Just been given a Spectrum+

For experts to discuss very technical stuff and newbies to ask why the Spectrum they bought off ebay doesn't work.
SpaceFlightOrange
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Just been given a Spectrum+

Post by SpaceFlightOrange » Sun Aug 11, 2019 8:36 pm

Hi.

I've just been given a Spectrum+, boxed and complete (I believe) thats been sitting in a loft for years (usual story)

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It does turn on (I checked and tested the PSU before this) and I get the correct values either side of the regulator

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But the keyboard isn't working. Most of the keys don't respond and those that do output the wrong values. The ribbons to the membrane don't look healthy:

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So I've ordered a new one from Rich at RWAP (via SellMyRetro) as directed to by Retroleum, and I've also ordered a recap kit and a new regulator.

I've not switched it on since and won't be until its recapped.

I know its too early to tell but the incorrect values from the keys, could they just be a symptom of the dodgy membrane? or could they point to a faulty ULA?

Thanks
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1024MAK
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Re: Just been given a Spectrum+

Post by 1024MAK » Sun Aug 11, 2019 9:11 pm

Given that the copyright screen comes up and it responds to at least some keys, that means that most (if not all) of the ULA is functional.

The black on the ribbon tails is not an issue, that’s just tarnish (the metal film conductors are an alloy). The normal cause of problems is either cracks in the plastic (which is the only thing with any strength) hence the conductive metal strips also crack / break. Or problems where the plastic clamps hold the different membrane layers together.

Which keys work, which keys don’t work, and which produce the wrong results?

Before you answer, keep in mind that the original Spectrum keyboard only had 40 keys, most of the extra keys on the plus keyboard are done by having a multiple layer membrane with two different pairs of electrical contacts that make when you press one of these keys. If part of the membrane is broken, instead of seeing two pairs of contacts being made, the keyboard scanning code will only see one, hence the following keys may well produce the wrong result:
  • TRUE VIDEO
  • INV VIDEO
  • DELETE
  • GRAPH
  • EXTEND MODE
  • EDIT
  • CAPS LOCK
  • BREAK
  • ;
  • .
  • ,
Mark
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SpaceFlightOrange
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Re: Just been given a Spectrum+

Post by SpaceFlightOrange » Sun Aug 11, 2019 9:22 pm

1024MAK wrote:
Sun Aug 11, 2019 9:11 pm
  • DELETE
  • CAPS LOCK
  • BREAK
Ha! That makes complete sense! None of the normal character keys worked and to be honest I can’t remember every key that produced an incorrect response, but I’ve left the ones I do remember in the quote above.

Thanks Mark. Sets my mind at ease.
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SpaceFlightOrange
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Re: Just been given a Spectrum+

Post by SpaceFlightOrange » Wed Aug 14, 2019 10:36 pm

HI.

An update for you.

I've recapped the Speccy, and fitted a new RWAP Membrane, and all good. keys all seem to work now, and the RF picture is a lot more stable.
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I tested the voltages on pin 1, 8 & 9 of IC13 and the 3 rails were within range.

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Ignore the names; Steve is the guy that gave me the Spectrum, and Ian is a fellow retro fan.

I tried a few times to get Bomb Jack II to load with no luck (tape has been in a loft for ages), so I flipped it over and Bomb Jack loaded no problem
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However, after playing Bomb Jack for a few minutes, I hit reset, and it started up, but with ghosting to the right on the screen and it was a little bit fuzzy (sorry no pic) I also noticed that on startup a couple of times, there was a delay. also the heatsink was red hot.

I left it off for about 5 minutes and tried again, and all seemed ok.

Now I am using the original PSU, which seemed to be ok. Should I ditch this and get a new one?

Also, any tips on how I can run a soak test?

Thanks
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Ast A. Moore
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Re: Just been given a Spectrum+

Post by Ast A. Moore » Wed Aug 14, 2019 11:23 pm

SpaceFlightOrange wrote:
Wed Aug 14, 2019 10:36 pm
Now I am using the original PSU, which seemed to be ok. Should I ditch this and get a new one?
Definitely get a regulated 9V 1.4A (at least) PSU. Also, it wouldn’t hurt replacing the 7805 voltage regulator. While they are fairly robust and rarely fail, they’re also cheap and easy to source.

Keep in mind that the power socket on the Spectrum is center negative. Either get a PSU with a center-negative wiring on the plug, or get one with a center-positive wiring (those are much more common) and rewire it yourself. (That’s what I did with mine.)
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Every man should plant a tree, build a house, and write a ZX Spectrum game.

Author of A Yankee in Iraq, a 50 fps shoot-’em-up—the first game to utilize the floating bus on the +2A/+3,
and zasm Z80 Assembler syntax highlighter.

