ZX Spectrum Z80 Keeps Resetting

Rob Jarratt robert.jarratt at ntlworld.com
Tue Jan 2 12:18:26 CST 2018

> -----Original Message-----
> From: Thorhallur Ragnarsson [mailto:thorh at ismennt.is]
> Sent: 02 January 2018 16:59
> To: rob at jarratt.me.uk; Rob Jarratt <robert.jarratt at ntlworld.com>; General
> Discussion: On-Topic and Off-Topic Posts <cctalk at classiccmp.org>
> 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" <cctalk at classiccmp.org>
> To: "General Discussion: On-Topic and Off-Topic Posts"
> <cctalk at classiccmp.org>
> 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

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