Diagnosing and fixing amplifier fault (loud crackling and popping)

[halfgaar]

Hi,

I've got an amp here for repair, which produces loud crackling and popping in the right channel, ocasionally. It has been doing this for years (5 or perhaps even more) sometimes, but it is getting worse now. I've already determined the fault exists somewhere in the poweramp, right side (left and right are on seperate PCB's).

I know this can be caused by degrading electrolytic capacitors, but tests would seem to indicate that this is not the cause.

My main question is: can transistors, and perhaps MOSFETs, be damaged in such a way that they will sometimes short or go open?

Also, perhaps somebody has other suggestions as to what it could be?

[mrshow4u]

...my guess is noisy differential stage transistors. you can freeze them and see if they get wild

[halfgaar]

I will get me some freeze-spray then

Is there a reason why your guess is in the differential stage, and not the voltage or current amplification stages?

[parsecaudio]

Check also the solders point.
If you do not have a multimeter whith a capacitor leakage measurement,you cannot measure well.
I can advise the substitution of the suspect cap.
What is the model of your amplifier?

[poobah]

halfgaar,

To add to the other suggestions; sometimes you can find bad components by tapping on them with a plastic (insulated) tool.

It does sound like a cap or a solder joint though.

[mrshow4u]

Is there a reason why your guess is in the differential stage, and not the voltage or current amplification stages?

It definitely could be the VAS too. Probably not current gain stage. Probably a stage where voltage gain is happening. I've repaired a lot of noisy amps and 9 times out of 10 it's the diffamp. If it turns out not to be the diffamp, it still sounds like front-end. maybe current source for the diffs, etc. I'd try spraying the diff transistors in isolation (Through a drinking straw, etc) and let them come back to temp, checking if they get noisy with the temperature transistion. Be careful when testing JFET front end's. If the input isn't loaded with a decent resistance (47 kohm, or so), the freeze-spray moisture might cause enough leakage to make the DC move around quite a bit.

[wrenchone]

Tap the offending board and see if it makes a noise. This may smoke out a problem with an intermittent solder joint. Once in a while, one will encounter a problem with an intermittent resistor as well (loose end cap, for example). I had one of those that drove me nuts.

[parsecaudio]

Hi mrshow4u
the same thing has happened to me in many Pioneer amplifiers whith the PNP dual transistor in input differential.Replacing this transistors pair the noise is ended.

[infinia]

Intermittents are the hardest to trouble shoot. I concur with freeze spray along with a careful heat gun can be the best tools. Most probably a problem with a semiconductor. I would guess Vas or a current source transistor if offsets at outputs look OK.

[mrshow4u]

Yeah, I don't know what the failure mechanism is with differential amps either. I don't know if it's silicon contamination or bond wire fails, but it definitely seems to follow "low-noise" transistors.

.....for the most part.