W
Watson A.Name - \"Watt Su
Guest
I finally got the ol' timer running. As soon as I got it I reamed out
the hole in the back and installed a modern socket like the one on the
back of an AT PS, which is zackly where I got the socket. Plenty of
room, so no problem. Took about an hour with a rat tail file and some
elbow grease.
I fired it up and it would put out a dozen volts but at no load, and
dropped to zero with any load. So I checked the 2N458A pass transistors
(2 in series) and they were bad. I looked for NTE Germanium subs, but
they cost $16.30 at Mouser, and at Fry's, not only did they not have
them, they didn't even have a peg for them. I guess the old timer
germaniums aren't in demand anymore.
So I bought some 2N5745 silicon PNP power transistors for a buck and a
quarter each on Ebay. I gooped 'em up with white goop and stuck 'em in,
but the PS still didn't work. Luckily I had received the schematic of a
later model 723A, with a few more bells and whistles than my ol' timer,
but similar otherwise.
I found that the schem called for a 220 ohm but for some reason the one
in the PCB was 2200 ohms. That's odd. Someone had fiddled with the
circuit in the past? Weird! So I replaced it with the right value.
When I fired it up this 220 ohm resistor started getting really hot, so
I checked the voltage and it was excessive. I figured that the reverse
voltage on the emitter to base of one of the pass transistors was a lot
more than 6 or so volts max that they can handle. Germaniums can handle
20 or more volts E-B, but not silicon. So I cut the trace on the PCB
and put a 1N4003 in series with the base so it wouldn't zener. I fired
it up, and I could plug in a 14V lamp and it lit it up and worked okay.
A bit of adjusting the trimmers took care of the current and voltage
maximums.
The only thing that's not quite right is that at low voltages below 5V,
the meter is off quite a bit. This has something to do with the bias
they put on the meter to compensate for the drop across the current
sensing resistors. I'll have to investigate to see if I can adjust it
somewhat.
The only thing left to do is replace the neon lamp in the front panel,
but I'm tired of working on it, and besides I gotta do my taxes, it's
that time of the year. :-(
--
@@F@r@o@m@@O@r@a@n@g@e@@C@o@u@n@t@y@,@@C@a@l@,@@w@h@e@r@e@@
###Got a Question about ELECTRONICS? Check HERE First:###
http://users.pandora.be/educypedia/electronics/databank.htm
My email address is whitelisted. *All* email sent to it
goes directly to the trash unless you add NOSPAM in the
Subject: line with other stuff. alondra101 <at> hotmail.com
Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
@@t@h@e@@a@f@f@l@u@e@n@t@@m@e@e@t@@t@h@e@@E@f@f@l@u@e@n@t@@
@@@@@@
the hole in the back and installed a modern socket like the one on the
back of an AT PS, which is zackly where I got the socket. Plenty of
room, so no problem. Took about an hour with a rat tail file and some
elbow grease.
I fired it up and it would put out a dozen volts but at no load, and
dropped to zero with any load. So I checked the 2N458A pass transistors
(2 in series) and they were bad. I looked for NTE Germanium subs, but
they cost $16.30 at Mouser, and at Fry's, not only did they not have
them, they didn't even have a peg for them. I guess the old timer
germaniums aren't in demand anymore.
So I bought some 2N5745 silicon PNP power transistors for a buck and a
quarter each on Ebay. I gooped 'em up with white goop and stuck 'em in,
but the PS still didn't work. Luckily I had received the schematic of a
later model 723A, with a few more bells and whistles than my ol' timer,
but similar otherwise.
I found that the schem called for a 220 ohm but for some reason the one
in the PCB was 2200 ohms. That's odd. Someone had fiddled with the
circuit in the past? Weird! So I replaced it with the right value.
When I fired it up this 220 ohm resistor started getting really hot, so
I checked the voltage and it was excessive. I figured that the reverse
voltage on the emitter to base of one of the pass transistors was a lot
more than 6 or so volts max that they can handle. Germaniums can handle
20 or more volts E-B, but not silicon. So I cut the trace on the PCB
and put a 1N4003 in series with the base so it wouldn't zener. I fired
it up, and I could plug in a 14V lamp and it lit it up and worked okay.
A bit of adjusting the trimmers took care of the current and voltage
maximums.
The only thing that's not quite right is that at low voltages below 5V,
the meter is off quite a bit. This has something to do with the bias
they put on the meter to compensate for the drop across the current
sensing resistors. I'll have to investigate to see if I can adjust it
somewhat.
The only thing left to do is replace the neon lamp in the front panel,
but I'm tired of working on it, and besides I gotta do my taxes, it's
that time of the year. :-(
--
@@F@r@o@m@@O@r@a@n@g@e@@C@o@u@n@t@y@,@@C@a@l@,@@w@h@e@r@e@@
###Got a Question about ELECTRONICS? Check HERE First:###
http://users.pandora.be/educypedia/electronics/databank.htm
My email address is whitelisted. *All* email sent to it
goes directly to the trash unless you add NOSPAM in the
Subject: line with other stuff. alondra101 <at> hotmail.com
Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
@@t@h@e@@a@f@f@l@u@e@n@t@@m@e@e@t@@t@h@e@@E@f@f@l@u@e@n@t@@
@@@@@@