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Old 26th Feb 2015, 9:46 am   #81
Heatercathodeshort
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Default Re: Marconiphone VRC74DA

There must be an easy explanation of this fault. I would not suggest you swap the RF units as this will probably introduce additional faults. Better to fault find the solvable problem [note I do not use the trendy word 'issue' that need only be talked about..] on the original unit. The superhet IF unit reflects standard radio procedures. The video output stage has very few components. I too am puzzled at the lack of response of feeding a high output audio signal to the control grid of the video amplifier valve. It has to work if the anode and screen voltages are anything near the ones quoted in the service data. Check the Z77 valve as these tend to run hot in the 1807 and literally die. I had three useless ones in one receiver. EF91's make a more reliable substitute. Hope you sort it.
[If not, uncle John has a Marconi VC53DA that badly needs that cracking tube. ] Regards, John.

PS Just a thought Mark. Is the internal audio oscillator in your E2 actually operating? I had an O/C modulation transformer in mine that created some headscratching.

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Old 26th Feb 2015, 12:50 pm   #82
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Default Re: Marconiphone VRC74DA

Hi John, I am hoping to find that it is something obvious O/C or shorted somewhere. There is no problem with the E2, it is working fine.
I will retest the Z77's, I did replace three that read low on the Taylor with EF91's. But a retest will confirm they are all ok.
I will go through all the resistors and replace any that are more that 20% high.

I only suggested trying the HMV RF deck as an absolute last resort!
This set has turned up more faulty components than any other set I have restored so far, but I am determined to get it working as well as I can.
If for no other reason than to demonstrate how bad it is compared with my Bush and Pye sets from the same era

Quote:
[If not, uncle John has a Marconi VC53DA that badly needs that cracking tube. ]
I doubt if it is as good as it looks in the photo's, but at least it does have some emission left!

Mark
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Old 26th Feb 2015, 4:38 pm   #83
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Default Re: Marconiphone VRC74DA

I have spent a couple of hours going through the chassis checking resistors, I have replaced four that were high.
I now have possibly found the problem, there are two black wirewound resistors that check open, but cannot identify them from the circuit diagram.
I am sure these had continuity when I checked through the resistors during recapping the chassis, both look physically fine with no signs of overheating.

One is connected from pin 5 on V5 (D77) to pin 1 on V6 (Z77). This is not on either of the circuits I have.
The other is from pin 5 on V6 to C29. I am using the circuit in post #59.

Mark
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Old 26th Feb 2015, 4:44 pm   #84
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Default Re: Marconiphone VRC74DA

They don't look like they have carried excessive current, so if thy are open the it must be corrosion.
I would lift one end to make doubly sure.
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Old 26th Feb 2015, 5:21 pm   #85
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Default Re: Marconiphone VRC74DA

I have had a look at my 1807 service info, and it appears they are inductors (L17 & L19).
They should read 300 ohms.
Any ideas what to replace them with?

Mark
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Old 26th Feb 2015, 7:30 pm   #86
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Default Re: Marconiphone VRC74DA

Mark, I have the full service data for this series. I will check them out but it's been raining all day and I don't fancy the trip to the summer house..Ha! Ha! John.
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Old 26th Feb 2015, 7:47 pm   #87
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Default Re: Marconiphone VRC74DA

Hi John, I know just what you mean! My temporary workshop is in the kitchen, as it is warm and dry and full of part dismantled telly

I hope I can replace these inductors and get the set working soon, as it has been in here for nearly two weeks now!

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Old 26th Feb 2015, 7:50 pm   #88
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Default Re: Marconiphone VRC74DA

It would certainly explain the lack of a picture if those are the correct references.
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Old 26th Feb 2015, 8:25 pm   #89
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Default Re: Marconiphone VRC74DA

Hello again Mark,
My therapist is paying an emergency visit tomorrow morning..
OK I have posted the vision IF chain and video amplifier minus the sound channel. The two resistors you mention are in fact inductors, L17 L19. Also check L18 on the top tag panel all in the video amplifier and should read S/C. I bet one or both of them is O/C. I had this recently with the earlier 1805.
An audio signal fed to pin 1 of V6 should produce black 'disturbance' bars on the screen that can be adjusted with the brilliance control. Start here and get this right first. The chokes can be shorted as a test. John
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Old 26th Feb 2015, 8:48 pm   #90
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Default Re: Marconiphone VRC74DA

Hi John, I have checked L18 and it is fine, will bridge L17 & 19 and fire up the E2
and see if this works.

Mark
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Old 26th Feb 2015, 9:03 pm   #91
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Default Re: Marconiphone VRC74DA

Hi John, Yes I now have the picture disturbance you describe, looks like we are getting somewhere!

Mark
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Old 27th Feb 2015, 8:14 am   #92
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Default Re: Marconiphone VRC74DA

Hang out the bunting! [Great!]
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Old 27th Feb 2015, 9:32 am   #93
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Default Re: Marconiphone VRC74DA

Still nothing from the Aurora though!
I did briefly try rocking the signal genny around the 34-37.5Mhz area, with the following results: V4 pin 1 gave patterning on screen. V3 pin 1 gave plenty of sound, but not much on screen. Pin 5 weaker sound, but some noise on screen.

I will get back to the set later today, I think a good start would be to list the voltages on the valves.

