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by 1024MAK
Sat Feb 08, 2020 2:02 am
Forum: Hardware
Topic: Does Anyone Recognize This Pattern?
Replies: 11
Views: 647

Re: Does Anyone Recognize This Pattern?

Update

I can now confirm that pin 39 on the vLA module is not used (I asked the producer of the module). So the only thing a ‘scope will pick up here is noise :!:

Mark
by 1024MAK
Fri Feb 07, 2020 7:22 pm
Forum: Hardware
Topic: Does Anyone Recognize This Pattern?
Replies: 11
Views: 647

Re: Does Anyone Recognize This Pattern?

The ULA had been working, but suddenly quit. Could a failing capacitor in the -5V power circuit do that? Only the 4116 DRAM chips use the -5V supply. So normally a failure of the -5V supply should not affect the ULA. Unless a 4116 DRAM chip fails short circuit internally, in which case it is possib...
by 1024MAK
Fri Feb 07, 2020 11:05 am
Forum: Hardware
Topic: Does Anyone Recognize This Pattern?
Replies: 11
Views: 647

Re: Does Anyone Recognize This Pattern?

ULA pin 39 on a Sinclair/Ferranti chip is rather sensitive and the capacitance from a ‘scope probe can affect it. On some of the recreated modern ULA modules, often the crystal oscillator circuit pin is either unused, or used for an additional function, as these modules often have their own crystal ...
by 1024MAK
Thu Feb 06, 2020 1:54 pm
Forum: Hardware
Topic: Help diagnosing issue with 48K Spectrum
Replies: 3
Views: 474

Re: Help diagnosing issue with 48K Spectrum

In your video, the Z80 is not running the start up code from the ROM. Before going any further, I have a bunch of questions: Which issue board is it (printed on the front edge)? What were the symptoms before you started work? What test gear do you own or have available? Have you tested all the volta...
by 1024MAK
Thu Feb 06, 2020 1:43 pm
Forum: Hardware
Topic: Does Anyone Recognize This Pattern?
Replies: 11
Views: 647

Re: Does Anyone Recognize This Pattern?

Before you go any further with your “recapping”, you should consider bringing the DC/DC converter circuitry up to the final Sinclair version, which increases its reliability. See my blog page for details . Then concentrate on getting the voltages on the +12V and -5V rails correct (with no 4116 DRAM ...
by 1024MAK
Thu Feb 06, 2020 12:35 am
Forum: Hardware
Topic: Issue 4a docs needed
Replies: 44
Views: 3357

Re: Issue 4a docs needed

On the 4A, remove TR4 from the board, and don’t fit any of the 4116 DRAM chips. Then power up the board and see what the current draw is now.

Mark
by 1024MAK
Wed Feb 05, 2020 9:43 pm
Forum: ZX Spectrum Lounge
Topic: Source of 200ohm speakers?
Replies: 11
Views: 924

Re: Source of 200ohm speakers?

XavSnap wrote:
Wed Feb 05, 2020 8:15 pm
Hi,

In case of 40ohm speaker, the resistor will pull up the ZTX450 main currant and will destroy it by overheated.
Xav, the issue two circuit does not use a transistor:
Issue two ear/mic/speaker circuitry
Issue two ear/mic/speaker circuitry
And the speaker is a higher resistance type.

Mark
by 1024MAK
Wed Feb 05, 2020 9:01 pm
Forum: ZX Spectrum Lounge
Topic: Source of 200ohm speakers?
Replies: 11
Views: 924

Re: Source of 200ohm speakers?

The reason that Sinclair changed the circuit (which occurred from the issue 3 board, but the schematic is wrong) is because the sound output was too low (too quiet). If you use a less sensitive speaker, such as a 40Ω one, and then wire a resistor in series with it, you will only end up making it eve...
by 1024MAK
Wed Feb 05, 2020 8:50 pm
Forum: Hardware
Topic: Does Anyone Recognize This Pattern?
Replies: 11
Views: 647

Re: Does Anyone Recognize This Pattern?

Ian, the vertical stripes look like a memory fault with the ‘lower’ 4116 (or equivalent) RAM. You say you have replaced all of them. But it looks like bit 2, which is provided by IC8 is not working correctly. First double check that all the pins are correctly in the socket. Then try swapping IC8 wit...
by 1024MAK
Wed Feb 05, 2020 7:44 pm
Forum: ZX Spectrum Lounge
Topic: 16k Issue 2 board with short.
Replies: 5
Views: 381

Re: 16k Issue 2 board with short.

Well done on unshorting it! :lol: :D

Mark