31 Dec
2017
31 Dec
'17
12:09 p.m.
For some time now I have had a couple of ZX Spectrum machines. Neither
works, each has a different problem. Today I got one of them out for another
look. The problem with it is that it seems to constantly reset itself, you
see the copyright screen briefly and then it disappears (the TV screen shows
the usual "snow" when there is no signal). It just cycles round doing this.
I put a logic analyser on it as well as a scope. The CPU Reset pin looks
fine, it goes high and stays high.
The logic analyser shows that it is happily reading code from the ROM during
the boot sequence and it will suddenly start fetching instructions from
address 0 again, this appears to be somewhat random. I don't have a lot of
experience with logic analysers, and I have found that some pod/probe
combinations cause the machine not to work at all, so the logic analyser is
not, unfortunately, above suspicion. However the behaviour I see appears
consistent with what the logic analyser is telling me.
My thoughts are that this must either be a bad Z80 CPU or a bad ROM. Neither
is socketed and I am reluctant to desolder ICs unless I really have to as
there is always a risk of damage to the board.
Has anyone seen a similar problem before? Could this indeed be the CPU or
the ROM, or could there be a different cause?
Regards
Rob
31 Dec
31 Dec
12:38 p.m.
On 12/31/2017 02:09 PM, Rob Jarratt via cctalk wrote:
For some time now I have had a couple of ZX Spectrum
machines. Neither
works, each has a different problem. Today I got one of them out for another
look. The problem with it is that it seems to constantly reset itself, you
see the copyright screen briefly and then it disappears (the TV screen shows
the usual "snow" when there is no signal). It just cycles round doing this.
I put a logic analyser on it as well as a scope. The CPU Reset pin looks
fine, it goes high and stays high.
The logic analyser shows that it is happily reading code from the ROM during
the boot sequence and it will suddenly start fetching instructions from
address 0 again, this appears to be somewhat random. I don't have a lot of
experience with logic analysers, and I have found that some pod/probe
combinations cause the machine not to work at all, so the logic analyser is
not, unfortunately, above suspicion. However the behaviour I see appears
consistent with what the logic analyser is telling me.
My thoughts are that this must either be a bad Z80 CPU or a bad ROM. Neither
is socketed and I am reluctant to desolder ICs unless I really have to as
there is always a risk of damage to the board.
Has anyone seen a similar problem before? Could this indeed be the CPU or
the ROM, or could there be a different cause?
It COULD be, but then it has to do a LOT of things right to
get the copyright message on the screen.
I'd be more likely to suspect a bad location or region of
the ROM than the CPU. Or, it could be noisy power
rails (maybe bad capacitors degrading) or some other
component in the address or data paths that
is no longer holding good logic levels or speeds or
something like that.
Jon
12:39 p.m.
On 31 Dec 2017, at 20:09, Rob Jarratt via cctalk
<cctalk at classiccmp.org> wrote:
For some time now I have had a couple of ZX Spectrum machines. Neither
works, each has a different problem. Today I got one of them out for another
look. The problem with it is that it seems to constantly reset itself, you
see the copyright screen briefly and then it disappears (the TV screen shows
the usual "snow" when there is no signal). It just cycles round doing this.
What is your PSU outputting? Sounds like it?s *just* borderline to feed the 7805 on input.
At least the 12v coil is OK so that?s one less thing to worry about - if it wasn?t you?d
not get a copyright screen at all. Remember the PSU is centre negative, it should be
putting out over 12VDC.
My thoughts are that this must either be a bad Z80 CPU
or a bad ROM. Neither
is socketed and I am reluctant to desolder ICs unless I really have to as
there is always a risk of damage to the board.
If either of those two are bad you?ll get static bars on the screen, they won?t cause a
reset like you?re seeing. I?d be checking power first.
Cheers
?
Adrian/Witchy
Binary Dinosaurs - Celebrating Computing History from 1972 onwards
1 Jan
1 Jan
3:46 a.m.
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
8:40 a.m.
What trigger evens does your analyzer have. Does it have a stop on trigger?
Dwight
________________________________
From: cctalk <cctalk-bounces at classiccmp.org> on behalf of Rob Jarratt via cctalk
<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
8:51 a.m.
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]
Sent: 01 January 2018 16:41
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
What trigger evens does your analyzer have. Does it have a stop on trigger?
