Restoring an HP 521A Frequency Counter: Part 1

We’re back into vintage HP stuff baby! In the previous HP video I asked y’all what piece you wanted to see next, and the while the voting was tight, the 521A Frequency Counter won out in the end. Honestly, I’m quite happy it won because it was the one I was most excited about! There’s all sorts of digital and analog magic going on inside of the purely tube instrument, so let’s dig in deep and see if we can’t figure out what’s going on inside!

Here’s my previous episode on the HP AC4 Decade Counter:
   • In-depth Look at the HP AC-4G Decade Counter  

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Music:
Artist: Ben Prunty
Title: Love Story
www.benpruntymusic.com

Ben, if you see this, thank you so much for the years and years of excellent music! You’re one of my personal music heroes and to have some of your music in one of my tiny little videos is absolutely mega. Thank you so much!

Intro Music adapted from:
Artist: The Runaway Five
Title: The Shinra Shuffle
https://ocremix.org/remix/OCR01847

Thanks for watching!

Chapters
0:00 Intro
1:57 Cleaning, painting and recapping
4:27 What I didn’t replace yet
5:43 Theory of operation
8:20 Amplitude discriminator
10:23 Crystal controlled time-base
12:16 AC-4A decade counters
17:55 First start
19:31 There’s going to be a part 2 isn’t there?
21:05 Outro
21:58 Bunny!