Murphy Barrel sets

[Murphyv310]

Hi.
Here is the explanation of the AGC circuit.
This is from the V530 manual, it's basically the same in the V410 & 510 bar the VHF FM dual conversion section.
   
   

[Geordie McBoyne]

Hi Trevor,
As I suspected it is a sync cancelled vision AGC system. I don't think this is quite as effective as a back porch sampling system which measures the amplitude of the sync pulses, but nevertheless it is much better than a simple mean level AGC circuit.

Geordie McBoyne.

[Murphyv310]

Hi.
The V410 developed a fault on switch on this afternoon. There was bad hooking on the top of the picture on both Aurora and Hedghog converters. One of the flywheel diodes had developed a leak, after replacing both with two 1N4148 diodes the hooking cleared but there was distortion on the verticals corresponding to the castellations. A quick look on the scope on the sync separator anode and video was leaking through. The sync coupling cap was new and the only resistor that was out of tolerance was the 150K anode load at around 260k, replacement was a cure.
There is no rhyme or reason to these resistors, some are well within tolerance and others wildly out, I think I'll need to more or less go through the lot before I do a final re-alignment.

[Terry]

A thought occurs to me, well two really, about that sync cancelled AGC system. It relies on the peak of the inverted video signal being black level and there being no blacker than black signal apart from the sync pulses.

However, on colour transmissions, the subcarrier can be blacker than black under some conditions - mainly saturated red and blue - so would one of these sets adjust the gain from scene to scene when they occurred or was there an HF roll off before the AGC sampling to stop this happening.

I saw very few Murphy sets, so the chances of watching one under the right conditions - time of day, transmission content - was extremely unlikely  and also, because 405-line NTSC was only ever transmitted from Crystal Palace, you wouldn't have been in a position to know, Trevor.

It while I was pondering this that the second thought came to me - what happened when the vertical sync broad pulses arrived? Or was there a long time constant involved that smoothed out these imperfection - both chroma and field sync - so that neither could cause any visible effect?

[Murphyv310]

Very good point Terry.
I'm lucky to have a NTSC 405 line source from the NTSC aurora, so I think I'll have a look at the AGC with NTSC enabled on the aurora.

[Geordie McBoyne]

Sync cancelled vision AGC was employed in many Ekco TV receivers, I suppose the system is a compromise between the simple mean level AGC and an a proper back porch sampling system.
Ultra used a vision AGC system which sampled the black level during the frame blanking period.
I guess that one advantage of negative vision modulation is that the sync pulse tips can be sampled for AGC purposes.

Geprdie McBoyne.

[ppppenguin]

I guess that one advantage of negative vision modulation is that the sync pulse tips can be sampled for AGC purposes.

They don't even need to be sampled. A simple peak detector will work.

[Murphyv310]

Hi.
I have been looking at a thread on the UKVRRR regarding a Murphy V310 restoration. There has been a lot of talk about dismantling the LOPT. A post was put on by Boater Sam regarding the use of a Jam jar. I think this is an excellent idea and if a suitable plastic lid can be obtained then why not, you can see the level of oil and of course the whole transformer is in an insulating material, one downside could be RF radiation but that is just a thought.