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<\/a><\/p>\nInside the H724 there are two Sprague metal-encased capacitor modules that are fastened to the rear panel. Each of these modules contains 2 x 0.1uF 1000 VDC capacitors. The terminals connect to the mains power (Active and Neutral) and the case connects to mains Earth. These capacitors are C500 and C501 and are shown on pages 81 (H724) and 82 (H724A) of the PDP-8\/e Computer Engineering Drawings<\/a>. The capacitors are not across the mains line full-time. They are between the relay K1 and the large power transformer (T1) and accordingly they are only in circuit once the front-panel key is in the ON position and the fans are running.<\/p>\n A copy of the H724 schematic, extracted from the PDP-8\/e Computer Engineering Drawings<\/em>, in available in the Links<\/a> section below.<\/p>\n I recently moved house and discovered that my PDP-8\/e instantly triggers the Residual Current Device that protects the single phase (230VAC) power outlets in my garage. If the RCD detects earth leakage (ie where the current drawn through the Active conductor does not match the current drawn in the Neutral conductor), the RCD trips and power is lost to the circuit breakers downstream of that RCD. The power outlets in the garage at my old house did not have this problem, as it was built before RCDs became mandatory where I live (Melbourne, Australia) for all new construction.<\/p>\n Kyle Owen was visiting me when this problem surfaced and was quick to identify the Sprague capacitors as being the cause. Kyle mentioned that he needs to use “unprotected” outlets in the U.S. for his 8\/e for the same reason. Using a Honda generator, we measured the earth leakage current to be 21mA on my H724A. The H724A earth leakage was sufficient to trigger the RCD protecting the garage, even though the RCD trip rating is 30mA. A quick Google search reveals that 30mA RCDs are required to trip by 30mA but may trigger at 15mA. I later discovered that my other 8\/e power supply (which is a H724 that I have converted to a H724A) had an earth leakage of 13mA at 230VAC and that this does not trigger the RCD.<\/p>\n I asked an Electrician friend of mine whether it would be possible to run an “unprotected” circuit to the garage to power my vintage computers. The answer was “No” – by law every new circuit installed must be protected by an RCD. This includes for example lights, washing machines, hot water services, ovens, air conditioners and machinery. No exceptions.<\/p>\n The simple solution is to remove the Sprague capacitors. This does however leave 11 partially-insulated 6.35mm spade connectors floating around inside the H724. I did this as a temporary fix, but decided something a little safer was needed.<\/p>\n So I’ve designed a small PCB that replaces the two Sprague modules. It is simple to fit, though it is necessary to remove the rear panel from the H724 to get adequate access. See picture on the right. Just disconnect and remove the two Sprague capacitors, and use the same screws and washers to mount the PCB. The 11 6.35mm spade connectors will align correctly with the PCB terminals (no need to bend or twist any of the wires).<\/p>\n The files needed to get this board manufactured (7 x Gerber files and 2 x Drill files) are available to download in the Links<\/a> section below. I had 10 boards made by PCBWay<\/a> for a total of US$8 plus US$20 delivery. They arrived from China 3 days after I placed the order.<\/p>\n The 6.35mm terminal are Multicomp PC250-836LT (Element14 1701613<\/a>) OR AMP TE Connectivity 726386-2 (Element14 4215618<\/a>).<\/p>\n I found a document by Evox-Rifa titled Capacitors for RFI Supression of the AC Line: Basic Facts<\/em><\/a>. It says that “… the Y capacitors generally must be kept to small values to limit the 50\/60Hz leakage current to ground. 4700pF is a typical Y capacitor value”<\/em>. <\/p>\n It is for this reason that I designed the PCB to optionally accommodate 4 x 4.7nF Class Y capacitors, on the underside. Keep in mind however that these are only 5% of the size of the original Sprague capacitors, so I query how useful they are at suppressing RFI, which I assume is the reason they were used in the H724. It is probably OK to just omit them.<\/p>\n The capacitors I used were EPCOS B81123C1472M289 (Element14 2797934<\/a>) which are 4.7nF 500V Y1 Safety Capacitors. It shouldn’t be too difficult to find other Class Y safety capacitors that can be used instead of this EPCOS part.<\/p>\n With the new PCB and 4.7nF capacitors fitted, the earth leakage current on my H724A dropped from 13.8mA to 0.7mA.<\/p>\nFixing the earth leakage problem<\/h4>\n
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