Help needed with two Spectrums

For experts to discuss very technical stuff and newbies to ask why the Spectrum they bought off ebay doesn't work.
Post Reply
Macross_VF1
Berk
Posts: 6
Joined: Sat Nov 17, 2018 10:41 am

Help needed with two Spectrums

Post by Macross_VF1 » Sat Nov 17, 2018 10:57 am

Hello Spectrum enthusiasts!

I am need of assistance concerning two ZX Spectrums in my possession; A standard 48k model and a plus model.

The 48k model was bought recently and I knew it likely would work poorly. And sure enough, the power supply needed some work and the faceplate was an awful mess. Also, in order to even get a picture I had to AV-mod the unit. However, the machine doesn't boot properly and show a number of strange artifacts. I tried the procedure in the following video and from that I'm fairly certain that the voltage regulator is faulty. Is it possible to replace it or is the entire unit a lost cause?



Second, my ZX Spectrum 48k+. This one I actually got a picture from using the standard RF connector, but I did an AV mod anyway to improve the quality. It worked great but after that the keyboard doesn't work properly and several keys are either dead or do not give the proper symbol. I know the most likely culprit is the membrane and that is a rather easy replacement, but could there be something else? Something in the unit have to interpret the signals from the membrane, right? And if so, could I possibly have damaged something when I did the AV mod?
0 x

User avatar
1024MAK
Manic Miner
Posts: 269
Joined: Wed Nov 15, 2017 2:52 pm
Location: Sunny Somerset in the U.K. in Europe

Re: Help needed with two Spectrums

Post by 1024MAK » Sat Nov 17, 2018 2:23 pm

For your rubber key model:
Can you post a photo of the image on the display please.

Do you have a multimeter? If yes can you measure the voltages as per this post.
Note that issue 4 boards and earlier may have a slightly different arrangement for the VBB -5V supply, so it may be between -4.3V to -5V.

If the VBB -5V is missing, or outside the range -4.3V to -5.5V turn off immediately.

For your plus model the most likely reason for the keyboard not working is the membrane. Can you please list the keys that don’t work, or which produce the wrong symbol (also state what you are expecting and what you get).

Apart from the physical part of the keyboard, reset switch/button and case, the rubber key and the Plus use the same series of PCBs. The keyboard for both is based on a 8 x 5 matrix. The eight ‘drive’ lines scan the keyboard. These are derived from the Z80 CPU address lines A8 to A15 via diodes. When reading the keyboard, the Z80 CPU reads the data from the ULA, which senses the state of the five keyboard ‘read’ lines.

So if the computer is otherwise working normally, the only other likely candidates are a diode failure (very rare) or a ULA fault. Original ULA chips are no longer available. There is a VERY limited number of ULA replacements available.

Before even thinking about buying any new parts, carefully look at the membrane tails. Look to see if there is any cracking of the metallic conductors, or of the plastic. If the membrane looks new, remove the tails from the connectors on the PCB and check to make sure that there are no broken bits of old membrane left in the sockets. Also check to see if the screws for the clamps on the keyboard are tight.

Mark
1 x

Macross_VF1
Berk
Posts: 6
Joined: Sat Nov 17, 2018 10:41 am

Re: Help needed with two Spectrums

Post by Macross_VF1 » Sat Nov 17, 2018 9:47 pm

Very helpful stuff, thanks in advance even if the unit turns out to be a waste of time.

I read through pretty much everything that could be found in the page you linked to and did tests based on that:

The edge connector
Pin 3B/+5V: +7.53V
Pin 4B/+9V: +9.22V
Pin 20A/-5V: -7.47V
Pin 22A/+12V: +9.21V

4116 DRam
Pin 1-16: 0V (yes zero!)
Pin 8-16: +8.46V
Pin 9-16: +7.55V

7805 Voltage regulator
Pin 1: +9.24V
Pin 3: +7.58V

Also, here's a picture of how the boot screen looks. The pattern changes/flicker somewhat but overall that's what I get. It also changes to a pink hue from time to time.

Image

As for my other speccy, I'm pretty convinced by now that it is the membrane that is faulty, though it's great to hear someone else confirming it for me.
0 x

Macross_VF1
Berk
Posts: 6
Joined: Sat Nov 17, 2018 10:41 am

Re: Help needed with two Spectrums

Post by Macross_VF1 » Sat Nov 17, 2018 10:08 pm

I should probably also mention that I'm using a high quality replacement PSU for the unit. The original one gives 13V out and I would rather avoid having it fry the computer.

In fact I have noticed a marked tendency for old console and computer (I currently own 153 machines) PSUs to produce a lot higher voltage than their stated output which is the reason I'm trying to replace them.
0 x

User avatar
Ast A. Moore
Manic Miner
Posts: 875
Joined: Mon Nov 13, 2017 3:16 pm

Re: Help needed with two Spectrums

Post by Ast A. Moore » Sat Nov 17, 2018 10:46 pm

Macross_VF1 wrote:
Sat Nov 17, 2018 10:08 pm
I should probably also mention that I'm using a high quality replacement PSU for the unit. The original one gives 13V out and I would rather avoid having it fry the computer.

