[Coco] CoCo 2 repair - Transformer?

[camillus.b.58 at gmail.com]

As far as I know the 12 volt is the output from the transformer after the bridge diode rectifier. Can be a bit more or less. Then all other voltages are derived from this 12V rectified source.


This being said, I’m not shure about the coco’s that had the old RAM who needed + and - 5v or 12 volt, but I was under the impression that only the coco I’s where such.


If your RAM chips work only on single voltage (5V) then you can even replace the transformer and power the coco with an 12 Volt DC 1.5 Amp. adapter. This needs a minor modification, which can also be undone as easy. But to explain this It would be better that you have a schematic of your board.


As for the capacitor, replace it, gives you ease of mind, the cost is not worth the ageny. And if it works with the capacitor you put in, with a little higher rating, just let it be. It would be close enough to  the 20% tollerance the capacitor has anyway.





Do you know what revision your board is?


Camillus


Sent from Surface





From: Chad H
Sent: ‎Tuesday‎, ‎December‎ ‎23‎, ‎2014 ‎7‎:‎48‎ ‎PM
To: CoCoList for Color Computer Enthusiasts





Actually I was going to consider that as an option "IF" I ever found a good service manual for the 26-3027/3127 models.  I know the unit needs both 12V/5V power supplies but I don't know if 12V+/5V+ is sufficient or if there are required 12V-, 5V-, etc. rails.  Heck I don't even know where I would even tie into the main board such a external power supply.   If I did, and it was a simple 12V+/5V+/GND source I would tack on a molex connector and hook in a Pico ATX power supply :)

-----Original Message-----
From: Coco [mailto:coco-bounces at maltedmedia.com] On Behalf Of Louis Ciotti
Sent: Tuesday, December 23, 2014 7:25 PM
To: CoCoList for Color Computer Enthusiasts
Subject: Re: [Coco] CoCo 2 repair - Transformer?

Why are you looking at replacing the transformer, From your last posts it sould like you found the problem with a bad ground lead. If I were looking at doing anything with the CoCo power supply I would do a total gut and replace.  Been a while since I looked at it, but I seem to remember that it was not exactly the best power supply design, even for the time it was produced.

On Tue, Dec 23, 2014 at 8:11 PM, Michael Graham <mkgraham at gmx.com> wrote:

> Hi!
>
> I think you're looking at the specs on the transformer in the wrong way.
> You're measuring 10.8V on each side of the tap, but that wouldn't make 
> it a 10.8V transformer, that would make it a 21.6V transformer.
>
> It probably is actually a 17.5V center-tapped transformer, but you're 
> measuring 10.8V instead of 8.75V on either side of the tap because the 
> CoCo isn't drawing the full amount of current for which the transformer is rated.
>
> --Michael
>
>
>
> On 12/23/2014 6:52 PM, Chad H wrote:
>
>> In trying to identify potential replacement sources for power supply 
>> parts I've yet to pin down one for the power transformer in my primary CoCo 2.
>> It
>> seems to be the same one in the 26-3027 as in the 26-3127 but there 
>> is no label on the transformer itself.  The technical reference I 
>> found for the
>> 26-3136 talks about a 17.5vac transformer but when I measured my 
>> 26-3027 secondaries I got approximately 10.8vac on each point to 
>> center.  I tested with the unit on and with it off, not much 
>> difference.  I was wondering if the following might be a possible candidate.
>>
>>
>> http://tinyurl.com/lcwp2hp
>>
>>
>>
>>    Can someone tell me if I'm on the right track?  Thanks!
>>
>>
>> -          Chad
>>
>>
>>
>
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>

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