[sdiy] EMS VCS3 II negative power supply problems

[Magnus Danielson]

Hej Mattias,

It's a classic over-current protection setup. So if there is too much 
current it will make the driver back off. If this is broken you god that 
happening you the supply.

Cheers,
Magnus

On 2022-09-16 23:53, Mattias Rickardsson wrote:
> Hej Magnus,
>
> yes, the power transistors are off board, and I've replaced that one. 
> Also the nearby metal can transistor. There's also a third transistor 
> in regular small plastic package close to them that I'm not completely 
> sure at the moment what it does in the negative power circuit...?
>
> /mr
>
> Den fre 16 sep. 2022 21:57Magnus Danielson <magnus at rubidium.se> skrev:
>
>     Hi,
>
>     Check the buffer transistor. That had blown on mine.
>
>     The Mark II PSU is nicer than the Mark I PSU for VCS3/Synthi-A.
>
>     About the same time as the Mark I and Mark II shift, they moved
>     the transistors off the PSU board. If they are on the board, move
>     them out as this remove heat and it will eventually fail. Guess
>     how I know.
>
>     Cheers,
>     Magnus
>
>     On 2022-09-13 21:50, Mattias Rickardsson wrote:
>>     Hey,
>>
>>     I'm trying to revive a VCS3 II where the power supply has issues
>>     on the negative regulation. With an incoming -25 V it puts out
>>     roughly -24 V instead of the supposed -9 V. Does anyone have
>>     experience with similar problems?
>>
>>     (The transformer was fried on the negative half, so I'm running
>>     it from a bench supply. I have also replaced the regulating 741
>>     and TIP3055 power transistor)
>>
>>     ((The phutney.com <http://phutney.com> site and its very readable
>>     schematics also magically disappeared, which didn't help)) :-)
>>
>>     Best,
>>
>>     /mr
>>
>>
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