Repair of Rockford Fosgate Punch 201s

[Jackson]

Hello Perry, sorry for the delay in getting back.
I'm still stuck with this amp...

I measured the voltage across R124, it seems like when going into clipping, the voltage drops. So, obviously the differential input stage will not drive the voltage amp stage hard enough.
The OK channel, also drops the voltage across R246, during clip, but not all the way like the failing ch.

I took some pictures of the oscilloscope during some measurements.


The above picture shows the signal at output terminal (high amplitude) and collector Q111. It can be seen that during negative clip, something strange happens to Q111. Positive swing seems OK though..



This picture shows how nice looking waveform it's at the OK channel, collector Q211.



During hard clipping, it's obvious the voltage amplifier stage gives too less drive for the output stage FET's.

I'm completely out of ideas ...

Regards
Jackson

[Perry Babin]

It may be easier to troubleshoot this from a DC point of view.

I tried this on the amp I have here. I used a 50k ohm potentiometer. One outer leg of the pot was connected to +15v. The other outer leg was connected to -15v. The wiper was connected to the input of the amp.

So that you can measure the voltage across the various resistors with a multimeter, drive the input (Q102) with the wiper of the pot. The voltage on the output should follow the wiper voltage precisely. If it doesn't, you can use your multimeter to measure the voltage on the various resistors to see where the problem is.

You'll likely have to remove C123 and D101 to prevent them from interfering with the drive voltage.

If you want to drive it to the rails (beyond the ±15v from the regs), solder a jumper in place of C123. Without the jumper, the gain of the amp is 1. With the jumper, the gain is 6.

Before you connect the pot, you may want to try jumping C123 and removing D101 to see if it makes a difference.

[Jackson]

Hi Perry!
I tried the DC troubleshooting. I connected a pot according to your instructions.
There is some strange behaviour on the negative side..
By the way, it seems the gain is about 2 without shorting C123 since the output stage gives a gain of 2. Anyway, when turning the pot, applying negative DC at Q102 base, the output starts to follow correctly. But it's restricted to 4 volts negative...
Positive is all ok, right up to the rail.
If turning the pot fast from zero till full negative, the output follows, and for 1/10 second the output follows full negative but returns to -4 volts again.
When applying negative input, it seems most of the current disappears from R124.
I can't understand. The input stage should really drive Q104/Q105 (current mirror) hard enough to drive Q111. Also, I measure voltage drop across R121, which goes next to zero aswell.
The current source seems to be somewhat constant, measuring the drop across R122.
It's waaaaay crazy!!

[Perry Babin]

I think you'll have to jump C123 to get linear response both positive and negative. For one polarity, the cap blocks DC and gives no DC gain. In the other direction, it will leak and cause you to have more gain. With it out of the circuit, you should see a gain of 1 both positive and negative.

Either jump or remove C123 to see if things change.

[Tyler123]

hey can you re upload the schematic of the the punch 201s the link doesnt seem to be workin

[Perry Babin]

Email me if you need the schematic.

babin_perry@yahoo.com