common winding on T12A ( LOPT ), to V6A ( PL38 pentode anode top cap). EY51 checks out o.k. on my AVO MK4.

I am not getting any resistance reading at all on this section, but the top cap of PL38 is connected to the top half of the winding ( primary at approx 55 ohms which is correct ).

If this is o/c, then it looks difficult to repair with all that potting - does anyone know of the best method to access this or maybe source a spare LOPT ?

Thanks, Paul

[Danny]

Hi ive restored many of these and never found a dud lopt i would carry on with the restoration and see what happens when it switched on. Danny

[Steve_P]

T5 Transformer, Winding 'b' I assume. Going off trader sheet 946.

Should be a reading from the top (power supply) to V16 anode. Also to EY51 Anode. If there is nothing between V16 Anode (EL38M - Top Cap) to V17 (EY51) then first clean all the contacts and carefully resolder.

Admittedly they don't fail very often, but there's always a first time. Check on all settings of the AVO to make sure though.

Cheers,

Steve P.

[radio1932]

Hi, Thanks guys,

I do not have trader sheet 946. And the Pye manual I am going off
does not have the winding breakdown details, only the schematic outline,
still, I think I am correct in my diagnosis.

Typical that the reference designators are different as well - is there anywhere I can get this trader sheet ?

In terms of cleaning contacts and resoldering - I guess that means the top cap for cleaning and resoldering the the EY51 on the solder pads ? The top cap is connected to supply through the primary winding, so it is clean'ish'. The EY51 solder pads are quite large and covered in black potting, so I can not see where the transformer wire is to the pad - if I get no joy powering up, I will suck off the solder to a clean pad and go from there.

I have done my final checks before switching on. During valve testing, One rectifier, PY31, V8A on my diagram which is on the output from the LOPT main secondary, not only exhibited fluctuating leakage but it also gave quite a remarkable lightning globe show on my AVO MK4 under load test !!
Never seen that before ! Closer analysis indicated loose flakes of material floating about inside the tube.

An EF50 was also replaced for leakage,

I powered up through a series 100W light bulb. No nasty pops or bangs at present and the valves are just about glowing. I am going to try a 150W bulb before commiting to direct mains supply - pop to the shop tomorrow - ( I may be over cautious, but I must get a variac sometime. )

I will keep you posted.

Thanks again
Paul

[Framer Dave]

Hello Paul,

I assume you will replace the boost diode PY31 before proceeding. The presence of loose material in the glass envelope suggests that the cathode has stripped due to excess current. It would be a good idea to check all the caps in the related circuit, especially the boost reservoir cap ( 25mf electro lytic). This is to be found close to the valveholder.

Regards,

Dave

[radio1932]

Hi all,

The PY31 has been replaced as a matter of course

- thanks to framer dave for the tip on excess current causing the material stripping - had not thought of that !!!

The 25uF 50V cap was essentially a leaky 22nF cap !! Replaced !!

After that one, I now have vision and sound with a good frame sync lock !!
It looks like the RF section requires no realignment - phew !!!!!

I am quite puzzled though, I still can not measure continuity between the
EY51 EHT rectifier anode and the PL38 top cap anode through the LOPT primary centre tap as illustrated in the diagram. If this was o/c, surely I should not be getting any EHT or a bright picture ?? Maybe the diagram is wrong or the LOPT has some other internal arrangement ? Any ideas ?

The picture require a little tidying up and the presets do not appear to have the range I think they should - a few more dud components in the vision section to route out I guess.

The picture can be made to almost fill the tube screen vertically - not quite reaching the bottom by an inch or so and starts to get a little squashed when extending the image down there. The width of the picture is fully across the screen and can not be made to draw in. The brightness control is hyper sensitive, no picture to maxed out in a little movement, with only a marginal adjustment for a reasonable balance.
the contrast control is quite smooth in operation but as the contrast is reduced, the test card exhibits increasing vertical ( left to right ) shadowing of the image. The line sync does not appear to do a lot of adjusting at all. The same goes for linearity.
Interestingly, the frame sync gets lost at extremes of brightness and contrast and requires readjustment, but it still operates well.

Now I have a landmark result, I am going to revisit the set on and off in the coming weeks - other things to catch up on I am afraid - but I will be back with more news. Please keep those ideas coming forth.

REgards
Paul

[Steve_P]

Check the power supplies and then the frame stages.

Check V19, R72, Smoothers C60 and C61. Then the frame stages - V9 and V10. Check caps in the area as well. Change all the waxies in the IF and RF stages.

For the brightness, check C58 and the Control. R36, R38, C21 and V13.

