Marconi VC76DA

[EF80TVVALVE]

Hi both

Thanks for your suggestions. PJL, I will have to dig the scope back out tomorrow and recheck this. The chassis is currently back in the set so I don't have too much access but I can get to the pins of the focus control easily. I connected my multimeter set to Hz and it seems to be measuring 27kHz before the 1k to the grid of the KT36. This may not be correct as the meter was somewhat struggling to give me a firm reading until it decided upon this value.

Halfabee, I've done as you said and temporarily snipped the MR4 diode but no difference can be noted running the set, I'll wait until tomorrow when I have the chassis back out and see what difference it makes voltage wise etc and see if there is any differences.

[EF80TVVALVE]

Today's work has involved trying to scope up and measure the frequencies on the grid of the KT36 which has proved to be quite difficult, I think PJL was correct that the capacitance of the scope probes is causing problems and my scope doesn't like to trigger very well to that waveform either

After playing around a little more I managed to get a better reading with my multimeter on the Hz setting which was varying around the 490Hz region, to me that sounds about right from what I can hear coming from the line stages. Circuit wise, around the B36 all seems to be okay but I do still have the old EMI mica caps in, none measure short and I normally find micas to be okay - but are EMI specials as bad as the rest of their caps?

[Halfabee]

Hi Bren,
Sorry to say that all EMI capacitors are to be viewed with suspicion.
If the line frequency is indeed around the 500Hz region then this is way out, it should be approx 10KHz!
In the line oscillator, the critical component is C33 (0.001uf). Replace this with a good quality mica component, double check R40(330k), R37(47K), R25(160 ohms) and of course the dreaded slider VR5. EMI loved potential dividers which could follow convoluted and confusing paths.
A possible course of action would be to isolate the line output stage, then concentrate on getting the oscillator frequency correct before proceeding. This could be achieved by disconnecting C41 and R53 the KT36’s screen grid resistor (important to avoid excessive valve current in the absence of line drive).
The best way to measure the oscillator frequency is with your oscilloscope, when running at the correct frequency the period of the waveform should be approx 10ms.
Good luck.

[EF80TVVALVE]

Well would you believe it! I was suspecting the 0.001uF capacitor as I've checked the resistors in that part of the circuit and was changing it as you replied! I found one in my spares and fitted it and it's shot into life, lovely healthy line whistle is heard now but still nothing on the screen as of yet. I'm going to get the aurora out now and do some fiddling around, hopefully I can get something on the screen this evening and update the thread.

[Heatercathodeshort]

Do you now have a spark at the anode of the U35? It is very difficult to see the filament of the U35. You will probably have to turn off all the room lights. It's similar to a 2v battery valve when energised. If you have EHT, check the voltage on pin 10 A1. If you have a few hundred volts present the ion trap may have been moved from it's correct position. J.

[Halfabee]

First light soon!

[EF80TVVALVE]

Yes John I have quite a good spark now, the set was dismantled before it was put in the loft last time around as it was out on my workbench then packed away for the last couple of years so some adjustments and setting up will be needed.

I now have this on the screen so far, the brightness has to be near full and contrast turned up to full to give this picture and the tube isn't very bright, I suspect it will come up with some use same as the pye LV20 that I repaired last year as that was awful for the first few hours. The voltage from MR4 to the crt seems to be around 260v DC which may be a little low, there is obviously still niggles and I'm sure more tweaking with the RF deck and probably some better valves too will have it going better but overall I'm very pleased to get this far and now have something on the screen. I just can't believe how much I overlooked that cap as it looked to be a fairly decent ceramic type.

[Halfabee]

Doubles all round!

[EF80TVVALVE]

Another photo after some fiddling, EHT is probably a little low at the moment or still a soft CRT, I need the focus control turned full and the focus magnet pushed right up the neck to get any decent adjustments but I'm going to run it for a bit and see how the CRT improves before setting everything up properly, otherwise it'll all need doing over.

[mark pirate]

Great to see you finally have a picture on it
I am sure the CRT will wake up with a few hours of use, you are lucky to have a Mullard tube rather than the Emiscope fitted to most of these sets.

There is always a compromise between focus & EHT, the amount of interaction between the presets is a standard feature of these sets!

I cannot believe the only thing securing the scan coils is a rubber band, in my set I used card shims to hold the scan coils in position.


Mark

[EF80TVVALVE]

Thanks Mark, there is still work to do to get it performing better but now I have the line stages working it's a big leap in the repairs. Strangely this set has been bodged on the RF deck, the picture displayed is actually on channel 3 although it's a midland set. If anyone recalls the first thread I started a few years back I noted how it had a Cyldon tuner fitted but only the B1 aerial socket was exposed, so there was no way of connecting a BIII aerial.