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Old 28th Feb 2015, 1:32 pm   #94
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Default Re: Marconiphone VRC74DA

The small chokes in the video amplifier are correction chokes to peak the high frequency capabilities of the stage. The effect of shorting them out will probably result in about .5mc/s lack of definition.
They can be rewound by hand using very thin 47swg wire in a single layer. Don't worry about the exact number of turns, just fill the complete length of the ceramic/glass former.
With the video amplifier now active you can check back the IF stages. With full RF output on generator at vision IF frequency connected to pin 1 of V6 [VA] you should see well modulated bars on the screen. IFT 6 is peaked at 36.5mc/s so rock your E2 a little either side of this with the modulation on. If not, check voltage on pins 5 and 7 of Z77. A much weaker signal should be observed when generator is connected to pin 5 anode. If zero, check D77 vision detector and make absolutely sure L17 is continuous. We need go no further back at this point. V4 V5 and V6 stages can then be cleared of suspicion.. [My therapist informs me I am a hopeless case but don't blame yourself.] Regards, John.
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Old 28th Feb 2015, 2:19 pm   #95
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Default Re: Marconiphone VRC74DA

IT LIVES
After methodically checking through the IF and checking voltages, I got 229v on the anode of V4, but the voltage on the anode of V3 was all over the place?
Moving a couple of components to get a closer look, revealed that the fine wire from IFT3 was corroded just up from where it was soldered to the valve holder.

A gentle touch and fell away!
I carefully cleaned the delicate wire and tinned it, I then soldered a single strand of wire to the valve holder and made a neat hook joint and soldered the two together.

On powering up with the Aurora attached, I was greeted with a picture at last!
It is somewhat disappointing, although it locks well both vertically and horizontally. The linearity is well out, but that will be partly due to the replacement frame output TX. It only gives a picture at one end of the contrast control. Backing it off whites out the picture and increases the sound.
Focus is horrible, but that is a standard feature of this set

At least I now have something to work with, and am over the moon to have finally found that elusive fault!

Mark
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Old 28th Feb 2015, 4:17 pm   #96
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Default Re: Marconiphone VRC74DA

I have just played the National Anthem on a 78 RPM record given to me by the manager of the Regal Wimbledon when it closed years ago..

Make sure the interfearance limiter is 'off '. Replace waxies in frame output stage with particular reference to C43 .1uf C41 .02uf C44 .0082 [weird value, use nearest] C45 .015uf C46 .1uf and finally the cathode bias capacitor, C56 160uf. It may be O/C but I suspect it's the miss matched transformer. Try reducing the value of the bias resistor to 100ohms [R25] then experiment with various sound output/frame output transformers till you get something acceptable. It should be 100% linear.
Stick to Plessey chassis and TV22's Mark. They can be repaired in a blacked out room...Great news! John.
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Old 28th Feb 2015, 4:39 pm   #97
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Default Re: Marconiphone VRC74DA

Brilliant Mark !
At last it's all coming together.

Looks like I'm in for lots of fun with my Marconi doesn't it

Marc.
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Old 28th Feb 2015, 4:56 pm   #98
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Default Re: Marconiphone VRC74DA

I took these pictures a few moments ago from the screen of my Marconi VC53DA. Same chassis with the same performance.
The full picture was taken in bright workshop lights and this is about as good as they get. My chassis does not have an interlace filter as can be seen from the close up. Even with a brand new B36 the timebases take a few minutes to stabilize but remain OK for an evenings viewing. Focus is 'fair' at mid way setting. This is operating at 240V input. A reduction of 10% makes the receiver unusable and that is with a good H.T. rectifier. EMI at it's very worst. Compare this with the Ferguson 941T and Bush TV12, both produced at the same period. John.
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Old 28th Feb 2015, 6:48 pm   #99
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Default Re: Marconiphone VRC74DA

Hi John, I don't think this set will give a picture anywhere near as good as your set, the picture is certainly nowhere as bright.
I only have around 5kV of EHT.

One thing I have noticed is the profound effect of fitting the valve screening cans on the RF chassis, some were missing on this set, but I borrowed some from my HMV.

Quote:
Replace waxies in frame output stage with particular reference to C43 .1uf C41 .02uf C44 .0082 [weird value, use nearest] C45 .015uf C46 .1uf and finally the cathode bias capacitor, C56 160uf. It may be O/C but I suspect it's the miss matched transformer.
All caps have been changed, bar the main electrolytic, the 2 1uf electrolytics and the 50uf on top of the chassis.
C44 on my set is a 6200pF, I have not replaced this yet.

I will try reducing the value of the bias resistor to 100 ohms, I need to replace the linearity slider with a rotary pot, as this is intermittent.

Actually, both H & V holds are stable.
I will carry on with this set after tea, and see if I can get some further improvements.

Quote:
Looks like I'm in for lots of fun with my Marconi doesn't it
You sure are Marc, this set has certainly thrown up a lot of problems
But I never thought it would be easy!
I have an HMV 2811 (same chassis, bigger tube) that I tried to restore last year, I am thinking of getting it back on the bench for another bash at it.
I must be mad, but it makes sense to tackle it after this one.

I will then get back to restoring a more worthy set


The photos below show the recapped chassis so far.

Mark
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Old 28th Feb 2015, 7:38 pm   #100
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Default Re: Marconiphone VRC74DA

The screening of the valves in these models is very important. Clean the locking bits of the can and base and make sure all cans are fitted as instability will result if there is any doubt in this direction. It is even more important with the London TRF models.
The EHT on mine is the same 5kv for focus so you may find it is reasonable. It is also necessary to fit replacement decoupling capacitors in exactly the same mechanical position if instability is still troublesome. My EMI notes state that the Paxolin valveholders gave poor contact in some cases. What did they get right? John
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