Punch 500aII rail voltage/Voutput

[brandes.cm]

Working on a 500aII that outputs 26Vrms on one channel and 22Vrms on the other. I have no load on the outputs and feeding a 1Khz sine to the inputs.
The rail voltages read +-49Vdc. When I got this amp one channel was completely blown, all fets were shorted. I have since got that channel operational, that one outputs the 26Vrms. Even loading the outputs with 3ohm load makes no difference.

It seems to me the output waveform should be closer to 30Vrms. Could sourcing from an mp3 player be causing some issue. I know Perry has mentioned that in the past.

Any thoughts

Cory

[Perry Babin]

I don't know if Rockford amps are sensitive to floating grounds. If the mp3 player is operating on battery power, you can connect a jumper from the shield ground to the chassis ground to eliminate the floating ground.

Does the amp clip at 22 and 26v?

If not, are the gains at the max position?

[brandes.cm]

No it won't clip at 26 or 22 and the gains are maxed out. I will look at the output of U104B and U204B.
I have not seen issues with the front end on rockford amps, maybe this one does have a problem.

[brandes.cm]

These readings are done with 1Khz sine input.

U104A
Pin3 707mVrms
Pin1 3.6Vrms (with gain at max)
Pin4 -13.1Vdc
Pin8 13.3Vdc

U204A
Pin3 707mVrms
Pin1 4.6Vrms (with gain at max)
Pin4 -13.1Vdc
Pin8 13.3Vdc

Schematic shows VCC at +-15Vdc

Cory

[Perry Babin]

Is the crossover set to full range and the bass and treble boost set to the minimum?

The ±13 is close enough.

[brandes.cm]

Crossover is set to full, treble and bass are set to minimum.

Could a leaky cap cause what I am seeing? What is the best way to test that, ohm meter and watch the meter rise?

[Perry Babin]

What is the AC voltage of the input signal on the RCA jacks?

Measure both channels, black probe on shield, red probe on the center conductor where it comes out of the back of the RCA jack.

[brandes.cm]

Scope reads 270mVrms, meter confirms. I've never had issue before with this setup. MP3 player won't be used anymore, see below!!

Swapped out the mp3 player, pulled the cd changer from the attic and will be using it from now on. I get roughly
2.2Vrms on the input now, the outputs can be driven to clip. They both clip around 35Vrms. Should I worry about the input difference with low amplitude?

Cory

[Perry Babin]

Measure the AC voltage at each non-inverting input for each op-amp in each channel and for each op-amp output in each channel. Compare the left channel to the right channel to find the point where the signal drops off. If I'm not mistaken, all non-inverting inputs should equal the outputs for each op-amp section except for the first op-amp (U101) in the circuit which will likely lose ~10% and U104/204 which will vary depending on the gain setting.

[brandes.cm]

After switching the source from mp3 player to cd changer, there is no difference in readings. With the gain pots set to low, I read 1.75Vrms through all the stages.

For the record I was using a Creative Labs Zen 30Gb.

Cory