Broken interesting Spectrum

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1024MAK
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Re: Broken interesting Spectrum

Post by 1024MAK » Sun Nov 05, 2017 2:06 am

axtelle wrote:
Sat Nov 04, 2017 10:50 pm
I wonder could you be trying to display a PAL-N (Argentinian) signal on a PAL-I (UK) TV set ?
If the TV does not recognise the colour encoding, it should still display a monochrome picture.

Mark

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1024MAK
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Location: Looking forward to summer in Somerset, UK...

Re: Broken interesting Spectrum

Post by 1024MAK » Sun Nov 05, 2017 2:13 am

Lardo Boffin wrote:
Sat Nov 04, 2017 10:57 pm
Thanks for the links!

I have a Spectra SCART which produces a picture by listening to RAM writes which presumably would ignore the tv output type? My only concern is that if this is just a dodgy computer it could damage my Spectra (which was rather expensive).
The SPECTRA will produce a picture based only on what the ULA baseband signal does (it gets timing information from this signal I believe) combined with the data written to the display memory. The SPECTRA then encodes the data as 625 line, 15625Hz horizontal, 50Hz vertical video, but outputs as red, green, blue video plus a composite sync signal.

And yes, it would be safer to get the computer working correctly before using the SPECTRA, as occasionally a faulty 4116 DRAM chip can leak +12V out to data or address lines :twisted:

A temporary composite mod using a 100uF electrolytic capacitor may be the best video connection during fault finding (assuming you are not going to do a permanent composite mod.).

Mark

axtelle
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Re: Broken interesting Spectrum

Post by axtelle » Sun Nov 05, 2017 2:13 am

Would bypassing the UHF modulator and viewing the CV output help ?

Lardo Boffin
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Re: Broken interesting Spectrum

Post by Lardo Boffin » Sun Nov 05, 2017 7:32 am

I will do a composite mod as will be doing this anyway. :)

Where do I pick up the voltages to test? I don’t really know a lot about the Spectrum... :oops:
ZX81 iss 1 (bugged ROM, kludge fix, normal, kit)
Iss 3 with ZXVid, another in nearly perfect box
TS 1000 iss 3, ZXPand AY, ZX8-CCB & ZX-KDLX
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Spectrum 48k
Toastrack + DIVMMC future

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1024MAK
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Joined: Mon Sep 26, 2011 9:56 am
Location: Looking forward to summer in Somerset, UK...

Re: Broken interesting Spectrum

Post by 1024MAK » Sun Nov 05, 2017 2:02 pm

The easiest way to test is either on the edge-connector contacts, or on the pins of one of the 4116 DRAM chips such as IC6. Note that sometimes Sinclair used equivalent DRAM chips, so the type number may not be 4116.

Note that some diagrams in some Sinclair documentation (including some of the manuals) show a "-12V" supply pin as well. Ignore this, the name is supposed to be "~12V" (where the ~ is a wavy line) as this is a AC waveform, not a DC supply. The supply rails that you should test are however, labelled correctly.

Test all of them with respect to a 0V/GND/ground connection or pin (-V/black meter lead connected to a 0V/GND/ground point).
If you have the case top removed, the metal tab (or heatsink) of the 7805 voltage regulator is a good place to use.

If you have not removed the case top, you can also use the outside contact of the UHF TV out connector for a 0V connection.

4116 DRAM chip pin-out:Image
(This pin-out is one of a number in a thread that is linked to in my signature ;) )
A copy of the full data sheet is here

Pin 1 is VBB, which is -5V (-5.5V to -4V)
Pin 8 is VDD, which is +12V (+10.8V to +13.2V) (although lower than 11.3V normally indicates a problem).
Pin 9 is VCC, which is +5V (+4.75V to +5.25V) (the output from the 7805 regulator is normally very close to +5V)
Pin 14 is VSS, which is 0V (ground/GND) (no need test this if using another 0V/GND connection)
(Voltage tolerances taken from here).

I also recommend that you read through the whole Service Manual. A HTTP version is here. There are also PDF versions available. But watch out for the OCR errors plus some errors in the original document.

Mark

Lardo Boffin
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Joined: Sat Nov 26, 2016 1:42 am

Re: Broken interesting Spectrum

Post by Lardo Boffin » Sun Nov 05, 2017 9:37 pm

I have checked and rechecked my figures but I am getting the following:-

Pin 1 is showing -0.02
Pin 8 is showing 2.22
Pin 9 is showing 5

Either I am doing something wrong or this this is somewhat broken!

Below is a photo of the setting I am using on my multi-meter just to ensure I am not making any schoolboy errors!

I measured relative to tab on top of the 7805.

