Spectrum+ No picture

For experts to discuss very technical stuff and newbies to ask why the Spectrum they bought off ebay doesn't work.
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spooney100
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Spectrum+ No picture

Post by spooney100 »

Hi guys

I'm having a problem with a Spectrum+ 48k, I got from Ebay. When its all plugged in it powers on but I'm not getting any picture at all. The RF cable and powerpack all work fine because I've used them on another Spectrum (48k Rubber keyed) with no problems, so its not the leads or the tv. I'm not tech savvy but did open the Spectrum to see if the Composite mod had been done but it hadn't and everything looks in order. I've tried pressing the reset button and other powerpacks (both the standard rubber keyed power pack and the + power pack) but nothing seems to work.

Can anyone help or is this Spectrum no good? :(
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1024MAK
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Post by 1024MAK »

Hello and welcome :D

Is the heatsink getting hot?

Is the ULA getting warm?

Are any of the other chips getting warm or hot?

Power it on and give it time before deciding. And by warm, I mean it’s warm to the touch, but not painful if you keep your finger on it. By hot, I mean you can’t keep your finger on it due to the heat. With chips, always test in the centre.

Can it be fixed? Almost certainly. But without knowing what is wrong, it’s not possible to say for certain. And finding out what is wrong is sometimes easy and sometimes rather time consuming. It all depends on what you want to spend on it in terms of time, effort and money.

Mark
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Re: Spectrum+ No picture

Post by dfzx »

Also, do you have any test tools? A multimeter? Or a logic probe?
Derek Fountain, author of the ZX Spectrum C Programmer's Getting Started Guide and various open source games, hardware and other projects, including an IF1 and ZX Microdrive emulator.
spooney100
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Re: Spectrum+ No picture

Post by spooney100 »

1024MAK wrote: Wed Jul 22, 2020 2:27 pm Hello and welcome :D

Is the heatsink getting hot?

Is the ULA getting warm?

Are any of the other chips getting warm or hot?

Power it on and give it time before deciding. And by warm, I mean it’s warm to the touch, but not painful if you keep your finger on it. By hot, I mean you can’t keep your finger on it due to the heat. With chips, always test in the centre.

Can it be fixed? Almost certainly. But without knowing what is wrong, it’s not possible to say for certain. And finding out what is wrong is sometimes easy and sometimes rather time consuming. It all depends on what you want to spend on it in terms of time, effort and money.

Mark
Hi Mark, thanks for replying. Unfortunately I'm pretty useless with electronics (but willing to learn). Is the ULA chip the big one in the middle? If so then yes it does get quite hot after a while to the point you have to remove your finger. I don't know where the heat sink is and I don't have any test tools at all, wheres the best place to get hold of them? I'm more then willing to try and fix it myself if its not too complicated. I know the keys work because when I press them I can hear them if that helps.
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1024MAK
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Post by 1024MAK »

Yes, the ULA (marked IC1 on the print on the board) is the only large chip that normally runs warm. The other large chips are the Z80 (IC2) and only on issue 5 and 6a boards, there is another large custom 40 pin chip (IC27).

I can’t tell you for certain what is marked on the actual chips, as it depends on which version or manufacturer Sinclair used at the time...

Which issue board do you have? It’s printed on the board near the front edge.

The heatsink is the large piece of aluminium bolted to the board.

Maybe you should take a photo of the board, use the ‘Add image to post’ link under the text edit box to upload and link to your photo.

Mark
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Post by 1024MAK »

spooney100 wrote: Wed Jul 22, 2020 4:05 pm I know the keys work because when I press them I can hear them if that helps.
Can you explain more by what you mean?
If the Z80 microprocessor is running the BASIC system from ROM okay, pressing a key (except the shift keys) will generate a key click from the speaker. Otherwise you may hear a difference caused only due to interference. This latter effect does not prove much on its own.

Mark
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Re: Spectrum+ No picture

Post by spooney100 »

Image

Yes the Heat sink gets quite hot too, the keys make a clicking sound when pressed which comes out of the Spectrum's speaker. The board is issue 45 or 4S?
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Post by 1024MAK »

That’s a 4S. You will likely find that on the bottom of the case, it will say made in the Republic of Korea by Samsung, like this one Image.

Be careful with the keyboard ribbon cables (membrane tails). They are easily damaged. The typical failure is the thin metallic conductor strips developing fractures and cracks, especially if the plastic is also suffering from going brittle.

So with the key click working, it appears the the Z80 microprocessor is running and the ROM is okay. That also means that at least some of the RAM chips are working.

For the next step, you really need to get a multimeter so that you can test some voltages.

Whereabouts in the world are you, and are you happy shopping online?

It would also help if you can take a close up detailed photo of the rear left hand side of the board in a well lit area. In order to help, we need to see the detail.

Mark
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Re: Spectrum+ No picture

Post by spooney100 »

Yes thats correct, made in the republic of Korea by Samsung is on the bottom.

Yeah I know the ribbons are fragile so I am being careful. I'm in the UK and shopping online is no problem for me.

I'm not sure if these pics are any good for you. Hopefully they are.
Image

Image
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Post by 1024MAK »

There are an amazing number of multimeters available. Unfortunately the really cheap meters available on trading sites like eBay are rather rubbish.

