I’d been using my dad’s old hi-fi tuner-amp without any problems for nearly thirty years, but recently it developed a super-loud low-pitched hum on the outputs, of the type that makes you go – shit, turn it off quick.

It’s been knocked about a bit over time so no photos of the outside, just the inside – if it was in decent nick it’d look like the example here. Here’s the inside from the top:

Lots of brown resistors! Barely any ICs! Some weird retro-futuristic green transistors! Complicated wiring!

Once I’d worked up the confidence to turn it all back on again, I pulled the output from my Soundcraft mixer all the way down: still humming. Turned the volume down on the Armstrong: slightly quieter, but still loud.

Straight-away I was thinking it would be due to a capacitor somewhere, but it’s easy to jump to conclusions, so thought I’d try and take measurements here and there.

The service document/schematic could be a lot clearer for a basic idiot such as me, but I managed to find the stereo amplifier inputs to check with my multimeter, and measured a load of AC there (with the output level from the external mixer set to 0).

The main internal rail voltage on the Armstrong is a chunky 82V, and I measured some ripple on that. Here’s the actual power supply board – the black slightly pointy blobs on the outside of the pair of green blobs are the diodes in the rectifier:

Actually it would have been a lot easier just to measure the voltage on the main smoothing capacitor; speaking of which, what’s this here?

That brown-ish crystalline blob that is growing out of the top of the capacitor just next to the blue wire looks very suspicious. OK, given that we’re seeing ripple on what should be a nice flat DC rail and the capacitor looks dicey, let’s just replace it.

Here’s the old one versus the new one – it’s definitely not as pretty a colour:

…but that sorted it.

Not the most exciting, but I was glad that this one was a boring easy fix.

The two fat 4000uF output caps are obviously the same vintage and will also need replacing. The cap I changed was apparently the first component change it’s needed in nearly fifty years, which is good going.

Really I should celebrate by listening to Radio 3 FM stereo broadcasts, which is what I think it was mostly made for, but recently it’s just been pure Intergalactic FM disco fetish all the way. Poor old Armstrong.