> -----Original Message----- > From: Thorhallur Ragnarsson [mailto:[email protected]] > Sent: 02 January 2018 16:59 > To: [email protected]; Rob Jarratt <[email protected]>; General > Discussion: On-Topic and Off-Topic Posts <[email protected]> > Subject: Re: ZX Spectrum Z80 Keeps Resetting > > Hello Rob. > > The power supply voltage might be a bit too high. >
Actually I tested that voltage with the PSU not under any load. I just tested it connected to the machine and the voltage is 10.15V. Still a bit more than 9V though. I'll give it a go with my bench PSU a bit later and see if it works any better at 9V. > Early Spectrums expected a "9V" DC power supply. > The DC/DC converter that makes +12V, +12VA, -5V and -12V(AC) in some early > versions (up to Issue 3, IIRC) stops working if the input voltage is too high. > > Happened frequently around midnight here in Iceland when one had typed in > (but not saved) a long program, because normal people then turned off their > lights, and the mains voltage went a bit up :) > > Best regards > > Thor. > > > ----- Original Message ----- > From: "Rob Jarratt via cctalk" <[email protected]> > To: "General Discussion: On-Topic and Off-Topic Posts" > <[email protected]> > Sent: Monday, January 1, 2018 11:46:13 AM > Subject: RE: ZX Spectrum Z80 Keeps Resetting > > Replying to my own mail to consolidate my answers to the two very kind > responses I got. > > In answer to Adrian: Regarding the PSU, I actually have two Spectrums, the > same PSU seems to power the other one OK. I quickly checked it and it is > outputting 13.4V and there is no ripple to speak of. So I think the PSU is OK. > > In answer to Jon: I did look at the power rails. The output from the 7805 > looks > absolutely fine and the inputs to some of the ICs looks fine. However the Vcc > input to the Z80 did look a bit noisy, I found there are quite a few spikes, > their > amplitude appears to be 600mV. I temporarily added a 3.5uF capacitor I > happened to have lying around, this reduced the amplitude of the spikes to > about 200mV, but didn't affect the behaviour. I am not sure if these spikes > could cause the reset behaviour though. I suppose the spikes could mean either > there is a faulty IC (finding that won't be easy), or there is a bad capacitor > somewhere. I did replace most of the electrolytic ones, but not all of them, > so > that is probably a good line of inquiry. > > I don't think it will be a bad memory location/region in the ROM though > because a lot of the resets occur in a loop, so it can read the locations, > although > I suppose it is possible that the logic levels on the address/data paths > could be > marginal and occasionally resulting in bad data. My next step was going to be > to discover how to get my logic analyser to capture the addresses *and* the > resulting data, but I think I will double check the capacitors first. > > Happy New Year! > > Rob
