Spectrum 128 ToastRack strange issue

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kbjunkie
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Spectrum 128 ToastRack strange issue

Post by kbjunkie »

Hi All,

I've just pulled my old toastrack 128 out after a couple of years gathering dust and it's faulty.

The +12V line was dead, replaced the 2 transistors near the coil and I've got all the voltages back and a screen display. The issues is that when it's cold and I power it up, it would just reset randomly, sometimes settling on the standard 48K mode, other times with the 128 menu screen with a bit of corruption. After about 30secs of powering it off and on, it would become more stable and I could use it for a little while (from the 128 menu).

I replaced the regulator for another, new 78S05CV (2A) regulator and recapped for good measure. Now it's worse.. but this is where it gets strange.

It always boots up to flashing crap all over the screen. If I replace the ROM with Brendon Alfords diagrom, it does the same.. the diagrom doesn't boot.

If I plug in a Retroleum smart card with Phil's diagrom, it runs fine, and reports all RAM fine. It also runs all of the 128 RAM tests just fine (I have both the V2 and new V3 smartcards). The only test that will NOT run is anything that accesses the ROM - so any of the ROM checks results in a garbage screen and a crash. I have replaced the ROM with a CY27C256 OTP ROM and a new image from the interwebs. Same problem. I have compared the original ROM contents with the file I downloaded and it's good.

It does have the UnRainer mod fitted (fitted a good while ago). It also had a new socket fitted for the ULA a long time ago as the original socket was loose - but it worked fine after that.

I have changed the ULA with one out of a Grey +2 (Amstrad 40056) - same issues as above.

Anyone have any ideas?

Cheers,

KbJ
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1024MAK
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Re: Spectrum 128 ToastRack strange issue

Post by 1024MAK »

Carefully examine the ROM socket contacts, you are looking to see if any are bent or distorted. Then check all the solder joints on the underside of the board.

If you can’t see any problems, the next stage is to buzz out (use a multimeter on it’s continuity range) the ROM chip connections. By this, I mean put the ROM chip in the socket. Then test between the relevant chip pin and the destination of the relevant signal.

Start at pin 1 and work your way through each one. You will need a copy of the schematic (circuit diagram). It’s best to have a printed copy so that you can use a coloured pen to mark the diagram as you test each pin.

Note that both pin 1 and pin 28 both connect to the +5V supply, so test each to the +5V pin of the Z80A.
Test all the address and data pins to the respective Z80A pins. Except for address A14, which goes to IC31 pin 12.
Pin 20 goes to the /ROM CS pin on the edge-connector and to resistor R38.
Pin 22 goes to /MREQ (MREQL) on the Z80.

Mark
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