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B&W TV Murphy V410

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PYE625
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Today, I plan to re-stuff the remaining wax capacitors on the side of the chassis that is hard to gain access to when the CRT is fitted. I know the tube is easy to remove, but I don't fancy taking it in and out like a fiddler's elbow as I would probably have to do so if fault-finding in the conventional sense. With so many capacitors including some Hunts moldseal type, I think this is a sensible idea.

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 25/07/2021 9:37 am
PYE625
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One such capacitor is C111, a 0.5uf that is mounted on the frame output transformer. This decouples the A1 supply to the CRT and it is suggested in the modification section of the manual to move the earthy end from ground to the HT connection. The reason is to improve the reliability of this component.

As this modification has not been done to this particular set, I won't bother. I am using a 630v rated capacitor inside the container of the old one and this will withstand the 480v of the A1 supply quite well I would expect.

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 25/07/2021 10:18 am
PYE625
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At every turn there is a wax capacitor lurking. Here below is the frame output transformer with the position vacant on top for C111. This lot is all mounted directly above and around V12 the 30P12 frame output valve so it is not surprising the wax will soften in the warmth during use.

It is all rather gooey and messy here in this area, more so than the photo tends to show. V12 is mounted directly below the lower wax capacitor, just out of shot.

IMG 6043 50

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 25/07/2021 10:57 am
PYE625
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Mind you, I wonder if anyone is really interested in all my ramblings.... Is it really worth putting all this down?

It's only yet another black and white set after all. But then I am enjoying it at least, and I hope it may be of some use to anyone embarking now, or in the future, on such a set as this.

Anyway, here is a view of the frame output valve V12 together with V16, the frame oscillator, directly below. To the upper left is V10, the audio output. The electrolytic capacitors also pictured have reformed well and are serviceable still.

IMG 6035 50

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 25/07/2021 11:11 am
PYE625
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Posted by: @pye625

It is therefore probably safe to assume that the set was used during it's service life in the Norwich region, the tuner frequencies will confirm this later however.

I can confirm this as the tuner biscuits are maked up for CH3 (BBC Tacolneston) and CH11 (Anglia Mendlesham). I cleaned the tuner switch contacts with Servisol as they were a bit grubby. The grease on the rotating parts of the tuner was perfectly fine and supple, so I left it alone.

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 25/07/2021 5:09 pm
PYE625
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Speaking of Anglia on CH11, I simply had to share this introduction to the service.....

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 25/07/2021 5:25 pm
Jayceebee
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Posted by: @pye625

Mind you, I wonder if anyone is really interested in all my ramblings.... Is it really worth putting all this down?

Please keep it coming, the 300, 400, & 500s were the first sets I was first let loose on when I started in the trade so this brings back lots of memories. I'd forgotten just how big the EHT caps were on these sets, would put some CTVs to shame.

The most common faults I remember were the little 1000pF screen decouplers in the IF and there was also to 1.5K resistors feeding the screen grid of the 30P4, change it to a 3K wirewound. A lot of people go on about the poor Mazda tubes in these but I changed very few.

John.

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Posted : 25/07/2021 9:36 pm
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Nuvistor
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@pye625

Famous for its use at the start of networked programmes to be seen in other UK areas.

Frank

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Posted : 25/07/2021 9:39 pm
Lloyd
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I certainly enjoy your ramblings, Andrew! Keep em coming!

 Regards

Lloyd

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Posted : 25/07/2021 10:25 pm
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PYE625
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Posted by: @jayceebee

The most common faults I remember were the little 1000pF screen decouplers in the IF and there was also to 1.5K resistors feeding the screen grid of the 30P4, change it to a 3K wirewound.

The resistors are R151 and R152. I will check these later, probably after initial powering up. Thanks John.

One such capacitor, C17 a decoupler in the IF stage, has previously been replaced by an RS red ceramic type. C17 is a torpedo type and is still in situ. It has clearly burnt out and the RS replacement tacked across it.

There are two or three other RS type that have clearly been used to replace faulty decoupler's. There is also one in the sound stage (C62 screen decoupler to V9)) and the core of L18 has been tampered with and is broken I notice. I bet it was twiddled at the time of a fault with C62 failing.

Hopefully, no other cores have been twiddled with.

Here is a view of the IF section, quite a number of suspect capacitors to check for sure. Including another torpedo type, looks like a 0.01uf, just below a wax capacitor middle top. (The two electrolytics have already been attended to).

IMG 6024a

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 25/07/2021 10:25 pm
Nuvistor
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@pye625

When we got the Murphy dealership these sets were about 10 years old and most were being replaced by CTV’s. However I did see one or two and one repair I did was no vision. The owner switched on the set set and there it was a white screen. Something suggested to me unstable final IF, small screwdriver and finger on G1 of the final video IF and on came a picture. 
It was one of those 10k caps that look like a resistor decoupling G2. Nice easy job and not in the house long. Probably replaced with a red Hi-K RS ceramic but can’t be sure now.

