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Old 11th Sep 2021, 9:21 pm   #61
PJL
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Default Re: Eddystone 750 wreck

I have a plan... clean off all the carbonised material and check insulation. Assemble the wafer onto the shaft, apply oil to the shaft and use epoxy putty to replace the lost material and bond it all back together. The epoxy should have better insulating properties than the lost paxolin.
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Old 12th Sep 2021, 2:25 pm   #62
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Default Re: Eddystone 750 wreck

After cleaning off the carbonised material I have 500V insulation between the rings so that bit is good. The bad news is the epoxy putty turns out to be a poor insulator, I guess no surprise as the filler was probably swept off some manufacturing floor somewhere in China.

So my new plan is to make my own 'mouldable glue' by mixing some plastic filings into a slow setting araldite.
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Old 12th Sep 2021, 6:51 pm   #63
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Default Re: Eddystone 750 wreck

That "select one, and short out the rest is" one of the most common type used in that style of Eddystone receiver, (there's some in my 840c.) Someone may have a "used" spare, and it's got fewer contacts than Ian Nutts' manufactured example shown on the EUG website. Is your "mouldable glue" going to be flame retardant?
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Old 12th Sep 2021, 8:43 pm   #64
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Default Re: Eddystone 750 wreck

In the end I have shaped a piece of thick paxolin to replace the lost section and glued it in with araldite.

I wonder how I can prevent this from recurring. There was a fair bit of black soot under the chassis but I suspect the fault is more likely due to the paxolin absorbing moisture. A water repellent spray might help but would probably evaporate off over time. Maybe varnish? It is the only wafer with HT on it.
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Old 24th Sep 2021, 9:10 am   #65
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Default Re: Eddystone 750 wreck

Been away but am now back and it's almost ready for the front panel to go back...

Does anyone know where I can find C63 (0,01uF Paper) that decouples the HT? It's the only paper cap I have not replaced.
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Old 24th Sep 2021, 10:03 am   #66
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Default Re: Eddystone 750 wreck

On my set, C63 was an almost cube-shaped dark brown Bakelite-cased job connected between the HT tag of the output transformer and an adjacent chassis tag (the same type as used for the output valve grid coupling cap). It measured infinity on a high-voltage tester, so it may actually have been a genuine mica dielectric type rather than the cheapy "Micamold" types but I changed it anyway as cheap insurance.
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Old 24th Sep 2021, 5:45 pm   #67
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Default Re: Eddystone 750 wreck

Ooops, it's still there. My excuse is I tested one of these types before and it was good but as it's across the HT I will do a leakage test and leave it only if OK.
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Old 28th Sep 2021, 8:16 pm   #68
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Default Re: Eddystone 750 wreck

It's back together and power has been applied but a few basic checks and I find the HT is 280V instead of the 235V in the service manual. I have fitted a NOS Brimar 5Z4G (and N78) but this HT voltage will cause the VR150 to exceed the datasheet 40mA current limit. The set is designed for 230V mains and the LT is 6.6V so 5% high is to be expected due to my mains voltage but the HT is 20% high.

Audio is working but no other signs of life yet but before I leave it powered up for long I am thinking I should fit a bucking transformer, a weaker 5Z4G, or a bit of extra resistance in the VR150 feed.
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Old 28th Sep 2021, 9:30 pm   #69
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Default Re: Eddystone 750 wreck

Interesting that you also found that- I found much the same with mine when I acquired it quite a long time ago now. HT was around 275V using a US-made metal 5Z4, I expected it to be a little high with typical c.240V mains into a 230V primary winding but not by that much! Once I'd fitted a 220ohm NTC inrush limiter on the primary side (something I like to do if possible on the various valve things around here to give things like switches and dial bulbs a gentler life), losing around 7V, and 2x 82 ohm 2W fusible film (series) between the HT centre-tap fuseholder tag and the reservoir capacitor negative tag, things were nearer the mark. To keep the VR150 operating current down after this, I replaced R7, 27k 1W, with a 12k 1W component now connected to the 150V supply (BFO switch tag) instead of the 235V supply- this diverts approximately 7mA from the VR150 and provides the second mixer, V4 ECH42, with a stabilised screen supply. The designer had provided the first mixer with a stabilised screen supply but not the second, presumably feeling that something connected to an oscillator running at up to 33.6MHz needed it but not the second mixer with its mere 1.535MHz oscillator. I've certainly found when tinkering with various sets that stabilising mixer screen supplies provides an extra dose of frequency stability at higher frequencies. I think it's worth doing something to "tame" the main HT as the N78 in particular is a toasty little valve at the best of times.

Good luck with it, it is a nice set to use when up and running properly.
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Old 29th Sep 2021, 4:50 pm   #70
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Default Re: Eddystone 750 wreck

I now know the Send switch is up for receive. Strangely, it only receives when the BFO is on but given the original condition I am expecting problems including shorted vanes on the trimmers.

I tested the valves and have one weak 6BA6 (I put that in the BFO and that is whistling merrily) and more seriously an ECH42 with some grid leak for which I don't have a spare.

Before I can do much more I need to sort the high HT out and have found a 24V transformer that I will wire as a bucking transformer to reduce the AC by 10%.
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Old 29th Sep 2021, 7:22 pm   #71
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Default Re: Eddystone 750 wreck

I forgot to mention that I also dug out a well used GZ30 but that made the HT a few volts higher.

The BFO switch shorts the AGC line out so my guess is the ECH42 grid leak is responsible. I have ordered a couple of replacements as there appears to be a glut at the moment.
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Old 30th Sep 2021, 7:58 pm   #72
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Default Re: Eddystone 750 wreck

My mains measures 248V with a newish fluke RMS DMM but the bucking transformer has successfully reduced HT to a more acceptable 248V. I can also confirm the ECH42 grid leak is pulling the AGC line high when the BFO switch is off so I need to wait for the replacements.
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Old 4th Oct 2021, 6:25 pm   #73
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Default Re: Eddystone 750 wreck

It's now working, IF alignment completed just BFO and RF to do.