6E2BE100-F942-4B45-8488-FE7B16CC1DF1.jpeg
6E2BE100-F942-4B45-8488-FE7B16CC1DF1.jpeg (604.02 KiB) Viewed 127 times
ZX81 iss 1 (bugged ROM, kludge fix, normal, kit)
Iss 3 with ZXVid, another in nearly perfect box
TS 1000 iss 3, ZXPand AY, ZX8-CCB & ZX-KDLX
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Spectrum 48k
Toastrack + DIVMMC future

Lardo Boffin
Posts: 623
Joined: Sat Nov 26, 2016 1:42 am

Re: Broken interesting Spectrum

Post by Lardo Boffin » Sun Nov 05, 2017 9:45 pm

Here is a photo of the underside while I have the case open.

DD8164D5-12F5-47FF-A6CA-6502ACE59750.jpeg
DD8164D5-12F5-47FF-A6CA-6502ACE59750.jpeg (1.13 MiB) Viewed 123 times
ZX81 iss 1 (bugged ROM, kludge fix, normal, kit)
Iss 3 with ZXVid, another in nearly perfect box
TS 1000 iss 3, ZXPand AY, ZX8-CCB & ZX-KDLX
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Spectrum 48k
Toastrack + DIVMMC future

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1024MAK
Posts: 1768
Joined: Mon Sep 26, 2011 9:56 am
Location: Looking forward to summer in Somerset, UK...

Re: Broken interesting Spectrum

Post by 1024MAK » Mon Nov 06, 2017 12:39 am

Lardo Boffin wrote:
Sun Nov 05, 2017 9:37 pm
I have checked and rechecked my figures but I am getting the following:-

Pin 1 is showing -0.02
Pin 8 is showing 2.22
Pin 9 is showing 5
The meter is set correctly. The 20V DC range is the most appropriate.
This indicates that transistors TR4 and/or TR5 are not working, most likely one or both are damaged.

Three things normally cause the death of these transistors, an expansion fitted or removed while the machine is powered on, or a 4116 DRAM chip has failed and overloaded the switching power supply that TR4, TR5 and the 'coil' form part of.
And sometimes a failed electrolytic capacitor.

So it's time to go shopping for some replacement transistors and replacement electrolytic capacitors. It's recommended that all the electrolytic capacitors be replaced with new ones of the same value (the rated voltage can be higher than that of the originals).

Once the electrolytic capacitors have been renewed, then the first thing to try, is to fit two new transistors:-
TR4 - use a ZTX651 or a ZTX653 (replaces ZTX650)
TR5 - use a ZTX750, or a ZTX751 (replaces ZTX213)

TR5 can also be replaced with a BC307 or BC557 but the pin-out of these is different.

Most suppliers can supply suitable replacements, but this supplier also has transistor parts:-
https://www.retroleum.co.uk/zx-spectrum-transistors

Get at least 3 of each, as you may well find that during repair, you have to replace them more than once...

See also the FAQ wiki
And the Spectrum Repair Guide

Electronic suppliers that I use or have used:-
http://cpc.farnell.com (part of Farnell, but the pricing is not always the same!)
https://www.rapidonline.com
https://www.esr.co.uk (Very helpful U.K. company)
http://uk.farnell.com
https://www.bowood-electronics.co.uk (I have not used this U.K. company much, but they are recommended by some people I know)
https://www.mouser.co.uk
https://www.tme.eu/gb/
http://uk.rs-online.com/web/

Mark

Lardo Boffin
Posts: 623
Joined: Sat Nov 26, 2016 1:42 am

Re: Broken interesting Spectrum

Post by Lardo Boffin » Mon Nov 06, 2017 6:45 am

Thanks Mark. I seem to be saying that a lot!!!

Caps are already on order. I have ordered the transistors.

I was reading the world of Spectrum repair guide last night - it appears top of the search when checking for common spectrum issues! :D
ZX81 iss 1 (bugged ROM, kludge fix, normal, kit)
Iss 3 with ZXVid, another in nearly perfect box
TS 1000 iss 3, ZXPand AY, ZX8-CCB & ZX-KDLX
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Spectrum 48k
Toastrack + DIVMMC future

Lardo Boffin
Posts: 623
Joined: Sat Nov 26, 2016 1:42 am

Re: Broken interesting Spectrum

Post by Lardo Boffin » Tue Nov 07, 2017 10:25 pm

I have recapped all but one cap / the one shown in light blue. I assume it is a 1uF cap but not sure of the polarity? Is it negative to the black stripe side?

D490E03B-E7CE-4F87-A419-ACAD20A5B3AB.jpeg
D490E03B-E7CE-4F87-A419-ACAD20A5B3AB.jpeg (645.23 KiB) Viewed 77 times
ZX81 iss 1 (bugged ROM, kludge fix, normal, kit)
Iss 3 with ZXVid, another in nearly perfect box
TS 1000 iss 3, ZXPand AY, ZX8-CCB & ZX-KDLX
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Spectrum 48k
Toastrack + DIVMMC future

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