I've had a Sansui AU-101 for 15 years - it's given great service. Over the last few months it has developed a really nastty intermittent crackling in the right channel. Using the pots also caused a low crackle but that was tolerable.
I've minimal electronics experience but I'm a technician by trade so I figured it was time to learn some new stuff. I bought the service manual from Rick at http://www.stereomanuals.com and got down to some study.
After several weeks of reading up - well over half of it on audiokarma - I had built up enough confidence to decide that the best start was to recap the amp.
I replaced 46 odd electrolytic capacitors, using Panasonic FM/FC. I also took out the pots, opened them up, cleaned and reassembled them.
When I put the amp back together again I found to my astonishment that a) it switched on b) it emitted no smoke c) it actually worked. Furthermore the "charming" low crackle that used to accompany any use of the bass/treble/volume controls was now gone. I felt pretty damn good.
But then after half an hour of use the nasty crackle returned. That burst my bubble pretty quick.
The symptoms are:
- right channel only
- tends to happen after 15-20 minutes of playing music. Can come and go but with time is happening more and more often.
- the bass, treble and balance controls all affect the crackle. i.e. turning the balance to the left makes it go away. Turning up bass/treble make it sound more boomy/trebly.
- the loudness switch, input selector and tape/monitor switch don't affect it - it crackles no matter if I'm playing a record, CD or whatever.
- the volume control does not affect the volume of the crackle. Sometimes, however, turning the volume control seems to start/stop the crackling. Only sometimes.
So here's what I need advice on:
I reckon that the controls that don't affect the crackle must be before the faulty component in the signal path.
And conversely the controls that do affect it must come after it in the signal path.
That makes sense, right?
So I take a look at the schematics for the pre-amp board - please see the attached image for an excerpt. This shows the section from the input selector switch (LHS) to the connection to the main amp.
As far as my untrained eye can understand the signal path goes
input selector -> loudness -> volume -> treble -> bass -> balance
Am I right on this?
If I am then this is really helpful because there is a clear divide between the components that I previously reasoned were before the faulty component
(input selector, loudness, volume) and those after (treble, bass, balance)
So the faulty component is most likely between the volume and treble pots.
How's my reasoning?
In between these components we have
- 3 electrolytic capacitors (already replaced)
- 6 resistors
- 1 transistor (2SC871R)
- and there are 2 mylar capacitors in series with the treble pot.
Have I any reason to suspect the transistor to be more likely to be at fault than the resistors?
I was thinking of running the amp and poking those particular components to see if that affected the crackle. Does using FreezIt help for this?
Would it be worth just replacing all 6 resistors and the transistor?
(and I'd do the same on the other channel to keep things balanced.)
What is a good replacement for the transistor 2SC871R?
Morden2004 (here and here) recommended the following:
NTE part no. NTE85
ECG part no. ECG85
Radio Shack part no. 276-2009
RCA part no. SK3122
2N4401
my favoured supplier (ie.farnell.com) seems to only have the 2N4401 in ready supply (some of the others are available but on special order - €20 surcharge). The following are available:
MULTICOMP - 2N4401
FAIRCHILD SEMICONDUCTOR - 2N4401BU
ON SEMICONDUCTOR - 2N4401G
Is there any real difference between these?
I've minimal electronics experience but I'm a technician by trade so I figured it was time to learn some new stuff. I bought the service manual from Rick at http://www.stereomanuals.com and got down to some study.
After several weeks of reading up - well over half of it on audiokarma - I had built up enough confidence to decide that the best start was to recap the amp.
I replaced 46 odd electrolytic capacitors, using Panasonic FM/FC. I also took out the pots, opened them up, cleaned and reassembled them.
When I put the amp back together again I found to my astonishment that a) it switched on b) it emitted no smoke c) it actually worked. Furthermore the "charming" low crackle that used to accompany any use of the bass/treble/volume controls was now gone. I felt pretty damn good.
But then after half an hour of use the nasty crackle returned. That burst my bubble pretty quick.
The symptoms are:
- right channel only
- tends to happen after 15-20 minutes of playing music. Can come and go but with time is happening more and more often.
- the bass, treble and balance controls all affect the crackle. i.e. turning the balance to the left makes it go away. Turning up bass/treble make it sound more boomy/trebly.
- the loudness switch, input selector and tape/monitor switch don't affect it - it crackles no matter if I'm playing a record, CD or whatever.
- the volume control does not affect the volume of the crackle. Sometimes, however, turning the volume control seems to start/stop the crackling. Only sometimes.
So here's what I need advice on:
I reckon that the controls that don't affect the crackle must be before the faulty component in the signal path.
And conversely the controls that do affect it must come after it in the signal path.
That makes sense, right?
So I take a look at the schematics for the pre-amp board - please see the attached image for an excerpt. This shows the section from the input selector switch (LHS) to the connection to the main amp.
As far as my untrained eye can understand the signal path goes
input selector -> loudness -> volume -> treble -> bass -> balance
Am I right on this?
If I am then this is really helpful because there is a clear divide between the components that I previously reasoned were before the faulty component
(input selector, loudness, volume) and those after (treble, bass, balance)
So the faulty component is most likely between the volume and treble pots.
How's my reasoning?
In between these components we have
- 3 electrolytic capacitors (already replaced)
- 6 resistors
- 1 transistor (2SC871R)
- and there are 2 mylar capacitors in series with the treble pot.
Have I any reason to suspect the transistor to be more likely to be at fault than the resistors?
I was thinking of running the amp and poking those particular components to see if that affected the crackle. Does using FreezIt help for this?
Would it be worth just replacing all 6 resistors and the transistor?
(and I'd do the same on the other channel to keep things balanced.)
What is a good replacement for the transistor 2SC871R?
Morden2004 (here and here) recommended the following:
NTE part no. NTE85
ECG part no. ECG85
Radio Shack part no. 276-2009
RCA part no. SK3122
2N4401
my favoured supplier (ie.farnell.com) seems to only have the 2N4401 in ready supply (some of the others are available but on special order - €20 surcharge). The following are available:
MULTICOMP - 2N4401
FAIRCHILD SEMICONDUCTOR - 2N4401BU
ON SEMICONDUCTOR - 2N4401G
Is there any real difference between these?
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