Peavey Max 112 - zener current

[g1]

This one seems a bit iffy on the design. +/-15V zeners (1N4744A) are rated for max. 61mA, but need to work at around 70mA (if I assume 10mA idle current total for the 3 IC's running off LV supply).
So it seems they can't keep up and voltage is about +/-17.5V instead.
One of them was shorted, but I think maybe the 250R5W voltage droppers are the wrong choice?

[clintronics]

BING AI - Hello, this is Bing. I’m not an expert on electronics, but based on the datasheets of the 1N4744A zener diode¹²³⁴⁵, it seems that you are exceeding its maximum power dissipation of 1000 mW. The power dissipation of a zener diode is given by P = Vz * Iz, where Vz is the zener voltage and Iz is the zener current. If you are using a 15V zener diode and a 70mA current, then the power dissipation is P = 15 * 0.07 = 1.05 W, which is more than the rated value. This could cause the zener diode to overheat and fail.

One possible solution is to use a higher wattage zener diode, such as a 3W or 5W one, or to reduce the current through the zener diode by increasing the value of the resistor. Alternatively, you could use a different voltage regulator circuit that does not rely on zener diodes.

I hope this helps

[Greg Robinson]

I'd be inclined to replace R42/43 with 470Ω 2W, that'll leave the zeners with ~30-32mA for regulation after the 2xRC4560's and LM3080, that should be plenty and keep them nice and happy dissipating about ~1/2W.
Will also keep the chips happier, if they're at +/-17.5V, that's precariously close to absolute max.

[Jon Snell]

The amplifier has lasted for 18 years so far, there are many out there working well and a design like this gives us all a job.

[Helmholtz]

What is the actual voltage before the series resistor?

[g1]

What is the actual voltage before the series resistor?

Matches schematic, +/-35V.

[g1]

The amplifier has lasted for 18 years so far, there are many out there working well and a design like this gives us all a job.

That was my first thought as well. I usually hate to second guess the designers. But 17.5V from a 15V zener just seems to high. The board is black under the zeners, the caps next to them were domed from the heat.
Haven't heard of this being a common fault for this model, but I do keep in mind that Fender finally gave in and adjusted the LV voltage dropper resistors in the Hot Rod style amps after many years. The Blues Jr. uses 750R in this position and is going from 8V less.
Peavey revised the Max 112 to use voltage regulators in 2012.

I think I will up the value of the resistors as clintronics Greg Robinson have suggested. Also will raise the zeners a bit off the board.