punch 500x bias voltage

[Hexhclass]

what voltage should the bias be in this amp?

i also have a small punch with no info on it.It is a two channel using 1 irf540 and 1 irf9540 per ch.Two PS FETs.I set this one at 0.1mv and it seems to be playing good.Any idea where it should be?

 

[Perry Babin]

This is how I generally set them for Rockford amps.
http://www.bcae1.com/temp/ausettingbias.swf

 

[Hexhclass]

The amp has 4 ports .If i set one at the right gage how would i no when the oder 3 is at the right gage?

 

[Perry Babin]

Each channel would show the same tiny increase in current draw.

 

[Hexhclass]

One of the channels is giving me a hard time to set.I set the other 3 ch @0.2mv but the next ch bias starts at 4.3mv and up.I change the port but no change.

Also the 2/4 ch input switch in making the amp draw more current when it is in the 2 ch position.where does this switch have to be when setting the bias?

I have a 50w light bulb for limmiter

 

[Perry Babin]

The switch shouldn't make a difference in the idle current. Don't have speakers or RCAS connected when setting the bias.

Did you replace the outputs in the channel with higher bias?

 

[Hexhclass]

I only replace 4 PS FETs
I think the owener of the amp turn up the bias trying to get more power from it.

The input switch is making the amp draw a little more amps when it is set to 2ch

 

[Hexhclass]

I found a leak on one of the FET in the same ch.I will change it and see what happen

 

[Hexhclass]

I used a next meter and FET is good

 

[Perry Babin]

Short the collector and emitter of the bias channel for the defective channel. Does that cause the bias voltage across the source resistors to go to 0.000v DC?

 

[Hexhclass]

Short which transistor?

IF i turn the port all the way counter clock wise the bias is 0.00v

If i try and set the bias it does not start at 0.01 It starts about 4.3mv and goes up and down.

 

[Perry Babin]

The bias transistor is the one between the output transistors.

0.00 isn't good enough resolution. You need to state voltage to millivolts.

 

[Hexhclass]

My meter shows mv only when it is set to auto range 0.0mv


With the bias port set fully counter clock wise.
Input switch set to 2ch and gain port full counter clock wise.
With emitter and collector short i get .0mv across emitter resistor
If i turn the gain port all the way up at full i get 13.5mv (unstable) across the emitter resistor.
If i set the input to 4ch i get .0mv even if i turn the gain port all the way to full
I think the input section have a problem

 

[Perry Babin]

It sounds like the amp may be oscillating.

If possible, set the defective channel to low pass to see if that makes a difference. Remove the jumper on the bias transistor.

 

[Hexhclass]

I try it in LP and HP no difference

 

[Perry Babin]

Are the legs for the frame of any of the front panes switches shorting to a trace? There was a problem with that on a few amps but I don't remember which ones. It caused strange problems.

 

[Hexhclass]

I check all switchs and did not find any short.

 

[Perry Babin]

Do you have a scope?

 

[Hexhclass]

not yet

 

[Perry Babin]

Do you have a large resistor that's about 2-4 ohms?