I wonder if anyone is familair with Arista Power Supplies

I have a faulty Arista 13.8Volt 7 amp regulated power supply.
Model number RPS 7138

TIA

Peter

"Peter" wrote in message
...
I wonder if anyone is familair with Arista Power Supplies

I have a faulty Arista 13.8Volt 7 amp regulated power supply.
Model number RPS 7138

TIA

Peter


I don't believe that I'm actually quoting Larry King, but here goes...
"WHAT'S THE QUESTION???!!!"

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)

Some days you're the dog, some days the hydrant.

The "short" answer to your question is YES, I am familiar with Aritsta
Power Supplies.
You stated that you have a faulty unit..... so what are the symptoms
(other than dead) and what prelimary troubleshooting, voltage readings
and component testing have you done?
Post the information and troubleshooting results and some repair
suggestions might be forthecoming.
electricitym

"Peter" wrote in message
...
I wonder if anyone is familair with Arista Power Supplies

I have a faulty Arista 13.8Volt 7 amp regulated power supply.
Model number RPS 7138

TIA

Peter
Arista web site (Fremont, CA)
http://www.aristaipc.com/contact_us.htm

Most linear power supplies use a regulator IC (such as LM723) and
pass transistors (NPN: 2N3055, 2N3771) for current handling.

A VOM or DVM can quickly take voltage readings, diagnose components to
resolve.
Need a DIY "How to guide" ? - Check the KBT DC Supplies web site
Menu of knowledge database, reference data sheets, "How TO" on left column
(gray)
http://www.kbt-dc-supplies.com/

g. beat

wrote in message
oups.com...
The "short" answer to your question is YES, I am familiar with Aritsta
Power Supplies.
You stated that you have a faulty unit..... so what are the symptoms
(other than dead) and what prelimary troubleshooting, voltage readings
and component testing have you done?
Post the information and troubleshooting results and some repair
suggestions might be forthecoming.
electricitym


OK thanks
The supply delivers 15 volts on the output (should be 13.8) but when I
connect a load, the voltage drops to 0, so there is no regulation.
The supply uses an IC numbered C324C which I'm assuming is a quad op amp -
LM 324 - configuration seems to confirm this.
Driver transistor is an SD880 driving a 2SC3281
Across the output on the board there was an open circuit 220mf electrolytic
and a burnt resistor which I thought was colored brown, brown, brown (110
ohms)not a preferred value - it measure 331 ohms. heat has perhaps made it
change value and alter the color coding?
What I'd like is a schematic.

Peter

"g. beat " @spam protected wrote in message
. ..
"Peter" wrote in message
...
I wonder if anyone is familair with Arista Power Supplies

I have a faulty Arista 13.8Volt 7 amp regulated power supply.
Model number RPS 7138

TIA

Peter
Arista web site (Fremont, CA)
http://www.aristaipc.com/contact_us.htm

Most linear power supplies use a regulator IC (such as LM723) and
pass transistors (NPN: 2N3055, 2N3771) for current handling.

A VOM or DVM can quickly take voltage readings, diagnose components to
resolve.
Need a DIY "How to guide" ? - Check the KBT DC Supplies web site
Menu of knowledge database, reference data sheets, "How TO" on left column
(gray)
http://www.kbt-dc-supplies.com/

g. beat
Many thanks for the tips and the link

Peter

"Peter" wrote in message
...
OK thanks
The supply delivers 15 volts on the output (should be 13.8) but when I
connect a load, the voltage drops to 0, so there is no regulation.
The supply uses an IC numbered C324C which I'm assuming is a quad op amp -
LM 324 - configuration seems to confirm this.
Driver transistor is an SD880 driving a 2SC3281
Across the output on the board there was an open circuit 220mf
electrolytic and a burnt resistor which I thought was colored brown,
brown, brown (110 ohms)not a preferred value - it measure 331 ohms. heat
has perhaps made it change value and alter the color coding?
What I'd like is a schematic.

Peter

Correct, the C324C is a LOW POWER QUAD OPERATIONAL AMPLIFIER (NEC part
number)
Spec Sheet
http://pdf1.alldatasheet.com/datashe...NEC/C324C.html

SD880 might be a NPN EPITAXIAL PLANAR TRANSISTOR

2SC3281 - POWER TRANSISTOR(15A,200V,150W)

Pull the 2SC3281 and check with VOM or DVM

"Peter" wrote in message
...

wrote in message
oups.com...
The "short" answer to your question is YES, I am familiar with Aritsta
Power Supplies.
You stated that you have a faulty unit..... so what are the symptoms
(other than dead) and what prelimary troubleshooting, voltage readings
and component testing have you done?
Post the information and troubleshooting results and some repair
suggestions might be forthecoming.
electricitym


OK thanks
The supply delivers 15 volts on the output (should be 13.8) but when I
connect a load, the voltage drops to 0, so there is no regulation.
The supply uses an IC numbered C324C which I'm assuming is a quad op amp -
LM 324 - configuration seems to confirm this.
Driver transistor is an SD880 driving a 2SC3281
Across the output on the board there was an open circuit 220mf
electrolytic and a burnt resistor which I thought was colored brown,
brown, brown (110 ohms)not a preferred value - it measure 331 ohms. heat
has perhaps made it change value and alter the color coding?

