[cctalk] Re: Repairing an Olivetti M24 PSU

-----Original Message----- From: Scott Baker via cctalk &lt;cctalk(a)classiccmp.org&gt; Sent: 29 August 2025 20:10 To: General Discussion: On-Topic and Off-Topic Posts &lt;cctalk(a)classiccmp.org&gt; Cc: Wayne S &lt;wayne.sudol(a)hotmail.com&gt;om>; Robert Jarratt &lt;rob_j37(a)hotmail.com&gt;om>; Scott Baker &lt;smbaker(a)gmail.com&gt; Subject: [cctalk] Re: Repairing an Olivetti M24 PSU I'm confused -- maybe I misread-- the original post says D134/D136 and the most recent post says D15/D16 and D18/D19. Which diodes do the oscilloscope traces go with? Scott content/uploads/2025/08/m24_psu_sche > matic-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 &lt;wayne.sudol(a)hotmail.com&gt; >> Sent: 29 August 2025 07:36 >> To: General Discussion: On-Topic and Off-Topic Posts < > cctalk(a)classiccmp.org&gt; >> Cc: Robert Jarratt &lt;rob_j37(a)hotmail.com&gt; >> 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&gt; >> 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_o >> f_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-o >>> n- >> 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 >