1980s EHX Stereo Memory Man - Remove internal power transformer

[clarisso11]

Hello everyone,

I recently got a 1980s EHX Stereo Memory Man in for repair, it is the US voltage version. As a first step, I wanted to convert the power supply to accept an external power adapter, removing the internal power transformer, and installing a standard pedal power jack.

I'm hoping someone can help with this, as I want to make sure I do it properly. The key things I am trying to achieve are:

1. Retain polarity protection through diodes
2. Retain power filtering
3. Choose the appropriate external power supply (24V maybe)?

I have attached a schematic for the unit. Where would be the optimal places to connect the new power jack to?

Any help would be greatly appreciated!

[dmeek]

The easiest way would be to use a DC-DC Converter. 9V in to +/- 15V out.
This one can handle 100mA: https://www.mouser.ca/ProductDetail/...8tWgSK0Q%3D%3D

TRACO Power TEC 3-0923
tec3_datasheet-3049618.pdf

[clarisso11]

Interesting solution, I can see the benefits as it would let me use the standard 9V power supply.

I do have a 24V power supply lying around and wouldn't mind using that for now, if that means I can get by with connecting the new jack directly to the board for the time being. I'm thinking along the lines of this article: https://www.fxdoctor.com/mxr-analog-...er-conversion/

Where would I connect the wires coming from the power jack to?

[Helmholtz]

Don't forget that the unit needs 2 separate supply voltages (+15V and -15V), so just connecting to a single external DC supply won't do.

What are secondary AC voltages with 120VAC at the primary?

[clarisso11]

Hi Helmholtz,
Thank you for the warning!
Unfortunately I don't have a step-down transformer to power the pedal at 120VAC. Not sure if that helps, but the transformer is marked: "R-811/POTRANS XA-0020"

[Helmholtz]

No variac either?

My idea was to supply the unit with an AC voltage corresponding to the full ACV of the unit's PT secondary (should be around 30V).
The external ACV voltage would be connected to the full secondary of the unit's PT.
Means that the internal PT is used as an autotransformer providing the necessary CT.

[clarisso11]

I see! No variac either I'm afraid.

I think I will be going the DC-DC converter route, using the converter suggested by dmeek. Should be easy enough to wire it together using some veroboard, attached to where the transformer currently sits.

[clarisso11]

The easiest way would be to use a DC-DC Converter. 9V in to +/- 15V out.
This one can handle 100mA: https://www.mouser.ca/ProductDetail/...8tWgSK0Q%3D%3D

TRACO Power TEC 3-0923
[ATTACH]n980895[/ATTACH]

When using this DC-DC converter, I'm assuming the ground and 9V wires from the power jack connect directly to the corresponding pins (GND, Vcc). Do the output pins (+Vout, -Vout) connect directly to the points marked as +15V and -15V on the schematic, skipping the diodes and 220uf filter capacitors?

[dmeek]

Bypass the regulator and zener but keep the 10uF cap like this

[clarisso11]

Bypass the regulator and zener but keep the 10uF cap like this

This is incredibly helpful, thank you!! I managed to find that exact part here in the UK, should come tomorrow. Will report back with results once I have it all wired up.

[dmeek]

Might be a good idea the add a resistor to pedal ground from one of the adaptor input leads, doesn't matter which, to prevent a common mode voltage at the input. anything from 100K to 1M would do it.

[clarisso11]

Will do, thanks!

[Helmholtz]

The nice little thing is switching at 100kHz.
Don't you need some filtering at the input to prevent RF radiation by the DC supply cable?

[dmeek]

Might be needed. Here is what they say tec3_emi_consideration.pdf

[clarisso11]

Might be needed. Here is what they say [ATTACH]n980940[/ATTACH]

Thank you, I will try to recreate this filtering circuit.
Do I still need the resistor to pedal ground and retain the 10uF capacitor, mentioned before? Or would the 10uF capacitor be replaced with whatever is recommended in the new filter circuit?