Sorry about the schematic, it is here https://rjarratt.wordpress.com/wp-content/uploads/2025/08/m24_psu_schematic-marked-up.jpg 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 <[email protected]> > Sent: 29 August 2025 07:36 > To: General Discussion: On-Topic and Off-Topic Posts <[email protected]> > Cc: Robert Jarratt <[email protected]> > 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 <[email protected]> > 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. > > https://rjarratt.wordpress.com/wp-content/uploads/2025/08/diodes-on- > 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
