How to Repair 0-100V DC Power Supply? Need Help

[Noam]

Hi,

I have an old 0-100V (0.2A) HP power supply (HP 6116A), which is
broken. The output is always 0.6V (when on). Nothing is obviously
burned or exploded inside.

The problem started when one of the people in my lab connected a
capacitor backwards across the supply's output, giving it a large
negative voltage spike.

Any ideas on what could be the problem? The supply has a transformer
and several largeish capacitors. However, the inner circuit is pretty
complicated. My guess is something near the output is broken, but what
could it be?

Any advice would be greatly appreciated.

 

[gb]

Hi,

I have an old 0-100V (0.2A) HP power supply (HP 6116A), which is
broken. The output is always 0.6V (when on). Nothing is obviously
burned or exploded inside.

The problem started when one of the people in my lab connected a
capacitor backwards across the supply's output, giving it a large
negative voltage spike.

Any ideas on what could be the problem? The supply has a transformer
and several largeish capacitors. However, the inner circuit is pretty
complicated. My guess is something near the output is broken, but what
could it be?

Any advice would be greatly appreciated.

Start with schematic.

gb

 

[Arfa Daily]

Hi,

I have an old 0-100V (0.2A) HP power supply (HP 6116A), which is
broken. The output is always 0.6V (when on). Nothing is obviously
burned or exploded inside.

The problem started when one of the people in my lab connected a
capacitor backwards across the supply's output, giving it a large
negative voltage spike.

Any ideas on what could be the problem? The supply has a transformer
and several largeish capacitors. However, the inner circuit is pretty
complicated. My guess is something near the output is broken, but what
could it be?

Any advice would be greatly appreciated.

Best guess would have to be the series pass transistor ( probably fairly
large and on a heatsink ).

Confirm with a voltmeter that there is a highish voltage on its input -
probably 120v or so and may be on the collector or emitter depending on
whether it's an NPN or a PNP type. If input volts are present, check for a
voltage on the base connection, which varies up to around 100v with rotation
of the set voltage pot.

If there is no input voltage, problem is back in the raw stage ie rectifier
/ smoothing / maybe surge limiter resistor.

If the variable voltage is missing, disconnect, base lead and check again.
If it's still missing, problem is back in control loop. If adjustable
voltage is present, and input voltage is present, check to make sure that
there are no shorts across the output of the transistor ( decoupling cap,
protection diode, overvolt zener ) then replace transistor.

Obviously, this is a fairly simplistic approach, and assumes that it is a '
conventional ' design linear supply, but should catch most common problems
for this type of fault. If problem does prove to be in the control loop,
then things will get rather more complicated, but suspicion should fall
first on any semiconductor devices ( transistors, control chip, diodes,
zeners ).

Arfa

 

[Noam]

No schematic available unfortunately. Do you know if it might be
available online somewhere? I checked the HP site, but no luck. It's
kind of an old power supply.

 

[Sam Goldwasser]

No schematic available unfortunately. Do you know if it might be
available online somewhere? I checked the HP site, but no luck. It's
kind of an old power supply.

HP wouldn't provide a schematic of a light switch.

Since you say it died when someone hooked up something incorrectly,
start by checking the semiconductors for shorts.

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header above is
ignored unless my full name AND either lasers or electronics is included in the
subject line. Or, you can contact me via the Feedback Form in the FAQs.

 

[Charles Schuler]

Check for a shorted diode across the output terminals.

 

[James Sweet]

No schematic available unfortunately. Do you know if it might be
available online somewhere? I checked the HP site, but no luck. It's
kind of an old power supply.

I'm not sure what good a schematic would be on a linear power supply,
normally they're not very complex, you can follow the circuit visually.

 

[Arfa Daily]

Best guess would have to be the series pass transistor ( probably fairly
large and on a heatsink ).

Confirm with a voltmeter that there is a highish voltage on its input -
probably 120v or so and may be on the collector or emitter depending on
whether it's an NPN or a PNP type. If input volts are present, check for a
voltage on the base connection, which varies up to around 100v with
rotation of the set voltage pot.

If there is no input voltage, problem is back in the raw stage ie
rectifier / smoothing / maybe surge limiter resistor.

If the variable voltage is missing, disconnect, base lead and check again.
If it's still missing, problem is back in control loop. If adjustable
voltage is present, and input voltage is present, check to make sure that
there are no shorts across the output of the transistor ( decoupling cap,
protection diode, overvolt zener ) then replace transistor.

Obviously, this is a fairly simplistic approach, and assumes that it is a
' conventional ' design linear supply, but should catch most common
problems for this type of fault. If problem does prove to be in the
control loop, then things will get rather more complicated, but suspicion
should fall first on any semiconductor devices ( transistors, control
chip, diodes, zeners ).

Arfa

Remind me again why I bother ...

Arfa

 

[Noam]

I checked the big transistor that was on the heat sink and it looks
fine. I took it out to test it, and also I checked in-circuit and
there are 130V across B-C and E-C while running (so I guess it's an
npn). There is also a capacitor with 20V on it and a capacitor with
130V on it.
I don't quite understand the stuff about the missing variable voltage.
How would I check for this?

