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Old 7th Mar 2021, 4:13 pm   #21
EF80TVVALVE
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Default Re: Marconi VC76DA

The TV22 I am also repairing proved useful to lend its PL38 to the Marconiphone for a while. Powering up I still have the same results as before with no improvements.
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Old 7th Mar 2021, 7:22 pm   #22
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Default Re: Marconi VC76DA

I have the Marconi version of this with the original Emiscope tube which is actually very good- presumably because the set did not run long enough to wear the tube out! Having replaced all the de-couplers, the most troublesome thing I found was the frame stage. I think it is a very marginal design and combinations of tolerance spread in different components (including the o/p valve) can give very strange results.
I have just finished restoring this set's successor an HMV 1816- an altogether better set but with the same nasty Hunts de-coupler issues. I'm just going to post something on the 'Tv in the Home thread'.
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Old 7th Mar 2021, 8:11 pm   #23
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Default Re: Marconi VC76DA

That sounds like a lucky find, maybe it went wrong all too often and the owners just gave up and bought another set I'm sure this one will be fine with the Mullard CRT, I just need to persevere and get the line stages working. I've just about checked every component and the circuit against the schematic and all seems as it should be, some resistors have drifted a bit but I've changed any that were badly out of spec. I also changed the valve socket for the U35 as it had signs of arcing to one of the mounting screws in the past, really just making sure that there is nothing left to chance.

I'm beginning to conclude that it's probably valve related so I'm going to order a 12SN7 to replace the B36 and a very good KT36 and see how it goes from there.
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Old 8th Mar 2021, 8:49 am   #24
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Default Re: Marconi VC76DA

I guess you could be unlucky and have the one LOPT in a 1000 that has failed?
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Old 8th Mar 2021, 11:13 am   #25
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Default Re: Marconi VC76DA

The lopt seems to be okay judging from the resistance of the windings, nothing is O/C and the resistances are close to those stated on the schematic. I think the bulk of the problem lies with the KT36, particualrly what is causing the grid to be around 14v positive. As John (HKS) said in an earlier post it should be around 20v negative. I have tried another KT36 that is really almost dead but that also gave the same results. A PL38 also did nothing but I didn't check the voltage on the grid whilst this was fitted so it may be worth me doing that tonight.

Looking at the schematic the first grid and cathode are linked (not directly) via a 470k resistor which I'm sure under fault conditions would push up the grid voltage. I wonder if any of the KT36's have heater-cathode shorts, and if this could be the cause of the problematic grid voltage. Looking at my earlier voltage readings the cathode voltage was 13v higher than the data suggests.
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Old 8th Mar 2021, 12:05 pm   #26
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Default Re: Marconi VC76DA

The cathode voltage given in the EMI manual for the line output valve is 14V. Adjustment of the focus control will vary the bias and the EHT from 2kv to 9kv. They do not give a voltage for the control grid but this will show a slightly positive potential due to the path of R50, the focus pot VR9 and R51 470k. This maybe a red herring and can be ignored for the present. The important bit is the voltage on the cathode controlled via the width and focus controls. It's a nasty arrangement.

I can only suggest you carefully check the focus and width preset controls. How does the cathode potential vary when you adjust the width and focus controls? I feel it is in this area that your problem exists. John
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Old 8th Mar 2021, 12:33 pm   #27
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Default Re: Marconi VC76DA

Thanks John, I'll have a look this evening and check over the controls. The focus control seems to be okay, resistance wise it checks out good and doesn't show any problems. The width control could be suspect, although I checked and it measured 250r it could be intermittent and be otherwise faulty. I'll see how both vary the cathode voltage.
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Old 8th Mar 2021, 1:45 pm   #28
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Default Re: Marconi VC76DA

