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Old 15th Jun 2018, 9:16 pm   #1
simpsons
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Default A640 LOPT transplant - Help!!

I need the help of those who know a thing or two.

Having a good “tar” primary and Plessey plastic EHT over wind, I thought that I would make a good LOPT between the two of them. Photo’s attached.

However, the “tuning coil” position use and connections are different on the Plessey to the Tar version and so is also the connection for the over wind. So, before I ruin one or other please read on.

Taking the Tar version, primary and over wind connections are as the schematic. That is Tuning coil, LOPT # 12 via brown lead to the 405/625 S correction switch, # 13 to earth and the coil is positioned inside the over wind coil between the core and the winding.

In the case of the Plessey over wind, the "Tuning coil" is situated above the overwind, # 12 is connected via the brown lead to the 405/625 S correction switch as above but #13 is connected to #2 of the primary then, as per the schematic, via a red lead to the line deflection coils ie NOT earth. I guess this is because the coil, no longer in the magnetic field of the overwind acts in a different way. I can live with this and assume I just wire it to #2 on the tar version. Am I right?

The Plessey over wind though is NOT connected via LOPT #10, but #9 which is then via 3L3 to the boost cathode which is something I have not come across before. Is it because the saturation coil SL4 is no required?

Just in case there is a doubting Thomas out there, I can assure all that the Plessey transformer wired as described worked fine until arcing between the paxolene and primary coil put an end to a Rediffusion badged TV175. Photo attached.

Help!!

Chris
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Last edited by simpsons; 15th Jun 2018 at 9:37 pm. Reason: proof reading
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Old 16th Jun 2018, 6:20 pm   #2
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Default Re: A640 LOPT transplant - Help!!

Just to say the plastic cased resin bonded overwind failed just as often if not more regularly than the early pitch covered component.

The Plessey version was probably wound to a different spec and this is probably the reason for the differences.

I might be wrong but your transformer appears to have been fitted with a replacement overwind 'insert'. These were windings that were manufactured and supplied by companies such as Papworth Transformers. You dismantled the transformer and fitted the new bobbins yourself.

You can probably get the original transformer rewound but due to the incredible amount of work it is not cheap.J.
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Old 16th Jun 2018, 7:28 pm   #3
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Default Re: A640 LOPT transplant - Help!!

Hi Heatercathodeshort

The EHT transformer is as purchased, which is why I am out of my depth as to why the "tuning coil" and overwind are treated so differently to the Tar version.

I have attached another photo which shows the construction. Without wishing to speak too soon, the primary is well constructed with the layers insulated from eachother by a polythene band. It was the failure of a paxoline insulation disk, placed between the top terminals and the coil which caused arcing and then carbonisation to the coil etc.

Now, as it so happens I decided to look at cleaning off the carbonisation on the cioil using switch cleaner. Having done that, I then poured quantities of high voltage varnish into the void.

I then made good the paxoline terminal strip and varnished the underside, made a new insulation disk from a thin sheet of paxoline. Connected the whole and varnished the HV terminal connection. All is now working well but I would still like to understand why this transformer is so different to the original.

Chris
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Name:	Murphy A640 LOPT 013.jpg
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Last edited by simpsons; 16th Jun 2018 at 7:30 pm. Reason: proof reading
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Old 17th Jun 2018, 9:23 am   #4
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Default Re: A640 LOPT transplant - Help!!

From your close up picture it appears your transformer is an original Plessey component.
The Mk1 pitch insulated TX was manufactured by RBM themselves.
I guess that Plessey had their own method of construction and this would explain the differences.

The resin encapsulated overwind gave trouble in all models that employed it. With regard to the A640 in particular, I have never known an underwind to fail. It is always deep inside the encapsulation where failure occurs.

The less disturbance the better is the rule for all flyback transformers but I wish you the best of luck with it.

Pictures show a NOS A640 transformer of Plessey construction dated May 1979. It came as a complete assembly from RBM and simply plugged in. They were very moderately priced at the time. I certainly replaced a few... The mains dropper was the only other regular failure, again a very cheap replacement. The A640 enjoyed a long service life. John.
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Old 17th Jun 2018, 2:04 pm   #5
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Default Re: A640 LOPT transplant - Help!!

Quote:
Originally Posted by Heatercathodeshort View Post
Pictures show a NOS A640 transformer of Plessey construction dated May 1979. It came as a complete assembly from RBM and simply plugged in. John.
The later type of LOPT supplied by the RRI service department employed different (superior) materials in its construction. For example, the blue plastic primary tagboard/bobbin moulding, the short thick white plastic sleeves on three wires and the clear plastic EHT rectifier valve socket holder. Another new addition is that thick plastic piece sitting above the LOPT, the function of which is not immediately obvious to me.
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Old 17th Jun 2018, 2:32 pm   #6
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Default Re: A640 LOPT transplant - Help!!

