Need to Fix an HP 723 Ol' Time PS

  • Thread starter Watson A.Name \"Watt Sun
  • Start date
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Watson A.Name \"Watt Sun

Guest
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
<at> 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.
 
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

"Watson A.Name "Watt Sun - the Dark Remover"" <NOSPAM@dslextreme.com> wrote
in message news:c25t7p$66han$1@hades.csu.net...
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
at> 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.
Click to expand...
 
John Walton wrote:

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
Click to expand...
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.

"Watson A.Name "Watt Sun - the Dark Remover"" <NOSPAM@dslextreme.com> wrote
in message news:c25t7p$66han$1@hades.csu.net...

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
at> 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.
Click to expand...
 
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 control circuitry, current
sources, etc. They start out with +/- 75 volt rails capable of handling 6
amps.

Jack
"Watson A.Name "Watt Sun - the Dark Remover"" <NOSPAM@dslextreme.com> wrote
in message news:c285df$672tp$1@hades.csu.net...
John Walton wrote:

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

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.

"Watson A.Name "Watt Sun - the Dark Remover"" <NOSPAM@dslextreme.com
wrote
in message news:c25t7p$66han$1@hades.csu.net...

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
at> 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.
Click to expand...
 
"John Walton" <jdwalton@comcast.net> wrote in message
news:eek:YKdnVnZvtS1ONrdRVn-hA@comcast.com...
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 control circuitry, current
sources, etc. They start out with +/- 75 volt rails capable of
handling 6
amps.
Click to expand...
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]
 
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