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Old 26th Aug 2021, 10:57 pm   #41
turretslug
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Default Re: Eddystone 750 wreck

Good luck with the front panel removal/replacement and the re-stringing- both need clear space, clear time and patience! The original pointer-drive wire on mine was at least intact, but refitting the thin springy steel wire back around the pulleys after a comprehensive dismantling and clean up of front panel components involved sore finger tips and a few choice words, in the end I wrapped each pulley in turn and temporarily secured it with electrical sticky tape while I dealt with the next one. I'm sure that would attract the derision of those who do it all the time but it helped me. Hopefully, no-one has glued the fingerplate in place, the one on my 750 thankfully dropped away cleanly but the one on the 670A here had been (why on earth do people do this sort of thing....), perhaps by the same person who had dangerously fouled up the chassis isolation measures on that AC/DC set. In the end, I used a cheese-wire to break the bond to avoid kinking the fingerplate, necessary to access the panel securing screws. Eddystone don't mention it, but I found that the ratio arm interface made a quick and convenient point to separate the tuning drive from the chassis- lifting the spring-loaded wire arms slightly allows the drive peg to disengage in order to lift the front panel casting away. When refitting, check that the two outer indented rollers engage properly with the wire arms, the middle one seating against the drive arm itself and rotating in the opposite direction. A nice piece of precise design.

I was sceptical of the audio coupling capacitor too, 10nF seems suspiciously like a Micamold sort of value but putting it on the 500V setting of a Vici VC60+ insulation tester said infinity. I still changed it, a modern 630V polycarbonate seemed like cheap insurance- same with the similar component that isolates the headphone socket from the output valve anode.
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Old 28th Aug 2021, 4:05 pm   #42
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Default Re: Eddystone 750 wreck

Thanks to your advice, the front panel is now is off. The floating audio coupling capacitor to the AF volume has never been soldered. The steel chassis will end up painted a silver colour I have as as I am too tight to spend money on colour matching. Lots to do...

Do you know what dial bulbs are supposed to be fitted? Mine had one 12V 4W whose bulb was too wide to fit through the slot.
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Old 28th Aug 2021, 11:40 pm   #43
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Default Re: Eddystone 750 wreck

I wonder if the previous owner got fed up with rustling noises in the audio output....

The original bulbs were 6.5V 0.3A MBC spherical types- the spherical envelope was important to avoid fouling the thin tuning pointer drive wire, there were a few suggestions in this recent thread;

https://www.vintage-radio.net/forum/...d.php?t=174602
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Old 31st Aug 2021, 11:56 am   #44
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Default Re: Eddystone 750 wreck

I am cleaning the dial mechanism and have a couple of questions aimed at hopefully avoiding removing the front panel again.

What serves as the dial stops, is it the gearbox plates or the tuning capacitor?
Is the pointer aligned to the 0 of the 0-2500 scale when on the left lock?
Does the dial run exactly to the 2500 point on the right lock?

The dial cord is soldered to the cord spools and to the sliding pointer. There is an adjuster on the pointer that allows a small amount of movement that I assume is the equivalent of one tooth on the spools.
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Old 31st Aug 2021, 2:32 pm   #45
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Default Re: Eddystone 750 wreck

Not entirely sure but I think the end stops are produced by the tuning cap. This link might help if you didn't spot it already: http://www.eddystoneusergroup.org.uk...%20Gearbox.pdf Cheers, Jerry
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Old 31st Aug 2021, 8:49 pm   #46
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I think I can make out some marks on the gearbox back that suggests it is the gearbox rather than the capacitor.

I could still really do with someone with one of these to confirm the correct method is to align the left end stop against the 0.
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Old 31st Aug 2021, 9:02 pm   #47
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Default Re: Eddystone 750 wreck

The end stops are set by the ratio arm drive peg bearing against the scalloped edges of the gearbox rear frame- another nice piece of Eddystone mechanical detailing. The fully-meshed position of the generously-sized gang capacitor is optically quite striking when the edges of the fixed and moving vanes are precisely parallel with each other, I wouldn't be surprised if this spot in the rotation is reproducible within minutes of arc. I used this as the point to set "00.00" on the logging scale, with the pinch-clutch starting to slip at "minus 00.02" as it were. At this setting, there are a couple more degrees of anti-clockwise rotation to reach the gang-capacitor physical end-stop but the clutch and drive-peg prevent this- IMHO, using the gang capacitor physical rotation limit is a really bad way to achieve end-stop even if everything from '50s battery portables to AR88s does it that way. Setting an external end-stop at actual full-mesh strikes me as far more mechanically sympathetic and it also eliminates the "backward tuning" syndrome that can happen when the gang capacitor begins to un-mesh again in that last morsel between full-mesh and physical limit. Certainly, setting LF scale-end this way gives accurate frequency correlation right along the length of the scale, so I assume that that's how it was intended to be done- with a relatively highly-resolving scale like the 750's, it's a bit irritating if things do go askew at some point along the length! In my set, the HF end drive-slip occurs at 25.22, but this probably varies slightly between individual sets, the main thing being that it occurs before the moving vanes press against the frame....
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Old 31st Aug 2021, 11:54 pm   #48
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Thanks again for the information. Still making slow progress but can only spend limited time on it.
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Old 1st Sep 2021, 11:52 am   #49
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Default Re: Eddystone 750 wreck

Take care when re-stringing the drive wire. My 680X (probably the same arrangement) came to me after this operation had been performed by a PO. It worked perfectly smoothly but the pointer went backwards! I did eventually correct it but, as has previously been stated, it needs time and patience.
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Old 2nd Sep 2021, 7:04 pm   #50
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Done it and it reliably locks at 0:00 without any noticeable slack. Only trouble I have is it has lost the fibre washer between the flywheel and bearing but I then discovered there is one on the knob side so I am going tape up the spools, lift the gearbox and put this washer on the flywheel side.

