Rockford Fosgate Punch 200.4

[Perry Babin]

Yes.

 

[seankane]

Sorry for the delayed response I have been so busy with other amps kinda just getting back to this one.

With the bias pot fully CW the resistors read as follows:

R352: 3.474 vdc
R330: 1.900 vdc
R331: 11.84 vdc
R339: -3.435 vdc

Does this seem normal?

 

[Perry Babin]

Is this the voltage 'across' the resistors?

If so, you don't need to give the polarity unless you stated which ends of the resistor you had the probes on.

Is R331 out of tolerance? If not, is it significantly hotter than R330?

 

[seankane]

Is this the voltage 'across' the resistors?

If so, you don't need to give the polarity unless you stated which ends of the resistor you had the probes on.

Is R331 out of tolerance? If not, is it significantly hotter than R330?

Yes this is the voltage across the resistors.

I didnt notice it getting hot, I will check the tolerance and temp.

 

[seankane]

R331 gets very hot but it reads 300.1 ohms in the circuit.

 

[Perry Babin]

What happens if you lift D302? Watch for excessive current draw.

 

[seankane]

Should I turn the bias down before doing this?

What happens? Do you mean will the channel bias and produce audio?

 

[Perry Babin]

Probably.

I'm relying on you for information to determine what to do next so post whatever you believe is relevant.

 

[seankane]

I turned the bias fully CCW and lifted D302 and powered up the amp.

There was no excessive current draw, the channel will bias and it produces clean audio on the scope.

The bias had to be turned up a little higher that the other channels though.

 

[Perry Babin]

I'd suspect Q215, 216 and 217. One is likely leaking or shorted.

Edit:

Sorry, those should be the 300 series parts.

 

[seankane]

I replaced Q215, Q216, and Q217 just for good measure. I re-installed D302 and same thing the channel will not bias and has distorted audio.

D302 is linked to something faulty huh?

 

[Perry Babin]

Is R336 within tolerance?

R337?

The protection circuit is engaged. It thinks there's excessive voltage across R336.

If you can't find the fault, lift D302 again and set the bias. Then measure the DC voltage across R336 and R349. They should be virtually identical. If they are not, the one with greater voltage is likely out of tolerance.

 

[seankane]

I had already replaced R337, and its still within tolerance.

I checked both R336 and R349 both tested within tolerance in and out of the circuit. I replaced them anyway and that fixed the problem.

The channel now will bias and produce clean audio.

I find it strange that components can test ok but still be bad, very strange.

I do remember a previous 4 channel amp you helped me with, something similair with a resistor that tested good and wasnt getting hot with excess voltage running across it but it was bad, even though tested good in and out of circuit.

Thanks as always Perry.

 

[Perry Babin]

It could have been a bad solder connection. On the SMD resistor, it could have been that the end plating that makes up the contact had pulled away from the resistive element. Testing it from the top could have made it appear OK.