I just replaced the last of the electrolytics, but the spikes on the Z80 5V rail have not gone away. They only exist on the 5V rail in close proximity to the Z80, other chips with a 5V supply don't have the spikes on the 5V rail.
The nearest capacitor is marked 473, which I assume means 47nF. It is not electrolytic and I believe not therefore susceptible to rising ESR with age. I don't know the actual type, it is axial, the centre of the case appears transparent. Surely this can't be the cause of the spikes, can it? Regards Rob From: Rob Jarratt [mailto:[email protected]] Sent: 01 January 2018 16:51 To: 'dwight' <[email protected]>; '[email protected]' <[email protected]>; 'General Discussion: On-Topic and Off-Topic Posts' <[email protected]> Subject: RE: ZX Spectrum Z80 Keeps Resetting Yes I believe it does. I am replacing some capacitors that I didn't replace the first time right now, so I will know soon if that was the cause of some of the noise on the Z80 Vcc rail. These boards are a nightmare for desoldering even simple through hole capacitors, the "eyelets" come away just from the suction of the desoldering gun. These must be cheap and nasty boards. Regards Rob From: dwight [ <mailto:[email protected]> mailto:[email protected]] Sent: 01 January 2018 16:41 To: <mailto:[email protected]> [email protected]; Rob Jarratt < <mailto:[email protected]> [email protected]>; General Discussion: On-Topic and Off-Topic Posts < <mailto:[email protected]> [email protected]> Subject: Re: ZX Spectrum Z80 Keeps Resetting What trigger evens does your analyzer have. Does it have a stop on trigger? Dwight _____ From: cctalk < <mailto:[email protected]> [email protected]> on behalf of Rob Jarratt via cctalk < <mailto:[email protected]> [email protected]> Sent: Monday, January 1, 2018 3:46:13 AM To: 'General Discussion: On-Topic and Off-Topic Posts' 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
