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HP 6236B win!

Saturday, February 18, 2017

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).

Pretty much exactly what I wanted. The lack of CC/adjustable current limit was a bit annoying, but frankly I'd never relied on it much in the old PSU anyway. More to the point, even with shipping amounting to double the sale price, it'd still be less than ½ the price of any comparable unit locally - new or used. And although they have some unobtainium parts (CA 3458T opamps, special "HP" versions of 2N2955/2N3055 transistors), there was nothing I didn't think couldn't be substituted with maybe a bit of circuit-tweaking (e.g. LM1558 or friends, MJ15003 etc.). In short, as long as it wasn't completely smashed to pieces with a blown transformer, it would be a worthwhile buy - either to be repaired, or to become a donor case/transformer/heatsink for a similar design. So I bought it.

Many people have bad things to say about eBay's GSP (and DAI Post, who handled it between Kentucky & Australia before passing it on to AusPost), but my experience was all good. Tracking was maybe slow to update, but no worse than 24 hours lag. Less than 2 weeks later, it arrived.

(There was a small kerfuffle, though it wasn't really a problem for me. The seller had two for sale at the same price but with vastly different GSP costs. Of course, I bid on & bought the one with the cheapest shipping - a later 1987 variant - while someone else bought the earlier 1977 variant. The seller mistakenly shipped us each other's supplies, which didn't matter one whit to me - the only real difference is the '87 one lacks a power-on lamp, which is itself unobtanium - but annoyed the other buyer who left negative feedback. When I figured out what'd happened. I contacted the seller offering a swap - while pointing out the minimum AU$90 shipping would make it uneconomical - who was grateful for the offer, but sensibly declined ;)

Opened it up, gave it a visual once-over, set the internal switches for 240V, tested the transformer & transistors for obvious opens/shorts/etc., swapped the US plug for an Aus one, plugged it in, flipped the switch, and …

BANG! (and a bit of smoke, some sparks, and a small flame…)

The X2 capacitor across the mains had decided to say goodbye to its career in interference reduction, and embark on a new and exciting (and short) life as a suicide bomber.

Ex X cap...
Failure of those early Rifa epoxy-yellow X2 caps isn't exactly unknown (Google "Rifa PME271M fail"), although in my previous experience they normally cook and stink for a while before blowing lots of smoke. This one, I guess, was in a hurry…

When something dies as violently as that, there's always a tendency to reflect: Were there any warning signs? Could I have done anything differently? What if I'd loved it a bit more? etc. But in this case, it's an X2 capacitor. It sits directly across the mains. Apart from a mains transient or internal degradation, there's literally almost nothing that can cause it to fail. Maybe if there was something wrong with the transformer primary it could cause a spike that broke down the capacitor … but that's a big if.

So I removed it, tested the transformer continuity & insulation (again), and replaced it. Powered it up, and …

It's fine. Everything works as it should, meter accuracy on all 3 rails is within a few mV across the scale, and output ripple is well within specs all the way up from zero to full load. So far I've used it for prototyping/testing an RF power amp (±15v @ 400ma/side), a 90v boost converter (4v to 7v @ up to 2A), and a couple of varicap tuned loops with low-noise RF signal amps (12v to 21v at anything from a couple to a couple of hundred mA) and it's been perfectly stable and quiet.

It's a nice little supply, and will do me quite nicely …

One thing about the whole experience I don't understand though: eBay's GSP pricing seems to be almost random. For instance: the seller had 2 near-identical supplies for sale, listed within seconds of each other, and with identical shipping weights set in eBay. And yet, eBay was quoting GSP shipping of ~AU$60 for one and ~AU$90 for the other. How does that work out?

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