Hi. I've just replaced all the electros and bias a Pioneer SA408. Nice little amp from late 70'.
There's a noise. It has two layers: an almost continuous "ptz" and another glitchy sound that comes in like every 2 seconds. Sometimes other artifacts can be heard. The noise is above the hum at the background. At night or at quiet moments it's annoying. That noise stays at the same level no matter the volume. The loudness and other switches don't affect it either. The bass & treble pots do affect it. At least, that's how it sounds like. I'm going to make some recordings. This noise was before all the recap and such, but I've always thought that it was due to aged components or so.
Cheers!
There's a noise. It has two layers: an almost continuous "ptz" and another glitchy sound that comes in like every 2 seconds. Sometimes other artifacts can be heard. The noise is above the hum at the background. At night or at quiet moments it's annoying. That noise stays at the same level no matter the volume. The loudness and other switches don't affect it either. The bass & treble pots do affect it. At least, that's how it sounds like. I'm going to make some recordings. This noise was before all the recap and such, but I've always thought that it was due to aged components or so.
Cheers!
Is it in only one channel, or in both?
If it is in both channels, is it literally identical noises in both channels, or distinguishable noises?
Unusually, the tone controls are in the nfb loop of the power amplifier, so they will certainly affect the noise.
https://www.hifiengine.com/manual_library/pioneer/sa-408.shtml
So, the noise is in the power amplifier, which is the entire amp (less the phono stage).
There's no line level amplifier.
If it is in both channels, is it literally identical noises in both channels, or distinguishable noises?
Unusually, the tone controls are in the nfb loop of the power amplifier, so they will certainly affect the noise.
https://www.hifiengine.com/manual_library/pioneer/sa-408.shtml
So, the noise is in the power amplifier, which is the entire amp (less the phono stage).
There's no line level amplifier.
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Both channels. It looks like this unit uses an ugly transistor: the 2SA798. Is known for this kind of problems. I have to replace them with two matched low noise PNP.
I found in the other forum what it looks like a good way to match them. By Alan354:
https://audiokarma.org/forums/index...ransistor-2sa798a-v-matched-ksa992fta.779018/
According to this:
What would be my Rin and Rb values? 2k2 and 6k8?
I found in the other forum what it looks like a good way to match them. By Alan354:
https://audiokarma.org/forums/index...ransistor-2sa798a-v-matched-ksa992fta.779018/
According to this:
What would be my Rin and Rb values? 2k2 and 6k8?
Kind of likely that a resistor may be noisy, or a transistor somewhere in the chain has a wonky junction.
The only real way to troubleshoot the problem is to use a signal-tracer in the area before the tone controls, and find the culprit.
Guessing, speculation, will only get frustrating and waste time.
If on both channels, then a power supply problem is likely, and also troubleshot with a signal-tracer, or O'scope.
The only real way to troubleshoot the problem is to use a signal-tracer in the area before the tone controls, and find the culprit.
Guessing, speculation, will only get frustrating and waste time.
If on both channels, then a power supply problem is likely, and also troubleshot with a signal-tracer, or O'scope.
The noise starts at the volume's wiper.
If I'm not wrong the problem must be either the pots (vol, balance) or the loudness switch (S2-3). Am I right?
If I'm not wrong the problem must be either the pots (vol, balance) or the loudness switch (S2-3). Am I right?
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I wouldn't be posting these questions if had all the equiptment and knowlodge. With all due respect, sir.
Having said that. I have a poor-man signal tracer. I can listen the signal along the path, but I don't have any kind of equipment like o'scope and such.
Before the vol pot there's no noise at all. but right at the wiper there it is the noise. I don't understand it, there's a capacitor blocking DC. Is it some kind of AC noise generated by the preamp/amp section?
Before the vol pot there's no noise at all. but right at the wiper there it is the noise. I don't understand it, there's a capacitor blocking DC. Is it some kind of AC noise generated by the preamp/amp section?
Even though there's no DC on the pot by virtue of blocking caps, the pot wiper's intermittent contact can still "gate" the audio off and on as is rotated. It's great to have lab equipment, but ears are useful, too.
Well, in that case, in order to be efficient enough and successful at doing repairs, the first thing is to aquire some test equipment, right?
A guy cannot build a house or work on a car without tools.
The noise is with the pot at minimum and it doesn't increase as the volume is being turned up.
I'm afraid I misconstrued that simply rotating pot generated noise. If noise is present with shorted wiper, anything preceding pot is probably not the culprit.
The only things ahead of the pot are the input jacks and some switches. Not likely to make that sort of noise.
I've desoldered the vol and balance pots, as well as the switch that manages the loudness and tape monitor functions. No luck.
Could be related with the power supply? I don't know...Could diodes (zeners or the ones in the bridge rectifier) make that noise?
Could be related with the power supply? I don't know...Could diodes (zeners or the ones in the bridge rectifier) make that noise?
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It's the 56pf cap to ground at base of Q2. I removed it from one channel and no noise whatsoever. What is doing that cap? Any problems if I live without it or should I try another value?
God, I've even matched some BC560 to replace the SA798(Q2) and it turns out that it was a small tiny cap.
God, I've even matched some BC560 to replace the SA798(Q2) and it turns out that it was a small tiny cap.
That's part of a low pass filter, unlikely to be bad, but maybe it wasn't soldered properly.
Probably should replace it at some point.
Probably should replace it at some point.
I think you have most likely just visualized a leak to ground (which must also affect the potentiometer), now you just have to compare with the other channel if it is still noisy and if not not the case, it is therefore that the problem is common to both channels which means that it probably comes from a power supply which serves the whole.
Both channels behave the same. With the cap there's noise. I increase it to 100pf and the noise becomes louder.
Without the cap, they are silent. I think I can very softly hear the noise when I put my ears touching the speaker. But I'm not sure if it's just my brain making it up. I have to check in a really silent environment. Even though, it would be negligible .
If there's a problem with the power supply, what should I check? I changed all electros, and before the change the noise was also there. I wonder if the diodes at the bridge rectifier could be the culprit.
Without the cap, they are silent. I think I can very softly hear the noise when I put my ears touching the speaker. But I'm not sure if it's just my brain making it up. I have to check in a really silent environment. Even though, it would be negligible .
If there's a problem with the power supply, what should I check? I changed all electros, and before the change the noise was also there. I wonder if the diodes at the bridge rectifier could be the culprit.
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