CISCO 2501 PSU issues

[JEWilson]

This PSU is a;

Computer Products - NFN40-7632E
P/N - 700184-002
Cisco P/N - 34-0625-02 Rev B0
Serial No. - ZKAI 1420
Date Code - ZH22N
Max. O/P - W
I/P Pwr - 100-240V 0.6-1.2A max. 50-60 Hz
Rail O/P - P1 + 12.0V
P2 COM
P3 + 5.0V
P4 + 5.0V
P5 COM
P6 - 12.0V

This PSU was DoA
Checked for any opens and shorts across the PSU connector at
the motherboard end - there were none.
Checked for any opens and shorts across the fan connector at
the motherboard end - there were none.

Took measurements across the PSU connector at the PSU end
when in cct.
When powered on, the 12V fan was ticking and moving slightly
Measured the voltage across the +12.0 v rail and it was measuring
positive and negative values ranging from 0.62 to 1.45V but these
varied.
Measured the voltage across the +5.0 v rail and it was measuring
positive values ranging from 0.64 to 2.35V, again these varied.
Measured the voltage across the -12.0 v rail and it was measuring
positive values ranging from -2.31 to -5.29V, again these varied.

Removed the PSU and opened, a number of small lytics, e.g. at the
PWM controller were measuring high (up to 14Z), replaced all of these (3 of) with Pana FC 100uF 35V.

Tested the ESR of the O/P filter caps - these all ok, e.g. NCC series KMH (primary) and LXF (output).

Removed switcher and secondary ICs.
Switcher - ST TRF8360 does not appear to have a short.

The secondary has;
1. 2 Rectification Diodes
These are single package diodes (2 pin only)
One is a Fairchild-MBR1660 for the +5.0V rail
The other is a Motorola MUR820 for +12.0V rail
2. Two three pin voltage regulators
One of these is a -12.0v reg., an ST-L7912ACV for -12.0V rail
The other is 1.5A low dropout reg., ST-L4940V12 for the +12.0V rail
3. A Thyristor SCR, a Motorola-C122F1

Checked the diodes for shorts - there were none
Checked the regulators for shorts - none apparent
Checked the SCR out of cct. - no problem apparent

In the first instance, as I don't have a scope, I checked if
the PWM controller (Motorola-UC3842AN) was s/c to gnd. No.

I tested all the discretes (resistors and diodes) - these all report
good.

My first replacement effort was to replace the PWM controller
(Motorola-UC3842AN as above) with a new part, an On-Semi
UC3842BNG. I rebuilt the PSU and test the O/P voltages of
the PSU when in cct.

First, the fan had stopped the ticking. Measurements recorded;
Measured the voltage across the +12.0 v rail and it was measuring
negative values (only) ranging from -0.46 to -0.8V but these
varied.
Measured the voltage across the +5.0 v rail and it was measuring
positive values ranging from 0.35 to 1.6V, again these varied.
Measured the voltage across the -12.0 v rail and it was measuring
positive values ranging from -1.29 to -5.76V, again these varied.

Odd, the +12.0V rail is negative
Also, the regulation is not up to spec.
Next, would it be appropriate to replace the -12.0v and +12.0V regs?
These may be malfunctioning

Any help and insight appreciated

[midibob]

Re: CISCO 2501 PSU issues

Hi JE,

I used repair loads of these, I even drew out a schematic which is knocking around somewhere. All the failures I ever had were down to a specific handful of components. Here's an extract of a mail I sent a few years back.

Anyway, as for the fixes....
Most occasions you will find that there has been a burn up of a couple of
resistors, not always though. This depends whether the Mosfet has gone
s/cct or not.

Here's a quick guide :-

Fuse OK, whistling, change C14 47uF 50V 105c (this should be changed
on all units as a matter of course).

Fuse blown, also R6,CR4 Q1, U1 and possibly R13, R4, R10, U2, CR5.
Change all parts (not forgetting C14).

A couple of gotcha's. When ready for testing always load the 5v supply
with about 9 to 10 Ohms and start it up with a variac if you have one or
series light bulb if you haven't. 40W will do just for testing.

The other one is don't be alarmed about the low resistance across the 12v
line (CR8) it measures 90 Ohms when good.

Depending on the source of manufacturer will depend on equivalents used
for some devices but if you have enough faulty units you'll get to see this.

I'll see if I can find the diagram if you haven't got one.

Good luck
Bob

[JEWilson]

Re: CISCO 2501 PSU issues

Bob, hi

Y - that would be very useful
Of the three bad PSU I have persently, they exhibit the same symptons;

I have scketched out the O/P and have attached these.
May prove useful to compare notes on these.

I have also attached the check list notes.

Thanks

[midibob]

Re: CISCO 2501 PSU issues

Here you go.
Not very good quality but I don't have any schematic drawing software (or the time to do it). I've marked some voltages as well which you may find useful.
I think some of your connections maybe incorrect?

Good luck
Bob

[JEWilson]

Re: CISCO 2501 PSU issues

Hi

Thx - you may well be correct
Thanks for the image

Regards