ampeg scrambler - short between battery leads

[Dr Ron]

I'm repairing an Ampeg Scrambler. It's a newer version with MPSA13 transistors and all NP caps (instead of polar electrolytics). The owner states that it was broken from the factory and was never used.

I measure a short circuit between the battery leads, and the battery gets very hot. The DC voltage (battery or PS) gets pulled down to near zero.

Does anyone know what to check?

I did check traces for a big solder blob, but everything looks fine.

There's no LED, so the switch doesn't have +9V.

Does anyone have experience with these?
Any guidance would be greatly appreciated!

[theehman]

If there's a polarity protection diode, I'd check it first.

[Dr Ron]

No polarity protection diode.

The battery + is connected to the large plane on the board, which is (I thought) supposed to be the ground plane (battery -).

Also, the battery short only appears when a coax cable is plugged into the input jack.

Could this be as simple as the battery is wired backwards?

The battery + goes to the pin on the side of the external DC jack.
The battery - goes to the middle pin on the coax input jack.
The DC jack pin closest to the outside goes to the circuit board (Q1 collector, etc)
The DC jack pin inside the board goes to th ground (or 9V?) plane

I think that's correct.

[Dr Ron]

There is a diode D6 that connects from ground to the emitter of Q3.
D6 isn't listed on the schematics I have. Maybe that's the one.
It doesn't look like the other diodes (it's smaller).

Could this be a 9 volt zener?

[Dr Ron]

D6 was it!
My guess is that it was a zener protection diode installed backwards.

Thanks for the help!!