CFXII - Black screen with it plugged in

About original Memotech hardware.
Martin A
Posts: 553
Joined: 09 Nov 2013 21:03

Re: CFXII - Black screen with it plugged in

Post by Martin A »

The schematic in both version of the manual have C53 as 470uf 16v. I'm pretty sure that was what came out too when I put the replacement in.

There's lots of baord photo's on Dave's site, including one of the internals of my MTX from before the repair, and none of them seem to have that capacitor oriented so that the values are visible.
acsi
Posts: 31
Joined: 16 May 2019 08:49

Re: CFXII - Black screen with it plugged in

Post by acsi »

Hmm service manual definitely says 100uF

CAPE R010025 100uf 25v Radial Cap 1 C53

Yeah I've tried searching images but none show the rating :(

Anyway after pulling the board out I found this under the heatsink which I don't think is factory

Edit: Also tried replacing the 74LS374N with a 74HC374N I had handy. No change.

Given both the sound and the CFXII stopped at the same time I am wondering on the possibility it could be the Z8430 chip?

Image
Martin A
Posts: 553
Joined: 09 Nov 2013 21:03

Re: CFXII - Black screen with it plugged in

Post by Martin A »

Interesting choice of insulation there! I beleive it's only thre TIP that needs insulation from the board.

I don't think it would be the CTC, the symptoms of a dead/missing CTC are a cursor that doesn't flash and the break key not working. I just tested the MTX/CFX-1 combination here and that was quite happy to boot and play the start up CFX tones with the CTC pulled. (control G works as well)

The sound chip defaults to making a noise until it's reset, which is what makes the black screen and tone such a common fault. No sound at all to me is an indication the problem is the PSG itself or somewhere between there and the output.
acsi
Posts: 31
Joined: 16 May 2019 08:49

Re: CFXII - Black screen with it plugged in

Post by acsi »

Thanks

I removed the metal plate BTW

I will order a replacement PSG. I've tested the path back from the audio out to the PSG and it only has the 1K resistor on the video card and the 100nF cap in the path which I believe is correct. Both the cap and the resistor test fine.

Mike
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Dave
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Re: CFXII - Black screen with it plugged in

Post by Dave »

Did you really mean the metal plate, or was it the dodgy BT Phonecard that you removed?

The metal plate should be there to help dissipate the heat from the regulators

Regards
Dave
acsi
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Joined: 16 May 2019 08:49

Re: CFXII - Black screen with it plugged in

Post by acsi »

That green plate under the board is metal, it had insulation tape between it and the board. Looks like someone cut it out of an electric motor ID plate.

The heatsink plate that the 3 regulators bolt too is still bolted on of course. ;)
acsi
Posts: 31
Joined: 16 May 2019 08:49

Re: CFXII - Black screen with it plugged in

Post by acsi »

Interesting. Never seen one of those before. Now that I have a good look at it yes its got a metallic finish but is actually plastic.
I gather someone installed it to support/protect the motherboard?
Maybe I should put it back.
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Dave
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Re: CFXII - Black screen with it plugged in

Post by Dave »

I don’t have many photos of the underside of the MTX board, but here’s one . . . .

Image

As you can see, the short bolts are usually the other way up. Maybe the user lost the washer that seems to be fitted to the board shown above. There is an insulated washer to protect the PCB traces next to the left hand bolt.

Does the board look OK under that phonecard?
acsi
Posts: 31
Joined: 16 May 2019 08:49

Re: CFXII - Black screen with it plugged in

Post by acsi »

Yep the board is fine. No sign of insulating washers but I'll find some.

In the meantime I've got a replacement PSG on order and the caps should be here soon.
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