Old EHX pedal, Input jack ground connected to V.Ref??

[Scruffie]

I posted this in another thread;

http://www.diystompboxes.com/smfforum/index.php?topic=108187.msg1032502#msg1032502

But figured it'd get more attention with its own dedicated post.

So I have an early EH Memory Man prototype with built in power transformer in for repair at the moment, got it working but from the way it's currently wired, the output pull down connects to V.Ref and the 'boost' switch which requires V.Ref to function, is hooked to the input jacks ground. So to function properly, the enclosure and input jack ground has to be hooked to V.Ref.

I'm wondering if this was just an outdated practice or if the previous owner has done some guess work when it came to wiring it back up and I should correct it...

Is having the power cord earthed to V.Ref going to lead to some issues too? Doesn't seem right to me.

There's no schematic that matches this model exactly to confirm against so some insight would be helpful.

[Cozybuilder]

Since this has an on-board power supply, then I assume it connects to the mains. Does it provide + and - supply voltages with the V-ref between them? If so, V-ref could very well be the same as ground on your other pedal power supplies.

[Scruffie]

Yup it connects to the mains but no it's not a bipolar supply.

If you look at the other thread, this is the voltage regulator supply

Then there's another OTA for the V.Ref.

[antonis]

Is there any magic to turn 23Vdc to 13.3 Vdc at Vdd or am I missing something..

[Scruffie]

The transistor's being used as a zener, other than that, no magic.

I think it's drawn a bit wrong too, but it's close enough to make sense of.

[anotherjim]

If the PT secondary, or either +ve or -ve DC rails have no direct connection to chassis or AC supply cord ground, it's perfectly feasible for Vref (if by that we mean the 1/2 supply reference) to be tied to signal/chassis ground. After all, if the power supply was bipolar, the Vref would be ground. Taking voltages in the circuit with the reference (neg meter lead) on ground would show the supply negative rail as a negative voltage.

[Scruffie]

I know it's virtual (yes we do mean 1/2 supply reference, or on this case like 1/3rd because of the odd set up) ground but it just seemed so wrong, never seen it done on a pedal before.

Oh well, guess you're right, i'll just hook it back up as is... although it might benefit from an input cap for anything prior to it.

[antonis]

The transistor's being used as a zener, other than that, no magic.

OK.. I give up..!!

(In your draw, transistor's base and diodes anodes should be at the SAME voltage but... "Vivere et discite"  )

[anotherjim]

Is there any magic to turn 23Vdc to 13.3 Vdc at Vdd or am I missing something..

It's noted on the drawing that the voltages were taken from different negative references. Looks like from Chassis ground out of the rectifier and from "node" 8 (Vref out of the regulator amp pin at the "Zener" transistor. Yes, it's the same power rail but from different reference levels.  The readings were taken before Node 8 got connected to jack ground yes?

[Scruffie]

Just to point out, that's not my drawing, I haven't traced the area as it's working fine and I get an expected 13V, I linked it just to show it's not a bi-polar supply.

If you go to the thread I linked in the first post that it came from http://www.diystompboxes.com/smfforum/index.php?topic=108187.msg1032502#msg1032502

PRR explains it quite nicely.