EHX Deluxe Memory Man issues

[jordan s]

Working on a repair for a friend. Basically from what I can tell the issue is that the unit is not getting powered. I seems pretty simple but I'm baffled. Schematic is attached. It goes- 24v input -> 150 1W resistor -> reversed 1N4003 -> LM7915 input. There is also a filter cap from the 1N4003 out to ground. Current stops at the diode. I don't really understand having the diode and filter cap reversed like this. I tried to mock up an exact power supply section completely separate from the pedal and I get the same result, nothing after the diode. What am I missing here?

 

[Khron]

Isn't that 24V supposed to be AC, by any chance?

 

[moamps]

Isn't that 24V supposed to be AC, by any chance?

No, it's DC but the trick is that the - pol is hot, + is common (ground).

 

[CJ]

replace the power jack.

that is after you check the diode and flip it if it is in backwards.

grounding is usually the culprit die to pwr jack internal sw.

that jack gets a lot of mechanical cycling stress.

 

[CJ]

if the guy is a pro jock then ask him to let you hard-wire the supply, eliminates two problem areas, the plug and the jack,

 

[jordan s]

Thanks for the suggestion, CJ. I'm actually already using an external bench supply clipped directly into the pcb which measures fine after the current limiting resistor but somehow that intentionally backwards diode stops the current dead in it's tracks. The schematic shows it being reversed so I can only assume there is a reason that is too advanced for me.

 

[PRR]

> bench supply clipped directly into the pcb .... that intentionally backwards diode stops the current dead in it's tracks.

This rig is POSITIVE GROUND!!

It would already be dead if not for that diode.

Wire the power the way EHX intended and power will go through the diode.

 

[jordan s]

Wow, thank you PRR. This is the first time I've dealt with a pedal that's positive ground, I didn't even know that was a thing. I've had no luck finding the power supply from EHX. Every distributor is out with no stock expected any time soon which is why I had to resort to my bench supply.

 

[CJ]

$10.99 in stock>

https://www.sweetwater.com/store/detail/24DC100--electro-harmonix-24dc100-24-volt-power-adaptor

 

[PRR]

Most pre-1955 cars were positive-ground; MG even later.

The 7915 exists only for positive-ground situations.

The issue here is, for mostly historical reasons, many BBD (delay) chips were documented as positive-ground due to their PMOS technology. Working as audio delays, they "can" be fed either way; EHX apparently kept with tradition.