Re: C128 Power Supply repair

From: Hegedűs István <>
Date: Tue, 17 Jul 2012 22:00:20 +0200
Message-ID: <>
Thanks Gerrit!

I am sure that this is not a 78M05 regulator. That regulation is done by the 
DIP IC inside the PSU.
This must be the thyristor for sure because searching the web had some 
results for TGA8542 which mentioned thyristor.
The C128 doesn't turn on with this PSU while it works with other C128 PSU (I 
have tested it with an other one cause I have 2 machines). As it is 
physically next to the 4700uF/16V capacitor, it has damaged that capacitor 
I have measured voltages on the connector and got the 5.1V on the proper leg 
when not connected to the machine. Probably it cannot feed the required 

The C128 hardware manual has some PSU schematics and it seems to be rather 
complicated for me... Of course I know the rectifier and filter cap part, 
that is easy. I will investigate your ebay item advise, seems promising.
Once I have upgraded my A1200 PSU with small switching PSU circuit using 12V 
input. That works fine but the C128 needs that 9V AC also...


-----Original Message----- 
From: Gerrit Heitsch
Sent: Tuesday, July 17, 2012 5:19 PM
Subject: Re: C128 Power Supply repair

On 07/17/2012 02:48 PM, Baltissen, GJPAA (Ruud) wrote:
> Hallo István,
>> I have a broken C128 power supply which has a burnt thyristor.....
> I never opened the PS of a C128 but is it possible it is a 78M05, the 3A 
> variant of the 7805?

The 78M05 is the 0.5A version of the 7805.

If the thyristor is burnt it might mean that the switching regulator is
shot. The Thyristor is part of the crowbar circuit which is there to
protect the C128 against overvoltage.

The PSU should work without it.

A good idea might be to just take out the whole regulator circuit and
just leave rectifier and filter cap. Then grab a small switching
regulator from ebay like this one:

It's good for up to 3A with some cooling and adjustable. Set it to 5V
(or 5.2V to compensate for cable loss) before connecting it to the C128
and you're done.


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Received on 2012-07-17 21:00:06

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