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Old 2nd Mar 2015, 9:11 am   #121
mark pirate
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Default Re: Marconiphone VRC74DA

Here are a few screen shots showing the problem on test card C.
The effect of increasing the brightness can clearly be seen, by the third photo the squares on the test card have disappeared totaly!

Mark
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Old 2nd Mar 2015, 9:53 am   #122
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Default Re: Marconiphone VRC74DA

Weird! It looks as if the contrast is too high and the brightness a bit low. With a low emission tube viewed in a darkened room you should be able to adjust the brightness and contrast to get a fully graded picture but with very little actual light output from the CRT. Your pictures appear to show what I call the 'soot and whitewash' effect usually produced by a video amplifier fault. Check the D77 detector, C22 4uf and the effect of disconnecting any connections to pin 1 of the D77 vision int. limiter section. C23 couples the video output to the cathode of the CRT and is worth a check if you have not done so.
Later, I will post some pictures from the screen of my VC53DA producing the effect of a low emission CRT. [Resistor connected across tube heater] Must do some work first! J.

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Old 2nd Mar 2015, 10:24 am   #123
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Default Re: Marconiphone VRC74DA

Hi John, The effect is weird, there is much more brightness available than my Baird T164. That set has a really low emission CRM 92. and as you rightly say, will produce a correctly contrasted picture, albeit incredibly dim.

C22 & C23 are new, so I will check the D77 (EB91) and the limiter circuitry.
I forgot to mention that turning up vision interference limiter actually improves the picture somewhat?

I ran the set up from cold last night. it needed a tweak to the line hold at first, but then ran fine for over two hours and apart from the picture issues, it was rock solid.

Mark
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Old 2nd Mar 2015, 10:57 am   #124
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Default Re: Marconiphone VRC74DA

Those earlier bridged chokes will have some effect ?
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Old 2nd Mar 2015, 12:40 pm   #125
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Default Re: Marconiphone VRC74DA

Quote:
Check the D77 detector, C22 4uf and the effect of disconnecting any connections to pin 1 of the D77 vision int. limiter section.
I have tried another D77 with no improvement. I have disconnected the limiter circuit from pin 1 of the D77. there is a slight improvement but the problem is still there.

Quote:
Those earlier bridged chokes will have some effect ?
I have checked the two on the HMV, one is O/C and the other looks ok, I suppose I could try the good one to see if it helps.

Mark
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Old 2nd Mar 2015, 12:43 pm   #126
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Default Re: Marconiphone VRC74DA

Hello Mark,
The limiter control in the 1807 series works in reverse. You need to turn it fully clockwise to put it out of action. [It is an 1807] The same goes for the contrast in my example and the wiring is original.
You can clip the wire to pin 1 of the D77 to disconnect the spot limiter circuit completely. John.
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Old 2nd Mar 2015, 12:56 pm   #127
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Default Re: Marconiphone VRC74DA

Quote:
You can clip the wire to pin 1 of the D77 to disconnect the spot limiter circuit completely.
Already done John, I have also replaced the choke from V5 to V6. Still no improvement though.

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Old 2nd Mar 2015, 12:58 pm   #128
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Default Re: Marconiphone VRC74DA

The effects of a low emission tube are demonstated here, the receiver being a Pye V4 that I have recently restored and was soak testing on the bench.
The heater volts were cut by simply placing a 25 ohm resistor across the tube's heater to reduce the available voltage to around 3v in the series heater chain.
As can be seen the picture lacks all 'punch', is grey in appearance but is in good focus generally.
The room lights were turned off for these pictures that are produced from a digital camera, many times brighter than they are on the actual screen.
Any attempt to increase brightness or contrast results in overdriving the weak emission turning the picture the classic negative appearance and giving it an overall 'satin' look. Compare with the picture of the CRT working normally.
The CRT in this case is a Mullard MW36-24 tetrode but the symptoms are identical in all receivers from 1936 to date including of course the EMI 3/16 employed in the 1807 series. Hope this clarifies the diagnosis.
PS. As Simon has suggested, have you repaired/rewound the video peaking chokes? John.
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Old 2nd Mar 2015, 1:56 pm   #129
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Default Re: Marconiphone VRC74DA

Quote:
As Simon has suggested, have you repaired/rewound the video peaking chokes?
I have replaced L17 from my HMV set, L18 is still bridged as L18 in the HMV is also O/C.
I will have a go at rewinding it, but I think the tube being low is the main problem.
It probably looks brighter in the photos than it actually is
I guess it as good as it is going to get. The picture is better with the limiter out of circuit, so I will leave it disconnected.
The photos below show the best picture I can Get, the second photo shows it with the brightness turned up.

Running the CRT heater from a !2v transformer gives a better picture, but not worth the risk of running it like that. Better to have a dimmer picture than an O/C heater!

I will try lashing up the CRT in the HMV to this set just for the hell of it, as I think it has reasonable emission.
At least it will be interesting to see if it is any better than this one, also it would prove if the set is worth restoring.

Mark
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Old 2nd Mar 2015, 4:08 pm   #130
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Default Re: Marconiphone VRC74DA

Hi Mark,
have you access to an oscilloscope? If you have then the waveform across R24, the vision detector load resistor, should be examined.
A common condition exists with positive modulation signals where the peak white is crushed when the signal levels are too high. When the contrast control is advanced the sync pulse height increases but the active video part becomes compressed. As mentioned earlier the contrast control is in wrong position where it is in the first RF stage. it should be the RF gain control and the contrast control should be further on in the vision RF stages.
Reason for the crushing of the video signal is caused by the use of a straight RF pentode in the controlled stage. Vari-mu valves are better.

