Trying to fix my ZxSpectrum+

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Hyuma
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Trying to fix my ZxSpectrum+

Post by Hyuma »

Hi guys, I bought a ZX spectrum+ on some flea market here in Italy and it requires some fix. I would like so much understand what's wrong... Here a picture of the issue:

https://m.imgur.com/gallery/AAWO968

The board it's an issue 6a, I only made the video mod and that's the result. Actually I don't have the multimeter with me, but when I came back home I could make all possible tests. Thanks!
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1024MAK
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Re: Trying to fix my ZxSpectrum+

Post by 1024MAK »

It could be a number of things at fault, including but not limited to:
  • ULA Faulty
  • RAM fault
  • Damage to the PCB
You may need more than a multimeter to fix this.

Mark
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Re: Trying to fix my ZxSpectrum+

Post by Hyuma »

Thanks! Next week I'll upload new pictures and keep you all updated on my step trying to fix that! Thanks again :D
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Muttley Black
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Re: Trying to fix my ZxSpectrum+

Post by Muttley Black »

Like [mention]1024MAK[/mention] says, it is not so simple. Anyway, the mainly problems that ZX Spectrum's have, are the faulty lower & upper ram IC's. It's very difficult to find a unit with no issue in ram IC's. In case you have faulty lower ram IC's, probably you have also faulty the transistors TR4 / TR5 or both. Except all that, we have also in game the ULA & CPU, that both appear to have frequent problems. Even if you find the problem, you should also replace all electrolytic capacitors to have a healthy unit. Not because that will fix your spectrum, but because 35+ years old capacitors need to be replaced. Also it is true that some issues start from there.

As you power on you machine for several minutes... before asking :) , we can skip part of initial tests but do some of them. i can drive you, but no too far. But there is some important questions first.
  • Do you have soldering iron / desoldering pump and if yes, what is your soldering skills?
  • Do you have an other working spectrum?
If you don't have the basic skills for this, it is better to take it to a technician. ZX Spectrum pcb's are not recommended to beginners on electronics. You can easily do big damage on traces if you need to work in there.
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Re: Trying to fix my ZxSpectrum+

Post by Hyuma »

Yes I have soldering iron and pump and I have fixed an old C64 replacing the pla and one voltage regulator. I'm thinking to buy some oscilloscope kit from Amazon as a starting kit before a real one oscilloscope :)
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1024MAK
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Re: Trying to fix my ZxSpectrum+

Post by 1024MAK »

Keep in mind that oscilloscope kits and cheap (inexpensive) new oscilloscopes are mostly not suitable for helping with the repair of microcomputers like the ZX Spectrum.

There is some guidance in this post ;-)

Beyond a good digital multimeter, the next items of test gear that I recommend are a logic probe (or you can build one) and a bench power supply unit (with a regulated adjustable output control and a current limit, better still, a adjustable current limit).

Mark
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Muttley Black
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Re: Trying to fix my ZxSpectrum+

Post by Muttley Black »

Good to hear.

Just for the story, before start the test's, keep in mind that the best way to check an unknown condition zx spectrum in the future, is to do some test before plug it on power supply. Cause you can easily do farther damage to it. That's tip for the next time as you already may pass that step.

Important:

So, if you left your spectrum on power, for several minutes, do the following test's. If you did the composite mod and just switch on your spectrum for several seconds to test the results and then switched of your spectrum immediately and never switch on the power again, skip the following steps and do ONLY the test's from the video you find at the end of the post.


Steps in case you left your spectrum on power for several minutes:

You need to test some dc voltages, and after that you post the results here.
  • First of all, test the output of your power supply "unplugged" from your spectrum . You should get 9+ volts with the negative probe on the center.
  • Then with your power supply plugged in, test the voltage regulator. With the negative probe to the center pin, measure the two other pins. You should get 9+volts and 5 volts.
  • finally test the voltages on IC6 (lower ram). With the negative probe to heatsink (the aluminium plate that absorb the heat from voltage regulator), check the voltages on pins,1,8,9 and 16.
You should get:

PIN 1: -5 volts
PIN 8: +12 volts
PIN 9: +5 volts
PIN 16: 0,00 volts

If you missing voltages on pins 1 and 8, you have faulty transistor TR4/TR5, or both. But simply change them with new ones, don't fix the problem, because missing that volts means also faulty lower ram IC's.

