Dragster Amplifier bad SMPS

[ppia600]

Someone buy that one on ebay so you can post the guts on here! I want to see, haha! Maybe I can convince one of the guys at work to buy it, they all of a sudden don't want to buy the orion or the ppi I bought and repaired.

[ben12345]

Well i'm going to resurrect my thread!

I forgot about this amp, but yesterday I thought i'd have another go at it.

I pulled out the mosfets for channel 1-4 and left the channel 5 fets installed. Channel 5 was the only channel whose output devices weren't replaced.

The amp powered up great and kicks bass out of channel 5 fine.


That got me thinking as to if the output devices were infact installed incorrectly as Perry mentioned. I checked again, and they are installed so that the source of the 640's are on the +ve rail and the source of the 9640's are on the -ve rail. By my reckoning, this is actually wrong. Can anyone confirm this?

[Perry Babin]

Which leg of the output transistor connects to the speaker terminal for the channel?

[ben12345]

The drain of both complementary output devices are tied together and connected to the speaker terminal

[Perry Babin]

if the output is taken from the drain, the P-channel (9640) source should be connected to the positive rail and the N-channel (640) source should be connected to the negative rail.

[ben12345]

Thanks Perry,

Thats obviously my problem! How can I individually remove 8 devices and reinstall 8 new ones in the wrong order while doing them one at a time??? I must have had a hangover or something.

Never mind... i'll do a bit more soldering this evening and see if we can get it fired up.

[Perry Babin]

Check the outputs carefully before reinstalling them. They may have been damaged.

Also, do one channel at a time in case one channel is defective. If you reinstall the outputs in 2 channels and the third gives you problems, you know the troubleshoot the third channel and won't waste time with the two you know to be working.

[ben12345]

Well we have sound through all channels! I reinstalled one channel at a time and all went fine, no problems.

I want to now set the bias.... on most amplifiers i've done this on, they say measure the voltage between points x and y and set voltage to z via trimpot.

Now, without the service manual, how do I know where to measure, and what the value should be?

[Perry Babin]

If you want to set it by measuring the DC voltage across the source resistors, you can set them to 0.001v (no signal, no load). That generally produces enough bias current for most class AB amplifiers.

Have the transistors tightly clamped to the heatsink before adjusting the bias current. After you have it set, let the amp idle for several minutes and recheck it. If it's still OK, drive the amp until the heatsink gets warm (at least to the point where it's too hot to hold your hand on it continuously) and recheck it. If it's not considerably higher than when cold, it's likely OK.

If there's a small transistor on the heatsink (generally between the outputs), make sure it's tight against the heatsink and that there is sufficient heatsink compound between it and the heatsink (enough so that there is no air gap).