Removing transformer and retaining on/off switch function

[ptstucky]

Hi all,

I am currently using a KAB PS-1200GX external DC powers supply. Using the PS-1200GX does not require the removal of the original transformer, however I have a Mike New bearing base plate that I would like to install (I have been sitting on it for over a year!) that needs the transformer removed for proper clearance. My concern is that I want to retain the function of the original on/off switch after removing the transformer.

I asked Kevin, and he replied with this:

"Just isolate the two wires coming from the power switch.
Wire them to the original power cable.
But, very important, place a 1A fast blow fuse and a (resistor = to the primary resistance of the transformer) in series with one of the wires."

I am keen to try this, but I want to make sure I don't mess things up, so I am hoping someone who has done this - or something similar - can help take me through the steps in a bit more detail.

Thanks in advance for any help or other ideas on a solution.

Paul

[AlexM]

I don't know the answer to your question, but i'm interested to try this for myself.

I externalised the transfomer, which is now hardwired to the board. It does take away a little from the user-friendly nature of the deck to have to grub around on the floor to turn it on and off. If the power switch is a simple two pole switch, I'd guess it just needs to break the positive side of the feed. Not sure why the lead needs a fast blow fuse or a resistor in series inside of the deck - I'd rather put it in the PSU case. Not sure if there are any issues with using a mains-rated switch for low voltage switching - contact resistance maybe?.

Regards,
Alex

[Simon_Nottingham]

These are the instructions for fitting the Origin Live PSU (https://www.originlive.com/images/pdfs/Instruction-Manuals/Technics-Owner-Manuals/Technics-Power-Supply-Instructions.pdf). Step 7 covers connecting to the switch - nice & simple; no mention there of a fuse & resistor, but different PSUs may have different internal protection, so I'd go with what the manufacturer recommends.

[AlexM]

Good one - many thanks. I think I will reconfigure my internal wiring to make use of the on/off switch, and mount an in-line fast blow 1A fuse internally on the PSU positive supply line.

Regards,
Alex

[Qwin]

A lot of hifi gear has a low value fast blow fuse on the PSU board or input socket to help protect the electronics.
The plug fuse is just to protect the mains lead from seeing the full ring main current in a direct short (trapped in a door etc) which could start a fire.
The Technics stock PSU board has a fuse (350mA).
As this board/fuse is being bypassed when using the external PSU, it may be the external supply does not have a fuse, so it would be wise to fit one?
Just guessing on the power supply fuse, but it would be a logical explanation.

[AlexM]

I'm using your PSU design, Ken. Where would you think the fuse is best placed? in the output from the PSU to the connector on the positive line? what fuse rating would be appropriate?.

Regards,
Alex

[Qwin]

From the Service manual, the Fuse (F1) is rated at 350mA and is on the mains input of the Transformer.

On Your external PSU, you could use an IEC mains input socket with built in Fuse drawer, that would be as good as anything.
Neat and easy to change, stick to the 350mA Fuse rating.

[AlexM]

Thanks - I already have an appropriate IEC and mains fuse.. Was wondering if an additional fuse on the low voltage side was needed because of the problem that could short the DC side.

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