what is this?

[Perry Babin]

It's unusual to have any DC on the input unless you're driving the input from the speaker outputs of a head unit. When driving the signal from a preamp source, you should have no significant DC.

You need to drive the input and follow the signal through the amp (meter set to AC volts). Begin with the signal on the center conductor of the RCA jacks. The signal will go from there to the op-amp nearest the RCA jacks to the preamp board and then to the amplifier. Measure the voltage on the output terminals of the op-amps.

Set the gain to the maximum position, set the crossover to the highest frequency.

 

[scampo77]

so is it ok that i use my computer as sound source? even though it lights up the 2 LED? or should i find a 12v radio/CD to use instead?

 

[Perry Babin]

The DC shouldn't be a problem. The input is isolated by capacitors.

 

[scampo77]

here is the AC voltages i am getting from the pre amp board. none are even close to what you said i should be reading.

all the legs that i left with just a "-" . i get the same thing, it seems to go up and down it goes from 0v to out of range, and it keeps going back and forth.

if i put jumper into the connector for the preamp board then i get sound and it seems to work fine. so does this mean that the op-amps are working on the main board? can i assume that everything is working perfectly fine on the main board?

i have 2 of these pre amp boards that i am fixing at the same time so i have something to compare. when i plug in the other pre-amp board i get similar results. the voltages are the same, either 1.25 or the 0-out of range. except where i find these values dont line up with each other. i mean, the locations where i find voltages dont corrospond with each board

 

[Perry Babin]

Did you set the input voltage and the crossover/gain/boost controls to the same as I did when I measured the voltage on the amp I had here?

 

[scampo77]

yes. all the controls the same and the test tone you sent me and the input voltage is .5v A.C.

 

[scampo77]

here are pictures of the 2 op amps that i replaced and the voltages i found AC with .50ACV into the rca inputs.

i am fixing 2 of these amps at the same time because i am still learning. so i have 2 of these preamp boards. both boards are measuring the same. every single leg of the 3 op amps on the preamp board are now measuring exactly the same. both boards have 1.3 ACV on every leg.

 

[Perry Babin]

What make/model multimeter are you using?

Where was the black probe?

There should be no AC on the power supply pins.

 

[scampo77]

http://www.canadiantire.ca/AST/brow...055 Mastercraft+Deluxe+Digital+Volt+Meter.jsp

that is the meter i am using. the black lead was on the ground shield of the RCA inputs. and the "sound" i was driving into the amp i adjusted to .5 ACV.

i had put 12v to the power of the amp.

 

[Perry Babin]

Try it again with the black probe on the chassis ground. For this amp, the only time you would use the shield as the reference (black probe) is when you're measuring the input voltage on the RCA jacks.

 

[scampo77]

black lead is now on the NEG terminal on the amp. when i put +- it means that it seems to be fluctuating back and forth and the meter tries to range it but it keeps moving around.

 

[Perry Babin]

The amp produces audio with the preamp board jumped out, correct?



Re-measure the AC voltage on the preamp board using chassis ground as the reference.

 

[scampo77]

yes the amp works with the jumper in and the preamp bypassed.

i got these AC voltages but it seems that when i put the meter on AC and the leads on the legs that are marked as 0v it starts at 2 or 3 volts then drops very quickly to zero. i tried both pre amp boards and both are reading very similar.

i know i have a cheap meter but is there something wrong with it?

and the ground lead this time was on the NEG terminal of the amp.

 

[Perry Babin]

Can you borrow a meter (preferably a Fluke)?