Check C51, R63 and it's worth changing C52. It's like that cap on a radio!

Any other help, you know where we are. All part no's on Sheet 946.

Just one check - is the tube body full of EHT on these?

Cheers,

Steve P.

[radio1932]

Hello Steve P,

Appologies, something has either got lost in translation or I am showing my ignorance.

I am not sure what - ''is the tube body full of EHT on these?'' - means.

Could you clarify ?

By the way, could you indicate where I could get a copy / photostat / scan image of the trader sheet 946 please, since your ref designators are different to mine.

Thanks again,

Paul

[Steve_P]

Most tubes have an EHT cap and the body is made of glass. Some are made of Metal and the whole lot is at 7kV or whatever.

Cheers,

Steve P.

[radio1932]

Well, It is like a bug, I just could not stay away from my set for more than a day !!

Having achieved first power up and a picture from a cursory inspection of the component list, I started to look at the detail. Working back from the most important end at this juncture, the vision output.

Ref: Trader 946/T5 kindly supplied by a forum member. Many thanks !

C59 was 60% too high, C57 was 95% too high, R69 was spot on. Steve P pointed out to look at C52 being akin to the radio output situation. He was right there !!!

TEsting the set again enabled me to cover the whole tube with raster. The brightness problem was still there though and the larger the image, the more fuzzy it becomes.

Being new to vintage tv, ( I have brought several radios back to life and this pye set is my first one ), I noted that a lot of vision output stage appears to be biased through that 10K brightness control pot. Ahhhh, very interesting !

Surely enough, 10k was above 50k !! So the biasing would be completely out. That would most probably also account for that whistling LOPT not being able to attain the correct operating frequency ! Please, Correct me if I am wrong.

The other issue - irratic brightness control. Well almost nothing to flat out, and the tube & LOPT look capable of producing a bright image, although washed out. Surely enough, on closer inpsection, the pot has a piece of winding poking out from it. Moving the wiper indicated that the feedback resistance to the crt grid fluctuated from a couple of 'k' to a few hundred and back to 50k.

NOT GOOD !

This is going to be my next priority, I now need to either find a replacement or resistance wire to rewind, or even breakdown another pot for the wire. Got nothing to do the trick yet, but will go searching. - ebay, etc..

Please look at the items wanted section shortly.

Keeping you posted.

Paul

[chipp1968]

Probly this fault that put the set out of service . Good news being a potentially good tube

[radio1932]

Hi Chipp1968,

I think you are right about the sets 'demise' from service, it had crossed my mind !

The wire break is not where I expected it to be, on the wiper contact where the all the wear and tear is, but on the opposite side !! Odd one.

Anyway, I thought, I had nothing to lose in trying to restore the pot to some sort of operational order. In any case, ( if the resistance is all to pot, pardon the pun ! ), I could always ballast it with a parallel resistor combination for the time being.

So out with the pot former ! A very easy job, since it just pops out of the retaining clips. A nice touch from yesterdays designers !!!!

The wire is as thin as a human hair and I could only see the stray end by placing a white sheet of paper on the table top in good light. I had to look very hard to spot its' counterpart end using a magnifying glass. There it was still clinging to the bobbin !!

The stray end was long enough, so using tweezers, I carefully lifted the other end and unwound one turn. Enough length to work with at both ends !!!

One problem to over come is that a loose windings on a pot is not a good idea, especially on the wiper contact side, so I carefully retracked the wire, ( one end at a time), into the void where it came from and tacked it with a minute spot of superglue either side of the wiped section. It sets in a couple of minutes and the wiper track looked as good as new, ( albeit missing one turn ), but who cares at this stage of the game ?

I was careful to ensure that the two ends met on the other side of the pot former, away from anything that could snag on it in use.

Resistance wire is generally not easy to work with, since it is made from more exotic materials than plain tinned copper wire. It is usually oxidised to provide inter-winding resistance, which can not be soldered unless this coating is removed. ( this also depends on the material type of the wire being used.)

Using a blunt penknife, I sat the pot former such that one stray wire end lay flat on the white paper and VERY gently rubbed the surface. Then gave the other stray end the same. treatment.

It was a very fiddly job twisting the two ends together, but it was done. I tested the resistance - should be 10k, measured 10.42k. So this was looking good. Would it solder ? Well I applied some and the ends were held together. I do not know if the solder has actually taken to the wire, but it is holding it together reliably enough anyway.

Clipping the former back into the wiper recepticle, I tested it and it now works across its' range !! ( Yes, there is a little flat spot, but not really noticeable and the full dark to light path glides relatively well )..

So another job done !!!

Keeping you posted.

Paul