Pioneer amp, no gain

[vincegironda]

Hi, I have a GM4200 which will not raise or lower the input gain when the gain pot is adjusted.

I have cleaned the gain pot, checked for dry solder joints and no change.

I tested the amp with both phono and din input, interestingly when I use a din input the amp plays louder but the audio is distorted unless I turn the source volume down.

Using din input has no affect on the gain pot either?

 

[JonSnell Electronic]

Faulty gain pot?

 

[Perry Babin]

Is your signal source ground connected to the amp's primary ground (12v supply ground)?

 

[vincegironda]

Is there anyway of testing the gain pot without removing it from the board, I have a feeling it's not the gain pot.

When the source vol is turned up the audio becomes distortedn, this amp should play clean even at full volume on low gain.

 

[vincegironda]

Hi Perry,

Yes the signal source ground and the amp's primary ground are the same.

 

[Perry Babin]

Is the source distorting?

 

[vincegironda]

No the source is working fine Perry.

I just hooked it up to a Kenwood KAC923 and the audio was crystal clear and much louder even at very low gain.

 

[vincegironda]

Could the NJM2068S dual op-amp be faulty?

 

[Perry Babin]

Where does it become distorted in the signal chain? If you don't have the service manual, the attached image should be close enough.

 

[vincegironda]

Thanks Perry, I assume I will need a scope to check the signal chain?
I don't have one at present.

 

[Perry Babin]

You can use an audio signal tracer. Are you in the US?

 

[Perry Babin]

Have you checked the various points in the audio signal path for excessive DC?

Have you confirmed that the muting is releasing?

 

[vincegironda]

How do I check the muting?

I did a quick experiment bypassing the input circuit and the amp plays as it should so maybe it is stuck in mute?

 

[Perry Babin]

The voltage on the muting transistors (base to emitter) will be 0v to pass audio (no muting) and about 0.6v to shunt audio to ground (mute).

 

[vincegironda]

Is there any way of bypassing the muting circuit?

 

[Perry Babin]

The simplest way is to pull the muting transistors.

 

[vincegironda]

I can't make out the Q numbers on the schematic for the muting transistors Perry, can you check them for me please? might be clearer on your side.

 

[Perry Babin]

You're not seeing it full size. Drag the thumbnail to a new tab in your browser then click anywhere on the image. Navigate with the scrollbars.

Or click the thumbnail to enlarge to fit the window and then click the arrows in the lower left corner. Navigate with the left mouse button (drag).

 

[vincegironda]

Great thanks Perry.