Lots of problems encountered on the way including:
Replaced 0.1uF x13, 0.01uF x16, 0.0005uF x2 paper capacitors. A mixture of types but all showed moderate or severe leakage when tested with maximum working voltage.
Replaced 30uF 15V electrolytic, x4 electrolytics reformed OK.
Replaced x9 resistors, all other resistors tested OK (within 20%).
Repaired burnt RF switch wafer, the only wafer that has full HT on it.
Repaired break in BFO coil lead-out (A failed bodge join in manufacturing).
Repaired C80 that was floating around due to a failed joint.
Serviced gear box and replaced dial cord.
Faulty 6BA6 with no RF gain and ECH42 (my stock) with grid leak.
Repaired damaged IF core.
Straightened bent vanes on RF trimmers.
Made new base plate.

85KHz IF was well out of alignment.

Very pleased I went with the full restoration test and replace up front as some parts are only accessible by stripping out many others. It is also extremely difficult to work on as the components are set deep inside the chassis and there is no access to the underside of many of the valve bases making fault finding very difficult. Removing/replacing the front panel is also a nightmare as nearly all controls are mounted to it.

I'll finish the alignment and post a couple of pics as it no longer looks a total wreck...
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Old 4th Oct 2021, 9:55 pm   #74
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Default Re: Eddystone 750 wreck

Wow, something of a marathon! Must be very satisfying to have re-vitalised what would have seemed something of a forlorn and neglected orphan of a radio. It's understandable that many would quail at the thought of a thorough and extensive overhaul like this on such a tightly-packed and inaccessible set, I'm sure that they must have been built up as modular chunks for final assembly and interwiring originally.

The HT electrolytics in mine are BEC made, which types I believe as having a good reputation in DAC90As and others- they certainly seemed to reform well and have given good service. I was going to replace them when I acquired the set, something I would normally think of as prudent but they're something of a feature in the set.

The front panel arrangement of this series of Eddystones adds to the awkwardness of servicing, too- I wonder what professional users thought of them at the time. As for working around the valve sockets in the RF box- aaargh!
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Old 6th Oct 2021, 5:52 pm   #75
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Default Re: Eddystone 750 wreck

One step forward...two steps back.

I have realigned the RF and sensitivity is pretty consistent across all wavebands.

However, the BFO core is also broken and stuck and this is not as easy as the IF where all I had to do was remove the bottom core and unscrew the top from beneath. To avoid stripping off the BFO module again, I will try setting the radio on a stand so I can work on it from underneath and hopefully gravity will help.
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Old 7th Oct 2021, 10:41 am   #76
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Default Re: Eddystone 750 wreck

Had to remove the BFO module and relocate a capacitor so I could get a screwdriver through the convenient hole in the valve socket to undo using the back slot.

I am now thinking it might be a good idea to araldite the head of a brass slotted screw onto the damaged end of these slugs so I don't end up without any working slots
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Old 8th Oct 2021, 4:23 pm   #77
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Default Re: Eddystone 750 wreck

Working really well now with no scratching from pots or other misbehaviour. The flywheel drive dial is a joy to use unless the station you require is at the opposite end when it is a bit laborious. Never realised just how many MW stations there are...
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Old 8th Oct 2021, 4:44 pm   #78
Electronpusher0
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Default Re: Eddystone 750 wreck

I just went back to post #1 to remind myself what it looked like when bought.
Wow! the difference is amazing. Are you sure its the same receiver?

Peter
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Old 8th Oct 2021, 8:27 pm   #79
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Default Re: Eddystone 750 wreck

Brilliant result, and must be very rewarding to have it working well after all that graft. Well done for rescuing it from its neglect. Did the seller know anything about the outer case's fate?- though something as nicely put together as this is almost too good to be hidden away!

I acquired my 750 opportunistically from someone living nearby, not being at all familiar with Eddystones and if anything a tad sceptical, but after working on it and then using it, I can understand Gerry O'Hara's enthusiasm for the 750 as a landmark product of theirs. I particularly like two features that are mechanical in nature- the variable ratio arm that linearises the tuning scale and the selectivity control with its coupled cranks and IFT coupling coil sleds.
Definitely a worthwhile classic. So far, I've resisted the temptation to acquire any more Eddys other than a 670A, which got in on the grounds of having LW.

Quote:
Originally Posted by PJL View Post
Never realised just how many MW stations there are...
I was impressed by the MW performance too- just goes to show how worthwhile a tuned RF stage can be, and I suppose that the second ECH42 could be considered as an IF stage that happens to have different input and output frequencies but still makes a worthwhile contribution to overall gain. It certainly pulls in the Spanish etc. stations in from early evening, and the precise tuning and 10kHz resolution come in very helpful here. Just a shame that the MW coverage only goes up to 1465kHz.

Last edited by turretslug; 8th Oct 2021 at 8:38 pm. Reason: Additional remark.
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Old 8th Oct 2021, 9:55 pm   #80
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Default Re: Eddystone 750 wreck

I asked about the case and RF cover plate but the guy at the carboot did house clearances and hadn't seen it. The switch wafer damage would have taken it out of action when it would have started smoking and the previous owner must have opened it up and decided it was beyond help. The BFO fault and dry joint were maybe due to poor storage.

I have a Decca Decola tuner with mechanical IF bandwidth in the pile of things to do. It was added to the pile when I saw how many of those 0.1uF bolt down capacitors needed replacement. Single conversion and octal valves so not going to be the same performance. Or I could have a go at the Racal RA17...
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