Wednesday, April 13, 2011

LA-2A back plate

I've managed to sneak in a few hours each night this week to work on the back plate of the LA-2A, and things are coming along nicely. 

First off, here's the final drill plan.  Actually, this version corrects a couple of minor issues with the version I used in order to improve spacing a bit.. for instance, the reservoir can capacitor has been moved a bit further from the top. But it's pretty close:

This is meant to be printed full sized and taped directly to the back panel.  Actually, the back panel itself was first covered with a layer of clear packing tape to protect the paint during the drilling step. Then the paper was applied, and another layer of packing tape applied to ensure that nothing would move around or tear during the drilling process:

 Pilot holes were drilled for everything with fine drill bit. Then, depending on the size of the opening,  larger drill bits, a unibit, or hole saws were used.  The small square opening for the IEC power filter was (carefully!) cut with a cut-off blade on a Dremel rotary tool.

Here's what the backplate exterior looked like when drilling was (nearly) finished and the IEC power filter being test fitted:

 Here's the interior. Burrs have been removed with a Dremel grinding wheel.  I've intentionally scuffed up the surface around most of the holes as well, since it is important that anything metal mounted in those holes ultimately have a secure electrical connection to the chassis (and therefore to safety ground). The anodized aluminum surface is not a good conductor. It's not particularly pretty, but it will all be hidden inside anyway:

Next up, we mount all the tube sockets, transformers, potentiometers, and other hardware:

Visible in the photo above is the vintage UTC-A24 output transformer I was able to find for the build.

Then I flipped it over, mounted the two circuit boards that attach to the backplate, and began wiring up what can be done at this point..mostly the heater wiring, a couple of runs to the tube sockets and some of the transformer connections: 

You can see that I've gone ahead and attached the backplate to the bottom plate here for a test fitting. The bottom plate circuit boards and transforemer aren't actually mounted here yet, I was just assessing.... and it looks pretty good!   You may have noticed that the anodization layer has been scrapped away on the edges of both the back and bottom plates... with a multi-section chassis like this it is important to ensure that the individual components are all electrically attached, again for the purpose of ensuring adequate grounding.


  1. Wow, great work Paul. It's looking awesome. I was thinking that it would be a really tight squeeze from the tube sockets to the circuit board, but now seeing it in folded up it looks like you'll have plenty of room (sometimes it's hard to visualize these things in 3D during the design phase).

    Where oh where did you happen upon that UTC transformer? I've bid on some on eBay but they usually go for ridiculous prices.


  2. The UTC A-24 was a fleabay purchase. It wasn't exactly cheap. With shipping it came in at $140.