Frank

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Posted : 26/07/2021 7:33 am
Nuvistor
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Thinking about another similar model I went to repair, it was in a farm house at the end of a long overhead power cable. It was about 4 or 5 pm, can’t remember the fault but fixed it then it broke with different symptoms. A mains voltage check was 200vac or there about. The farmer had just come home and said it does that every tea time when the cooker is on, I just adjust the voltage disc until after tea. 😀

I suggested one of the valves could be weak, had a similar problem at another location different set make and the LO would fail, new PCF80 sorted that without recourse to voltage settings. The farmer decided against further work, no point spending brass when I can twiddle the control.

Frank

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Posted : 26/07/2021 9:49 am
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Cathovisor
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@nuvistor

A friend of mine recounted a similar tale, whereby a customer discovered he could eke out a bit more life from his tired tube by turning the voltage adjuster.

This worked well..

... until the day he turned the adjuster a click too far and put it on the "DC" settings - which bypassed the rectifier...

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Posted : 26/07/2021 12:36 pm
PYE625
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It was a bit silly to have the voltage selector so easily accessable and adjustable. May have been better if it was covered by the rear panel with perhaps just a small viewing hole to check the voltage selection.

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 26/07/2021 5:18 pm
Katie Bush
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Posted by: @pye625

It was a bit silly to have the voltage selector so easily accessible and adjustable.

Especially if you had curious kids in the house! - I was lucky enough to grow up with a Bush TV75, where you could not screw around so easily with the voltage settings, but I (silly bugger me) turned and twiddled the living daylights out of every accessible control knob.  If we'd had a Murphy back then, I'm pretty sure this silly 6 year old would have fried it for certain!

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Posted : 26/07/2021 6:58 pm
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PYE625
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As far as the LOPT goes, the leakage is still coming down. I have decided against having it in the oven and prefer to run a small current through the windings as before. I guess either method works just as well, provided the vent tube is not sitting below the oil level blocking free movement of air.

We are now over the range of the meter, so above 2000 megs at 500v on the EHT, almost 500 megs on the main winding. This is a distinct improvement from when I first begun the process.

I knew this would take some considerable time and it is not convenient to have it in the oven for days on end.

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 26/07/2021 9:15 pm
PYE625
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As the leakage has come down, I thought it might be a good idea to test the LOPT in the set. So this evening, after watching a thunderstorm, I fitted it back into the chassis.

I am not completely convinced I have solved the problem of leakage because in my Murphy V280C, that LOPT has totally zero leakage even when warm. However, that can had been opened and was thoroughly dried out.

This one, when warm, does still indicate a measured leakage though. However, in the set, it would appear to work ok. I need to check the EHT voltage, but even though the main HT in the set is 20v down on what it should be, the results seem promising. This could be the metal HT rectifier, but futher investigation is needed here.

After the usual minor adjustments to the line, frame hold and height, plus some frame linearity tweaking, we have a reasonably good picture. The sound is quite low and will need investigation at some point. Voltages on the CRT base are not a million miles out, A1 being about 70v low. Not helped by the slightly low HT I imagine.

The tuning is spot on for both ITA and BBC, no adjustment needed here. No noticeable drift during warm up either. A damn good design of tuner indeed.

Anyway, here is a screen shot and yes, a party trick of increasing the contrast too high does indeed result in a completely negative picture with no loss of sync.... You were right John 😉

IMG 6044 50
IMG 6046 50
IMG 6047 50

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 28/07/2021 11:16 pm
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PYE625
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I am not happy with the LOPT. Although it would appear to work with no apparent defects such as reducing width or a ballooning picture associated with falling EHT, the case get's very warm and there is slight oil leakage around some of the cable outlets on the neoprene top. When tested again for leakage whilst still warm, it has increased to similar values as when first tested with it cold. Also, there was a slight bulge in the top so pressure inside must have increased. I carefully tilted the LOPT so when I removed the stopper, only a little air came out and not oil shooting all over the place.

I am now doing what I should have perhaps done to begin with. I have drained out as much oil as possible and then will warm the LOPT in the oven for a few days at 60-70 degrees. The measured leakage is still there with no oil. Hopefully, any moisture will now be better able to escape. The oil itself is being heated to just over 100 degrees to hopefully boil off any moisture content. It was cloudy to begin with, but now is clear as it is getting hot.

Here below I have fitted a small spout into the stopper hole of the LOPT to see better the progress of oil flow. The oil itself is being heated in a Pyrex jug in cooking oil.... And it smells. It is steaming very slightly too.

IMG 6051 50
IMG 6052 50

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 29/07/2021 6:26 pm
PYE625
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Unfortunately, I overheated the oil as it has now turned a very dark brown. I left it on the hob unattended and came back to find the temperature had reached just under 140 degrees. Obviously, whatever kind of oil this is, it is probably safe to assume it must not be heated over 100 degrees.

I was in doubt as to re-use the oil to begin with. I certainly won't use it now.

IMG 6053 50

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 30/07/2021 10:55 am
Nuvistor
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@pye625 
That’s a shame but it happens, what oil can you use as a replacement? I checked online and only found transformer oil available in gallon or 25 litre quantities and it was expensive.

Frank

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Posted : 30/07/2021 11:30 am
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