Author Topic: CTM640 went POP  (Read 853 times)

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Offline JonB

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CTM640 went POP
« on: 17:58, 26 April 19 »
Hi

I had an episode with a CTM640 today - I'd connected it to a 6128 with Bryce's tiny 5v-12v adapter card (CPC5V - http://www.cpcwiki.eu/forum/amstrad-cpc-hardware/5vcpc) installed and it all went fine, until I attempted floppy drive access, then the lights went out. I wonder if I had it wired incorrectly or it drew too much current and overloaded the 5v rail in the CTM? The 3A fuse in the monitor's plug, as well as the 2A PCB fuse had both blown. I've tried replacing these with the same result. I suspect that I'd put the power plug onto the adapter board the wrong way round and caused a short circuit.

Looking at the schematic I see the 5v line is generated by a LM78MG, a four legged adjustable 5v regulator.

http://www.cpcwiki.eu/imgs/5/5c/CTM640_Schematic.png

Problem is, I can't find one for sale anywhere. I believe it has failed and gone closed circuit. Can anyone point to a place where I can get one - or has one spare?

Thanks
JonB
« Last Edit: 18:33, 26 April 19 by JonB »

Offline Bryce

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Re: CTM640 went POP
« Reply #1 on: 15:29, 27 April 19 »
Hi JonB,      it's extremely unlikely that the input of an LM78MG would go short and blow both of those fuses as it has internal thermal and current overload protection. Connecting the 5VCPC the wrong way also shouldn't cause a fault like this. I suspect that something else in the monitor failed. My suggestion would be to remove the LM78MG and re-try without anything connected to the 12 and 5V outputs. If the fuses blow again something else has failed. If it doesn't fail, I may have some replacement LM78MG parts here (not new, taken from scrapped monitors).

And just for information: I'm pretty sure I mentioned in the 5VCPC thread that the device won't work with a CPC monitor due to the current requirements.

Bryce.