Uploaded April 2025 | Updated September 2026, 2 weeks ago
Workshop Porject #46 - HP 721A Lab Power Supply
The HP 721A is a tiny DC lab power supply that can deliver 0-30V DC with some preset current limits up to 225mA. So a whooping 6.75W... certainly nothing pretty much any modern PSU couldn't do. But the form factor is just too cute to ignore! I never saw a professional PSU that is so small. The manual I found online has a copyright of 1959, but since this unit is ever so slightly different, I'd say it might be early 1960s? So a very early transistorized device. The power transistor that is the linear regulator is a germanium one, which seems to throw off contemporary testers.
I love the form factor and just how compact this thing is. The output is reasonably accurate, but again: don't compare it with modern devices; it does drift a bit in the millivolt range. But for regular voltages up to 30, I still consider that spot on!
0:00 Intro / Overview
1:35 Disassembly
4:37 Isolation Summary / Case Repair
6:58 Desoldering Wires...
8:00 PCB Details
9:14 Recapping!
11:28 Meter Cleanup
12:36 Switches Cleanup...
14:46 Switch "Change"?
15:55 Potentiometer Cleanup...
17:16 Power Switch Cleanup...
19:02 Assembly Start: Meter and Power Controls
20:08 Binding Post Cleanup / Potentiometer and Switches
22:30 Mains Transformer Rewiring
23:38 Power Transistor Checkup and Mounting
25:09 Filter Cap Mounting / New Power Cord
26:34 Soldering Front Panel Components
28:06 New Fuse / First Power Up Test / Isolation Issue
29:25 Potentiometer! / PSA
31:13 PCB Connections
33:03 First Try! / Grub Screw Cleanup / Knob Fitting / Calibration
34:45 New Grommet / New Feet / New Plug
35:37 Testing Voltages / Summary / Credits / Outtake
Music (in order)
My Train's A Comin' - Unicorn Heads
Nine Lives - Unicorn Heads
Wrong - Dan Henig
Town of 24 Bars - Unicorn Heads
Light Expanse - Unicorn Heads
(YouTube Audio Library)
You can support me on Ko-Fi: ko-fi.com/atkelar
and also on Patreon: patreon.com/Atkelar
and now also here by using the "Super Thanks" feature!
Thanks to everybody who signed up already!
Workshop Porject #46 - HP 721A Lab Power Supply
The HP 721A is a tiny DC lab power supply that can deliver 0-30V DC with some preset current limits up to 225mA. So a whooping 6.75W... certainly nothing pretty much any modern PSU couldn't do. But the form factor is just too cute to ignore! I never saw a professional PSU that is so small. The manual I found online has a copyright of 1959, but since this unit is ever so slightly different, I'd say it might be early 1960s? So a very early transistorized device. The power transistor that is the linear regulator is a germanium one, which seems to throw off contemporary testers.
I love the form factor and just how compact this thing is. The output is reasonably accurate, but again: don't compare it with modern devices; it does drift a bit in the millivolt range. But for regular voltages up to 30, I still consider that spot on!
0:00 Intro / Overview
1:35 Disassembly
4:37 Isolation Summary / Case Repair
6:58 Desoldering Wires...
8:00 PCB Details
9:14 Recapping!
11:28 Meter Cleanup
12:36 Switches Cleanup...
14:46 Switch "Change"?
15:55 Potentiometer Cleanup...
17:16 Power Switch Cleanup...
19:02 Assembly Start: Meter and Power Controls
20:08 Binding Post Cleanup / Potentiometer and Switches
22:30 Mains Transformer Rewiring
23:38 Power Transistor Checkup and Mounting
25:09 Filter Cap Mounting / New Power Cord
26:34 Soldering Front Panel Components
28:06 New Fuse / First Power Up Test / Isolation Issue
29:25 Potentiometer! / PSA
31:13 PCB Connections
33:03 First Try! / Grub Screw Cleanup / Knob Fitting / Calibration
34:45 New Grommet / New Feet / New Plug
35:37 Testing Voltages / Summary / Credits / Outtake
Music (in order)
My Train's A Comin' - Unicorn Heads
Nine Lives - Unicorn Heads
Wrong - Dan Henig
Town of 24 Bars - Unicorn Heads
Light Expanse - Unicorn Heads
(YouTube Audio Library)
You can support me on Ko-Fi: ko-fi.com/atkelar
and also on Patreon: patreon.com/Atkelar
and now also here by using the "Super Thanks" feature!
Thanks to everybody who signed up already!
