Hi Jon I restored a PDP-8 straight 8 that had a similar PSU. I am here in New Zealand with 230v 50Hz. I used a step down transformer 230v -> 110v but of course retained the 50hz. It worked just fine. I did replace the capacitor in question however. They contain PCB’s so use gloves and dispose with care.
Kind regards Brendan --------------//---------------- [email protected]<mailto:[email protected]> +64 21 881 883 From: Jon Elson via cctalk <[email protected]> Date: Thursday, 17 July 2025 at 02:50 To: Tom Hunter via cctalk <[email protected]> Cc: Jon Elson <[email protected]> Subject: [cctalk] Re: Ferroresonant transformer mystery On 7/16/25 09:13, Tom Hunter via cctalk wrote: > Recently I got a nice and complete PDP-8/s from the US. The power supply > uses a ferroresonant transformer which in addition to the standard primary > and secondary windings has a separate 2.3H winding connected in series to a > 2uF 660VAC capacitor forming a resonant "tank" circuit. The transformer's > secondary side and the resonant circuit are operated in saturation. There > is a magnetic shunt to prevent the primary side going into saturation as > well. It accepts a wide input voltage range, but is very sensitive to the > input frequency of 60Hz. This is quite a nice if not elegant design for the > period in question, but maybe not the most efficient. > > As I live in Australia I get 240VAC and 50Hz as opposed to the US 115VAC > and 60Hz. > > I can easily convert our 240VAC to 115VAC with a step-down transformer, but > cannot easily supply 115VAC at 60Hz. So I was considering using a step-down > transformer to get the 115VAC, but modify the resonant "tank circuit" for > 50Hz. > > Unfortunately there is some magic I don't understand. The resonant > frequency of a LC circuit with L=2.3H and C=2uF is about 75Hz not the > expected 60Hz. > > Otherwise I could just solve the standard LC resonant circuit formula for C > and plug in 50Hz and 2.3H to get the required C. > > Obviously ferroresonant transformers are more complex than this former > software engineer can grasp. Could any experienced EE with relevant > transformer knowledge please chime in and help me understand how to > redimension the tank circuit to use 50Hz instead of the original 60Hz input. > > Obviously I could replace the entire power supply with two modern switch > mode supplies to create the two rails, but it would be really nice to keep > the original supply and just reversibly adapt it for 50Hz. That L is probably different when the transformer is excited at full mains voltage and near saturation. Of course, this seems like it will reduce L and therefore drive the resonance higher! What I might do is make up a cap bank that is 1.2 X larger than the 2 uF and power it up. Then, measure the output voltage, and if it is within range just use it like that. If you want to get fancy, put it on a Variac and sweep the input voltage. You will note a reverse slope, as input voltage rises through the range, output voltage will decline. Jon
