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Old 19th January 2013, 09:40 AM   #41
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This amp doesn't use conventional source resistors so you can't set the DC voltage across them.

These amplifiers are unstable without the bias being set. You can use a combination of the current draw and the 'noise' on the output of the op-amp driving the channel. As you reach the proper bias, the output of the op-amp will become cleaner (less high fr3equency noise).

The bias is very touchy so you need to make sure that the bias pots are not intermittent. If they are and the bias creeps up, it can cause the channel to run hot or fail.
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Old 19th January 2013, 06:28 PM   #42
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I have one channel that was not repaired, that starts to draw current as soon as you move the bias pot. It does not make alot of noise when its all the way down like the other channels. The channel works fine, but I want to be sure I dont have any bias issues.

What to check?

There is an 82 ohm flame proof resistor looking like it connects to sections of the amp together. It has been clipped in half. I have seen this on other 650's as well. Some are hooked up and some are clipped. Whats it for?

I hooked it up and the current draw of the amp started slowly getting higher and higher. I clipped it and left it out of the circuit. The amp seems to work fine without it but I really want to know what it does.

How many amps should this amp draw at an idle?

May have caused the issue with the other channel?? Dont know???
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Old 19th January 2013, 08:21 PM   #43
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Which two sections of the amp? Photo?
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Old 19th January 2013, 09:01 PM   #44
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This is the 82 ohm flame proof resistor. I have seen it clipped in alot of 650's and some I have seen still in tact.

The bias seems fine now I dont know why it was acting up.

I just want to know what this resistor is for or does?
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Old 19th January 2013, 09:16 PM   #45
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It appears that one end connects to the shield ground and the other end goes to the primary ground. They may have cut it because it was causing engine noise in some installations.
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Old 19th January 2013, 11:20 PM   #46
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Ok so its not going to hurt anything left unconnected then.

I have another Power 650 for repair. I will start a new post if needed.

It draws 20 amps current. The 75 ohm resistors next to the D40/41 are burned. Pulled it apart to find someone had replaced the outputs with NTE2371's on 3 of them.

There is also a bunch of mis matched date codes on the other channels FETS as well. Looks like this one will need all new outputs........am not looking forward to changing them all there are so many, and its delicate to not ruin the solder pads on the board.
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Old 20th January 2013, 02:24 AM   #47
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Apply enough new solder to allow the soldering iron to bridge all 3 terminals, lay the iron across all 3 terminals and slide the outputs off of the pads.
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Old 29th January 2013, 11:00 PM   #48
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Perry changed the NTE fets out to IRF9540 just to test, replaced the burned resistors and the amp powers up stable, has no DC on the output wires and produces clean audio.

Question......These amps are very noisy without the bias set properly and I was viewing the output through the output pins of the op-amps in the feedback section to clear up the noise with the bias. The other three channels bias settings are close to the same. The channel that was worked on the bias has to be much higher to get rid of the noise. Its not drawing any current being higher.

Do you think its because I have the original N channel FETS installed with the newer P channel IRF9540's?

I am going to change all the outputs in that channel, I was just curious. The amp seems to work great.
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Old 29th January 2013, 11:17 PM   #49
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The difference in the position of the bias control is likely due to the different threshold voltages for the various transistors. It's not really an issue.
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Old 30th January 2013, 05:08 AM   #50
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I have played the amp on the bench for about 45 minutes, its getting pretty toasty to the touch and the fan has not came on yet.

I tested the fan on my supply before the cover was installed and it worked fine. Its been so long since I have ran one of these I dont remember how long it takes for the fan to come on.

Should I be concerned and which component controls the fan?

I set the fan trim pot and it was working then but not now.

I know its one of the 2 components that sits up against the heat sink, not sure which one of the two I should check?
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