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Need to Fix an HP 723 Ol' Time PS

I bought a HP 723A PS off Ebay, and it came DOA, which was not unexpected since it was advertised as "as-is". So now I have a weekend project to fix it up once I get the manuals from EWd Matsuda (ematsuda earthlink.net) But did accomplish something. I opened it up and removed the old time three pronged power connector on the back and took a rat tailed file to the hole, so it now holds a regular power connector from an old AT power supply. Makes it a lot easier to find a cord for the 0ld Timer.

But now I have the problem of getting germanium replacement transistors for it. It has 2 2N458A transistors for the pass transistors, and they're not HP part numbers, so apparnetly it has already had those replaced at some point in the past. And apparently they couldn't handle the abuse, and gave out. I'm guesstimating if they're bad, I haven't checked them yet, but the meter will register a dozen or so volts but at open circuit, and it drops to nothing as soon as a load is put on it. So I figure the pass transistors are probably open.

But I need to get somethign a bit more heavy duty to replace them. I'm going to look at some NTE replacements, in the TO-3 case. I have some

2N457As but they're too low voltage. I was thinking about replacing them with PNP silicons, but they tend to go into oscillation since the bandwidth is so much greater. Has anyone done such a thing before to a power supply? Maybe I should consider a big old power FET?

There are other really weird devices in there, too. I hope they're not bad. I saw that the zener looked like it had been unsoldered in the past, probably replaced. I've got a HP crossref, but those old parts are hard to find, even tho they're 2N or 1N type parts. Thanks for any advice.

Reply to
Watson A.Name "Watt Sun - the
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the 2n458 is an 80V 5 amp PNP -- you should be able to use something like a TI41B, or an MJE13005 (TO-220 devices), or MJ15004 -- why not scan the schematic and post the schematic to see what other suggestions you get. you may have to twiddle with the bias with the new transistors, but they are much more forgiving than germanium devices.

i have fixed dozens of HP supplies -- even pressed two of them into service as audio amplifiers (with a lot of tinkering!).

jack

Reply to
John Walton

Thanks for the info. I haven't received the manuals for it yet, so I can't post anything other than a pic of the PS or of its inner works. I've thought about putting Si pass transistors in it, but I don't know how well it is going to tolerate the wider bandwidth transistors. I had an old audio amp that someone gave me, and it had Ge output transistors replaced with Si ones. It was oscillating at above the AM broadcast band!

Someone left a schematic of an audio amp that used a LM317 as the output. Well, that's fine, but it's probably not as cheap or as efficient as a couple power transistors.

Reply to
Watson A.Name "Watt Sun - the

If it oscillates you have to tame the error amplifier.

The HP supplies like the HP6129C are basically quasi-complementary amplifiers anyway -- I replaced the voltage c>

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Reply to
John Walton

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I ordered the manuals for a similar HP 723A, and they should be here in a few days. These are old timers from the '50s using all Germanium parts, maybe even the rectifiers on the HP 721 PS. I bid on Ebay and won some PNP Silicon power transistors, good for 70 or 80V at 15 amps. I'll look into trying them out when I get the schematic.

Actually, when you think about it, maybe an oscillating pass transistor might be more efficient if the oscillations at the output could be filtered out. Just like a switching PS.

[snip]
Reply to
Watson A.Name - "Watt Sun, th

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