Tell us the results from your test's.


Steps in case you left your spectrum on power for several seconds:

[media]https://www.youtube.com/watch?v=IzgCmldm2H4[/media]
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Muttley Black
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Re: Trying to fix my ZxSpectrum+

Post by Muttley Black »

Muttley Black wrote: Sun May 19, 2019 10:54 am
  • First of all, test the output of your power supply "unplugged" from your spectrum . You should get 9+ volts with the negative probe on the center.
Because that step may be confuse you. When i say "9+ volts" i mean 9V or greater rather than +9 volts. The original zx spectrum power supply is unregulated and the output voltage depending on how much current the computer draws. So, with the power supply not connected, the output voltage is between 12V and 16V. With the power supply connected to the computer should drop around 10 volts and 12 volts, but not below 9 volts.
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Re: Trying to fix my ZxSpectrum+

Post by Hyuma »

Ok, on Monday or tuesday I'll do all tests and post here the results! There is a guide to build the logic probe? I would like to do that too!
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Re: Trying to fix my ZxSpectrum+

Post by Hyuma »

Made some preliminary tests with multimeter:

Image

Image
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Ast A. Moore
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Re: Trying to fix my ZxSpectrum+

Post by Ast A. Moore »

Either the voltage converter circuit is borked, or one (or more) of the lower RAM ICs is messing with the -5V and +12V rails. Test TR4 and TR5 (ideally, out of circuit). Replace, if faulty, and retest the voltages.

What’s your board revision, Issue 3 or 3B?
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.
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Re: Trying to fix my ZxSpectrum+

Post by Hyuma »

this is my video mod i've done:

Image
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Re: Trying to fix my ZxSpectrum+

Post by Hyuma »

Ast A. Moore wrote: Mon May 20, 2019 1:12 pm Either the voltage converter circuit is borked, or one (or more) of the lower RAM ICs is messing with the -5V and +12V rails. Test TR4 and TR5 (ideally, out of circuit). Replace, if faulty, and retest the voltages.

What’s your board revision, Issue 3 or 3B?
Mine is 6a, how can I test the TR4 and TR5 and which are some equilavents parts? Thanks!
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Muttley Black
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Re: Trying to fix my ZxSpectrum+

Post by Muttley Black »

Hello.

Quick post cause I am from mobile and at work. Check the video I posted on my previous post. He saw you the way how to test TR4 & TR5 transistors. Post here the results.
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Re: Trying to fix my ZxSpectrum+

Post by Hyuma »

TR5 ----> 0.77V
TR4 ---> 11.94V
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Re: Trying to fix my ZxSpectrum+

Post by Muttley Black »

You need to do them with your multimeter set on "diode test". See the test on the video carefully. You should post 4 measurements for every transistor. Zx spectrum should not be connected with power supply.
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Re: Trying to fix my ZxSpectrum+

Post by Hyuma »

Image

TR4:

.628
.003
1.018
.003

TR5:

1.343
1.125
.740
.745

I tested without power connected and multimeter configured as in picture. I made the 4 measurements for each transistor by first putting the red probe in the center and I measured on the other two pins. then I repeated the operation putting the black probe in the center.
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Muttley Black
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Re: Trying to fix my ZxSpectrum+

Post by Muttley Black »

  • Much more technical educated members, can correct me if i am wrong.


Both your transistors TR4 & TR5 are defective. This happens cause of one, or more faulty 4116 (lower ram) IC's. You can replace TR4 with ZTX651 transistor & TR5 with ZTX751 transistor. You can buy them from Retroleum.