There is a remote possibility that the U35 EHT rectifier heater has fractured and shorted to the anode. Try disconnecting the top cap anode and run the chassis and see if you now have a spark. Just a thought but the 'groaning' you describe is typical symptom of this fault. The U35 is a very reliable rectifier but you never know! Another possibility is the EHT smoothing cap, the EMI one with rubber ends. Again this is a reliable component but if shorted will completely damp the line timebase especially as the U35 has a directly heated filament and will produce EHT almost immediately the timebase starts. J.
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Old 8th Mar 2021, 2:28 pm   #29
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Default Re: Marconi VC76DA

I'll add it to the list, if I remember correctly I also tested these and they were fine, I have another in my spares that I also tested but I will double check just in case. I did consider the EHT smoothing cap and although it measured okay on a multimeter I've seen before similar HV caps short under working conditions. I replaced it yesterday as I have some new that I use in place of visconols if they're needed.
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Old 8th Mar 2021, 10:13 pm   #30
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Default Re: Marconi VC76DA

I've spent some more time one the set this evening, being as I have a spare 250r slider control in good condition I decided to remove the width slider from the set and just swap it over, upon removal it showed varying resistances when measuring across the resistor. The one now in the set is a good one and even feels much better when sliding.

I've checked the voltages and the width control varies the cathode voltage from 11.2 to 30v but the focus control has virtually no effect at all on the cathode voltage at all.

I disconnected and can draw a small spark from the anode/top cap lead for the U35, nothing substantial just a small blue spot when rubbing against a screwdriver.

Cheers
Bren
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Old 8th Mar 2021, 11:31 pm   #31
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Default Re: Marconi VC76DA

Further investigations reveal the Height control is also faulty so this will have to be replaced, unfortunately I've now ran out of spare sliders so I'll have to hope that either someone has a spare or try to repair the one I have with a wire wound as I seen on this forum some years ago.
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Old 9th Mar 2021, 4:57 pm   #32
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Default Re: Marconi VC76DA

This line output stage is nasty! All this trouble just to save a fully adjustable focus magnet.

Out of curiosity I pulled my Marconi version off the shelf and took a few voltage readings. The old metal rectifier has been replaced with a 25ohm surge limiter and a BY127 silicon diode. This gives an H.T. rail of 260V

The focus control gives a strange action. Fully anti clockwise gives a reading on the control grid [DVM]of 1.6v+ slowly turning the control clockwise increases the voltage to 3.5v+ until you reach the two thirds point when further advanced the voltage reduced to zero then turns negative to .66v- for the remainder of the track. The picture focusses at 1.15v+.

The grid pin is incredibly sensitive virtually collapsing the line output stage when measured with a DVM or the 10-1 scope probe connected to pin 5. The cathode voltage on pin 8 is 11v+. The screen grid voltage to pin 4 is 150V.

The valve holder is of good quality but I would imagine the slightest leak from pin 5 to any other point including chassis ground would have a considerable effect on the operation of the line output stage.

Remember when checking the voltages that my H.T. line is 260V and incorrect readings to some extent may not indicate a fault. It is considerably sensitive and no wonder these were chucked out early in their life especially with those impossible to adjust presets and the unstable mains voltage at that time. I can't think what else to check. As mentioned the mains dropper and line output transformers really are bullet proof.
It's a shame really, just a couple of pounds extra spent on the design would have produced a better receiver. I think it would have sold at a slightly increased price considering it had the HMV pedigree. Hope you make some sense of it. Regards, John.
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Old 9th Mar 2021, 5:19 pm   #33
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Default Re: Marconi VC76DA

Thanks John, I really appreciate your help with it! It's a shame it's being so reluctant to work, maybe it's just been enjoying its retirement a little too much. A further thanks for checking your own and giving me some figures, I'll compare this with mine and keep head scratching and see what I can work out.

I think I will change all of the sliders for potentiometers as they are actually in a poor state after close inspection, maybe this will give some improvement but I'm getting used to the idea that it's not going to be coaxed into life easily. Still I'll keep pursuing and hopefully get some life from it soon.
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Old 10th Mar 2021, 6:48 pm   #34
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Default Re: Marconi VC76DA

Quote:
I think I will change all of the sliders for potentiometers as they are actually in a poor state after close inspection
A very wise decision, I replaced the sliders in my set & it certainly helped with the lack of EHT I was experiencing.

