Heathkit TM-1626 Stereo Microphone Mixer (UNBUILT):Unboxing, Assembly, Demonstration, Circuit Theory

I used to love building Heathkits, and my experiences with them profoundly influenced and benefited my life and career. I really miss building them, and I decided to "go for it" to outbid other contenders for one of the rare UNBUILT Heathkits that occasionally show up for auction on eBay. I would have preferred something like a piece of test equipment, but no such luck (yet). I landed an UNBUILT TM-1626 Stereo Microphone Mixer (mixing console), a model that I had once owned (purchased used and already assembled) decades ago, and briefly used in my budding on-site concert/music recording side business.
I wanted to do my usual treatment, usually reserved for documenting Heathkit test equipment, but ALSO give enough of a taste of the Heathkit building experience to share with my viewers. Added to all that, because this kit is around 50 years old since it left the factory, I had to deal with past-their-shelf-life components, dried out adhesives, corroded electrical terminals, potentiometers and switches. All of that is included in this video. And I wrap it up with my usual study of the schematic diagram and fairly detailed circuit analysis (circuit theory of operation).
Table of Contents in Timeline:
00:00 Introduction/History
07:25 Unboxing
13:24 Heathkit Manual
25:33 Dealing With Corrosion
31:55 Making Missing Pieces
36:25 Printed Circuit Board Assembly
1:03:23 Chassis Assembly
1:26:20 Chassis Wiring
1:57:11 Initial Tests
1:58:28 Audio Tests
2:09:26 Demonstration
2:11:54 Final Assembly
2:15:00 Beauty Shot
2:15:12 Overview of the Mixer
2:21:58 Schematic Diagram/Circuit Theory

[@marka1986]

Most of the time the primary of a mic input transformer has a grounded center tap to reject common mode signals. That might be where some your hum is coming from. Remember seeing one cheaper mixer using a pair of 1k resistors across the primary with the center to ground to create sort of an artificial center tap. Also thanks for taking the time to make these videos.

[@youtuuba]

Marka1986, hmmmm, I used to design custom equipment for radio stations, and I don't recall ever seeing or using an unbalancing transformer with a center tapped primary. Certainly there are circuit topologies that use center tapped transformers, but a simple winding provides common mode noise rejection without a center tap to ground. Even a quick check online shows many such circuits.
As for the hum in my demonstration, none of the mic wiring was near a source of 60Hz magnetic field, so I doubt that the cable was picking up much of anything. I suspect poor foil routing in the Heathkit PCB, probably sharing a ground foil between the mic preamp and the adjacent power supply circuit. Heath dumped this product pretty quickly, probably only made one production run of them, and this might have been a contributing factor.

[@marka1986]

@@youtuuba I did some reading and looks like either way is fine. Thanks

[@Stumpbeefknob]

Wow!

[@DougForce]

“Life like” I’m guessing? Ahh they fixed it later.