HP 6236B: Adding multi-turn pots

Yeah, I know, this is so simple it hardly deserves writing up, does it? But today on a fairly popular blog I read a post about changing a knob that was little more than "loosen grub screw, replace knob, tighten grub screw", plus nearly a dozen "before/during/after" pictures 😳. So I figured 'hey, just this once …' 😊

Nice as my HP 6236B is, one minor complaint is that setting the voltage is just a little finicky. The manual states the "resolution" as ±70mV for the 20V rails (±20mV for the 6V rail), and that's certainly achievable if you have a deft touch - but, in practice, it's more like 150~200mV. That's more than good enough 90% of the time, and it's rock-stable - but sometimes you want just a little better precision.

So I decided to change out the original 300° single-turn pots for some multi-turn units…

Original pot on left, replacements on right (apologies for pic quality all-round today!)

The original pots are old-fashioned large (body 1⅛" diameter & ⅝" deep) 10kΩ, 2 watt, linear, board-mounted units. Ideally, I would've liked to find drop-in replacements; unfortunately, that physical size is pretty much unavailable - heck, it's starting to get hard to buy good quality 24mm / 0.95" body pots these days! The other problem is available depth - there's not much free space behind the existing units, and though it might be possible to squeeze some 18-20mm deep 10-turn pots in there, it'd be tight for at least one of them. 5-turn pots, if shallower, would be ideal - but they're also strangely hard to get these days. 3¾-turn pots, on the other hand, are common.

Original pots in-situ

I ended up buying some Bourns 3856A-286-103AL (10kΩ, 2W, 3.75-turn, continuous rotation, PC-mount sealed cermet) pots and panel-mounting them. That isn't as dodgy as it sounds; the existing tracking pot & meter voltage/current switch are both panel-mounted, and the voltage pots in the almost identical earlier model HP 6236A were also panel-mounted. On top of that, apart from panel-mounting, that model of Bourns pot is a drop-in replacement - both shaft & mounting diameter & length are perfect, so no extra hole-drilling or shaft-cutting is required. All that's needed is an extra nut so the pot can be mounted on the panel at the correct depth.

Truth be told, I should've bought Bourns 3856A-282-103AL pots - the "6" in "286" identifies the ones I bought as having PC pins (the "28" identifies it as having the required ⅞" shaft length). But I got a good deal from a reliable source - becoutlet / Electronic Component Super Store on eBay - which meant I could buy 4 incl. shipping for the price of 2 excl. shipping here, so beggars can't be choosers.

So, on to the actual job!

Parts

• 2 x 10k multi-turn pots (e.g. Bourns 3856A-282-103AL or similar)
• If you choose a different pot, as well as checking there's enough mounting depth it needs to be a ⅜" mounting hole type with a ¼" shaft ⅞" long.
• 2 extra pot nuts (i.e. 4 all up) to suit (e.g. ⅜-32 UNEF)
• Insulated hookup wire
• Heatshrink to suit

Tools

• Philips #1 screwdriver
• Hex driver, 1.3mm
• Flat open-ended* potentiometer spanner (e.g. ½", or to suit)
• Soldering iron
• Desoldering tools (e.g. solder sucker, desoldering wick, etc)
• Hot-melt glue or similar non-corrosive adhesive (see step 6)

Procedure

1. Prepare the pots by attaching ~1" of hookup wire to each terminal, & insulate the terminals with heatshrink.
2. Remove the "VOLTAGE +6V" and "VOLTAGE ±20" knobs from their shafts on the front panel by undoing the two grub screws (1.3mm hex driver) on each.
3. Remove the top and bottom panels (4 screws each), followed by the front panel (4 screws).
4. Remove the side panels:
• screws along the LH & RH side of the circuit board (most of mine were missing; YMMV)
• 2 screws holding the LH side to the transformer
• 2 screws for each side in the rear heatsink
5. Pull the front panel clear of the board
6. Desolder the existing 10KΩ potentiometers
• On either side you may need to remove the plastic/rubber board supports. These are press-fit through the board; both of mine were hardened with age & the mounting stud crumbled when removing. Since there is not much else supporting the board at this location, I re-attached them with hot melt glue.
7. Mount the new potentiometers on the front panel
• Use a single mounting nut on the front, and a washer and nut on the back. Mount so that the end of the threaded mounting bushing is level with the front of the front nut. To prevent scratching the front panel, using the spanner tighten the rear nut only.
8. Solder the wires from the new potentiometers to the appropriate holes.

New ±20V pot fitted below original tracking pot

Reassembly is, as they say, the reverse of disassembly. You'll find if using the suggested Bourns pots that the knobs fit back exactly as they did on the originals.

End result? The supply is a lot nicer to use, especially when you want to dial in with 100mV accuracy.

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(* Comedy of errors story, from the days when it was socially acceptable to ride a motorbike with a guitar slung across your back: I once repaired a guy's amp, minutes before a gig, using the punched-sheet-steel ½" spanner from his ancient British 'bitser' bike's toolkit. Of course it was a closed spanner, and of course I was in a hurry - so, after repairing it & then having to disassemble it again to get the spanner on the nut to tighten it, I assembled it for the second time - then realised that there was no way to get the spanner off! He had to go on, I had to go off, so I figured I'd catch up with him the next day to do it properly & retrieve his spanner.
Of course, on the way home he hit a puddle which went straight up his air cleaner. Stopping to take the cover off & wring it out, he reached into his kit for the ½" spanner, and …

Needless to say, he wasn't a happy chappy…)