Issue 4a docs needed

eccles
Posts: 26
Joined: Tue Dec 24, 2019 10:37 pm

Re: Issue 4a docs needed

Post by eccles »

Not getting too far here - not convinced the cheap and nasty Hantek USB oscilloscope I purchased is either working or any good, since it doesn't ever appear to show square waveforms, only a constant DC power level - will see if I can test it on something known good

But some voltages:
ULA IC1 pin 14 - 4.94v (pin 13 is 3.6v if relevent)
14,15,16 on LM1889 IC14 all 11.6V

R64, LHS 4.74v, RHS 4.94v

Also attached, long awaited images of the 4a (bad lower ram/dc-dc), and the 4s (repaired dc-dc, no image)
Attachments
4s.JPG
4a.JPG

Lardo Boffin
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Joined: Sat Nov 26, 2016 2:42 am

Re: Issue 4a docs needed

Post by Lardo Boffin »

Can’t really help with the repairs (and that 4S will never work with all those chips missing! :lol: ) but there are people who can repair these for you. I have used Mutant Caterpillar Games quite a few times over the years.
Also you can get a replacement ULA from here (https://www.retroleum.co.uk/zx-spectrum-chips) if it is faulty.
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
Iss 3 ZXVid
TS 1000 iss 3, ZXPand AY, ZX8-CCB, ZX-KDLX & ChromaSCART
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Spectrum +3

eccles
Posts: 26
Joined: Tue Dec 24, 2019 10:37 pm

Re: Issue 4a docs needed

Post by eccles »

Good news on the 4S

Armed with clues from other posts from other people, I checked clock pulses and found I had none.
I removed the RF modulator and cleaned some corrosion from the underside, and replaced the X1 crystal.

Hey presto, clock pulse returned, and this 4S now boots

As you can see from the image attached, even with a composite mod, it's a bit of a wobbly picture (though I don't know how much of that is the unit, and how much the old TV - it's an ancient CRT)

The 4S is giving me absolutely no sound, so more work is clearly needed.

I haven't worked more on the 4A yet, though it's clock pulses are present - I am worried at the electrical "hum" from the 4A, and the low 12V line (10.8V) - any suggestions appreciated
Attachments
spectrum_output.jpg

McKlaud
Posts: 277
Joined: Tue Dec 19, 2017 10:02 pm
Location: St Albans, UK

Re: Issue 4a docs needed

Post by McKlaud »

eccles wrote:
Mon Jan 27, 2020 9:53 pm
I am worried at the electrical "hum" from the 4A, and the low 12V line (10.8V) - any suggestions appreciated
I would not worry much about the hum, it seems that coil's windings became loose. I would worry about very low 12V line. As a first guess I would replace TR4 (ZTX650 type) with new one or ZTX651 alternatively. In the same time I would do both 100uF capacitors (C44 & C45) replacement and then power up the board and check the voltage on the 12V rail.
Claudius
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eccles
Posts: 26
Joined: Tue Dec 24, 2019 10:37 pm

Re: Issue 4a docs needed

Post by eccles »

TR4, and all electrolytic caps on the 4A have already been replaced.
I do have spare ZTX650s, so I guess I could double check the caps out of circuit and replace TR4 again just in case

McKlaud
Posts: 277
Joined: Tue Dec 19, 2017 10:02 pm
Location: St Albans, UK

Re: Issue 4a docs needed

Post by McKlaud »

Looking at the issue 4a schematic, there are other parts worth checking like TR5 and both diodes D15 & D17.

BTW is any of memory chips getting hot?
Attachments
2FDB70F1-30A2-40AD-9AD9-D99CFB96DAC2.jpeg
Claudius
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1024MAK
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Re: Issue 4a docs needed

Post by 1024MAK »

For the low +12V:
  • Test the +5V rail at the emitter of TR5 to confirm that it is 5V,
  • Switch off and disconnect and remove TR5,
  • Test the value of resistors R59 and R60 (you may have to desolder and lift one leg of one of them),
  • Fit a new transistor in TR5 position.
ZTX213 can be replaced with ZTX750 in TR5 position. Or you can use a BC307 or a BC557 but the orientation/pin-out/lead-out is different.

Mark

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1024MAK
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Re: Issue 4a docs needed

Post by 1024MAK »

McKlaud wrote:
Mon Jan 27, 2020 10:15 pm
eccles wrote:
Mon Jan 27, 2020 9:53 pm
I am worried at the electrical "hum" from the 4A, and the low 12V line (10.8V) - any suggestions appreciated
I would not worry much about the hum, it seems that coil's windings became loose.
If you think the whine is loud, try an issue 1 or issue 2 board, the coils on these don’t appear to have any varnish, so the coils vibrate loudly. If the whine is too annoying for you, carefully apply some PCB or nail varnish to the windings. Then allow to set overnight.

Mark

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1024MAK
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Re: Issue 4a docs needed

Post by 1024MAK »

No sound
Test and if needed, replace D9, D13 (both 1N4148), TR7 (ZTX650, ZTX651 or ZTX653) and the speaker (40Ω).

Mark

eccles
Posts: 26
Joined: Tue Dec 24, 2019 10:37 pm

Re: Issue 4a docs needed

Post by eccles »

I did a composite mod to the 4A to ensure the RF modulator wasn't in the way.

Initially I had a screen (white border, funky corrupt content) which I expected given i had no lower ram installed, then it went away :-)

further inspect finds R59 and R60 OK, but the clue is... i'd already replaced TR5, but used a BC213 which I now find is not fully compatible, so i'll clear off and get a ZTX751/753 and report back

Still awaiting a TR7 for the 4S with no sound - tested D9 and D13 in circuit and they're both OK. If I have no luck after, i'll test the speaker seperately (it's the correct impedence, if nothing else)

Thanks for the great help so far

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