In fact I have noticed a marked tendency for old console and computer (I currently own 153 machines) PSUs to produce a lot higher voltage than their stated output which is the reason I'm trying to replace them.
That’s normal. They use liner (transformer) unregulated PSUs. Under load, their voltage drops to a reasonable value. Of course, having a modern switch-mode regulated PSU is usually beneficial, provided it’s a low-noise, high-quality one. It’ll take a great deal of load from the voltage regulator, and the latter will have to dissipate much less energy as heat.

That said, the voltage on the output of your 7805 is too high.
0 x
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.

User avatar
1024MAK
Manic Miner
Posts: 269
Joined: Wed Nov 15, 2017 2:52 pm
Location: Sunny Somerset in the U.K. in Europe

Re: Help needed with two Spectrums

Post by 1024MAK » Sat Nov 17, 2018 10:52 pm

Your 7805 voltage regulator is defective. Either replace it with a new 7805 voltage regulator (still requires a heatsink) or replace it with a RECOM or Traco Power DC/DC converter.

As the +5V supply to all the important chips has exceeded 6V, a number of these may have been damaged (nearly everything connects to the +5V supply).

Because the +5V is out of specification, the on board DC/DC converter/inverter formed around TR4, TR5 and ‘the coil’ will not work correctly, and it is very likely that TR4 and TR5 have been damaged.

As the -5V supply is missing, it is likely that some or all of the 4116 DRAM chips have been damaged.

If you need help with part numbers, shout out.

Mark
0 x

User avatar
1024MAK
Manic Miner
Posts: 269
Joined: Wed Nov 15, 2017 2:52 pm
Location: Sunny Somerset in the U.K. in Europe

Re: Help needed with two Spectrums

Post by 1024MAK » Sat Nov 17, 2018 11:14 pm

Ast A. Moore wrote:
Sat Nov 17, 2018 10:46 pm
Macross_VF1 wrote:
Sat Nov 17, 2018 10:08 pm
I should probably also mention that I'm using a high quality replacement PSU for the unit. The original one gives 13V out and I would rather avoid having it fry the computer.

In fact I have noticed a marked tendency for old console and computer (I currently own 153 machines) PSUs to produce a lot higher voltage than their stated output which is the reason I'm trying to replace them.
That’s normal. They use liner (transformer) unregulated PSUs. Under load, their voltage drops to a reasonable value. Of course, having a modern switch-mode regulated PSU is usually beneficial, provided it’s a low-noise, high-quality one. It’ll take a great deal of load from the voltage regulator, and the latter will have to dissipate much less energy as heat.

That said, the voltage on the output of your 7805 is too high.
I do wish people would stop using ‘linear’ to describe PSUs. The external Sinclair 9V PSU is a just a simple transformer/rectifier with a smoothing capacitor. There is nothing linear about it.

With this type of DC PSU, with no load connected (no computer) the electrolytic smoothing capacitor will charge upto the peak AC voltage available from the transformer secondary. With no load, the AC voltage on the transformer secondary will also be higher, between 10% and 20% (depending on the quality and size of the transformer). So 9V + 20% is 10.8V AC. Converting to the peak of the AC waveform √2 x 10.8 = 15.3V DC. This is the voltage that the electrolytic smoothing capacitor charges up to. So this is the voltage a meter will read when connected to the unloaded output plug.

When this type of PSU is connected to a load, the output voltage will drop down. How much this voltage drops, very much depends on how much current the load is using. At the rated maximum output current, the voltage should be no lower than 9V (but this is an average value, AC ripple not removed by the capacitor may cause the voltage to dip lower as the mains AC waveform drops as it crosses zero).

None of the above matters to the ZX Spectrum. Because inside on the main board, there is additional power supply circuitry. Namely the 7805 series pass voltage regulator. This provides a nice well regulated and stable 5V rail (well, it would, but yours is broken). Then TR4, TR5, ‘the coil’ and associated components then provide a semi-regulated +12V rail and a -5V regulated rail for the 4116 DRAM chips. The +12V rail is also used by the colour encoder chip.

[/RANT ENDS]

Mark
0 x

Macross_VF1
Berk
Posts: 6
Joined: Sat Nov 17, 2018 10:41 am

Re: Help needed with two Spectrums

Post by Macross_VF1 » Sun Nov 18, 2018 10:23 am

Nice rant! :D However you are probably far more correct than I could ever hope to even understand so I won't begrudge you it. ;)

Anyway, it certainly seems like the voltage regulator is doing unspeakable things to the computer. For now I'll shelve this unit and treat is as spares, but I might try to do some repairs in the future.

Thanks for the help!
0 x

User avatar
1024MAK
Manic Miner
Posts: 269
Joined: Wed Nov 15, 2017 2:52 pm
Location: Sunny Somerset in the U.K. in Europe

Re: Help needed with two Spectrums

Post by 1024MAK » Mon Dec 03, 2018 4:45 am

0 x

Post Reply