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replacement for DEC AC input Y-capacitors, aka common-mode capacitors, aka bathtub capacitors

gwiley

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Nov 12, 2021
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Location
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Does anyone remember this post? There was a post somewhere on the forum about replacement parts for the AC input common mode filter capacitors, or Y-capacitors, or "bathtub" capacitors, in an older DEC machine. I think it was by someone who posts often, and they recommended that the filter capacitor module be replaced because it's prone to failure. There was also a link to a specific replacement part on Mouser or some other parts distributor, which is what I'm looking for. The thread might have been related to a PDP-8/L or 8/I, but it could have been about some other DEC chassis. The thread topic may have been about something else, but there was a comment about the capacitors with a link to the replacement part. I think the H724 supply in an 8/E may use the same or similar Y-cap, C500 & C501.

I've tried the advanced search function but either come up with nothing or too much.

I think the post might have also mentioned that some rather spectacular failure modes have happened. I'd rather just replace them if parts are readily available, or I could just disconnect them until they've been verified.

Here's a photo & schematic to clarify what I'm talking about. There are a pair of these capacitors in an 8/L which I've not yet connected to AC power.
input Y capacitors.jpg
This is the back side with quick connect terminals to AC hot and neutral. The common point between the two capacitors is connected to the case.
input Y capacitors terminals.jpg
It's C1 and C2 in this schematic
Power Supply 718 schematic.jpg
 
I understand that they are "there for a reason" in Enterprise/Industrial applications, but what about just disconnecting them?
Is there enough of an EMI issue (etc.) to be concerned about in a Hobbyist application?

Pete
 
Does anyone remember this post? ...

Malcolm MacLeod made a small replacement PCB with the same foot print as the original "bathtub" capacitors used in the PDP-8/e. It is populated with 16 spade connectors and 4 Y-class filter capacitors nicely replacing the two original side-by-side mounted "bathtub" capacitors. The originals all have excessive leakage current to ground and trip the RCD circuit breaker (safety switch).

Malcolm has a nice write-up including gerbers at: https://avitech.com.au/?page_id=3585

If the 8/L side-by-side dimensions of the "bath-tub: caps don't exactly match the PCB, you could just cut two of the PCBs (cut a bit off-center) such that it you end up with two individual dual cap PCBs and with the narrower left over PCB section discarded.

JLCPCB makes these quickly for very little money and you get them in a few days via DHL.

Tom Hunter
 
Safety capacitors are readily available today (well, as available as any electronic component, given the component crisis and all) and should be replaced with working ones.
 
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