Replacing the transistors:

Now, replacing both transistors with new ones, while the "faulty" 4116 IC's are still one the board, for me it's a bit risky. Even a single "faulty" 4116 IC can draws about 1 amp of current. This means that many faulty 4116 IC's can easily blow the new transistors again. What i do, is to remove all the lower ram IC's from the board first. Then, replacing the new transistors and with none lower ram IC's fitted on the board, I recheck the voltages, this time on pcb that Pin's 1,8,9,16 was fitted. With the voltages back in game, (-5 and +12) i am ready to test the IC drams one, by one.


Testing the 4116 ram's:

With multimeter: With IC's out of the circuit as i mentioned above and with you multimeter "set" at 20k ohms, test pin1 with pin16. You should get more than 11k ohms.. Next, test pin8 with pin16. You should get more than 20k ohms.
Any values below (in both cases), means that dram is probably faulty.

With Arduino tester: In case you want to make one.


About replacing the faulty Dram IC's:

I go a bit back in the step of desoldering the IC's. I need to say that this part is a bit tough. "Cheap" ZX Spectrum PCB's don't easily forgive. Removing old IC's its a pain even for experienced people. You can damage trace's if you are not carefully enough. Some times after the desoldering pump, i use desoldering air gun and IC puller for making the extracting easier. Put sockets on every new IC! If you are ready to solder the new lower ram iC's, don't forget to put sockets. Next time that you may have a faulty ram, you can replace it in seconds!

You have three options for lower ram replacements. New 4116, modded 4164 and final a Lower RAM Replacement Module. We can discuss that when you are ready.


That's for now. After the new lower ram & transistors fitted we see what next.
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1024MAK
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Re: Trying to fix my ZxSpectrum+

Post by 1024MAK »

One known cause of damage to TR4 is an expansion (such as a joystick interface) being disconnected from, or connected to, the edge-connector while the ZX Spectrum is powered up.

As said above, a faulty 4116 (or equivalent) DRAM chip (forming the “lower RAM”) can also damage TR4.

In fact, any overload of the +12V supply rail can damage TR4.

As well as TR4, the transformer (known as “the coil”) can also be damaged.
If replacing TR4, it is recommended to replace TR5 at the same time. Also renew electrolytic capacitors (if not already renewed):
C44 100uF 16V
C45 100uF 16V
C46 1uF 50V or 63V
C79 1uF 50V or 63V
C80 22uF 25V

A further problem, is that if the computer is powered up and the -5V supply is missing or out of specification, this can damage the 4116 chips.

As it is easier to renew two transistors compared to removing eight DRAM chips, and the transistors are low cost, I recommend renewing TR4 and TR5 first.

Either get someone to help you, or use a suitable meter probes/clips so that you can monitor the -5V voltage when you power up the computer. At the same time, place a finger on the new TR4 transistor. Power up and watch the -5V supply on the meter. If this does not go to -5V (between -4.75V and -5.25V) after power up, or if you feel TR4 getting hot, switch off / disconnect the power immediately.

Failure of the -5V or TR4 getting hot are indications that either there is a short circuit fault (likely one or more faulty 4116 DRAM chips), a short on the +12V supply somewhere, or the “coil” is damaged.

If after power up, the -5V supply is okay and TR4 does not get too hot to touch, wait for a bit (say 60 seconds) to check. Then (making sure you don’t accidentally short anything out), test the +12V supply.

If both the -5V and the +12V are now okay, see what display is being generated.

Note that fault finding the issue 6A board is slightly different to issue 1 to 4 boards.

Mark
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Re: Trying to fix my ZxSpectrum+

Post by Hyuma »

Ok guys, it's been a while since last time I posted something. Today I removed the two ZTX

Image

ZTX 650 and ZTX 213. So I have to buy Ztx651 and Ztx651 right?
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1024MAK
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Re: Trying to fix my ZxSpectrum+

Post by 1024MAK »

No.

TR4 (which was a ZTX650) should be replaced with ZTX651 transistor or a ZTX653 transistor. All these are NPN type.

TR5 (which was a ZTX213) can be replaced with a ZTX751 transistor. Although many small signal TO92 cased PNP transistors will also work in this position, but the pin-out of the legs may be different).

It is important not to mix up the NPN and PNP types (unless generating magic smoke is your thing!)

