28 Dec
2013
28 Dec
'13
10:12 a.m.
Afternoon,
I've managed to acquire an LA120! Woohoo!
However, it had some issues...as was discussed when I got it. ;)
I've replaced the blown fuse, the there is continuity for the pico fuse on
the PSU...but I have no idea if that indicates it works or not. The only
thing that runs is the fan.
What are some test points I can try? Also...what is the value for the
picofuse? I have 125V 1.5A fuses...but I think it needs higher current.
--
Cory Smelosky
http://gewt.net Personal stuff
http://gimme-sympathy.org Projects
28 Dec
28 Dec
10:29 a.m.
On Sat, 28 Dec 2013, Cory Smelosky wrote:
Afternoon,
I've managed to acquire an LA120! Woohoo!
However, it had some issues...as was discussed when I got it. ;)
I've replaced the blown fuse, the there is continuity for the pico fuse on
the PSU...but I have no idea if that indicates it works or not. The only
thing that runs is the fan.
What are some test points I can try? Also...what is the value for the
picofuse? I have 125V 1.5A fuses...but I think it needs higher current.
Okay. I tested. These voltages are preset:
+5
-12
-24.
No +12 or +24.
--
Cory Smelosky
http://gewt.net Personal stuff
http://gimme-sympathy.org Projects
10:41 a.m.
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org]
On Behalf Of Cory Smelosky
Sent: Saturday, December 28, 2013 12:12 PM
To: cctech at classiccmp.org
Subject: LA120 PSU fault
Afternoon,
I've managed to acquire an LA120! Woohoo!
However, it had some issues...as was discussed when I got it. ;)
I've replaced the blown fuse, the there is continuity for the pico fuse on
the PSU...but I have no idea if that indicates it works or not. The only
thing that runs is the fan.
What are some test points I can try? Also...what is the value for the
picofuse? I have 125V 1.5A fuses...but I think it needs higher current.
--
Cory Smelosky
http://gewt.net Personal stuff
http://gimme-sympathy.org Projects
The manual is here:
http://manx.classiccmp.org/details/1,1601
Cindy Croxton
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29 Dec
29 Dec
11:14 a.m.
I'f I'm reading the printset (on bitsavers) correctly, it appears there
were 2 versions of the PSU.
The older one used a constant-voltage-transformer ('CVT', ferroresonant
transformer) and a PCB containing some regulators for the +5 and +/-12V
lines. The later version was a switcher. The printset contains the
scheamtic of the switcher and of the PCB for the older version, but for
some reaso nthe CVT circuit is not there.
In both cases, the +12V line is produced from the +24V line by a 7812
regulaotr, so if you're missing the +24V line, it's not surprising that
the +12V line is missing too.
Ifyou have the switcher, there's a not a lot that can remvoe the +24V
line. An open-circuit rectifier would be the most likely problem I think.
If it's the CVT, then I would guess the +24V line is obtained by
rectfying and smoothing a secodary winding on said transformer, so again
I'd check the rectifier. Trace the +24V line backwards to find it.
I just hope it's not transformer trouble!
-tony
12:28 p.m.
On Sun, 29 Dec 2013, Tony Duell wrote:
I'f I'm reading the printset (on bitsavers)
correctly, it appears there
were 2 versions of the PSU.
The older one used a constant-voltage-transformer ('CVT', ferroresonant
transformer) and a PCB containing some regulators for the +5 and +/-12V
lines. The later version was a switcher. The printset contains the
scheamtic of the switcher and of the PCB for the older version, but for
some reaso nthe CVT circuit is not there.
In both cases, the +12V line is produced from the +24V line by a 7812
regulaotr, so if you're missing the +24V line, it's not surprising that
the +12V line is missing too.
Ifyou have the switcher, there's a not a lot that can remvoe the +24V
line. An open-circuit rectifier would be the most likely problem I think.
If it's the CVT, then I would guess the +24V line is obtained by
rectfying and smoothing a secodary winding on said transformer, so again
I'd check the rectifier. Trace the +24V line backwards to find it.
I found an open 270Ohm resistor on the board and I've found some caps I
don't trust. The resistor was between a diode and "C4 NEG"
I just hope it's not transformer trouble!
-24V and -12V are present so I doubt it's the transformer.
-tony
--
Cory Smelosky
http://gewt.net Personal stuff
http://gimme-sympathy.org Projects
30 Dec
30 Dec
10:52 a.m.
I found an open 270Ohm resistor on the board and
I've found some caps I
don't trust. The resistor was between a diode and "C4 NEG"
Which type of power supply do you have?
I just hope it's not transformer trouble!
-24V and -12V are present so I doubt it's the transformer.
11:28 a.m.
On Mon, 30 Dec 2013, Tony Duell wrote:
H7110.
So do I, but an open-circuit secondary winding could casue the lsos of
+24V only I think
It was a loose fuse holder, believe it or not. I got the +24V rail back.
Next comes the board repairs to bring the +/-12V back. (They went out
after I removed the busted resistor)
I found an
open 270Ohm resistor on the board and I've found some caps I
don't trust. The resistor was between a diode and "C4 NEG"
Which type of power supply do you have?
I just hope it's not transformer trouble!
-24V and -12V are present so I doubt it's the transformer. -tony
--
Cory Smelosky
http://gewt.net Personal stuff
http://gimme-sympathy.org Projects
3778
days inactive
3780
days old
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