M7891-DC 2a fuse on +12?

From: Gordon JC Pearce <gordonjcp_at_gjcp.net>
Date: Thu Feb 5 08:57:31 2004

On Thu, 2004-02-05 at 14:25, Brad Parker wrote:
>
> I had a spare moment yesterday so I decided to try and troubleshoot my
> bad M7891-DC (128kw) in an 11/34a.
>
> Looks like I have no +12v. The 12v regulator seems to be working as I
> have 12v up the 2A fuse. But nothing after that.
>
> I'm assuming the fuse has blown. Easy to replace, but I also suspect it
> blew for a reason. (and replacing it might just yield another blown fuse
> :-)

Hmm, usually the transistors blow to protect the fuse...
 
> Anyone seen this before? Do they sometimes blow due to transients?

Fuses do just blow sometimes. You could try replacing it and seeing
what happens, but make sure it's got the same rating. Another old audio
amp trick is to use a car indicator bulb (12v, 21W) or tail light bulb
(12v, 5W) across the fuse - if it's drawing a clearly silly current, the
bulb will light.

> I'm tempted to apply 12v after the fuse with a bench supply that has a
> current regulator and see what it draws. It's been a while since I
> worked with "plain old DRAMs" but I assume the 12v is for them (Vdd).

Be careful - my bench PSU, when set to 12v, actually produces 6v-0-6v
with chassis earth (and hence mains earth) being the 0v centre tap.

What type of DRAM chip is it? I seem to recall that the 4116 DRAMs used
in the ZX Spectrums tended to fail short-circuit between 12v and 0v.
This usually stopped the chopper PSU that provided +12v and -5v from the
nominal 9v input. This caused mad black-and-white flickering, because
the RAM was reading out random garbage and the 12v supply also fed the
colour encoder.

HTH
  Gordon.
Received on Thu Feb 05 2004 - 08:57:31 GMT