DFWB.
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Old 2nd Mar 2015, 4:41 pm   #131
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Default Re: Marconiphone VRC74DA

The first picture now lacking the 'soot and whitewash' effect looks a lot better with the limiter [picture spoiler..] completely out of action. The second is giving the classic symptoms of a worn CRT. I agree, I think that is about as far as you can go with this one.
The 3/16 has a 13v heater but early version may have been 8.5v so you have to take care here. A few were 10.5v to add even more confusion. They tend to go O/C without warning and are very bright when operating at the correct voltage.
Well at least you got a picture on the beast! John.
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Old 2nd Mar 2015, 4:47 pm   #132
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Default Re: Marconiphone VRC74DA

Hi John,
We should introduce a topic sometime in the future entitled "Improving HMV 1807".

DFWB.
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Old 2nd Mar 2015, 4:50 pm   #133
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Default Re: Marconiphone VRC74DA

Hi David, Yes I do have a scope, It certainly is worth checking.

Quote:
As mentioned earlier the contrast control is in wrong position where it is in the first RF stage. it should be the RF gain control and the contrast control should be further on in the vision RF stages.
It is a truly horrible design, what were they thinking?
Worse still is the focus arrangement varying the EHT!
In fact all the presets controls interact with each other, making adjustment a real pain.

I am in the process of connecting the chassis to the CRT in the HMV 2811, so this will be a good test of the chassis and the tube.
In a way I am hoping the CRT in the HMV is low, as I won't have to restore another 1807A chassis set!

Mark
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Old 2nd Mar 2015, 4:59 pm   #134
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Default Re: Marconiphone VRC74DA

Hi John,
Quote:
The 3/16 has a 13v heater but early version may have been 8.5v so you have to take care here. A few were 10.5v to add even more confusion. They tend to go O/C without warning and are very bright when operating at the correct voltage.
Mine seems to be a 10.5v version, although there is no indication on the label!
Running from a 12v transformer did prove that the tube is tired.
It will be interesting to see if the 3/18 in the HMV produces a better picture when hooked up.

Mark
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Old 2nd Mar 2015, 6:51 pm   #135
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Default Re: Marconiphone VRC74DA

The best way to check which voltage heater your tube is fitted with is to measure the AC voltage when in the heater chain. My 'Bertha' made a decent job of reactivating the one in the VC53DA and that was was a lot dimmer than yours. It's a real gamble as you can finish these EMI tubes off so easily.All EMI tubes post TA10/15 were short lived. The 14" and 17" rectangular ones in the later models were even worse, 3 years if you were lucky! [4/15 SE17-70 etc] J.
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Old 2nd Mar 2015, 7:01 pm   #136
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Default Re: Marconiphone VRC74DA

I have lashed up the Marconi to the HMV's CRT and scan coils, the results are interesting.
The CRT is definitely better, more definition and brighter. But still behaves the same when either brightness or contrast are increased.
I think it is time to fire up the tube tester to compare emission.

Mark
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Old 2nd Mar 2015, 9:15 pm   #137
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Default Re: Marconiphone VRC74DA

The local oscillator looks slightly off tune. It is L5 [operation 7] on the Newnes circuit but better left alone if you don't feel confident about adjusting it. It's normally adjusted manually for maximum sound with freedom from sound on vision. If you are ok with this have a go, it will only require a tweak but take care so as not to damage the coil. Under no circumstances adjust any other tuning coil! John.
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Old 2nd Mar 2015, 11:44 pm   #138
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Default Re: Marconiphone VRC74DA

Hi John, I will give it a tweak I received 5 volumes of the Newnes red books today, I have been reading through the 1870A info.
It also has the service info for the dreaded Murphy V150 that still lurks in the roundtuit pile

It is a real shame the CRT is low, I have had it running for 4 hours this evening.
It is as solid as a rock, no adjustments necessary!

Mark
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Old 3rd Mar 2015, 12:50 am   #139
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Default Re: Marconiphone VRC74DA

Hi Mark,

Well I know it's a bit low on emission but it still looks quite good to me.
At least it's a bit of inspiration to have a crack at my set.

Cheers,
Marc.
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Old 3rd Mar 2015, 10:06 am   #140
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Default Re: Marconiphone VRC74DA

I would give it a shot Marc, the LOPT and dropper are very good quality items that never seem to fail. Most of the problems were with the RF deck and the horrible slider presets, and of course the Emiscope CRT's going flat.
The Z77 valves were also notorious for shorts, but can be replaced with EF91's

It would be worth checking out the following items that were faulty on my set.
The frame output tx had an open primary.
Chokes (L17, L18) were also open, as was the 2.2 meg resistor on the RF deck.
All the presets apart from the width pot were open.

I would definitely recommend removing the RF deck, it is so much easier to work on with it out of the main chassis.

Having proved that the valves are ok and CRT is reasonable in my HMV set, I will have a go at getting it working.

You may be lucky with yours, not all the CRT's will be as flat as mine!
The biggest headache on mine was finding the fault on the RF deck.

I will hopefully finish of the remaining jobs on the set today, fit the back and call it done. I will keep a look out for a basket case set to harvest the CRT and chassis. this set really does deserve a better tube.

Mark
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