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TonyDuell · 27th Apr 2020, 11:49 am

Yes, an earth problem, but not in the Tandy (which was fine). The problem was an open signal ground (pin 7 of the DB25) connection at the wall socket. The terminal server had signal ground earthed to mains earth, as did the VT100, so the circuit was completed that way. The Tandy Model 100 runs off internal batteries so it wasn't earthed. The bench supply did indeed have the -ve side connected to mains earth, so that also completed the circuit when I ran the buffer off that supply. But as you guessed the wall wart was isolated.

Once I'd realised what was going on, I unscrewed the DB25 plate from the wall, spotted the wire hanging off, and resoldered it.

27th Apr 2020, 11:49 am	#27
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,225	Re: Maplin Z80 SBC Yes, an earth problem, but not in the Tandy (which was fine). The problem was an open signal ground (pin 7 of the DB25) connection at the wall socket. The terminal server had signal ground earthed to mains earth, as did the VT100, so the circuit was completed that way. The Tandy Model 100 runs off internal batteries so it wasn't earthed. The bench supply did indeed have the -ve side connected to mains earth, so that also completed the circuit when I ran the buffer off that supply. But as you guessed the wall wart was isolated. Once I'd realised what was going on, I unscrewed the DB25 plate from the wall, spotted the wire hanging off, and resoldered it.