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Old 24th Mar 2019, 9:51 pm   #21
Davewantsone
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Default Re: Pye V4

The can covering the LOpT may say V4 or VT4 but they are identical. There are 4 screws on the underside holding the Tx in position. On the top two screws hold the line linearity and width controls. It is then just 4 wires on the Tx (4 and 5 o'clock) The wire (line scan 9pm) and other side going to top cap near PY81.
If possible leave the line and width adj. with the transformer.Thank you.
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Old 25th Mar 2019, 12:42 pm   #22
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Default Re: Pye V4

Well, mine's an Ekco T.283 which may be equivalent to the Pye but maybe someone with better chassis knowledge than me might confirm (or not).
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Old 25th Mar 2019, 1:17 pm   #23
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Default Re: Pye V4

Thanks for the offer, but the EKCO Tv would not be suitable for me. You may be able to offer it to someone else.
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Old 26th Mar 2019, 9:22 am   #24
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Default Re: Pye V4

The PYE V4 VT4 V7 VT7 V4C etc is a one off transformer with feedback windings. It gets cooked inside the close fitting screening can leading to stress. It had quite a high failure rate back in it's hey day. John.
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Old 26th Mar 2019, 4:43 pm   #25
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Default Re: Pye V4

I have only a fraction of the knowledge of John or David but I would agree that the LOPT failure rate was high. Every V4 I bought from jumble sales at 3/6d at time in my youth had a duff LOPT.

When I returned to old tellies it was my quest to have a working V4. Hard work but worth all the effort (spurred on by seeing young Stephen's (Freya) V4 lovingly restored). I have a spare V4 with (last time I fired it up) a working LOPT. If you would like the complete set or just the transformer you would be most welcome.

We hope to be at the NVCF in May if that helps. (East Kent is along way from Wigan!)

Regards Steve.
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Old 26th Mar 2019, 7:23 pm   #26
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Default Re: Pye V4

Yes thanks for your kind offer. I will PM you.
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Old 4th Apr 2019, 4:51 pm   #27
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Default Re: Pye V4

First of all thanks to Steve (beltinge bore) for the V4 chassis.
I will give some feedback as to what hasbeen happening with the set.
I gave Steves LOpT 3 days near a radiator before I removed it from his chassis. I did a resistance check on the windings they were OK apart from the EHT winding it only read 370 ohms (should be 455). My LOpT read 480 ohms but the scan coil winding was only 13 ohms (should be 17) Steves LOpT was 17 ohms.
I wired his LOpT into my set but unfortunatly the result was no EHT.
I removed the LOpT and decided to bake it in the oven for 20 mins, it must have been hot as some of the plastic casing that secured the EY51 started to melt!
I then measured the EHT winding and it had now increased to 450ohms. Whilst still warm I put it back in the set. I was rewarded with a picture which was just satisfactory but if brightness was advanced it ballooned!. slowly over around an hour the picture got worse as the transformer cooled down. It has now stabilised around 1 inch narrow on both sides (still balloons) and is a lot darker.
Switching on today the picture appears but is the same narrow and a bit dark from switch on. But hey I do have a picture!
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Old 4th Apr 2019, 5:53 pm   #28
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Default Re: Pye V4

3 days is not enough to drive out the moisture, it will take several days connected to a regulated power supply. The EHT winding needs dried thoroughly before using in the set.
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Old 4th Apr 2019, 6:21 pm   #29
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Default Re: Pye V4

I 100% agree with the statement in the above post.

If you don't dry that transformer out properly and you keep on using the set with reduced picture size and EHT, your're going to end up with another duff transformer.

Do NOT power the set up again until you've dried the transformer out properly by running current from a limited and regulated power supply through the windings while monitoring current and temperature - and it'll take as long as it takes!

If I'm totally honest, I'm a bit shocked that after finding a such a rare item as a replacement transformer, that you'd risk keep powering up the set knowing that it's going the same way as it did before.
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Old 4th Apr 2019, 7:53 pm   #30
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Default Re: Pye V4

Ok so what voltage and current do I put through it?
What do you mean by as long as it takes? What am I looking for?
Its already had two goes in the oven! Which got it going!
The previous time it was not the EHT winding but the line scan coil winding that went low resistance!
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Old 4th Apr 2019, 8:34 pm   #31
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Default Re: Pye V4

Looking back at my notes, I used 30 volts at 65mA for three days and nights.
Is yours still pitch coated after its oven bake.
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Old 4th Apr 2019, 8:52 pm   #32
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Default Re: Pye V4

