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Old 5th Jan 2021, 9:57 am   #21
mark pirate
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Default Re: My newly found Marconi 707

Quote:
My tube appears to have odd rings around the face of the tube.
This is probably ion burn, I have seen the 5" model working, I reckon two thirds of the tube face had ion burn, but still displayed a watchable image, if a bit dull.

I think you have been very fortunate with this set, it does look remarkably untouched.
Hopefully the TV section has had little use, as there was only a couple of hours a day of broadcasting, and of course the transmitter was shut down at the outbreak of war.

I suspect the radio has seen the most use, it would be very interesting to test the valves, this would give a rough idea of how much use the set has seen in TV mode.


Mark
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Old 5th Jan 2021, 10:04 am   #22
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Default Re: My newly found Marconi 707

Hi Ed,
Well I'm currently restoring a prewar set for someone else and I have been using the process of cutting open aluminium electrolytics using a pipe cutter. For the smaller caps that have a thread at the bottom end and a connection at the top end, I used a pipe cutter to cut through the upper end, where the case pinches in below the top insulator.
For the larger electrolytics which have the connection within the mounting bolt, I used a starrett hole saw (with the centre drill removed) after first fitting a suitable sleave over the screw thread. The smaller caps were glued back together with epoxy resin. The larger caps were stuck back together with hot melt. The hardest part was digging out the old insides from the larger caps, each one took hours. I can post pictures if you like. The main smoothers were missing in my HMV905 (it had those grey sleeved RS axial caps in it) , so I used the ones from the BVWS spares debt, they are the right diameter, but much shorter than the originals.

I used the following tip for the toggle switches. I connected a 1A current regulated dc bench supply across the switch contacts, which were in the 'On' position. I wound up the voltage (mine goes up to 30V) and noticed when the voltage collapsed, which is when the tarnish on the contacts had broken down. I then operated the switch a few times with the psu still connected. This would be a bit too extreme for say a wader switch, but on these bulgin toggle switches it works a treat.


Cheers
Andy
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Old 5th Jan 2021, 11:21 am   #23
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Default Re: My newly found Marconi 707

Well done on finding your 707 Ed.

With any luck you won't have severe ion burn but here's a good example.

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Old 5th Jan 2021, 5:59 pm   #24
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Default Re: My newly found Marconi 707

Here I am currently testing the EHT and trying to see if I can get a spot to appear on the tube. All bleeding resistors apart from one read open, surprisingly the rest in that chain are just under 1meg, so on the money!

Iíve also checked all my AC Volts apart from EHT, and seems fine.

Thankfully managed to repair the switch last night but called in to bed once I had realised I put it on backwards and was wondering why it still wasnít coming on!

Iím having a little bit of trouble getting the right amount of EHT though. No where near 2KV but about 1.5KV at 230 and not able to get much of an indication on the Emiscope tube face either.

R86 which feeds the Anode of the CRT states a value of .15m, and R59 feeds C76 from C77. What does M mean, meg? Not quite familiar with reading values written out like this, or confident with reading such old resistors!

R86 reads over 2megs, which doesnít appear right to me.

As Iím writing this, I decided to ramp it up on the veriac again, a lot less voltage this time... I think my EHT rect might have failed. More to follow soon.
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Old 5th Jan 2021, 6:27 pm   #25
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Default Re: My newly found Marconi 707

Hi Ed,
R86 is 150K, 0.15M is just another way of writing it. It is a bit like the way that a 100nF decoupling capacitor is often labled 0.1uF.
The Low EHT is probably failing EHT caps, you might want to try disconnecting them to give the poor old transformer a chance. A gassey rectifier can also be a problem, but it would glow purple though.
R59, 350K is another one to check.

Ultimatley I would reccomend that you replaced the beader chain with something more trustworthy. You will find that apart from the 1/4W and 1/2W values, EMI resistors are not very good.

For the EHT smothing caps I used a couple of RS no. 384-7795

Cheers
Andy
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Old 5th Jan 2021, 7:07 pm   #26
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Default Re: My newly found Marconi 707

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Originally Posted by beery View Post
Hi Ed,
R86 is 150K, 0.15M is just another way of writing it. It is a bit like the way that a 100nF decoupling capacitor is often labled 0.1uF.
The Low EHT is probably failing EHT caps, you might want to try disconnecting them to give the poor old transformer a chance. A gassey rectifier can also be a problem, but it would glow purple though.
R59, 350K is another one to check.

Ultimatley I would reccomend that you replaced the beader chain with something more trustworthy. You will find that apart from the 1/4W and 1/2W values, EMI resistors are not very good.

