Sunday, 11 February 2018

EL84 Amp II: Progress Post 4

A bit more activity on the amplifier as time permits, and a few trials and errors later... we have a power supply board.

In the previous post, I showed the design. I made the board, using my new hand-made UV exposure box with 120 UV LEDs in it, and after etching and drilling, began to build the circuit.

I began with the low-voltage parts: the delay switch-on and LED colour reverser.

Long story short, it didn't work. Due to two errors on my part.
  1. I'd omitted the reverse-biased diode across the output of the 555 IC (which I've never used in previous designs, and thus far gotten away with. This time it didn't work)
  2. I'd accidentally used a relay with a 9V coil voltage instead of the required 5V for the LED colour reverser.
So, it didn't work. And worse, in soldering and de-soldering components to test it, I ended up stripping some of the tracks off the board.

So it was back to the design. Make up a new board that rectifies these omissions (I ordered a relay with 5V coil and of course its pin spacing was different)

So, a new board was designed and exposed, developed and etched.

I use the Mega / Farnell UV-sensitive boards, and Ammonium Persulphate as an etchant. These boards are not specified for this solution so while it works, it's very slow, etching a board takes around 30-40 mins. Of course during that time, the etchant bath cools down and the process slows as a result.

//TODO: Buy an etching bath heater!

Anyway, my UV exposure lightbox gives a much more consistent light than my previous approach, which frankly is too embarrassing to describe here. So the boards produced with it look a lot better.

After etching, it was to the drillpress to drill around 145 holes of various sizes, then back to the soldering bench.

First test was to stuff and solder just the components for the low voltage circuit. Make sure the revised delay switch-on and LED colour reverser was working.

Success. It worked as planned! This meant I could then continue to stuff and solder the rest of the components.

This is now done and the results are in the pictures.

Bit of a squeeze to get everything on the board - the size dictated by the chassis. During testing I'll be watching carefully for any heat stress

Closer view of the low-voltage switching section

Dimensions: 140 X 75mm, about the size of your smartphone

47,000µF - to smooth the DC heaters for the phono stage. Because bigger is always better

How clean is this?!

Next steps: Metalwork - case drilling - and a bunch of connectors to make up.

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