Yes, though the plastic supports that held the metal disc that surrounded the EY51 had melted a bit! I gather you mean to feed this current through the EHT winding (PL81 TC to EY51 anode?) If so can I do it insitu? So I Just connect 30V via a 470 ohm resistor and set current limit to 65mA? Or do I set current limit through EHT winding to 65mA and wait till voltage rises to 30V (455ohms X0.065A = 29.6V)?
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Old 4th Apr 2019, 9:14 pm   #33
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Default Re: Pye V4

No reason to take it out again, I did not use resistor, just connected to the two ends of the overwind. For an experiment I had the whole lopt in a large clear zip bag which did form condensation on the inside. Proved a point at the time.
The reason it takes so long is the pitch which at the time it was supposed to prevent ingress of moisture but over the many dormant years it creeps back in.
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Old 4th Apr 2019, 9:25 pm   #34
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Default Re: Pye V4

OK. I will give it ago tomorrow. Any suggestions on the other LOpT that is low resistance on the line scan coil winding 14 ohms should be 17 ohms?
Thank you.
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Old 4th Apr 2019, 9:43 pm   #35
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Default Re: Pye V4

First of all, do not expect LOPT resistance values to match those in the service manual. They differ widely due to changes in wire gauge, number of turns and the use of resistance wire to lower the capacity. These can all change during a long production run.

The dampness needs to be driven out from the INSIDE. No amount of external heating will rectify this. Warming the overwind with a current through it is very reliable. It needs to be warm to the touch and as Stephen has mentioned must be carried on for around 3-4 days, checking that the temperature of the overwind does not increase. It should be stable warmth.

Do not use the V4 until you have done this, it will lead to total failure of the overwind. It has taken decades for the hydroscopic pitch to absorb the dampness and to rid it takes a little time and patience! Good luck with it. John.
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Old 5th Apr 2019, 9:42 pm   #36
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Default Re: Pye V4

I have had the LOpT connected to a 30V power supply delivering around 65mA for over 12 hours. If I touch the EHT overwinding it does not feel warm. Is this correct and I just leave it for 3 days? The problem for me is that my power supply only delivers a maximum of 30V so no way of increasing voltage unless I put a car battery in series with power supply!
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Old 5th Apr 2019, 10:10 pm   #37
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Default Re: Pye V4

Would you mind taking a picture of the bottom of the lopt showing the tags and the connections you are using please.
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Old 5th Apr 2019, 10:31 pm   #38
Davewantsone
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Default Re: Pye V4

I am using the EHT overwinding pin 6 to anode of EY51. Resistance is around 440 ohms, so current around 65mA at 30V which means around 2W of power. My digital Power supply reads 30V 65mA. I have now put a 12V battery in series with power supply at 42V current now 93mA or around 4W of power. I would have thought for a winding of around 2.5 inches in diameter a power of around 10W would be needed to get it warm.
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Old 5th Apr 2019, 10:47 pm   #39
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Default Re: Pye V4

Hi Dave,
With regard to the current and voltage required for drying out the overwind, it's the power, that's the important factor. That's what causes the heating effect. I usually find about 2 Watts (from memory) is about right for chunky overwinds so that they maintain a temperature hot enough to just keep your fingers on. That probably equates to 40 to 50 deg C.

The resistance of the overwind determines the combination of current & voltage needed to achieve this. You can only alter the current by changing the applied voltage, so I use a variable power supply and have an ammeter in series with the overwind.

I start with a low voltage, measure the current taken and work out the power with Watts = volts x amps. As the winding heats up, it's resistance will increase causing it to draw less current, so you need to monitor the temperature and alter the voltage accordingly until a stable temperature is reached.

If the overwind has quite a low resistance, the required current is alarmingly high to achieve the desired heating effect. For example, to get 2W dissipation in a 50 Ohm winding needs a current of 200ma!

Good luck and it's definitely the way to go to avoid failure.

Cheers
Nick

Reading your post above, maybe I was wrong at 2W. It needs to get warm or it won't do any good, so you will need to increase the voltage to a point where it does generate some warmth.

Last edited by 1100 man; 5th Apr 2019 at 10:56 pm. Reason: extra text added
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Old 5th Apr 2019, 11:01 pm   #40
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Default Re: Pye V4

This is odd because I have just connected my bench 1amp supply to my working V4 lopt and at 65mA it is getting to between 40-50 degrees within 15 mins at 30 volts.

Just a thought, have you disconnected all the other tags whilst connected to the DC ?
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Last edited by Freya; 5th Apr 2019 at 11:09 pm.
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