The pike wire would not accept solder so I tightly twisted some thin tinned copper wire in and soldered to that. Next job is to finish off the paint work.
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Old 2nd Sep 2021, 7:59 pm   #51
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Default Re: Eddystone 750 wreck

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Originally Posted by PJL View Post
The pike wire would not accept solder so I tightly twisted some thin tinned copper wire in and soldered to that. Next job is to finish off the paint work.
My solution to this has been to extract the brass-barrel-with-two-screws from a 5Amp "choc block" connector and saw it in half to yield two short-barrels-with-a-single-screw which can be used to clamp the wire.

On my 840A I soldered the choc-block-barrel to the scale-pointer, then strung the drive-wire, before setting the pointer to the right place and tightening the screw. This makes fetting the match of tuning-capacitor/drive traverse and tuning-pointer traverse properly matched a _much_ easier task.
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Old 2nd Sep 2021, 7:59 pm   #52
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Default Re: Eddystone 750 wreck

Nice work I had the same trouble with pike wire on my eddy 730 .
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Old 7th Sep 2021, 12:38 pm   #53
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Default Re: Eddystone 750 wreck

The wreck is still progressing, the steel PSU above chassis components have been stripped off and along with the steel braces have been repainted in more of a rust ridding than quality finish. Once dry I will start the next phase of rebuilding the PSU and move onto the RF box.

I set out to reform a 30uF 15V electrolytic from a 15V source with series resistor, it started at about 7mA slowly reducing but after a few minutes went abruptly dead-short...just as well I tested it.
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Old 10th Sep 2021, 11:31 am   #54
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Power supply section rebuilt. Although removing the rust was an extra chore and has added to the effort, most of my time has been spent accessing parts for testing and replacement. These sets are not easy to work on and I was thankful my bid at a local auction for an S504 just lost.

Last stage now, the RF module that requires coils to be stripped out. The screen bias chain resistors all needed replacement, the ones for V2 seemed to be underrated particularly as they are off the neon stabilised supply.

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Old 10th Sep 2021, 9:06 pm   #55
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Default Re: Eddystone 750 wreck

I think that part of Eddystone's problem was that, being a relatively low-volume manufacturer, they kept case size variations to a minimum- thus, the 670A and 750 here have the same case volume, the 670A with 5 valves is easy to work on, the 750 with 11 valves isn't....

The V2 screen feed resistor power ratings (particularly R3) are as you say very marginal, perhaps it was felt that its minimal length leads soldered to broad tags would help but nonetheless, it's a half-watt resistor called on to dissipate half a watt- not good for long term stability or reliabilty.
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Old 11th Sep 2021, 6:57 pm   #56
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Default Re: Eddystone 750 wreck

Dissaster...one of the switch wafers is a burnt out mess. I had noticed R2 was burnt out but thought it was the 0.1uF capacitor.

Seems unlikely I will find another 5 way make/5 way break so I will have to try a repair by patching in a replacement centre section taken from another switch.
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Old 11th Sep 2021, 7:35 pm   #57
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Default Re: Eddystone 750 wreck

Just the other day, I was clearing out loft boxes and was slightly surprised/pleased to find several dozen NOS switch wafers that I thought had been thrown out many years ago- I think they were from sackfuls of MoD surplus cleared out of my school physics lab in turn, so a long time ago now! If you have no luck, I'll have a sift through and see if anything is suitable, I know that some had features like shorting rings.
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Old 11th Sep 2021, 8:12 pm   #58
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Default Re: Eddystone 750 wreck

Probably not the most sensible design as it has HT on the wiper ring. I may struggle to repair this as there is probably a short between the rings as they have tags that pass through to the opposite side with not much more than 1mm clearance.

I will check the coil windings are all good before setting about finding a solution.

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Old 11th Sep 2021, 8:51 pm   #59
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Default Re: Eddystone 750 wreck

Quote:
Originally Posted by PJL View Post
Dissaster...one of the switch wafers is a burnt out mess. I had noticed R2 was burnt out but thought it was the 0.1uF capacitor.

Seems unlikely I will find another 5 way make/5 way break so I will have to try a repair by patching in a replacement centre section taken from another switch.
Check the Eddystone User Group website. Ian Nutt can possibly reconstruct wafer switches http://eddystoneusergroup.org.uk/spa...one-receivers/
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Old 11th Sep 2021, 8:59 pm   #60
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Default Re: Eddystone 750 wreck

Later versions of NSF 'Oak' switches were fitted with nylon centre rings, which gave much better insulation compared to the original paxolin. It may be that someone has a wafer of this type that you could use to fit the original rings. Sadly the two wafers with nylon insulation I bought on ebay a few years ago have been used on reviving a Dynatron T69 tuner and a Ferguson 401RG radiogram, both with w/c switch shorting HT to AGC line faults.

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