For the EHT smothing caps I used a couple of RS no. 384-7795

Cheers
Andy
Thanks Andy for letting me know and your suggestions. Yes I fear you're right about the caps, but Iím suspecting a tired EHT valve first. Made an error with the earth connection for my probe, but still the highest I seem to get is 1.7KV.

Disconnecting the top cap, lots of AC is there!

I donít know if I have a spare U17, Iíll have to check my pile. Itís a little too old for my work.

Is there any reproduction or old-looking resistors which I could purchase for the bleed chain?

I would like to put a 50ma fuse in line with the EHT, just in case something does short. Dreading this transformer going up in smoke.

I want to confirm the EHT and my CRT are good before I continue with the rest of it. If my tube is tired/bad, I might as well put it back together until I can find a suitable replacement.

Iíve disconnected the smoothing caps, not much of a difference really!

Before I declare the EHT caps bad, Iíll swap the rect valve with a known good one. I have a few replacement cans here which I can use to replace them with anyway if need be.
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Old 5th Jan 2021, 7:35 pm   #27
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Default Re: My newly found Marconi 707

We now have a Nice EHT!

The U17 wasnít conducting properly I guess. For the mo it has a HRV2 which Iíve fitted for now just for testing.

The spot is quite bright!

Now I know these parts are good, I will proceed with the rest of it.

I will also be changing the tired old smoothing caps. I donít want to risk any damage to anything!


Mark, Iím going to be testing valves in due course. Unfortunately I donít have my tester here with me, itís currently in London, so I wonít be able to get there due to the lockdown.
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Old 5th Jan 2021, 7:38 pm   #28
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Default Re: My newly found Marconi 707

Quote:
Originally Posted by beery View Post
Hi Ed,
Well I'm currently restoring a prewar set for someone else and I have been using the process of cutting open aluminium electrolytics using a pipe cutter. For the smaller caps that have a thread at the bottom end and a connection at the top end, I used a pipe cutter to cut through the upper end, where the case pinches in below the top insulator.
For the larger electrolytics which have the connection within the mounting bolt, I used a starrett hole saw (with the centre drill removed) after first fitting a suitable sleave over the screw thread. The smaller caps were glued back together with epoxy resin. The larger caps were stuck back together with hot melt. The hardest part was digging out the old insides from the larger caps, each one took hours. I can post pictures if you like. The main smoothers were missing in my HMV905 (it had those grey sleeved RS axial caps in it) , so I used the ones from the BVWS spares debt, they are the right diameter, but much shorter than the originals.

I used the following tip for the toggle switches. I connected a 1A current regulated dc bench supply across the switch contacts, which were in the 'On' position. I wound up the voltage (mine goes up to 30V) and noticed when the voltage collapsed, which is when the tarnish on the contacts had broken down. I then operated the switch a few times with the psu still connected. This would be a bit too extreme for say a wader switch, but on these bulgin toggle switches it works a treat.


Cheers
Andy
Would love to see some of your work Andy. Have you got a thread on here about it?

Thankfully I managed to get the switch working again!

Still unsure what the best approach for smoothing caps will be. My first task will be to undo all the added ones in the chassis, and remove them for closer inspection.
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Old 5th Jan 2021, 7:41 pm   #29
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Default Re: My newly found Marconi 707

Hi Ed,
Sometimes the transformer's EHT winding is intact, but it is the winding that supplies the rectifier's heater that breaks down. You can check this by disconnecting the heater connections and using a solid state diode instead. One from a microwave oven should work.

You can use a wire ended Mazda U25 at a push. You will had to add a resistor to one of the heater connections to drop 4v down to 2V.

Cheers
Andy
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Old 5th Jan 2021, 7:41 pm   #30
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Default Re: My newly found Marconi 707

I was relieved to hear your EHT came up. I had a similar problem with a mains EHT Murphy. It turned out that the EHT winding was O/C but it still managed to produce a significant voltage. I presume it was arcing across the break.
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Old 5th Jan 2021, 7:44 pm   #31
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Default Re: My newly found Marconi 707

PS You could also leave the original rec in place for show but fit a solid state diode. The advantage of this is you can get the eht off the heater winding, and so remove a major source of breakdown on these transformers.
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Old 5th Jan 2021, 8:06 pm   #32
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Default Re: My newly found Marconi 707

Quote:
Originally Posted by em536716 View Post
The spot is quite bright!
In a word, "Wow"! No sign yet of an ion burn, at least from the photo.

