Re: cbm 8032 motherboard + atx power supply

----- Original Message ----- 
Sent: Friday, August 19, 2016 3:22 PM
Subject: Re: cbm 8032 motherboard + atx power supply

> On 19/08/2016 20:28, Steve Gray wrote:
>> +5V, +9V, +12V, -5V regulated to the main board
>> +9V, -9V, +16V unregulated to the expansion power connectors
>> according to page 11 of the schematics.....
I think you got it a bit wrong,
there is no regulated +9V,
ohny unregulated.
-- 
Christian Dirks
Toast_r@Idealine.info

----------------------------

I don't think everyone is aware that unlike Apples and in fact most other computers, the older PETs do not actually have a discrete power supply per se.

There is a transformer and a filter cap that plug into the main board and everything else (bridges, diodes, regulators, filter & bypass caps etc.) is on the board.

The board is supplied with 9VAC and 16VAC; from those it generates: 

+5V regulated (the main supply, around 4-5A), using one or two regulators.
+12V regulated for the DRAMs
-5V regulated, also for the DRAMs
+9V unregulated for the cassette motors

and it supplies +9V, -9V and +16V unregulated on the power expansion connectors.

If you don't mind some drilling, cutting traces and adding jumpers on the main board then a slightly modified AT or ATX supply would do the job; as mentioned elsewhere, if it doesn't have the -5V it's easily derived from the -12V. There are several approaches, more or less neat and with more or fewer modifications.

If you want a plug-in (no mods to PCB) switching solution then you would need three voltages:

+9V at around 4-5A 
+16V at around 1A 
-9V also around 1A or less.

They're not critical and don't have to be regulated, so you might get away with merely disabling/modifying the regulation on an AT(X) supply and using that.

m

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