Mark
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Re: Trying to fix my ZxSpectrum+

Post by Hyuma »

Removing ram was a nightmare and there were serious damage:
the RAMs hascrumbled when I pulled them out and some of the tracks were broken.

Image
Image
Image
Image
Image

what can i do, now?
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Muttley Black
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Re: Trying to fix my ZxSpectrum+

Post by Muttley Black »

Sad! The sure think is that now you know how much careful you have to be with spectrum boards. You put a lot of force to pull the IC's out from the Pcb. In most case's there is solder even in the components side of the board. That's mean in IC leg's by looking it from the component's side of pcb. IC legs need to be clean of solder in both ways.

Anyway DONT PANIC! I did the about same damage in my first attempt fixing a ZX Spectrum. You can fix that.
Below you can read what i did, but some other may have a better way. So please be patient and wait for more opinions.
  • First thing to do is to cut with "Hand-held Paper Cutter" the traces that are "unglued" from the pcb. Just to that point! No farther!
  • Desolder again every IC hole, by put a bit of solder on them and use your desoldering pump to suck every hole, to clean them good. Need to have every hole clean and open.
  • Clean good the area with soft toothbrush and isopropyl alcohol, both solder & components side, and let it dry.
  • Buy "AWG 30 wrapping wire" take of the plastic insulation and start fixing the broken traces by hole to hole. Keep the wire a bit stretch under the board, that's help you work better when you want to "drive" the wire in old trace path.

Image
  • Take a very small paintbrush and an "transparent nail varnish" and with this way glue the traces with the pcb. Need to be careful here. Don't put nail varnish on the hole's area and also your "new" traces don't touch other traces.
  • After a good dry, take a continuity test with your multi-meter. Also check for sort's if any.
  • When you are sure that all is as need to be, start putting the sockets. In this case you need to put dual wipe sockets to be able to fit with no problems the IC leg and the wire.

Image
  • Now you just need to solder the sockets and the edge's from the wire's.
Image


Here is the board finished.

Image



This is the "hard" way. The easy way is to fix this damage on the solder side of the board. But i don't like that way.
If there is a step that you didn't understand please ask. I say that cause my English sucks! :)
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Re: Trying to fix my ZxSpectrum+

Post by Hyuma »

thank you! I understood your step, next days i'll try to fix the mess! If anyone use discord i'll glad to chat with someone just to have a moral support :(
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1024MAK
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Re: Trying to fix my ZxSpectrum+

Post by 1024MAK »

Just to add to what Muttley Black posted.

When I am replacing missing / broken tracks, I use “5A fuse wire”. This is uninsulated tinned copper wire that has a diameter of 0.2 mm. In the U.K. it is sold in electrical stores on a cardboard card. Or you can extract the strands from insulated flexible stranded equipment wire (sold as 7/0.2mm, 16/0.2mm or 32/0.2mm types where the first number is the number of wire strands and the second number is the diameter of each strand)l

As said above, cut off any bits of track (trace) that are free and which could move and cause a short circuit. I use a sharp “cut-off” knife for this.

I then thread the wire through the hole of the chip/component, pass it over the board, then back down the next chip/component. I use a drop of superglue to hold the wire where it has to bend (use a tool such as a flat bladed screwdriver to hold the wire, and a wood cocktail stick to position the small drop of glue). Then after the glue is dry and I have fitted all the wires, I use a PCB lacquer pen to hold and protect the wires. However these are very hard to get hold of and are not cheap :( This small bottle dispenser appears to be the nearest equivalent product.

Note that if the component hole has lost its through hole plating, leave the wires long enough to wrap around the legs/pins of the socket. Also if the pad/track on the top side is intact, but the pad/track on the bottom side is missing, the wire will need to be carefully soldered to the top pad/track using just enough solder, but without getting any solder into the through hole.

I myself would fit turned pin sockets. Looking at the state of the old 4116 DRAM chips, I would not bother trying to reuse those. Either obtain some new old stock, or some “pulls” (preferably chips that have been tested), use a RAM replacement module, or modify the board and use 4164 DRAM chips.

More later...

Mark
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