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!)

Congratulations to the Yahoo DRMNA Forum…

… he's your idiot now 😂.

Oh, he's been there for years - since 2005, I think - but now, thanks to his main haunt disappearing, his special brand of half-baked belligerent ignorance is all yours.

Contact me if you want some background. Good luck. You're going to need it…

… and then a miracle occurred …

I've mentioned before the RF noise that plagues my location - weird 'nodes' of noise/hash & re-radiated IM all around my suburb that pretty much wipe out the LW/AM/lower SW bands on normal receivers, with one particularly bad 'node' centred around the main switchboard & pay TV distribution boxes for my block that are right underneath one corner of my unit.

And now it's gone.

HP 6236B win!

Well, it can't all be about radios...

I've been after a decent general-purpose power supply for a while, to finally retire the old 40V 3A EA/DSE kit I've been using for the last …mumble… years. Although it wasn't one of EA's greatest designs I'd managed to tame / live with its worst issues, but I'd decided it was time to look for something better.

Unfortunately, when you can't justify multiple $100's for your hobby there ain't much in the way of choice. Affordable new ones are cheap & nasty, switchmode, or both - problems when using it for/near RF & sensitive electronics. I toyed with the idea of designing & building one, but by the time you add up the basic parts (case + transformer(s) + heatsink(s)) it starts heading into expensive territory. Second-hand, the local used tools & equipment economy seems to work on the "buy high, sell even higher" principle and prices are ridiculous. I'd resigned myself to living with the old kit for a while longer - when I saw a HP 6236B on eBay USA.

±0-20V @ 0.5A. 0-6V @ 3A. "Powers on, untested". ~AU$30 + ~AU$60 postage (via eBay's Global Shipping Program).

Titus II DRM receiver

I posted some info about this radio on the DRM receiver list page a few days ago just hours after it was revealed at HFCC B16 in Miami, but I think it's worth a front page post:

The TWR / PantronX Titus II AM/SW/FM/DRM receiver

The Philips 513 AN

This is an interesting one. I bought this radio mostly sight unseen on eBay because it looked terrible in the pics, and had a fair bit of cabinet damage that'd been poorly repaired. I was looking for something to try out a few ideas I had about Bakelite repair, and this seemed like a suitable candidate. After clicking the button at the right time, a short drive and $30 later it was mine.

Locally-made Philips sets aren't exactly thin on the ground here, and over the years more than a few European models have made their way here too, so I wasn't concerned that I couldn't identify the model of this set. But once I got up close and had a good look, I discovered why…

AWA 565MA

1950's AWA Radiola (not mine!)

I quite like these little 50's AWAs. There's something about the minimalist yet remnant Art Deco styling of local sets of this era which is almost quintessentially Australian. In some ways it's 10 or 20 years later than the rest of the world - but, then again, I don't think the rest of the world produced a style quite like it…

Capacitor Restuffing

When repairing/restoring vintage radios it's pretty common to end up replacing all the electrolytic, wax-paper, and molded paper capacitors, as well as several carbon-composition resistors. Usually it's simply a case of 'out with the old, in with the new' (while keeping any above-chassis components in place but disconnected to preserve the 'look' of the radio), but sometimes you might want to keep things looking as original as possible above and below the chassis.

That's where restuffing comes in handy…

Retr0bright & radio collecting

A lot has been written about Retr0bright in the last few years - most of it aimed at vintage computer / games collectors, but also by & for groups as diverse as telephone collectors & Imperial Stormtroopers - but it seems there's been surprisingly little discussion about it amongst radio collectors. I suspect that's partly because most collectors focus on older wood or Bakelite sets, partly because of an assumption that it really only works on white or other light coloured ABS components, and partly because of the long-held belief that yellowed plastic = the set belonged to a chain smoker who sat in front of it all day and is therefore irretrievably tainted…

But there's a fair number of collectable sets with ABS cases out there (many 1950's & 60's mantle radios come to mind), it's not only white/light-coloured ABS that suffers from bromine yellowing, and cigarette tar is actually fairly easy to clean off*.

Have a look at these before-and-after photos of a grey 1955/56 AWA 565MA I've been working on:

An Intro

Welcome to the new blog!

For a whole stack of reasons I needed to re-organise things in my life, so I'm (slowly) shuffling several blogs and accounts into different themes. This one, which will eventually hold all my radio, TV, and electronics hobby related stuff, is the first.

Oh, and it's not connected in any way to the old Australian Radio, Television and Hobbies magazine. If you're looking for that, go hassle Leo at Silicon Chip magazine. I hear he likes the Internet.