Share:
Notifications
Clear all

B&W TV Murphy V410

Page 5 / 5
 
PYE625
(@pye625)
Famed V-Ratter Registered
Posts: 4773

I think with a Murphy set of this era, there is always going to be a strong possibility of LOPT trouble. OK, this can apply to almost any set of any era, but being an oil filled affair makes it extra difficult. Certainly easy to open up the can, but a potential nightmare to seal it effectively again afterwards. I have managed it here, but it is a real pain to be quite honest.

It would be nice to have a facility that could make a new can and have the tooling to close it up afterwards in a similar fashion to the original. Oh well.... dream on.

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

ReplyQuote
Topic starter Posted : 15/08/2021 10:31 pm
PYE625
(@pye625)
Famed V-Ratter Registered
Posts: 4773

This evening I was, about 5 minutes after switch on, greeted with another fault. The nature of this one is sudden loss of frame and line sync. The controls were adjusted for a floating picture as photographed below, but there is no lock. It will, however, be perfectly fine when the set is cool and will not occur until after a few minutes. This is repeatable with sudden loss of sync again. Tomorrow evening, I shall investigate further.

IMG 6082 50

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

ReplyQuote
Topic starter Posted : 17/08/2021 9:37 pm
PYE625
(@pye625)
Famed V-Ratter Registered
Posts: 4773

Looking around the sync separator valve, V8 30FL1, the fault comes and goes at the slightest touch. Barely tapping the valve causes the same thing, so I replaced V8. The fault would seem to be clear now and refitting the old valve brings it back, so it is almost certainly a faulty internal connection within the valve.

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

ReplyQuote
Topic starter Posted : 18/08/2021 5:24 pm
Nuvistor and Lloyd liked
PYE625
(@pye625)
Famed V-Ratter Registered
Posts: 4773

Faults keep coming out of the woodwork because I have been having trouble with drifting line lock. This has been getting worse and today, it was impossible to achieve lock from cold. Also, there was twitching verticals from cold consistant with dodgy line hold. As the set warmed, until after about 15 mins, the line hold needed constant re-adjustment. It would then be stable.

An investigation was needed. I checked all the voltage conditions in the line oscillator stage, thinking it was possibly a HT feed resistor drifting in value. Nothing dramatic was found amiss. I Swapped the line oscillator valve V13, but this was not the trouble. The line hold control itself and associated components all seemed stable too. However, the frequency was all over the place at one point just after switch on. All DC seemed ok around V6, the flywheel phase splitter triode section, so this left the flywheel discriminator circuit.

I attempted to check the discriminator diodes in circuit, but could not get a sensible reading. After carefully removing them both, they each measured about 370k in one direction on the ohms range of a DVM. This is perfectly fine, and they were stable too. Just in case one was breaking down, I decided to fit a couple of 1N4148 diodes in series with 18k resistors to eliminate the original selenium diodes.

At this point, I discovered the true nature of the fault! When desoldering a connection at the junction of MR3b and R73, the end of R73 simply fell away from the resistor. Upon removal proper, it could be seen that the internal end connection of the resistor was green and corroded. In circuit, these resistors (R72/3) checked out a bit high in value at about 140 and 170k respectively. Anyway, I replaced them both with a couple of similar type 100k resistors. These both are around 125k each in any case, so are fine to use.

I re-fitted the original selenium diodes and now the set is perfectly stable with the line hold. The second pic show's the refitted diodes in their plastic sleeves, last pic the remains of R73. It fell to bits when removed due to internal corrosion.

IMG 6083 50
IMG 6085 50
IMG 6084 50

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

ReplyQuote
Topic starter Posted : 22/08/2021 1:41 pm
Lloyd liked
Katie Bush
(@katie-bush)
Illustrious V-Ratter Moderator
Posts: 5078

I'm wondering now if that might have been behind the apparent fault with V8 - How about, just for fits and giggles, refitting the original V8, and see if it was actually at fault in the first place?

