Nakamichi PA504 Car Amp

[kds9591]

Working on a Nakamichi PA504 4 channel car amp. Has no sound output, outputs and power supply components ok, no obvious shorts or burnt components. Voltages on the TL494 ok, pin 5 has the sawtooth, 5VDC reg, ect. Seems to stay in mute mode, only draws about 250mA, may be a protection circuit problem.

I Never worked on a Nakamichi car amp so I'm not familiar with them at all. Anyone have ideas or has a schematic or know where I can get one. Any help would be greatly appreciated.

[Perry Babin]

Do you have rail voltage?

250ma is extremely low current for a 4 channel class AB amp that's simply muted.

[kds9591]

No rail voltages. A picture of the inside is at AmpGuts, link below.

http://ampguts.realmofexcursion.com/Nakamichi_PA-504/

[Perry Babin]

Do you have pulses on pins 9 and 10 of the TL494?

[kds9591]

Yes, both 9 & 10, square wave 10VPP 40KHz.

[Perry Babin]

Do you have the same signal on the gates of the FETs?

[kds9591]

I rechecked the FETS and they are shorted, the signal was on one side of the 100 ohm gate resisters. The FET's have heat shrink tubing on the leads and a clip to hold them to the heat sink, so my probes must not have made direct contact with the leads when I was checking for shorts the first time.

The owner of the amp said the amp just cut out when driving, normal volume, not being pushed at all. FET's- BUZ102S -hard to read the number, is that a good number, equivilents?

I hate to replace the FET's only to have them short out again after power-up because the transformer has some shorted turns.
Sure sounds like that may be the case, some loose turns with spotty varnish may have shorted together.

Your opinion Perry?

[Perry Babin]

That's a good number but it appears to be obsolete. If you can't find them, I'd suggest IRFZ44s. If you want to use something rated for more current, you'll probably have to use 68 or 47 ohm gate resistors.

When you power up the amp, do so through a current limiting resistor or a 10 amp fuse. Have the transistors clamped when you apply power.

If you don't change the gate resistors, check each one of them to make sure they're within tolerance (±5%?). Also check the PNP driver transistors. I'm not sure what they're using but it will be in the circuit between the 494 and the gate resistors.

Before reinstalling the FETs, make sure you have a clean signal at the gate pads.

It's possible that the transformer is shorted. The fuse or limiting resistor will help protect the new FETs.

[kds9591]

Thanks for all your replies Perry. I do have some new IRFZ44's on hand.

I always use a 3 or 5 ohm current limiting power resister in series when I first power up a amp & clamp the FET's to the heatsink, it's just that sinking feeling you get if the idle current climbs over 2 amps and knowing if you power-up directly the FET's you just installed will be toast.

I'll press on the transformer windings while watching the current for changes and hope the current stays stable.