output

[acebaker]

I asked this question in the solid state forum now I'll put it here. Just starting to learn about amps so bear with me. PPI PC2150 in working condition but when i went to adjust the gain i noticed that the left channel operated at about 2.25VAC less that the right using a Fluke 75 multimeter, test tone cd ( 1khz) and alpine 7995 deck. I swapped RCA's on the deck to make sure that wasn't the prob. I opened the amp up and found a blown gate resistor on the one of the pwr supply fets. changed it and it blew again. changed it again along with the fet (IRFZ 44N) and all seems well in the pwr supply dept. No change in the output. There are no other obvious (visual) defects. What can I look for. I cannot hear a difference when playing the amp and it appears to play clear but all the other amps I have have nowhere near the voltage spread btween channels. Thanks

[Perry Babin]

Are both large 10 ohm resistors near the RCA jacks within tolerance?

Is the mismatch of level constant at all gain levels?

Is the mismatch of level constant at all input (volume) levels?

Have you worked all switches and pots to see if any are intermittent or in need of cleaning?

[acebaker]

Resistors are at 10.8 and 11ohms both on the Fluke and the cheapo meter. At 15 on the 7995 and min gain the difference is less than .5V. At 30 it's 1V. Deck at 15 and gain at min the diff is less than .5V but at 1/2 gain it's 2.3V. Cleaning had no effect. Is the gain pot pooched? Where can I get a suitible replacement? I'm going to break down and buy your DVD Perry. I have a Tek 2235 that i need to learn how to use too. Pawnshop, cheap. Thanks

[Perry Babin]

Do you have an oscilloscope?

[acebaker]

Yes i do have a tektronic 2235 but as I am in the infancy of electronic knowledge I still don't know how to use it yet. However i took the board off the amp and fluked the gain pot. I set the meter at kohm's and i'm assuming minimum gain is maximum resistance. There are two sets of three pins with each set having two with a common connection. There was no continuity between the two rows just between the three pins on each row. Putting the leads on the outside pins I got 0 kohms on both then at half way one had 1.12 kohms and the other 1.51 kohms. When i went to the other end it measured 2.73 kohms and 4.76 kohms. I'm assuming it is just a simple double potentiometer. Before i rip another amp apart to compare it to, could this be the problem? Thanks.

[Perry Babin]

Look at the speaker level signal for both channels at ~1/2 power (or where you get the 2+v difference). Is the signal symmetrical (positive and negative) for both channels?

Set the scope's timebase to 0.5ms/div and the vertical amp to whatever v/div makes the waveform vertically fill ~3/4 of the display. This assumes that you're using a 1k test tone.

[acebaker]

Problem solved. Gain pot was the source. Changed it now have .2V difference in output voltage. Going to install it in car this week see how it performs.