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Murphy V230

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MurphyV310
(@murphyv310)
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Hi.

One of the sets Norman brought me is a Murphy V230. It is a bit tatty round the edges and the original CRT suffered from an intermittent O/C cathode. Although the set works it has bad frame speed drift after 15 minutes. The usual valve replacements (20L1, 6L18) were initially tried with no improvement. The caps in the fame stage had already been replaced so I went through the resistors, most were in tolerance but as usual some were way out. Frame circuits usually need to have the resistors close to tolerance or not too far out as it will effect linearity or frame speed. The ones out of spec were changed, only around four or five though. This made no difference to the fault. Application of the hair dryer and freezer showed up the top tag strip a likely leak. Removing leads adjacent to an HT connection I found 70-80 volts on the empty tags. So in the meantime I have removed the suspect tags an literally hung the components in the air. This made a difference with no drift for half an hour, after some more time with heating and cooling It was noted that the valve base could be suspect on the 6L18. I then read 8 meg ohms between two connections on the base with the wiring removed. Not having a spare B8A base I discussed the issue with David Boynes, he said that the 6/30L2 with both triodes paralleled was an Ediswan mod for the obsolete 6L18. I duly fitted a ceramic B9A base and wired in the 6/30L2. On switch on after adjusting the hold the picture was spot on with no adjustment to height or linearity, so this mod is a good one for replacing the old 6L18. An hour soak test proved all seemed OK. On re-assembly into the cabinet a slight frame speed drift was noted after 20 minutes and this turned out to be the 4700pf frame cross coupling cap not liking the heat, it was an RS Polypropylene yellow axial. I replaced it with a 60's Mullard Mustard and it now is rock solid with no drift. 

The picture is very good, the CRT is bright and sharp and I've welded the cathode with EHT.

IMG 20190323 151152(1)
IMG 20190323 151140

Cheers,
Trevor.
MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member

 
Posted : 23/03/2019 8:02 pm
Lloyd, crustytv, PYE625 and 9 people reacted
PYE625
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Excellent fault-finding there and the results do you credit. ? 

Great video too.

I would be too scared to make any video of my stuff lol.

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

 
Posted : 23/03/2019 9:08 pm
Norman-Raeburn
(@norman-raeburn)
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Hi Trevor, Good to see your back into the shed again. I hope the collection of sets I brought over are keeping you amused. I hope I didn't put any faults on the set. Good result and a good outcome for the crt. Keep up the good work. Norman

 
Posted : 23/03/2019 9:09 pm
crustytv
(@crustytv)
Posts: 11869
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Hi Trevor,

That's an excellent picture you've got there, great work as ever ? 

I've got one of these, its been sitting under the store bench, after sorting out a power supply fault I obtained first light but left it alone after and there its sat for more years than I can remember. If I recall its got a "Nulife" regun tube fitted. I half expected fireworks from the notorious * Mazda U801 but it was fine.

* For the uninitiated; the 4 anode U801 valve has two anodes strapped together as an HT rectifier and the other two strapped as the efficiency diode. According to many reports this caused an often spectacular failure.

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Posted : 23/03/2019 9:13 pm
Nuvistor
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Didn’t some sets fit resistors in the anode circuits of the U801 Valve, one resistor would burn out putting all the load on the other anode and blue flashes ensued.

Nice work Trevor.

Frank

 
Posted : 23/03/2019 9:18 pm
MurphyV310
(@murphyv310)
Posts: 453
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Hi.

The U801 is notoriously bad, I've a NOS one that might be used one day. Funnily enough the one in this set looks original as is the smoothers. Although a bit tatty round the edges I would say it's been a reliable set during it's life. I'd say it was unused after the CRT gave trouble but perhaps been stored in a damp place. Valve bases are now a constant issue in 50's sets and not just the paxolin ones.

Thanks for all the encouragement, still struggling especially during the evening, far too quiet here now, maybe organise a meet up this year...... we will see.

Cheers,
Trevor.
MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member

 
Posted : 23/03/2019 9:27 pm
Till Eulenspiegel
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The suggestion for replacing the Mazda 6L18 triode for the Mazda 6/30L2 comes from one of those pocket size Mazda valves and picture tube data books.

 The characteristics of the 6L18 are: gm 7.6mA/V. Mu 17. ra 2.25Kohms. PA(max) 4 watts.

 The 6/30L2: gm 3.4mA/V.  Mu 18. ra  5.3Kohms. PA 2watts.

So it can be seen that when the two sections of the 6/30L2 are connected in parallel the characteristics will be almost the same as the 6L18.

There is no equivalent for the Mazda U801 and having that 0.2amp 80 volt heater doesn't much either.  In some sets such as the Murphy V230 one half of the U801 functions as the line boost diode and the other the HT supply rectifier. Perhaps the only substitution for this will be to go all solid state, a pair of 1N4007 diodes for the HT rectifier and say, an UF5408 for the boost diode.  A 400 ohm 20 watt resistor replaces the valve heater.

In the same Mazda book the suggested replacement for the 6P28 line output is the EL504 which according to substitution notes is an export valve and only available to special order.

Till Eulenspiegel.

 
Posted : 23/03/2019 10:17 pm
MurphyV310
(@murphyv310)
Posts: 453
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Hi.

I've tried silicon diodes as boost diodes in valve sets before and don't work. The wave shape is such that silicon diodes get really hot. The ones I've tried in the past are proper recovery diodes, very fast types. 

Cheers,
Trevor.
MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member

 
Posted : 24/03/2019 5:05 pm
MurphyV310
(@murphyv310)
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Hi.

I have popped the set into the loft in the meantime as I want to do something with the mask and case soon. The intermittent O/C cathode on the CRT has re-appeared so the "welding" wasn't a lasting success. One tap on the neck and its fine till you turn the set off. I think I can tidy the mask with Isopon or cellulose putty (if you can still get it) and the case will come up with sprayed gold Hammerite, the finishing pieces at the top and bottom of the case will need to be cleaned up and sprayed too, as will the speaker grille.

Next thing is to get back into the loop aerial in due course.

Cheers,
Trevor.
MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member

 
Posted : 26/03/2019 9:27 pm
Nuvistor
(@nuvistor)
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The regun CRT, I wonder if the replacement electron gun was made Mazda, an O/C cathode was a common fault with Mazda BW tubes.

 

Frank

 
Posted : 26/03/2019 10:00 pm
MurphyV310
(@murphyv310)
Posts: 453
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Hi Frank.

It was me that tried to "weld" the cathode with EHT, with the heaters fed from a 12v supply, i've seen it done years ago with a certain amount of success. it did last a few days on this set but it is back to a tap and its fine. Sad thing is the emission is really good on the tube.

Cheers,
Trevor.
MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member

 
Posted : 26/03/2019 10:08 pm
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