I wonder if this set moved away from the Midlands and someone attempted to get it working on channel 3 before giving up and fitting the Cyldon tuner. This probably also explains the poor contrast along with the sound and vision interference. On channel 4 all I get is a blank raster with some sound bars appearing. I'll have to retune the RF deck but this may be tricky without an RF sig gen.

[EF80TVVALVE]

This evening I've attempted to tune the set back into channel 4. I have good sound on channel 4 with very little vision interference but I can't seem to get any vision at all and only sound modulating the raster, almost like I can't tune it up to the right frequency for the vision. I presume there may be some awol capacitors again in the tuning circuit that aren't too great and have caused the tuning to drift further out that the trimmers can adjust to.

[mark pirate]

It does appear that it may have been got at in the past to make it work on channel 3.
I would check the RF deck against the circuit diagram to see what has been removed/added.

Two of my sets are actually marked Midland, god knows how they ended up down here on the south coast?


Mark

[EF80TVVALVE]

I think mine has travelled around a bit too, I bought it from a local second hand shop but obviously the addition of a tuner and the attempts to retune the set show that it must have travelled elsewhere before returning to the Midlands. Retuning to Channel 3 is obviously not something that can be done well on this RF chassis hence why they gave up and fitted the tuner.

I'll inspect the RF chassis and see if anything has been changed, I only changed what was necessary on this, from memory just the heater caps so it should be largely untouched.

[mark pirate]

All the wax caps & nasty little brown Hunts will need replacing before any realignment is attempted, I could not get any signal through my sets until all these were changed, also the Z77 valves will need testing/replacing.

I removed the RF chassis for recapping, it is simple to get out, only half a dozen wires to desolder, it makes it much easier to work on.

It would be a good idea to disconnect the tuner, you can always reinstate it at a later date once the set is working properly.


Mark

[EF80TVVALVE]

I had a look under the chassis last night, I have changed all fo the small brown hunts and wax caps. Nothing was altered circuit wise so I set about aligning it as per the instructions in the service manual. Its almost there, I have a picture and sound on channel 4 now but I still need to finish it off, it must have been getting on for midnight before I switched off.

I do need to check the RF deck as it seems as though there isn't much gain/output of the video signals. The contrast is quite weak and the control has to be all of the way clockwise, if it's backed off then the screen very quickly goes blank. It's probably due to a poor Z77 or resistor I suspect but I'm yet to investigate.

[mark pirate]

It's probably due to a poor Z77 or resistor I suspect but I'm yet to investigate.

I had to replace several Z77's on all three of my sets, not only were most low emission, but some had internal shorts.

EF91, 6AM6, 8D3 & CV138 are all more reliable replacements for the awful Z77.
Glad to hear you are making good progress, these sets can be a real challenge!


Mark

[Heatercathodeshort]

Yes I agree! The Z77 is not the best of the 'EF91' series. I serviced a couple of 1805 series for the Museum and discovered three with zero emission. They gave confusing symptoms.
No sound or vision then no sound all due to completely duff Z77s.

To be fair they were all around 70 years of age but you never have this problem with the Brimar or Mullard versions. I think they also suffered control grid to screen grid shorts burning out feed resistors. They always appear to have very untidy internals. John.

[EF80TVVALVE]

Yes, I have quite a few Z77 and tested all I have including all from this set when I begun the repairs. I did make up a good set, 'good' being no shorts but emission varied, some were very good and some were quite poor.

I've now found the box of tele valves I keep and there are a good amount of mullard EF91's in there so I'll go about changing the ones in the set that were poor emission and see how that improves things. I still need to measure the EHT and see how that is, I suspect it is still a little low but I may need to run the set more to liven the tube up too.

I don't want to finalise the alignment until I'm confident that everything else is working properly but I've got to a good stage to get everything else working before doing the last of the tuning up.

[EF80TVVALVE]

Spent some more time this evening on the set, I've managed to find some better valves that have improved the performance of the RF stage a bit. I'm still struggling to get the alignment sorted and it's proving difficult to get good vision and sound together. I'm thinking of just purchasing a signal generator suitable for alignment but I wondered if anyone can suggest which type would be best for the set? Having never done RF allignment on a TV before I'm unsure as the information attached states to feed modulated and unmodulated signals. Would a normal RF sig gen be okay or will I need one specifically for TV use? If someone could recommend one then that would be great, although I doubt it would see much use after this!