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Old 23rd May 2021, 11:28 am   #1
Superscope
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Join Date: May 2017
Location: St Austell, Cornwall, UK.
Posts: 1,018
Default Maas SPS-4128A Switch-Mode PSU

Hi Forum,

Does anybody have a Circuit Description or Schematic for
this Switch-Mode PSU ?
Or indeed any information on it?
I know the basic principles, but I'm no expert on SM-PSU's

Doesn't appear to be anything on-line that I can find.

Picked this up a few Months ago and it had a fault.
You know the Type, Spares or Repair but was working fine when last used!
Yes of course it was!


Would turn on, Fan started to run and Voltage rose to around
10 Volts, before falling back to around 2.5 Volts within a
couple of seconds and the Fan would go off.

On closer inspection, I could see one of the electrolitics
bulging slightly and decided to change it out.
3300 uF 16V which I changed out for 3300uF 25V
The replacement is arrowed in the 3rd photo.
Being a 25v, it was bigger than the original and was a bit of a squash
getting it in.
16 Volt looked a bit low spec, but without a Circuit who knows!

To my surprise, it then started working as it should.
I wasn't honestly expecting the fault to be that straightfoward.

Obviously the PSU had detected something not quite right and
rolled back.

It may have just been a Cap failure, or it may be
something else that has caused it to fail over time.
Hence a Schematic or even a Service Manual should one exist
for the future would be potentially useful.

Otherwise, it seems to be a fairly well built unit and very quiet
in the Rf department.



Ian
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Old 23rd May 2021, 12:06 pm   #2
WME_bill
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Default Re: maas SPS-4128A Switch-Mode PSU

Switched mode power supply.
Looking at your photographs, it looks very like most computer power units. May I suggest you do a search on the web. There are lots of circuits and several people who have adapted a computer PSU to a variable lab power supply.
I could post a few circuits which I have collected over the years, but you will do better to go to the original.
wme_bill
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Old 25th May 2021, 9:56 am   #3
Superscope
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Default Re: Maas SPS-4128A Switch-Mode PSU

Thanks Bill,

But it's specific information relating to this actual PSU I was interested in.
I wonder if this PSU is just a maas badged Clone, or they actually made it?

I'm surprised there is no information about.
At least none that I can find at the moment.
maas don't seem to be particularly small players in the PSU market!

Ian
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Old 25th May 2021, 10:50 am   #4
MelJon66
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Default Re: Maas SPS-4128A Switch-Mode PSU

Hi Ian,

This looks suspiciously like the same unit. They give no info but it might assist your search if LokoPower is the OEM.

http://www.lokopower.com/english/pd/SPS-4128ACH.htm

Cheers
Mel
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Old 31st May 2021, 5:28 pm   #5
Superscope
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Default Re: Maas SPS-4128A Switch-Mode PSU

Thanks Mel,

That certainly looks like the same PSU, although there is no mention of the 6A secondary output which the Maas one has on the back. No photos of the back to check either.

Ian
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Old 31st May 2021, 6:14 pm   #6
SEG7RAL3
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Default Re: Maas SPS-4128A Switch-Mode PSU

That supply is the same as the Watson W-25SM and the MFJ-4125 look the same just missing the add on mains choke.


Andrew
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Old 31st May 2021, 6:30 pm   #7
SEG7RAL3
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Default Re: Maas SPS-4128A Switch-Mode PSU

You can find the diagram for the W-25SM on here
https://g3zjoradio.wordpress.com/201...lies-15536757/

Andrew
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Old 6th Jun 2021, 11:20 am   #8
Superscope
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Default Re: Maas SPS-4128A Switch-Mode PSU

Thanks Andrew,

I have downloaded the Schematic for the W-25SM.

Not had time to actually go through it yet and check,
but hopefully it's the same circuit.

Worryingly, there seem to be reports of these PSU's going
high Voltage without warning.

Anybody on here had this problem with the Watson, maas or the clones?



Ian
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