29 Aug
2025
29 Aug
'25
2:03 a.m.
Sorry about the schematic, it is here
https://rjarratt.wordpress.com/wp-content/uploads/2025/08/m24_psu_schematic…
but it is just a marked up version of the schematic that is already on BitSavers.
I attached the probes to the anodes of the diodes marked D15/D16 and D18/D19 in the marked
up schematic and to the GND output.
Thanks
Rob
-----Original Message-----
From: Wayne S <wayne.sudol(a)hotmail.com>
Sent: 29 August 2025 07:36
To: General Discussion: On-Topic and Off-Topic Posts <cctalk(a)classiccmp.org>
Cc: Robert Jarratt <rob_j37(a)hotmail.com>
Subject: Re: [cctalk] Repairing an Olivetti M24 PSU
Your schematics are not accessable unless you’re a vcf member.
The rigol outputs look like there no rectification going on as there’s both a
positive and negative going wave.
Where did you attach the scope probes?
And can you make the schematic available to the genetic public?
Sent from my iPhone
On Aug 28, 2025, at 23:26, Robert Jarratt via
cctalk <cctalk(a)classiccmp.org>
wrote:
My Olivetti M24 was humming away nicely until I
switched it off. It then
failed to power on again. The PSU is not outputting any voltages on any of its
outputs. The technical manual is here
https://bitsavers.org/pdf/olivetti/m21_m24/Olivetti_M21_M24_Theory_of_O
peration_Nov84.pdf. The PSU is described from p188 with an outline
schematic on p194. The full schematic is on p274. A schematic useful to
anyone with the actual board in front of them has been created in the
following forum post https://forum.vcfed.org/index.php?t...ower-supply-
replacement.1237316/#post-1299248, the schematic is this one:
https://forum.vcfed.org/index.php?attachments/m24_psu_schematic-
jpg.1252584/.
I have done some verification with the scope but I have reached a point
where I am not sure what signals I should expect. I have attached a dummy
load on the +12V, +5V and +15V outputs to meet the specs for minimum
current.
I see a 20KHz square wave on the inputs to the output transformer T438
(pins 1 and 2). The manual says that the diodes on the output rectify a half
wave. I assume these are the ones labelled D134 and D136.
The following image shows the signal on the two diodes marked D134 on
the schematic for the 12V output, measured from their anodes to 0V output.
12v-output.png
For the other two diodes, labelled D136 for the 5V output I get this:
https://rjarratt.wordpress.com/wp-content/uploads/2025/08/diodes-on-5v-
output.png
Do these look correct? The peaks seem to be too low to even begin to
produce the required output voltage.
>
> Thanks
>
> Rob