Ground Zero 24 volt amp repair

[Perry Babin]

I'd add pulldown resistors between pins 9/10 and ground. This may also help the drivers to operate at a lower temperature.

Why is it only producing a drive signal of ~12-15v when you are operating at 24v? That could be part of the reason the drivers are heating up.

[PQatPIT]

Added 820 ohm resistors from pins 9 and 10 to gnd, and took out gate resistors between MPSA56's and gnd, see pic. Better?

The 24 volt line is feeding a 220 ohm / 1 watt resistor, after that there is a 16 V zener diode to gnd and this goes to TL494 pins 8 and 11, here I measure about 16,6 volts when supply voltage is 27,3 V. I have understood that fet conducts at saturation even when it is conducting higher voltage than there is on gate, correct?

I took out MPSA56's and replaced them with ones with longer legs, should help with some cooling.

Thanks!

[PQatPIT]

There is also a 220 ohm 1 watt resistor with some zener to gnd between remote line and pin 12. I guess that TL494 was never meant to see 24 volts, only 16 V or so.

[Perry Babin]

They regulate the voltage to prevent having more than 20v drive going to the gates. Above 20v (voltage from gate to drain), the FETs could be damaged.

As long as the Vgs is 10v or more, the FET will be fully on (saturated), no matter the circuit voltage (Vds).

The drivers will probably run hotter with longer leads. With short leads, they can sink some heat to the board. No matter the length of the leads, they should be reliable.

Are the FETs running cooler with the pulldown resistors on 9 and 10?

[PQatPIT]

Hello and Thank for your help!

This amp is now playing, pic shows gates of power supply FET's. They get warm but not too much. MPSA56's get quite hot, can touch them for few seconds though.

Still it draws about 3 amperes when voltage is 27 V and with no signal, after ten minutes of playing easy background music. This is too much I think.

No other transistors get hot, only warm. Sound is ok, as far as cheap amps usually can provide. +-15 volt line is steady, just as +-36 volt power supply lines, no noise or other oddities there.

PQ