SpaceFlightOrange
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Re: Just been given a Spectrum+

Post by SpaceFlightOrange » Wed Aug 14, 2019 11:58 pm

Ast A. Moore wrote:
Wed Aug 14, 2019 11:23 pm
Definitely get a regulated 9V 1.4A (at least) PSU. Also, it wouldn’t hurt replacing the 7805 voltage regulator. While they are fairly robust and rarely fail, they’re also cheap and easy to source.
Thanks. I'll get one ordered. I've got a Traco TSR 1-2450 regulator. I'll fit it next week. It was getting a bit late to do it last night.
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1024MAK
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Re: Just been given a Spectrum+

Post by 1024MAK » Fri Aug 16, 2019 3:36 pm

SpaceFlightOrange wrote:
Wed Aug 14, 2019 11:58 pm
Thanks. I'll get one ordered. I've got a Traco TSR 1-2450 regulator. I'll fit it next week. It was getting a bit late to do it last night.
If you’re fitting a DC/DC converter (like a Traco) in place of the 7805 voltage regulator, you don’t need to replace the Sinclair ZX PSU.

The heatsink gets hot, because it’s doing it’s job of removing heat from the 7805 voltage regulator, and dissipating the heat into the air. Something that feels hot to a human hand may not be too hot for a semiconductor, which can operate with their internal temperature up to between 100°C and 125°C.

The nominal 9V Sinclair PSU is an unregulated type, hence when not at the full rated load, it’s output voltage will be substantially higher than 9V.

Typically when powering a ZX Spectrum, the voltage from the PSU will be between 10.5V and 12V. Most of the circuitry in a ZX Spectrum runs off a +5V rail. The 7805 voltage regulator does not have a problem as long as the input voltage is between 7.5V and 30V, as long as it can get rid of the heat it produces (that’s where the heatsink comes in).

7805 voltage regulator produces a nice clean, smooth and constant +5V by “cutting out” 5V from the input it is supplied with. The difference between the input voltage and 5V is “dropped across” the regulator. Typically a ZX Spectrum consumes between 550mA and 800mA. At say 600mA, width an input of 11V, that means the regulator has to get rid of (11V-5V) X 0.6A so the lost power is 3.6W hence why the heatsink gets hot. So this is well known :lol:

To improve on this situation, you can either use a regulated DC PSU, in which case we feed the Spectrum with a lower input voltage, then the 7805 and heatsink does not have to dissipate as much heat.

Or a DC/DC converter can be fitted. This uses switch mode power supply technology, and is far more efficient (better than 85%) compared to a 7805 running at 0.6A. This also means you don’t need a heatsink. The DC/DC converter does not care about the input voltage being between 10.5V and 14V, as it does not have to dissipate hardly any heat.

Mark
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Re: Just been given a Spectrum+

Post by SpaceFlightOrange » Tue Aug 20, 2019 12:30 am

Hi.

Thanks for the Advice, Mark.

I've replaced the regulator with a DC/DC converter. Nice and cool now!

However I noticed a few things im not sure about, both before and after.

Prior to the regulator replacement, I tried using a switch mode PSU (multi-voltage, set to 9v and centre neg) and it powered the Spectrum on, but the screen was a bit off-colour (This is a program from the manual):

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Swapping to the Sinclair psu resolved this:

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But when I replaced the regulator, the same thing happened, only once after about 15-20 minutes of use, during a power-cycle. Pulling the plug and leaving for a few seconds resolved this. Around this time I did discover the Sinclair PSU had a break in the DC cable. I've swapped the entire cable out now.

I loaded BombJack and noticed these lines on the cyan background, which I hadn't seen before:

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Am I reading too much into this? Bear in mind that this is still an RF output, I haven't done a composite mod yet.

I'm surprised at how hot the ULA can get. I'll order a heatsink for it, I think.

Sorry if I'm asking stupid questions.
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1024MAK
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Re: Just been given a Spectrum+

Post by 1024MAK » Tue Aug 20, 2019 2:08 pm

First, the easy one, the ULA will run hot. That’s normal and not something to worry about. What is hot to your finger, is not too hot for the semiconductor chip inside the ULA. Semiconductor chips can operate without problem with internal temperatures up to between 100°C and 125°C.

If you do decide to fit a heatsink, keep in mind that it will not have any benefit unless there is free airflow over it / through it (depending the the design). Certainly the rubber key case is poor in terms of airflow. The plus case is slightly better, but still not great.

Intermittent power connections can cause all sorts of strange behaviour.

Cheap switch mode power supply units (SMPSU) can have high frequency noise on the output, this can mess up the video circuitry. Worse, they often don’t have very good electrical safety.
Only use quality SMPSU that have been bought from a reputable supplier/ shop in your own country.

The 16K, 48K, and plus models have two separate oscillators, one for the ULA and one for the colour encoder chip. This along with the inexpensive circuit design causes colour distortion in between vertical lines with different colours. This will still be present on a composite video output.

Mark
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SpaceFlightOrange
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Re: Just been given a Spectrum+

Post by SpaceFlightOrange » Wed Aug 21, 2019 7:46 pm

Cheers Mark.

So, roughly translated: "Stop worrying, James, and just enjoy the thing!" :lol:

You are absolutely right. I worry about these things too much, in fact the RF output is far superior to my Beeb which has terrible flare whenever there's white on the screen! I use RGB so I'm not concerned.

The PSU issue was odd. It's a Supply from Maplin. Admittedly a few years old, but I've never had any issues with it.

I ordered a Smart Card V2 from Retroleum which arrived today, so I ran the memory, ULA, ROM and Z80 tests and all came back fine.

learning to have fun with it now!! :D

Thanks again.
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