Dwight
_____
From: cctalk <cctalk-bounces at classiccmp.org
<mailto:cctalk-bounces at classiccmp.org> > on behalf of Rob Jarratt via cctalk
<cctalk at classiccmp.org <mailto: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
9:22 a.m.
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
10:07 a.m.
On 01/01/2018 11:22 AM, Rob Jarratt via cctalk wrote:
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.
It could be caused by bus contention, which may also be the
cause of the resetting. Possibly some of the
decode logic has failed, and is allowing two devices to
drive the data bus at the same time. I've seen this
before, and it does cause spikes on the 5V rails.
Jon
10:34 a.m.
-----Original Message-----
From: Jon Elson [mailto:elson at pico-systems.com]
Sent: 01 January 2018 18:07
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
On 01/01/2018 11:22 AM, Rob Jarratt via cctalk wrote:
two
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.
It could be caused by bus contention, which may also be the cause of the
resetting. Possibly some of the decode logic has failed, and is allowing devices to drive the data bus at the same time.
I've seen this before,
and it
does cause spikes on the 5V rails.
That is interesting, I hadn't considered that. Given that it gets a fair way
into the boot sequence it would have to be an intermittent failure.
Regards
Rob
12:35 p.m.
Is there a random element to the rebooting, or is it a fixed specific
pattern?
Consider, . . .
if some high order bits of the address are non-functional, then when it
goes to location xxx000 , it starts over at 000000.
Not necessarily the most likely cause of the problem.
1:02 p.m.
-----Original Message-----
From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Fred
Cisin
via cctalk
Sent: 01 January 2018 20:36
To: General Discussion: On-Topic and Off-Topic Posts
<cctalk at
classiccmp.org>
Subject: RE: ZX Spectrum Z80 Keeps Resetting
Is there a random element to the rebooting, or is it a fixed specific
pattern?
Consider, . . .
if some high order bits of the address are non-functional, then when it
goes to
location xxx000 , it starts over at 000000.
Not necessarily the most likely cause of the problem.
Unfortunately it is random. To a point. When I did my trace with the logic
analyser I could see it was in a RAM testing loop and would fail part way
through the loop, failing on an address that previously worked.
However, it manages to get past that bit and gets to the point where the
copyright message is displayed. But it always resets at that point. I have
not been able to capture that point in the address trace to see if it is a
consistent address, but maybe that is something I can try with a trace that
triggers on a stop condition. I will try that this evening.
Regards
Rob
12:45 p.m.
devices to drive the data bus at the same time.
I've seen this before,
and it
does cause spikes on the 5V rails.
That is interesting, I hadn't considered that. Given that it gets a fair way
into the boot sequence it would have to be an intermittent failure.
12:56 p.m.
-----Original Message-----
From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Adrian
Graham via cctalk
Sent: 01 January 2018 20:45
To: General Discussion: On-Topic and Off-Topic Posts <cctalk at classiccmp.org>
Subject: Re: ZX Spectrum Z80 Keeps Resetting
It could be caused by bus contention, which may
also be the cause of
the resetting. Possibly some of the decode logic has failed, and is
allowing two
I?d be interested in putting my diagnostic ROM on that just to see what it
throws up. In the absence of having it in front of me so I can do that is the ROM
socketed? You can easily burn your own if so.
The ROM is not socketed. I have found that every time I desolder something from this board
I end up damaging the through holes, just the suction from my de-soldering gun was enough
to remove the "eyelet" today when removing a capacitor. I could socket it though
as that would be useful anyway, and I can check the ROM contents and make sure it reads OK
multiple times.
Regards
Rob
devices to drive the data bus at the same time.
I've seen this
before,
and it
does cause spikes on the 5V rails.
That is interesting, I hadn't considered that. Given that it gets a
fair way into the boot sequence it would have to be an intermittent failure. 
2 Jan
2 Jan
8:58 a.m.
Hello Rob.
The power supply voltage might be a bit too high.
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
10:18 a.m.
-----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
3 Jan
3 Jan
10:35 a.m.
I am beginning to wonder if the problem might be the ULA.
The reason I say this is that when I set up the logic analyser to *stop* on a zero ROM
address, it didn't. Which means it isn't resetting in the way I thought it was.
When I traced the instruction addresses during boot up I could see lots of resets back to
address zero, and assumed that somehow it was getting further into the sequence to show
the copyright screen, but then resetting again, because the screen would display the
"snow" after showing the copyright screen.
