Presto 1-JE

[jjwharris]

So I've been using this as a bit of machining practice with the mill I bought over lockdown. I drew up the design in CAD over the last 18 months from pausing a lot of YouTube videos and reading a lot on here.

Back plate/knife holder/spring holder is aluminium, pole pieces/magnet clamps/armsture are mild steel and the spring holders for the armature are brass.

I think I need to remake a couple of parts, the omega springs are piano wire and I have a feeling I could go up a gauge, and the knife edge seems to be off kilter. Haven't wound my coils yet, but will hopefully get onto that soon, I'm thinking maybe foregoing the bobbins and just doing a bare coil, RCA style, hopefully to get some more turns in.

The armature with stylus holder and brass spring holder weighs in at 1.9g, which I *think* is less than the original prestos.

The plan for dampening is an acrylic paint/acetic acid silicone blend. The armature won't be screwed to it, but there will be a tight fitting hole the armature sits in, then the whole thing is compacted against the body and case.

That's my thinking at the moment at least.

It's been fiddly work but immensely enjoyable tbh.

[jjwharris]

I finally got this assembled this weekend. I haven't used omega springs, I still think this will require some experimentation.

Hoping to test it out in the following week.

When I began milling this, I decided to leave quite a large margin for error as I had never milled anything before. Now it's assembled I've realised that that margin is causing a few problems in terms of air gap and coil position, I've managed to shunt the coils away from the armature by packing and gluing the coils in place, but the next version will have tighter tolerances, so more of it should clip into place.

You'll notice I misunderstood the layout of the omega springs which caused some serious issues.

I've also left the piano wire to fit snug coming out of the holding block - the only m2 grub screws I could find were too hard to mill centre holes into, I had some m2/5mm grub screws but they wouldn't allow for a case, and I couldn't find Allen keys to fit.

[tragwag]

looks awesome!

[Estrada]

Looks great. Will be interested to hear some samples at some point

[jjwharris]

Little update - it's a nightmare!

I think my downfall is the springs. I can probably upload some of my CAD drawings if anyone is interested.

Everything is scaled from disassembly photos - if anyone could share the read dimensions of the armature etc, that'd be pretty helpful - making the torque springs is a real nightmare.

Can anyone tell me if the armature is straight, or does it narrow at the top/bottom? I'm imagining this would give it a larger EMF, yet the effective mass would be reduced as the tips of the armature are thinner/there is less weight being thrown around - I've noticed the fin on the RCA MI-4887 is thinner at the top than it is at the bottom..

I've put it on the shelf for the time being and I'm working on a RCA MI-4887/Presto 5C Hybrid. It looked daunting last year, but seems a bit more attainable after messing around with the Presto 1-D Design.

[piaptk]

I have some extra springs that I had made a while back. I would also be very interested in getting a few armatures made if they are made to the same dimensions as a 1c. I've got a graveyard of parts and the only thing keeping me from putting together 2-3 good heads is a lack of armatures.