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Old 29th Sep 2018, 6:19 pm   #1
itainthard
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Default Pye LV20 Help?

Hello,
I have recently acquired a Pye LV20 and it looks to be complete and un molested.

I have got a service sheet for it and was about to start hunting down a set of capacitors to replace all the existing before trying to apply any power.

Although i have a component list for it it does not seem to give the capacitor voltage ratings and i cant read all the original caps so was wondering if anyone could help? and also any recommendations for suppliers?

Thanks in advance

Joe
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Old 29th Sep 2018, 7:07 pm   #2
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Default Re: Pye lv20 Help?

Hmm. I'm surprised that you're unable to read the ratings on the original caps. Is it an access issue? If so snip them out and use a magnifier. Is it that they're waxies? If so scrape the wax off. Most old caps can be read if you remove them and get up close and personal.
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Old 29th Sep 2018, 7:17 pm   #3
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Default Re: Pye lv20 Help?

Good evening
The waxies can be replaced by the yellow axial ended polyester types. 630V rated ones should suffice most places. The BVWS sell these plus 500V rated electrolytic capacitors. The service sheet should show you the location of the various components and then the component list will list the values of all of the capacitors and resistors. The flat mica ones should be ok and shouldn't be disturbed.

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Old 30th Sep 2018, 9:09 am   #4
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Default Re: Pye LV20 Help?

https://www.vintage-radio.net/forum/...d.php?t=145614
You may find this helpful.

Please do not do a mass replacement of the capacitors or carry out any frantic cleaning. You will only introduce man made faults that are a nightmare to trace and cause possible damage to fragile coils etc.

If the chassis is original it should be able to resolve something on the screen with the minimum of replacements.

I would suggest the order of play. Power supply. Line timebase, Frame timebase followed by RF, video and sound stages.

I have repaired many of these and have never discovered the main electrolytic to be faulty. Once you have something visible on the screen you can then use the information displayed to work through the chassis.

The Mullard tube will be OK but the LOPT is a weak area. I would suggest you pass a current of around 40M/a through the EHT section for a couple of days to dry it out. [Top cap of PL38 to anode of EY51] Good luck with it but take it slow and easy. John.
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Old 30th Sep 2018, 10:27 am   #5
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Default Re: Pye LV20 Help?

In my opinion and indeed experience of restoring many such sets, replacing the wax caps will simultaneously rid you of the same amount of faults. They're undeniably the major cause of faults in vintage radios and TVs from the late 40s and 50s. We all have different methods of restoring sets. Some enjoy the challenge of locating each and every fault piecemeal, others including myself prefer to eliminate as many as possible by substitution then get down the inevitable and more difficult fault finding caused by less obvious faulty components and gremlins. For the same reasons I also test all valves and recondition/clean all switch, pot and valve base contacts. To ME that makes final restoration much easier as any further fault finding can be more focused and not masked by a myriad of other, more easily eradicated ones.
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Old 30th Sep 2018, 10:31 am   #6
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Default Re: Pye LV20 Help?

Like John, I have restored several sets using this chassis. All the values on the capacitors should be readable, I use a squirt of WD40 on the old yellow wax caps to clean the grime off and make the value & voltage readable.

I have an unrestored set that just required the main smoothing cap to be replaced to bring it back to life, the metal cased Sprague caps usually hold up well.

I try to power up sets with minimal replacement, use a lamp limiter & a variac if you have one.

This thread shows the restoration of one of my sets: https://www.vintage-radio.net/forum/...d.php?t=116290
Hope it helps.

Mark
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Old 2nd Oct 2018, 2:04 pm   #7
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Default Re: Pye LV20 Help?

Hello and thank you for all your replies,

I have decided to heed some advice and have set up the chassis on a shelf near my wood burner to warm through so i can bung it on the variac in a week or two.

I shall let you know how it pans out.

Thank you, Joe
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Old 2nd Oct 2018, 2:44 pm   #8
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Default Re: Pye LV20 Help?

Its not so much the chassis as the winding on the transformer that needs to be expelled of the moisture that has crept in over the dormant years. As John says, it takes the small current to drive out the moisture. The EHT coil should be slightly warm when doing this.
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Old 2nd Oct 2018, 3:33 pm   #9
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Default Re: Pye LV20 Help?

Hello Steven,
What sort of voltage would you suggest? I have a variable voltage / current limited power supply

Thanks Joe
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Old 3rd Oct 2018, 8:32 am   #10
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Default Re: Pye LV20 Help?

Hello Joe,
There is no definite voltage. You need to pass around 40M/a AC or DC through it and check after say an hour that it has a slight warmth to the feel. The current can be measured with your test meter of course. If all is stable, leave it for a couple of days and this should expel dampness from the INNER core of the winding. That will be a very good start. John.
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Old 3rd Oct 2018, 6:16 pm   #11
itainthard
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Default Re: Pye LV20 Help?

Hello John,
Lovely,I've currently got it running 24v dc at 40ma ish running through it so I'll leave it for a few days,its definitely just warm.

Thanks Joe
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Old 8th Oct 2018, 2:40 pm   #12
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Default Re: Pye LV20 Help?

Right, after a few days drying out I spent yesterday winding up the variac to see what happened.

All seemed to go well and after a few hours we were at 230V with no smoke or loud bangs.