ReplyQuote
Posted : 22/08/2021 4:05 pm
PYE625 liked
PYE625
(@pye625)
Famed V-Ratter Registered
Posts: 4773

Hi Marion, V8 caused a slightly different fault, total loss of sync, both frame and line. I have just re-tried it and it is certainly intermittant. I briefly tried in in the sound output position and it gave hum with crackling sounds, so I think it has a dodgy heater cathode insulation. It is now in the bin.

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

ReplyQuote
Topic starter Posted : 22/08/2021 4:52 pm
PYE625
(@pye625)
Famed V-Ratter Registered
Posts: 4773

No real updates here to report, other than the set is still working very well.

The next step is to remove the CRT to clean the faceplate and escutcheon. The rubber surround band holding the CRT will need to be replaced as it is rock hard and affording little grip at the moment.

Not had much spare time lately, but must get around to it.

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

ReplyQuote
Topic starter Posted : 12/09/2021 8:43 pm
PYE625
(@pye625)
Famed V-Ratter Registered
Posts: 4773

I have removed the CRT and cleaned the mucky surround and replaced the hardened rubber of the CRT holding band. As it was previously a loose fit, the CRT had moved too far forward and was resting on the front glass.

IMG 6094 50

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

ReplyQuote
Topic starter Posted : 03/10/2021 10:22 am
Lloyd liked
PYE625
(@pye625)
Famed V-Ratter Registered
Posts: 4773

The next task has been to remove the control cover, mains switch lid and speaker assembly from the top of the cabinet. This is because it is quite loose after many years of use and it all needs a good clean. The hinge will need cleaning and minor lubriction upon re-assembly. The wooden cabinet is in very good shape and needs little more than a clean.

IMG 6098 50
IMG 6097 50

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

ReplyQuote
Topic starter Posted : 03/10/2021 11:32 am
Lloyd liked
PYE625
(@pye625)
Famed V-Ratter Registered
Posts: 4773

With the above jobs complete, the set is now re-assembled. Whether or not any further faults develop by testing the chassis in it's cabinet remains to be seen. Obviously, it won't get the kind of use that it would have done when it was in service. All in all, this has turned out to be a nice set to work on and I enjoy it.

IMG 6101 50
IMG 6099 50
IMG 6100 50

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

ReplyQuote
Topic starter Posted : 04/10/2021 4:24 pm
Lloyd liked
Lloyd
(@lloyd)
Noble V-Ratter Registered
Posts: 1394

That’s come up good! It’s a really nice design overall, I’d love to give one house room!

 Regards

Lloyd

ReplyQuote
Posted : 05/10/2021 10:30 am
Till Eulenspiegel
(@till)
Famed V-Ratter Registered
Posts: 4463

Not so lucky with my Murphy V410. The line output transformer has gone short circuit to chassis.

Till Eulenspiegel.

ReplyQuote
Posted : 05/10/2021 10:45 am
PYE625
(@pye625)
Famed V-Ratter Registered
Posts: 4773
Posted by: @till

Not so lucky with my Murphy V410. The line output transformer has gone short circuit to chassis.

Till Eulenspiegel.

I think it has to be said that the oil filled LOPT is the most difficult thing to deal with. Ok, this applies to many non-oil filled LOPT set's too, but it is plain to see that being oil filled does nothing to improve the reliability of the things. I for one would have thought that the oil would make it more reliable, be a great insulator, never a breakdown of insulation, nor damp ingress. But alas, no.

I wonder if the the V410 would be adaptable to a Thorn Jellypot ?

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

ReplyQuote
Topic starter Posted : 05/10/2021 5:25 pm
dawson5
(@dawson5)
New V-Ratter Registered
Posts: 3

One of the most attractive and best performing sets in fringe conditions. They were in their time pretty reliable.

So easy to get cabinet off for servicing.

Just had to watch for oil leaks. Don’t remember changing many LOPT’s.

ReplyQuote
Posted : 05/10/2021 9:39 pm
Page 5 / 5