 

[giroup01]

I checked the big transistor that was on the heat sink and it looks

fine. I took it out to test it, and also I checked in-circuit and
there are 130V across B-C and E-C while running (so I guess it's an
npn). There is also a capacitor with 20V on it and a capacitor with
130V on it.

Post a link to pictures of the unit. Check for an open (power)
resistor somewhere in the feedback loop.

 

[Arfa Daily]

I checked the big transistor that was on the heat sink and it looks
fine. I took it out to test it, and also I checked in-circuit and
there are 130V across B-C and E-C while running (so I guess it's an
npn). There is also a capacitor with 20V on it and a capacitor with
130V on it.
I don't quite understand the stuff about the missing variable voltage.
How would I check for this?

Assuming then that it is NPN, if you have 130v on the collector, you would
expect the emitter to be the output. To check the control voltage, you need
to measure volts between ground and the transistor base, whilst turning the
set volts pot up and down. You should see a variable voltage between 0 and
100v, and the emitter should follow the base, less around 0.7v.

But before doing this, switch your meter to ohms, and just check the
resistance between the emitter and ground. There just might be a short
across the output from a diode, as I, and another poster suggested. If you
don't see a short, go ahead and check the base volts, as detailed, and let
us know.

BTW, try not to top-post, as it makes the thread difficult to follow for
people ;-)

Arfa

 

[Noam]

Thanks for all the advice so far. Here is the update:
There are no shorts across the output. Also the emitter, base, and
collector of the big power transistor are not shorted to ground. But
here is something that seems wrong: there is no variable voltage on the
base of the big power transistor when the set-voltage is varied. So
there is some problem before the big power transistor. This is all I
have so far.
Any advice?

 

[Arfa Daily]

Thanks for all the advice so far. Here is the update:
There are no shorts across the output. Also the emitter, base, and
collector of the big power transistor are not shorted to ground. But
here is something that seems wrong: there is no variable voltage on the
base of the big power transistor when the set-voltage is varied. So
there is some problem before the big power transistor. This is all I
have so far.
Any advice?

That would be the reason that there is no output. What you now need to do,
is follow the base lead back into the circuitry, and see where it comes
from. There are several ways that the adjust voltage may be produced, from a
simple pot on the base of a driver transistor with a reference from a zener
diode in the emitter circuit, to more elaborate schemes using a chip, often
a uA723 ( or any '723 equivalent ). You might try also checking that there
is some kind of varying voltage on the wiper tag of the set volts pot, when
it is rotated. This voltage may actually be quite small - of the order of a
few volts. Would it be possible for you to take a couple of photos of the
control pcb and post them up somewhere ? Might give the experienced eye a
clue as to how the regulator is designed. If you don't have anywhere to post
them, you could always send them to me direct, and I'll see if I can help
that way.

Arfa

 

[Noam]

Thanks. I ordered the manual on ebay so I'll post again when I get it.

 

[Noam]

Thanks for the suggestions. The power supply is now fixed. The
problem was one of the transistors controlling the big series power
transistor. The faulty transistor was part of a feedback loop
comparing the output voltage to some set reference.

The suggestion that helped the most was to order the manual on ebay.

 

[Arfa Daily]

Thanks for the suggestions. The power supply is now fixed. The
problem was one of the transistors controlling the big series power
transistor. The faulty transistor was part of a feedback loop
comparing the output voltage to some set reference.

The suggestion that helped the most was to order the manual on ebay.

Glad you sorted it, and we were able to be of some help to you

Arfa

 

[Michael A. Terrell]

HP wouldn't provide a schematic of a light switch.

You're right about HP, the computer supplier, but Agilent is in the
process of collecting and making availible scanned HP manuals of older
products, as well as software and user's manuals.


http://www.home.agilent.com/agilent/facet.jspx?lc=eng&cc=US&t=80039.k.1


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[David Harmon]

HP wouldn't provide a schematic of a light switch.

You're right about HP, the computer supplier, but Agilent is in the
process of collecting and making availible scanned HP manuals of older
products, as well as software and user's manuals.[/QUOTE]

HP a computer company? I thought they are an ink cartridge company?

 

[Sjouke Burry]

On Sun, 19 Mar 2006 19:04:05 GMT in sci.electronics.repair, "Michael



HP a computer company? I thought they are an ink cartridge company?

Psssst... you are not wearing your smily!!!!!

 

[Franc Zabkar]

You're right about HP, the computer supplier, but Agilent is in the
process of collecting and making availible scanned HP manuals of older
products, as well as software and user's manuals.


http://www.home.agilent.com/agilent/facet.jspx?lc=eng&cc=US&t=80039.k.1

Do you mean to say that a company like HP goes to the trouble of
producing a 500 page technical manual and then doesn't bother to keep
an electronic copy of it? What does that say about the value of their
IP?

- Franc Zabkar