Thank you both Heatercathode Short and Dazzlevision.

So, the tagstrip version is a later version RBM issue, which this thread addreesses, I hope it should last the test of time.

Getting back to the subject, Both the locartion and different electrical connection of Pitch or Plastic designs has no doubt to do with fundamental mathematics for designing LOPT's to work at 405 and 625 line frequencies. I am still interested in why the over wind is connected to the boost diode cathode etc.

The reason is that I have a Philips over wind with, you guessed it, a coil on top of it. I would like to couple it to the working good Pitch primary I have but I'm not in the business of setting alight tar covered LOPT's or damaging the chassis!!.

Chris
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Old 17th Jun 2018, 5:41 pm   #7
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Default Re: A640 LOPT transplant - Help!!

This document might help. It lists the LOP stage differences between the three RBM silicon hybrid mono TV chassis that are very similar, especially the dual standard A640 and (the later "compacted" version of the A640) Z146. The A793 is a single-standardised version of the A640.
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File Type: docx DIFFERENCES BETWEEN A640 A793 and Z146 chassis.docx (16.4 KB, 101 views)
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Old 17th Jun 2018, 5:54 pm   #8
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Default Re: A640 LOPT transplant - Help!!

Thank you dazzlevision for the details.

You will note from my earlier post that the grey over wind, clear primary version has its own peculiarity which I will document in a similar format.

Are your notes from a RBM document or self written?

Chris

Last edited by simpsons; 17th Jun 2018 at 5:55 pm. Reason: Added details
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Old 17th Jun 2018, 6:37 pm   #9
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Default Re: A640 LOPT transplant - Help!!

Quote:
Are your notes from a RBM document or self written?
Self written, based on studying the relevant RBM service manuals.
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Old 20th Jun 2018, 9:27 am   #10
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Default Re: A640 LOPT transplant - Help!!

I always had problems with replacement LOPT's on the RBM A640 chassis they always seemed to suffer with poor line linearity.
The best replacement LOPT that I used was from a company called Telepart which did away with the DY802 and used a TV18 stick instead. You could wire it up for dual standard operation, single standard 625 or single standard 405 operation.
It gave perfect results and the whole TV seemed to run a lot cooler with this LOPT fitted.
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Old 20th Jun 2018, 10:37 am   #11
dazzlevision
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Default Re: A640 LOPT transplant - Help!!

Telepart was a brand name used by Willow Vale Electronics Ltd.

Here's a picture of the LOPT I think you are referring to.

Looking at the darkening of that thick piece of plastic above the LOPT, I reckon its purpose might be to reduce the amount of radiant heat from the LOPT valves reaching the LOPT....?
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Old 21st Jun 2018, 9:25 am   #12
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Default Re: A640 LOPT transplant - Help!!

Its a long time ago but the LOPT I bought only had one winding presumably the EHT overwind was somehow incorporated on the same core as the line output winding.
It did work well with good EHT regulation and line linearity. Sadly the Murphy V1914 that I fitted it in I sold on which I have regretted ever since.
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Old 23rd Jun 2018, 8:01 am   #13
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Default Re: A640 LOPT transplant - Help!!

Thank you all for your feedback.

From what I have read so far there are several versions of the 405/625 LOPT which, from looking at the photo's, not only have different construction techniques but also electrical characteristics.

Attached is the amended circuit diagram and sketch of the paxoline version I have and shown below, text describing the electrical connection.

A640 (404/625) chassis replacement clear plastic primary, paxoline tag strip
LOPT AXXX (RBM/Plessey?)
LOPT core connected to chassis (via tag 3)
LT supply derived from separate winding around LOPT core (tag 1)
LOPT desaturation choke 3L4 fitted with ac coupling components 3C20 and 3R76
EHT over wind connected to tag 9 not tag 10
P/S6-3 goes to system switch 3S2c (625 contact) on the timebase panel
3R27 (EHT rectifier heater ballast) not fitted
One end of secondary to primary (on 625 only) coupling winding
(tag 12) is connected to primary winding tag 2 not tag 9
3R77 (line pulse feed to Boost HT stabilisation circuit) is 27KΩ
3C25 (420pF) and 3R79 (200Ω) connected in series between P/S6-1 and P/S6-4, on timebase panel.

The initial use of a coil within the body of the overwind and the later external coil and different circuit is, as they say, above my paygrade.

Chris
Attached Files
File Type: pdf Scan_Doc0106.pdf (671.6 KB, 76 views)
File Type: pdf Scan_Doc0107.pdf (390.3 KB, 79 views)

Last edited by simpsons; 23rd Jun 2018 at 8:02 am. Reason: edit
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