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About original Memotech hardware.
Steven.G
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Joined: 18 Jul 2020 14:03

Re: No picture.

Post by Steven.G »

As I'm waiting for the fuses it'll give me a chance to scrounge a 40w filament bulb from somewhere, but yes I can assemble that!

Steve..
Steven.G
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Re: No picture.

Post by Steven.G »

Amazing what a new fuse can do!! :roll:
It Lives.JPG
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Steve..
stephen_usher
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Re: No picture.

Post by stephen_usher »

Yeah I had a similar issue with the MTX512 I got on eBay a few years ago. Changed the regulators and did more investigations, then discovered that I'd bought fuses which were an order of magnitude too small and that was why they kept blowing! D-oh!
Steven.G
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Re: No picture.

Post by Steven.G »

This one was hidden in the PSU the sneaky blighter! :?
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Dave
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Re: No picture.

Post by Dave »

Hi Steve,

it appears that you have been very lucky that the failed fuse has not caused damage to the Video RAMs.

If my power supply simulation is anywhere near accurate, with the fuse blown, you would have exceeded the maximum voltage on Vbb of the VRAMs. The voltage there should be -5V, with a limit of -5.5v and likely reached around -6V with the blown fuse. (The -V supply rail would appear to have been running at around -28V! (Mark previously commented that the blown fuse, "severely affects the voltages inside the computer [and] has unintended consequences for the negative supply rail".) Fortunately, with no peripherals connected, nothing is actually connected to the -V line, so no damage will have been done there.

However, the 4116s VRAMs are a very common cause of video faults (corruption) on the MTX and are VERY sensitive to out of tolerance voltages. For the ongoing health of the machine, particularly in the event that the fuse blows again, you might want to consider removing its holder completely - or fitting the nail that I mentioned previously :lol:

regards
Dave
Steven.G
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Joined: 18 Jul 2020 14:03

Re: No picture.

Post by Steven.G »

OK I'll remove the fuse holder from the circuit and replace the DIN plug too.
The 40w bulb to test the PSU is due tomorrow.

I have only one question,,, why do you think the fuse is there? I'm pretty sure the PSU case hasn't been opened before.

Steve..
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Dave
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Post by Dave »

I’m afraid I’ve no idea why it’s there.

If it WAS factory fitted, it may be something specific to Euro models, but I can’t think why. Euro plugs don’t have fuses like ours do, so I could see a need to provide safety protection in case the transformer developed a short and overheated, but you’d have expected a fuse to have been put on the primary side, and anyway, the thermal fuse Is there. I suppose a fault could pull more current than the cable could handle without tripping the thermal fuse though, an internal fuse could protect against that, but surely it should be on the primary side.

I do have a Euro PSU, but don’t really want to break the case trying to get into it.
Steven.G
Posts: 42
Joined: 18 Jul 2020 14:03

Re: No picture.

Post by Steven.G »

Well whatever the reason it's gone now. :!:
Steven.G
Posts: 42
Joined: 18 Jul 2020 14:03

Re: No picture.

Post by Steven.G »

Why the shocked face? You said to remove it!
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