Before signing off the tube as definitely good, it might be worth checking for grid-cathode and heater-cathode leaks. I suggest using an Avo or similar on the high resistance range and test 'cold', with the tube unplugged from its base. The situation can unfortunately change when 'hot' but I would tend not to run this tube without proper operating voltages and scans because the (pristine?) phosphor could so easily be damaged. Just my opinion.

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Old 5th Jan 2021, 8:07 pm   #33
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Default Re: My newly found Marconi 707

Quote:
Originally Posted by beery View Post
Hi Ed,
Sometimes the transformer's EHT winding is intact, but it is the winding that supplies the rectifier's heater that breaks down. You can check this by disconnecting the heater connections and using a solid state diode instead. One from a microwave oven should work.

You can use a wire ended Mazda U25 at a push. You will had to add a resistor to one of the heater connections to drop 4v down to 2V.

Cheers
Andy
Thankfully I think I have some solid state diodes I could try once Iím ready to do a test run. The Valve base looks stuffed well in there. I should have a spare U25 as well. Scavenged quite a lot of spares over the years.
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Old 5th Jan 2021, 8:08 pm   #34
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Default Re: My newly found Marconi 707

Quote:
Originally Posted by Niechcial,Steve View Post
I was relieved to hear your EHT came up. I had a similar problem with a mains EHT Murphy. It turned out that the EHT winding was O/C but it still managed to produce a significant voltage. I presume it was arcing across the break.
Thanks for your advice. As Andy mentioned, I might end up trying this if the heater winding breaks down!
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Old 5th Jan 2021, 8:13 pm   #35
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Default Re: My newly found Marconi 707

Quote:
Originally Posted by Panrock View Post
Quote:
Originally Posted by em536716 View Post
The spot is quite bright!
In a word, "Wow"! No sign yet of an ion burn, at least from the photo.

Before signing off the tube as definitely good, it might be worth checking for grid-cathode and heater-cathode leaks. I suggest using an Avo or similar on the high resistance range and test 'cold', with the tube unplugged from its base. The situation can unfortunately change when 'hot' but I would tend not to run this tube without proper operating voltages and scans because the (pristine?) phosphor could so easily be damaged. Just my opinion.

Steve
I do have an old AVO somewhere. I was speaking to a fellow restorer who mentioned this to me.

Iím aware of the presentable damage to the phosphor, was just a quick test I wanted to carry out. Saw a lot of good discussion on testing, actually, I under powered the EHT with 1KV just in case I did manage to ruin the face of the tube. I was surprised to see it was that bright in all honesty! The tube will go in a box locked away until itís testing time. Last thing I want to do is get carried away with it!
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Old 5th Jan 2021, 8:18 pm   #36
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Default Re: My newly found Marconi 707

Later on, you might consider adding a switch-off burn protection circuit too. This can be done with just a few components and is described on Hugo Holden's 'worldphaco' site.

Steve
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Old 5th Jan 2021, 8:20 pm   #37
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Default Re: My newly found Marconi 707

Now Iíve proved my Emiscope tube appears to conduct properly and my EHT transformer is OK. Iím going to move my focus to the radio portion of the receiver and power supply.

Does anyone have any ideas on how to free up the tuning shaft? Tuning control moves freely but the outer part which has the little chain on it appears very stiff. Is this a form of grease??


Thanks all for your support so far.

Ed.
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Old 5th Jan 2021, 8:21 pm   #38
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Default Re: My newly found Marconi 707

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Originally Posted by Panrock View Post
Later on, you might consider adding a switch-off burn protection circuit too. This can be done with just a few components and is described on Hugo Holden's 'worldphaco' site.

Steve
Thanks for the tip Steve. Iíll do some research into this.
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Old 5th Jan 2021, 9:29 pm   #39
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Default Re: My newly found Marconi 707

Another Hugo inspired tip, regarding smoothing block restuffing.

Rather than using a pipe-cutter, you can use a file to partially remove the rolled seam at the base. You can then clamp the solder tags in a vice and pull the can off leaving the "gubbins" behind.

I have done this with several smoothing blocks, and when the paxolin is cemented back into place after reshuffling it's difficult to tell it's not original.

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The 1mm drilled holes are so the new capacitor wires can covertly exit and connect to the solder tags.

If your smoother uses its can as the common ground, you also have to sneak an earth wire out.
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Old 5th Jan 2021, 10:12 pm   #40
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Default Re: My newly found Marconi 707

That appears to be a full emission tube with no cathode impurities. At least that gives you encouragement.
Don't be surprised if the EHT transformer fails after a few hours/days. You may be lucky.
The insulation employed in 1937 was poor at high voltages and with only 2500v EHT had to withstand over 5kv. They can be rewound successfully. John.
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