It was probably orange-orange-brown and 330 Ohms to start with, so it's
likely almost spot on in value.


What I'd like is a schematic.



A schematic? If you have even a basic grounding in electronics (enough to
interpret a schematic) you should have no trouble at all diagnosing a simple
linear power supply like this!

There should usually be around 15.2V on the base of the driver transistor,
and around 14.5V on the base of the output pass transistor to give an output
of 13.8V (0.7V B-E drop on each transistor). All you need to do is check
this off load and on load and see where the fault is. If the base of the
driver drops to 0V or thereabouts under load then you have a problem in the
control circuit or current limiter.

Check both the driver and output transistor have unregulated voltage on
their collectors as well- I'd expect 18V or more likely higher. Check if
there's a current sense resistor in series with the output (usually ground
side) and that it isn't open circuit. It will be a fraction of an ohm so a
basic continuity check will suffice.

The supply has quite a beefy output transistor so maybe the driver failed?
IME linear supplies often take out the driver when the outputs are shorted.
Either way it should take about 5 minutes with a multimeter to at least
track down the region of the fault to either control, driver, pass or
current regulation sections.

Dave

"Dave D" wrote in message
...

"Peter" wrote in message
...

wrote in message
oups.com...
The "short" answer to your question is YES, I am familiar with Aritsta
Power Supplies.
You stated that you have a faulty unit..... so what are the symptoms
(other than dead) and what prelimary troubleshooting, voltage readings
and component testing have you done?
Post the information and troubleshooting results and some repair
suggestions might be forthecoming.
electricitym


OK thanks
The supply delivers 15 volts on the output (should be 13.8) but when I
connect a load, the voltage drops to 0, so there is no regulation.
The supply uses an IC numbered C324C which I'm assuming is a quad op
amp - LM 324 - configuration seems to confirm this.
Driver transistor is an SD880 driving a 2SC3281
Across the output on the board there was an open circuit 220mf
electrolytic and a burnt resistor which I thought was colored brown,
brown, brown (110 ohms)not a preferred value - it measure 331 ohms. heat
has perhaps made it change value and alter the color coding?

It was probably orange-orange-brown and 330 Ohms to start with, so it's
likely almost spot on in value.


What I'd like is a schematic.



A schematic? If you have even a basic grounding in electronics (enough to
interpret a schematic) you should have no trouble at all diagnosing a
simple linear power supply like this!

There should usually be around 15.2V on the base of the driver transistor,
and around 14.5V on the base of the output pass transistor to give an
output of 13.8V (0.7V B-E drop on each transistor). All you need to do is
check this off load and on load and see where the fault is. If the base of
the driver drops to 0V or thereabouts under load then you have a problem
in the control circuit or current limiter.

Check both the driver and output transistor have unregulated voltage on
their collectors as well- I'd expect 18V or more likely higher. Check if
there's a current sense resistor in series with the output (usually ground
side) and that it isn't open circuit. It will be a fraction of an ohm so a
basic continuity check will suffice.

The supply has quite a beefy output transistor so maybe the driver failed?
IME linear supplies often take out the driver when the outputs are
shorted. Either way it should take about 5 minutes with a multimeter to at
least track down the region of the fault to either control, driver, pass
or current regulation sections.

Dave

The transistors tested out as OK - voltage readings were very close to what
you advised.
I replaced the (open circuit)220 MF electro and the 330 Ohm 1 watt resistor
as well as 100nf ceramic which were across the output. All three looked heat
stressed.
After that the supply performed faultlessly - delivered 5 amps into a dummy
load fine.
Wondering how the o/c electro could have caused the voltage to sag. Also
thinking that the 330 ohm resistor runs a bit hot - one can see that the
board is heat stressed around this component. It would be passing 40 ma.

Many thanks.
Peter

"Peter" wrote in message
...

Wondering how the o/c electro could have caused the voltage to sag. Also
thinking that the 330 ohm resistor runs a bit hot - one can see that the
board is heat stressed around this component. It would be passing 40 ma.


It was probaby there to stabilise the current sense, and without it the
current limiter was kicking in when a load was connected due to the sudden
output voltage dip.

Just a guess mind, it's hard to say without seeing it.

Dave