Good luck with the troubleshooting, I am sure you will get there in the end!


Mark
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Old 10th Mar 2021, 8:29 pm   #35
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Default Re: Marconi VC76DA

Thanks Mark! Ed Dinning has been so kind as to sort me a complete set of potentiometers for it so I'm having a few days off until they arrive, then I'll get them all replaced. Hopefully that changes things a bit and I start to see life from the damned thing. It can't be too far off now and I'm sure once I get the line stages working then it'll only be a few bits here and there to get it spot on.
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Old 10th Mar 2021, 8:44 pm   #36
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Default Re: Marconi VC76DA

Hello,
I have just finished an HMV1807. Spurred on by Matt's BVWS article and knowing advice from the legends that populate this Forum would be available to me.
I shamelessly copied all young Matt's mods. (Better EHT decent focus magnet etc). I had the same EHT problems. I changed all the sliders for rotary Pots (including the width control) and although every switch on has me fearing an O/C tube heater it now gives a cracking picture.
I have a scrap very very rough 1807 chassis with a LOPT should you need it.
Regards Steve.
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Old 12th Mar 2021, 12:34 pm   #37
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Default Re: Marconi VC76DA

Thanks Steve, I'll keep you in mind should there be a problem with the lopt. I'm really hoping that replacing these sliders will give it some life. I've also bought a NOS KT36 as both of mine are quite low on emission as well as an RCA 12SN7 to replace the troublesome B36 so fingers crossed this will help.
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Old 13th Mar 2021, 8:54 pm   #38
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Default Re: Marconi VC76DA

Hi all,

So today the potentiometers were delivered (thanks Ed!) so I've set about fitting them. After a couple of hours lining up, cutting, removing the old sliders and fitting the replacements I now have a good full set of controls. Unfortunately this has made zero difference to the set and it still refuses to give me any life

Upon powering up the 'groaning' low freq noise is much louder now, probably to do with the replacement pots, it changes in frequency more easily with adjustments to the height and vertical hold controls. I also replaced the B36 with a 12SN7 and bought a good NOS KT36, both have made no difference.

The voltages on the KT36 are still similar as before, adjusting the width I get between 10-34v on the cathode and 0.2-23v on the grid but the focus control still has very little to no effect at all on the grid voltage.

I've probably looked at it enough for today but thought I would update the thread to where I am now. If anyone has any ideas on what to keep checking then let me know, otherwise I'll crack on tomorrow and see if I can work out what's going on.

Just out of interest, my set has two westector type diodes, a small one that is usually in all of these types of chassis but the other is a larger type used in the sets with mullard screens, on my data it is MR4 as photographed. If any of these are faulty am I likely to get similar problems to what I'm facing now? Please excuse all the scribbles on the schematic!
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Old 13th Mar 2021, 10:15 pm   #39
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Default Re: Marconi VC76DA

On your scope traces, what is the frequency and amplitude on the grid? (although based on John's comments, the probe capacitance may be messing up the grid signal)
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Old 13th Mar 2021, 11:05 pm   #40
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Default Re: Marconi VC76DA

Hi Bren,
The purpose of MR4 and C60 is to supply the first anode voltage for the Mullard tetrode tubes. They do this by rectifying the waveform at the line oscillator's anode (pin 5 of the B36) and adding it to the HT voltage, producing a voltage of 300-350v for said first anode.
If MR4 or C60 were to be leaky or short, it would probably dampen the oscillator resulting in low line drive. May be worth disconnecting MR4 at the pin 5 end to see if this results in increased KT36 activity.
MR4 and C60 aren't present in sets fitted with Emiscope tubes, as these are simpler triode types.
As always best of luck and keep at it, you will get there!
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