So if it isn't resetting then something may be just interrupting the video display,
and the machine just thinks it is showing the copyright screen and waiting for input. This
is further evidenced by the fact that the ROM addresses when it is in this cycle all seem
to be in the keyboard input area, based on a PDF that documents a disassembly of the ROM
that I found on the web.
If that was the case I thought it might respond to keypresses, but I checked and it
doesn't, and the same happens with a second keyboard from another machine. However,
the ULA is responsible for the video signal and for the keyboard input, so it suggests the
ULA, or something around it, could be the problem.
I have a second Spectrum (with a different fault that I think is the memory) that is a
later design. It has the 6C001E-6 ULA, the older machine (which appears to reset) has a
5C112E ULA. Looking at this page http://www.bytedelight.com/?p=41 the 6C001E-7 (not -6)
ULA can be used on older machines.
Does anyone know if, just as a test, it would be safe to put the newer 6C001E-6 ULA in the
older machine? Fortunately both ULAs are socketed, so this would be an easy test.
Thanks
Rob
4:30 p.m.
On Wed, 3 Jan 2018, Rob Jarratt via cctalk wrote:
The reason I say this is that when I set up the logic
analyser to *stop*
on a zero ROM address, it didn't. Which means it isn't resetting in the
way I thought it was. When I traced the instruction addresses during
boot up I could see lots of resets back to address zero, and assumed
that somehow it was getting further into the sequence to show the
copyright screen, but then resetting again, because the screen would
display the "snow" after showing the copyright screen.
As I recall with the ZX Spectrum there wasn't any "snow" on TV with a
subsequent reset (as when pressing the reset button on the Plus
variation).
Moreover DRAM refresh continued to work (presumably being driven by the R
register of the Z80 CPU according to its machine cycles, although at this
point I'd have to double check the details as my brain actually seems to
be missing the R register altogether), so unlike with a power-up reset,
where you'd initially get random rubbish on the screen, with a subsequent
reset you'd see a black rectangle (with thin vertical stripes IIRC,
meaning a pattern with a single bit set) instead as the part of memory
used for video data gets cleared in the DRAM initialisation process,
followed by the copyright notice you mention.
That you see "snow" instead is I believe indeed a strong indication that
something else goes wrong beyond (or instead of) just an intermittent
reset. And given that the ULA produces video output right away after
power-up with no need to program any device registers is I think an
indication that either the chip is broken or something else causes it to
malfunction, just as you have concluded.
FWIW,
Maciej
11:21 p.m.
As I recall with the ZX Spectrum there wasn't any
"snow" on TV with a
subsequent reset (as when pressing the reset button on the Plus
variation).
Moreover DRAM refresh continued to work (presumably being driven by the R
register of the Z80 CPU according to its machine cycles, although at this
point
I'd have to double check the details as my brain
actually seems to be
missing
the R register altogether), so unlike with a power-up
reset, where you'd
initially
get random rubbish on the screen, with a subsequent
reset you'd see a
black
rectangle (with thin vertical stripes IIRC, meaning a
pattern with a
single bit
set) instead as the part of memory used for video data
gets cleared in the
DRAM initialisation process, followed by the copyright notice you mention.
That you see "snow" instead is I believe indeed a strong indication that
something else goes wrong beyond (or instead of) just an intermittent
reset.
And given that the ULA produces video output right
away after power-up
with
no need to program any device registers is I think an
indication that
either the
chip is broken or something else causes it to
malfunction, just as you
have
concluded.
Thanks Maciej. Since I never had Spectrum when they were new, I didn't know
that this was how it behaved on a reset. I am glad that my analysis seems to
be on the right track (given what I know at the moment). I am going to
concentrate on the ULA and the video output, but given that the keyboard
also doesn't seem to work the ULA has to be the prime suspect at the moment.
I am still hoping someone will know if I can try swapping the ULA with a
newer one from a later model I have. I may also look at getting a NebULA as
they are not expensive.
Regards
Rob
4 Jan
4 Jan
3:04 a.m.
On 4 January 2018 at 08:21, Rob Jarratt via cctalk
<cctalk at classiccmp.org> wrote:
I am still hoping someone will know if I can try swapping the ULA with a
newer one from a later model I have. I may also look at getting a NebULA as
they are not expensive.
Possibly helpful...
https://spectrumforeveryone.com/technical/zx-spectrum-ula-types/
As well as NebULA, there's also SLAM:
https://www.sellmyretro.com/offer/details/slam,-zx-128-ula-replacement-boar…
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