RE: CBM Power Supplies

From: Geoff Oltmans (
Date: 1999-07-23 00:47:13

On Thu, 22 Jul 1999, Bo Zimmerman wrote:

> That's interesting.  I took what you said back to funet and looked at the
> 8050 power supply schematics.  It seems that the big nasty brick actually
> outputs 8.2V AC and 16.2 V AC in that schematic.  Wierd numbers, but
> although I'm not a very good schematic reader, I guess its after the trip
> through the green monster that it becomes 9V DC and 5V DC, eh?  Reason I'm
> asking is because I'm still trying to figure out what to find to replace the
> brick inside this 8280 to get 110V.  With the smaller drives it was easy
> because they all use the same PS as the 1541, and '41s are cheap.  The 8280,
> however, has a four-plug output from the brick, which is different from
> other CBM drives (including the 8050 on funet), and certainly from the PETs.

That's right. On all of the PET stuff I've seen, there's the big AC
transformer, and then it's converted to DC on the board through some
rectifiers, and then it uses a set of voltage regulators (75xx stuff I
believe, looks like a big three prong transistor) to step that voltage to
the proper 9V and 5V DC that all the components need.

I'm a little confused as to what you're trying to accomplish though. :) 
Is this like a 220-240V 50Hz to 110-120V 60Hz conversion or vice versa, or
just a simple repair? At any rate, if the thing has a four wire output
instead of three, it may just be that it has an isolated secondary
transformer, in which case you'd need the extra wire. The 1541's do this
too I believe. Those voltage levels you mention are fairly common to find
in AC transformers...finding a BIG enough suitable replacement may be a
bit of a challenge, and then again may not be. 


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