However although I can hear the EHT side whining away I have nothing on the screen and no sign of the heater element at the base of the CRT.

Does anybody know the pin configuration for this and or what voltage this should have at it as the service book doesn't help?

I have tried putting a meter across the various used pins once powered down and so I think the heater coil maybe open circuit but I am not sure.

Thanks Joe
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Old 8th Oct 2018, 7:29 pm   #13
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Default Re: Pye LV20 Help?

Good evening,
The heater chain if I can remember correctly, all the valve heaters are in series with the CRT being the final heater before it returns to chassis potential. If the line stage is whistling away check that all of the valves are lit but not excessively bright.

It sounds like a short somewhere along the heater chain, Possibly a shorted decoupling capacitor. This would cause some valves to light excessively but every other valve further down the chain including the CRT heater will not receive any or very little heater current hence why the CRT heater might not be lit.

With some valves lit, The HT rectifier is at the top of the chain so HT would be present, some of the stages in the television will be powered and trying to work if the valves are lit. This could include the timebase circuits.

Christopher Capener
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Old 11th Oct 2018, 2:31 pm   #14
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Default Re: Pye LV20 Help?

Hi Joe,
I was going to give you a fairly straight forward answer as to where to look next, but...
I have just found that the PYE manual downloaded from Jon Evan's site shows the CRT heater connected in the middle of the RF section of the heater chain, but trader sheet no. 946 shows the CRT heater at the end of the chain with one end connected to chassis. The trader sheet probably covers later revisions (as it also covers the LV20), so your chassis could be wired either way.

Of course you can't see if the red EF50 valves are lit, but the EB41 valves can give a clue.
If no valves are lit then there is a break in the chain. If some are lit brightly and others are not then there is a short across the chain to chassis at some point.

Breaks in the heater chain are usually due to poor contact with the valve sockets. Both the octal and the EF50 sockets are of fairly poor quality in this set. The heater chain mains dropper could also be at fault, though it is usually reliable in this set. you can quickly rule it out with a quick test using the 230V tapping set. The heater surge limiting thermistor could also be at fault, but it is fairly unlikely.

Shorts across the heater chain are usually do to heater decoupling capacitors in the RF stages. In the PYE manual they are C2S, C2T, C2U - however trader sheet 946 shows four capacitors, C63, C64, C65, C66 - all 0.0015uF. If one of these capacitors looks like it has been getting hot then you can cut one end of it as a temporary measure to see if the heater chain starts to work ok.

I hope that helps for now.

Cheers
Andy
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Old 11th Oct 2018, 8:12 pm   #15
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Default Re: Pye LV20 Help?

Hi Joe,
Me again.
Gosh, you can tell I've not recovered yet from Malaysian time as I did write my last post in a rather hap-hazard way. Maybe I win the prize for the most use of the word 'set'.
Anyway, I forgot to mention that the MW22-14 CRT has its heaters between pins 1 and 8 if that helps.

Cheers
Andy
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Old 11th Oct 2018, 9:04 pm   #16
itainthard
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Default Re: Pye LV20 Help?

With some help from Stephen I have had a trace through the heater chain and have found 6.3v at all the heater pins on the valves exept the crt which is only 2v.
I then hooked the crt heater (pins 1and 8) to a 6.3vac supply but still no heat or light, I can read the current as my good meter is with a friend so I couldn't measure the current draw but when I put an ohm meter across the crt heater coil it is around 1200 ohms so I think the heaters knackered. I will have to try and find a replacement.

Thanks Joe
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Old 11th Oct 2018, 10:31 pm   #17
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Default Re: Pye LV20 Help?

There is an inconsistency there.

The CRT should be an MW22-14 and the heater should be 6.3V at 0.3A. 1200 ohms is very high and when in series in the heater chain it would result in virtually all of the voltage showing across the CRT pins.

I haven't got the circuit diagram but it is possible that there is a capacitor in the heater chain that is shorted out. The more likely scenario is the CRT had a partial heater short and putting 6.3V on it has caused the remaining section to fuse but leave a residual resistance.
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Old 12th Oct 2018, 3:28 am   #18
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Default Re: Pye LV20 Help?

Hi Joe,
Let's be sure we have the right CRT...

I've attached a picture of what the rear of an MW22-14 looks like.

If the set has had a replacement tube fitted then it may well be like the attached picture of an MW22-18, in which case the heater is between pins 1 and 12.

1200 ohms on a CRT heater is very high. Make sure you measure on the CRT itself rather than on the socket. The MW22-14 has pins molded in the glass base, so a high resistance measured here is bad news. Later CRTs had lead out wires soldered onto a bakelite base, so sometimes a high reading just indicates a dry joint on the tube base.

Cheers
Andy
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Old 12th Oct 2018, 6:52 am   #19
itainthard
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Default Re: Pye LV20 Help?

Hello Andy,
From the pins it is a mw22-14. I have the socket removed and am testing directly at the pins.
Joe
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Old 13th Oct 2018, 11:51 am   #20
itainthard
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Default Re: Pye LV20 Help?

Hello All,
Right I have tried applying voltage to the heater whilst measuring the current draw and it appears the heater is open circuit.
This is strange how it shows some continuity on a meter but I now conclude I need a new CRT, I will place a threat in the wanted section and hope someone has one up for grabs so I can continue with this television.

Thanks for all the suggestions.

Joe
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