ZX Spectrum Z80 Keeps Resetting

[Rob Jarratt]

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:robert.jarratt at ntlworld.com] 
Sent: 01 January 2018 16:51
To: 'dwight' <dkelvey at hotmail.com>; 'rob at jarratt.me.uk' <rob at jarratt.me.uk>;
'General Discussion: On-Topic and Off-Topic Posts' <cctalk at classiccmp.org>
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:dkelvey at hotmail.com> mailto:dkelvey at hotmail.com] 
Sent: 01 January 2018 16:41
To:  <mailto:rob at jarratt.me.uk> rob at jarratt.me.uk; Rob Jarratt <
<mailto:robert.jarratt at ntlworld.com> robert.jarratt at ntlworld.com>; General
Discussion: On-Topic and Off-Topic Posts < <mailto:cctalk at classiccmp.org>
cctalk at classiccmp.org>
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:cctalk-bounces at classiccmp.org>
cctalk-bounces at classiccmp.org> on behalf of Rob Jarratt via cctalk <
<mailto:cctalk at classiccmp.org> cctalk at classiccmp.org>
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