Peavey JSX Mini Colossal

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Peavey JSX MiniPeavey JSX Mini This amp came to me with a very low output problem from a music store. All the power supply voltages were fine, but the output was low and it seemed like the amp was loaded down because the output transformer was fine. Since the amp was a "Power Soak" feature, I disconnect that to see it it was the issue. It was not. Upon futher disassembling the amp, it was apparent that the output wire from the output transformer was pinched. The power soak PCB was pinching the wire against the chassis. It was probably fine at the factory, but over a period of time and jostling around, it ended up a partial short. A little heat shrink tubing and re-routing the wire fixed things right away.

Then I went on to some mods... I wanted to improve the low end. I made 4 modifications:

  • Increase coupling capacitor C1 to 0.069 uF up from 0.022 uF. I did this by adding a 0.047 uF cap in parallel.
  • Increase coupling capacitor C1 to 0.069 uF up from 0.022 uF. I did this by adding a 0.047 uF cap in parallel.
  • Increase bypass capacitor C7 to ~500 uF up from 22 uF. I did this by adding a 470 uF cap in parallel.
  • Short out unneeded resistor R38. Al this does is form a voltage divider with R14 and you lose about 1/3 of your signal here.

All these mods were done in a way to make them easily reversible and to keep the original components in place. I found that my mission was accomplished, especially when hooked up to a larger speaker cabinet. The included 8" speaker does not do it justice, the amp really comes to life on a more transparent cabinet such as my Marshall 4x12.

The modified schematic is attached below.

AttachmentSize
PV_JSX_Mini_mods.PNG78.61 KB

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