No reply from him as yet so I assume he's figured it out. It's also quite likely he posts here/reads the site. He buys off my site and if you hunt on google for this stuff, my threads on the mods come up before my site does, so people click through from here. Like I say this is a regular buyer too. His name has come up time and time again on my eBay sales in the past.
I mean no offense to this person and I understand the frustration when something like this happens. A while back every single PCE I was modding ended up producing nothing but severe noise and interference on both TVs in the house, green screening and buzzing. I tested a lot of equipment and chucked stuff out in frustration. They were doing it through SCART and the native AV. Then I noticed the fault was also on my unmodded stuff. It happened with different power supplies, different TVs, different cables, different power strips, different soldering irons/none at all in the case of unmodded. I wrote a bunch of consoles off as potentially unfixable as I could not find the source of the problem. I went into a state of absolute panic, and was hardly coherent in testing the stuff, nor did I have time to do thorough checks. I was in the middle of a huge order to be completed for a gamestore, and it was intense. Five consoles were "broken" by this fault. I gave up on plugging any more in for testing. I went to bed highly miserable. A day later, they were all working again! To this day I do not know what caused this ridiculous phantom problem. Why it only occurred on PC Engines I have NO clue, everything else was working. It's very easy isn't it, to assume the fault was caused by your TV, then your power strip, then your soldering iron, your cabling, you blame everything and anything and it's severely frustrating. So I can see why he would assume it was the mod, or the console. It's just impossible though. Obviously he isn't as aware of the fact it is impossible, or I imagine he would be taking his consoles apart and tinkering with them himself. The signal from the EXT port can't damage a CD ROM unless there's a short circuit somewhere, and as they share the +5V and ground when plugged in together, that short would blow the fuse on the console before you ever plugged it into the CD ROM. So if it's working fine by itself it can't wreck the CD ROM. Even if the short was on the PCE and you plugged it into the CD ROM before testing it by itself, the fuse would go anyway and there would be no damage done. Only way you could damage is if you replaced that fuse with a wire. The console would still fail to run by itself though!
I have nothing against my buyers of consoles, but this sort of stuff was why I took a long hiatus from selling consoles. PCEs are so modular, that if the fault is on the person's equipment the blame WILL go on your console, as you modded it. If you sell it on eBay it's especially bad, the buyer won't know you, will come out of the blue and be instilled with all kinds of "sellers on eBay are scammers" bias despite the fact that Paypal makes it so that most scammers on eBay are well, buyers. It made selling CD consoles completely impossible as 1/10 of drives would break thanks to overseas postal service. You could still sell and make a profit, replacing the 1/10, but was the threat of negative feedback worth it? Nope. I STILL don't sell CD consoles. The buyers can purchase a Core or a white engine and go get a Super CD ROM2 elsewhere off somebody braver than me. I have that philosophy, then something like THIS happens! Saying that though I never had a Super CD ROM2 break on me, but the weight posting them from overseas STILL cripples your profit and the tax on bringing them to the UK from a supplier kills the resale profit anyway. Oh well.