28 Jun
2009
28 Jun
'09
4:12 p.m.
Can anyone offer any guidance on an 11/23 switchmode power supply I'm attempting to
repair ?
It failed in use and started blowing fuses. I diagnosed a failed bridge rectifier in the
primary side and replaced it. That resolved the problem with the fuses blowing.
However, when I monitor the +12V line, I can see it peak around +11.5V on power up and
then it slowly sags (off-load), all the way down to +4.5V over a period of a few minutes.
With a small dummy load connected the 12V line drops down to 4.5V in a matter of less than
a second. I can see why it stops at 4.5V; there's a diode, normally reverse biased
when the voltages are correct, that's connected between +12 and +5. The +5V line is
rock solid at 5.1V.
The power supply has a single pair of transistors driving the primary side of T2, and +5
works so I am inclined to think that the problem is somewhere on the secondary side. I
pulled the pass transistor (Q3) for the 12V line and checked it with the Huntron Tracker;
it appears to be serviceable.
The schematic for this is on p75 of
http://bitsavers.org/pdf/dec/qbus/MP00740_1123_schem_Oct81.pdf.
Is there a common cause for 'sag' in output voltages in switchmode supplies ? I am
wondering if this is regulation issue or a problem with drive to the pass transistor ?
Thanks,
-Dave
29 Jun
29 Jun
10:04 a.m.
From: david_comley at yahoo.com
Can anyone offer any guidance on an 11/23 switchmode power supply I'm attempting to
repair ?
It failed in use and started blowing fuses. I diagnosed a failed bridge rectifier in the
primary side and replaced it. That resolved the problem with the fuses blowing.
However, when I monitor the +12V line, I can see it peak around +11.5V on power up and
then it slowly sags (off-load), all the way down to +4.5V over a period of a few minutes.
With a small dummy load connected the 12V line drops down to 4.5V in a matter of less than
a second. I can see why it stops at 4.5V; there's a diode, normally reverse biased
when the voltages are correct, that's connected between +12 and +5. The +5V line is
rock solid at 5.1V.
The power supply has a single pair of transistors driving the primary side of T2, and +5
works so I am inclined to think that the problem is somewhere on the secondary side. I
pulled the pass transistor (Q3) for the 12V line and checked it with the Huntron Tracker;
it appears to be serviceable.
The schematic for this is on p75 of
http://bitsavers.org/pdf/dec/qbus/MP00740_1123_schem_Oct81.pdf.
Is there a common cause for 'sag' in output voltages in switchmode supplies ? I
am wondering if this is regulation issue or a problem with drive to the pass transistor ?
Thanks,
-Dave
Hi
The schematics are not real clear as to what wires
are connected to what wires from board to board.
There seems to be a separate 555 that pulses the T4 that drives
Q3. On page 80 there is several points you might probe.
You might check the voltages and signals at Q1, E1, Q5,
E2 and Q6.
The decaying voltage is most likely related to the start-up +12.
It is only there for a short time to get the regulator running.
Once running, it should provide its own voltage which in
this case it doesn't seem to be.
Dwight
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1:37 p.m.
Can anyone offer any guidance on an 11/23 switchmode power supply I'm
attempting to repair ?
It failed in use and started blowing fuses. I diagnosed a failed
bridge rectifier in the primary side and replaced it. That resolved the
problem with the fuses blowing.
However, when I monitor the +12V line, I can see it peak around +11.5V
on power up and then it slowly sags (off-load), all the way down to
+4.5V over a period of a few minutes. With a small dummy load connected
the 12V line drops down to 4.5V in a matter of less than a second. I can
see why it stops at 4.5V; there's a diode, normally reverse biased when
the voltages are correct, that's connected between +12 and +5. The +5V
line is rock solid at 5.1V.
The power supply has a single pair of transistors driving the primary
side of T2, and +5 works so I am inclined to think that the problem is
somewhere on the secondary side. I pulled the pass transistor (Q3) for
the 12V line and checked it with the Huntron Tracker; it appears to be
serviceable.
I've not looked at the schemaitcs, but I assume there's one chopper
transformer that provides both the +5V and +12V output. In which case,
the problem has to be on the secondary side.
Now, in most of these supplies, the 5V line provides the regulation
feedback, and the other lines essentially tag along. DEC were better than
most in that they provided separate regulator circuits, often linear, for
the other supplies.
My first question is 'do you have enough load on the 5V line?'. If you
don't, the chopper circuit is running very 'lightky', and probably the
12V side isn't getting enough power to work properly. I've certainly had
DEC supplies where the 12V output is low untill I loaded the 5V output.
-tony
2:17 p.m.
On 6/29/09, Tony Duell <ard at p850ug1.demon.co.uk> wrote:
My first question is 'do you have enough load on
the 5V line?'. If you
don't, the chopper circuit is running very 'lightky', and probably the
12V side isn't getting enough power to work properly. I've certainly had
DEC supplies where the 12V output is low untill I loaded the 5V output.
There's a load-board for the uVAX2000/VS2000 box for use if you don't
have an internal hard drive. AFAIK, that's why it's needed - to
ensure enough load on the +5V line for the +12V regulator to work.
While I've seen it, I haven't used one myself (thus can't check
details) - all of my uV/VS2000 boxes have drives.
-ethan
5584
days inactive
5585
days old
3 comments
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participants (4)
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ard＠p850ug1.demon.co.uk -
david_comley＠yahoo.com -
dkelvey＠hotmail.com -
ethan.dicks＠gmail.com