The TENMA 72-7770A (also branded as the UNI-T UT33D) is a reasonable quality low price basic multimeter that at least has some consideration of the users safety (unlike a lot the lower cost meters).

https://www.ebay.co.uk/itm/203028025814

https://cpc.farnell.com/tenma/72-7770a/ ... Multimeter (cheaper, and know good U.K. company, but unless you buy some other items, you will be subject to the handling charge).

There is also a used UNI-T UT33D being auctioned on eBay with a low starting price.

Mark
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Re: Spectrum+ No picture

Post by 1024MAK »

Before you buy a multimeter, have you tried using a phono to phono lead from the TV out on the ZX Spectrum to the composite video input (the socket normally coloured or marked yellow) on your TV?

And have you popped the metal cover off the modulator? The modulator is the silver tin can in the rear left hand corner with the TV out socket on it. To remove the cover, use a flat bladed screwdriver to lever up the top part of the metal case. If you need help, let us know. Never lever against any other component or against the PCB.

Mark
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Re: Spectrum+ No picture

Post by spooney100 »

I've tried phono leads to no avail. I have checked inside the modulator and it looks ok to me but I wouldn't know if anythings up with it to be honest so I've attached a pic for you to check.

My friends getting a multimeter, not sure how good its gong to be but its a BEVA digital multimeter apparently.

Image
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1024MAK
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Post by 1024MAK »

The photo is fine.

That modulator has definitely not been bypassed, so no composite video modification has been done.

Definitely need the services of a multimeter. If your friend has a logic probe or better yet, an oscilloscope, tell him that they would be helpful as well...

First, whatever the measurement, the negative / black meter lead / probe has to connect to a 0V/GND (ground) point (ground in this context does NOT mean mains earth, but the circuit common). Good places are the metal tab of the 7805 voltage regulator (the thing that the metal heatsink is attached to) or the case of the modulator.

Set the multimeter to the 20V (or equivalent) DC range. The computer does need to be powered up for these tests.

With the positive / red probe, carefully (don’t slip) test the following points:
  • IC14 (LM1889) pins 14, 15, 16 (expected voltage is between +11V and +12V). DIL chips (as fitted to a ZX Spectrum) count anti-clockwise, see this topic for an example showing the Z80A microprocessor.
    We don’t have a board layout for the issue 4S board, but it is based on the earlier issue 4A, here is the layout for the issue 4A
  • Positive lead of capacitor C44 (expected voltage is +12V). This type of capacitor is an electrolytic, they have a black or white band marked with minus signs (-), the band will have arrows pointing to the negative lead of the capacitor. There may be a plus symbol marked on the PCB next to the positive (+) lead.
  • Positive lead of capacitor C45 (expected voltage is between +11V and +12V).
    C44 and C45 are in the front centre area of the board.
  • Wire going into the modulator nearest to the front of the board (expected voltage is +5V). This wire is on the left hand side of the board / modulator case.
  • Wire going into the modulator nearest to the back of the board. Again on the left hand side.
Report back the voltages that you actually have.

Mark
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Re: Spectrum+ No picture

Post by spooney100 »

Hi Mark

I've tested the connections and it looks like theres a few issues. the Voltage in each are as follows.

IC14 :- 14 - 11.55, 15 - 3.53, 16 - 11.51
C44 :- 11.88
C45 :- 11.55
Modulator front Wire :- 3.52
Modulator Rear wire :- 0
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Re: Spectrum+ No picture

Post by spooney100 »

Ignore my last response Mark, I checked the wring pins on the IC14.

The correct readings are below.

IC14 :- 14 - 11.53, 15 - 11.53, 16 - 11.53
C44 :- 11.88
C45 :- 11.55
Modulator front Wire :- 3.52
Modulator Rear wire :- 0
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1024MAK
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Post by 1024MAK »

So the 12V supplies and devices are fine.
But the analogue 5V is low and there is no video signal.
Modulator front Wire :- 3.52
Modulator Rear wire :- 0
I would expect between 4.5V and 5V on the front wire to the modulator, for example on a issue 3 board, I get 4.94V. This is the 5V supply to the modulator and to the video output stage transistors.

The video to the modulator is via the rear wire and should be around 2 to 3V DC when using most multimeters.

Measure the voltage on both leads of resistor R64 (15Ω). It’s around the middle of the left hand row of resistors.

Same on resistors R51 (2.2kΩ) (near TR1) and R48 (4.7kΩ) (in the same row of resistors as R64).

Are TR1 and TR2 cool or is one warm? They are near the modulator.

Mark
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Re: Spectrum+ No picture

Post by spooney100 »

Readings are as follows:-

R64:- L - 4.90, R 4.77
R51:- L 3.75, r 0.79
R48:- L 1.95, R 1.39

TR1 is cool but TR2 is hot.
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1024MAK
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Post by 1024MAK »

TR2 should not be hot.

If you can use a soldering Iron, carefully disconnect both wires going into the modulator from the PCB. Then power up and see if TR2 still gets hot. If it does, there is a good chance that it’s defective. In which case it will need to be replaced with a new transistor.

Mark
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spooney100
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Re: Spectrum+ No picture

Post by spooney100 »

Hi Mark

Yeah it looks like TR2s defective, I've disconnected the 2 wires going into the modulator and its still getting hot. I'll have to get it replaced. Thanks for all the help. Its massively appreciated.
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Re: Spectrum+ No picture

Post by 1024MAK »

Once TR2 has been removed, with the power disconnected from the computer, use the resistance ranges on your multimeter to test the various resistors in that area as well.

Mark
:!: Standby alert :!:
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :dance
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