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Old 4th May 2019, 2:48 am   #1
Autozavod
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Default Pye LV20 - Diagnostic assistance

Hi everyone,

I recently purchased a Pye LV20 as a display piece, but after taking her home and peeking inside decided to restore her based on the chassis' excellent condition. A date stamp on a cap states Oct 50.

Going through the usual process by running a low current through the LOPT for a few days, then soaking the electrolytics. My first impression was right, the set is remarkably clean with only period resistor replacements.

Time for the variac. After a stately warming period, up came the line whistle and a half decent picture, marred only by a chronic lack of frame synchronisation. Testing of the related caps showed then to be all high, tho not leaky (except the metal clad caps which have been spot on). These were quickly replaced, the set fired up, and a perfectly stable picture attained.

Fiddling with the focus ring produced a stunningly good image, with only minimal ion burn.

However, there are two remaining problems, which may be related. The first one is after about 20 mins the EHT starts to drop, accompanied by a blowing up of the image and lack of focus. At a pinch it is still watchable, tho nothing compared to what she is capable of when cold. I tried swapping the PL38 with no difference. The EHT seems to drop to a level then remain there.

The other problem is the sound. Adjusting the volume control changes the picture size, leading it to shrink as the volume is increased. At a point halfway on the volume adjustment the sound disappears. Putting a meter over the volume pot shows inconsistent values, so will be replaced when a new one arrives.

Does anyone have any advice? Other than mentioned above I haven't touched any other components.

Matt
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Old 4th May 2019, 3:53 am   #2
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Default Re: Pye LV20 - Diagnostic assistance

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Originally Posted by Autozavod View Post
Hi everyone,


The other problem is the sound. Adjusting the volume control changes the picture size, leading it to shrink as the volume is increased. At a point halfway on the volume adjustment the sound disappears.

Matt
One way the volume control could likely alter the picture size is if the volume control was altering the bias of a valve that was drawing significant current from the power supply, and this would imply a leaky coupling capacitor between the volume control and a valve grid.

The EHT drop with time would be possible to diagnose, but it would likely require a scope and meters, possibly a high voltage probe and having the schematic in front of you. I am not familiar with this model set, but others may have seen this issue before.
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Old 4th May 2019, 7:05 am   #3
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Default Re: Pye LV20 - Diagnostic assistance

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Originally Posted by Autozavod View Post
Testing of the related caps showed then to be all high, tho not leaky (except the metal clad caps which have been spot on).
Presumably you were just testing the caps with the capacitance range on a multimeter? This is a classic problem....most digital meters will measure a leaky cap as 'high' simply because of the way they work. All those caps will have been very leaky had they been measured on a proper cap tester or subjected to their normal voltage and the leakage measured. The metal-clad caps may fare slightly better but it's likely that they are all well below par as well. Any caps used for audio coupling should be replaced irrespective of what the multimeter says....it's likely that they are breaking down when subjected to voltage....something that a multimeter can never show.
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Old 4th May 2019, 9:13 am   #4
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Default Re: Pye LV20 - Diagnostic assistance

Your picture symptom is typical of a saturated line output transformer. There has a lot been written about this over the years. Running a current through the windings particularly the EHT overwind may improve matters. It may take three or more days to drive the damp out of the pitch covered windings. J.
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Old 4th May 2019, 9:13 am   #5
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Default Re: Pye LV20 - Diagnostic assistance

Thanks gents,

The EHT should be about 6.5kV. On the electrostatic meter it shows 6.5, then gradually drops to a stable 5kV.

Don't worry, I'm using a Wayne Kerr Universal Bridge to measure the capacitors, those little electronic meters are unreliable for measuring in circuit.

Matt
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Old 4th May 2019, 10:09 am   #6
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Default Re: Pye LV20 - Diagnostic assistance

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Testing of the related caps showed then to be all high, tho not leaky (except the metal clad caps which have been spot on). These were quickly replaced
I have restored several of these sets and have found that the metal clad Sprague caps are still fine, but always replace them for peace of mind.

I would strongly agree with John about passing a current through the LOPT, it has cured the EHT drop symptoms in many sets I have restored.
Simply baking the LOPT in an oven does not do the job.

Mark
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Old 6th May 2019, 11:01 am   #7
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Default Re: Pye LV20 - Diagnostic assistance

Replacing the grid coupling capacitor fixed the sound and varying picture size problem. I’m leaving the lopt for another week with 30V at 0.15A to dry out more. I’ve also positioned a small plastic bag to catch some of the moisture purely for interest’s sake.

I’m disappointed my WK universal bridge didn’t detect the capacitor leakage. I shy away from bulk capacitor replacement based on the good advice from this forum, it really does help your understanding when you replace only one or two targeted components at a time.

Thanks gents ��
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Old 6th May 2019, 7:35 pm   #8
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Default Re: Pye LV20 - Diagnostic assistance

I don't think you will catch any actual 'water' in the plastic bag. It is very fine moisture dissolved in the air and takes very little to upset early flyback EHT transformers.

I hope your treatment has a beneficial effect. It should help. John.
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Old 6th May 2019, 7:48 pm   #9
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Default Re: Pye LV20 - Diagnostic assistance

It's certainly one for the survival manual, that: unusual sources of drinking water include the line output transformer of an old CRT TV, the older the set and the worse the picture the better the chance .....
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Old 6th May 2019, 10:04 pm   #10
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Default Re: Pye LV20 - Diagnostic assistance

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Originally Posted by Autozavod View Post

I’m disappointed my WK universal bridge didn’t detect the capacitor leakage. I shy away from bulk capacitor replacement based on the good advice from this forum, it really does help your understanding when you replace only one or two targeted components at a time.

Thanks gents ��
Best way to check a capacitor is by passing a dc voltage through it and measuring the current....as the voltage increases so will the leakage. It's extremely unlikely you will get anywhere near it's rated voltage before the leakage becomes unacceptable. If you put a 1K (wirewound) resistor in series with the capacitor, connect a variable DC voltage across the capacitor and series resistor, measure the voltage across the 1K and then work out the current by ohms law. Remember you can get a rough idea by knowing that 1 volt across 1K = 1mA.....in fact you don't really need to go that far since if the voltage across the resistor capacitor combination is (say) 50V and your meter across the 1k reads 1 volt...the capacitor is useless anyway.

You can make a variable DC supply easily with your variac....just use a 1N4007 rectifier and an 8 or 16uF 350V (or higher voltage) capacitor for a degree of smoothing and you can vary the DC voltage from 0 to whatever your variac allows.
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Last edited by Sideband; 6th May 2019 at 10:11 pm.
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