Diagnosing crackle on AU-101

scytale

New Member
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?
 
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I'd bet it's a small signal transistor. These are sweet sounding amps. I'll have a look and get back to the thread.
 
Crackle could also be a bad tube, when they go microphonic the sound can crackle like you describe. Try swapping tubes from one channel to the other and see if it follows the tube swap. Try one set of tubes at a time, begin with the small signal/preamp tubes first. Let it play for a while... before swapping other tubes.

Also take a look at all your solder joints and be sure there is no greyish spidery looking solder joints, they should ALL look shiny silvery. It wouldn't hurt to just hit any of the solder joints with the tip of your solder iron just to be sure they are good (only touch for like 3 seconds.. count 1 thousand 1. 1 thousand 2, 1 thousand 3.. .lift off the solder tip and repeat.

Cheers,
Bob
 
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+1 on the transistor.

if you have a freeze spray, you can verify this easily: freezing the 2SC871R should stop the crackle momentarily.
 
Following the advice of Echowars and MarktheFixer, I order the KSC1845 and KSA992 by the hundreds and use them pretty much anywhere a small signal transistor goes. They usually match up.

I haven't checked the datasheet for the 2sc871, but I bet the KSC1845 would be a good sub. They usually make a very good improvement in phono stages. And they are dirt cheap.

I get them from Mouser.
 
Following the advice of Echowars and MarktheFixer, I order the KSC1845 and KSA992 by the hundreds and use them pretty much anywhere a small signal transistor goes. They usually match up.

I haven't checked the datasheet for the 2sc871, but I bet the KSC1845 would be a good sub. They usually make a very good improvement in phono stages. And they are dirt cheap.

I get them from Mouser.

thanks for the advice.

any idea if there's much of a difference between 2sc871 and 2sc871r ? (I have the latter).

anywhere I can find transistor equivelance tables online? my searches so far just bought me to datasheets for individual transistors or to references to printed matter.
 
Crackle could also be a bad tube, when they go microphonic the sound can crackle like you describe. Try swapping tubes from one channel to the other and see if it follows the tube swap. Try one set of tubes at a time, begin with the small signal/preamp tubes first. Let it play for a while... before swapping other tubes.

it's not a tube amp!

Also take a look at all your solder joints and be sure there is no greyish spidery looking solder joints, they should ALL look shiny silvery. It wouldn't hurt to just hit any of the solder joints with the tip of your solder iron just to be sure they are good (only touch for like 3 seconds.. count 1 thousand 1. 1 thousand 2, 1 thousand 3.. .lift off the solder tip and repeat.

Cheers,
Bob

yeah I spent a lot of time inspecting joints. they seem to have help up well - all shiny.

how does touching them with a soldering iron show if they're good?
Does it help strengthen the bond?
 
any idea if there's much of a difference between 2sc871 and 2sc871r ? (I have the latter).

The difference should just be gain or hfe. If you do each pair it shouldn't matter.

Hitting them with the soldering iron is called reflowing the joint. I add a little solder too.
 
The difference should just be gain or hfe. If you do each pair it shouldn't matter.

gotcha.

there are another 2 pairs of these in the phono stage. what kind of lifetime do these things have? as in, should I be thinking of replacing all of them?

Hitting them with the soldering iron is called reflowing the joint. I add a little solder too.

ah understood.

oh and I've been asked to add this: :banana:
 
Well, they're semiconductors. They work till they don't.

If it were mine, I'd put the replacements I said above, in all of it. But since you are in Europe and it's harder to get them (Mouser has a Belguim office) I'd just put in the new 871s that you have.

These amps are worth fixing and even upgrading. I was blown away when I fixed one that I found in the trash.
 
Crackle could also be a bad tube, when they go microphonic the sound can crackle like you describe. Try swapping tubes from one channel to the other and see if it follows the tube swap. Try one set of tubes at a time, begin with the small signal/preamp tubes first. Let it play for a while... before swapping other tubes.

Also take a look at all your solder joints and be sure there is no greyish spidery looking solder joints, they should ALL look shiny silvery. It wouldn't hurt to just hit any of the solder joints with the tip of your solder iron just to be sure they are good (only touch for like 3 seconds.. count 1 thousand 1. 1 thousand 2, 1 thousand 3.. .lift off the solder tip and repeat.

Cheers,
Bob

The AU-101 is a 15watt per channel all solid state amplifier.....
 
If it were mine, I'd put the replacements I said above, in all of it. But since you are in Europe and it's harder to get them (Mouser has a Belguim office) I'd just put in the new 871s that you have.

don't actually have any 871's. Or any replacements.

and yes seems to be hard to get replacements here.

farnell offer best shipping for me - but they only have 2N4401 - that a good replacement?

with Mauser (less good shipping rates) the closest to KSC1845 I've found so far is KSC1845FTA from Mauser. That look close enough?

Mauser also have NTE85 - which I read elsewhere is also a replacement.

These amps are worth fixing and even upgrading. I was blown away when I fixed one that I found in the trash.

YES! I've loved the sound out of this amp for many years and I'm very happy to be able to restore it. The design is very nice - it's easy for a beginner like me to follow they circuitry - it's very modular, simple and nicely laid out.

thanks
 
Hi,

The 101 is a nice little amp, I have several of these and they a great performers,

Another alternative for the 2SC871 is either a BC549 or BC550 (both are low noise transistors). The only issue with these is that the lead pin outs will be different, so you would need to take this in to account when mounting them to the PCB.

Cheers

John
 
don't actually have any 871's. Or any replacements.

and yes seems to be hard to get replacements here.

farnell offer best shipping for me - but they only have 2N4401 - that a good replacement?

with Mauser (less good shipping rates) the closest to KSC1845 I've found so far is KSC1845FTA from Mauser. That look close enough?

Mauser also have NTE85 - which I read elsewhere is also a replacement.


thanks

oh thought you said you had some...

yes that is the transistor I am talking about. the 1845...

The thread linked below says the 4401 will work too. Be aware the pinout is different...

Do not buy NTE transistors...

Let me know how much the shipping charges are that you are seeing.

Looks like you're right. An advanced Google search on "2sc871" comes up with this:

http://www.audiokarma.org/forums/showthread.php?t=228031

I am a MarktheFixer fan!
 
The thread linked below says the 4401 will work too. Be aware the pinout is different...

that's probably what I'll go for since farnell have it.

Let me know how much the shipping charges are that you are seeing.

mauser and digikey both charge €20 for shipping on orders up to €60-70.

farnell give free shipping with min order €20, so obviously makes most sense for smaller orders.

I have another couple of amps and receivers that need maintenance so I guess I could work out a parts list to get me up to the farnell threshold. It means waiting a while, which is a pity, but I really don't feel like wasting €20 on shipping for a small bag of transistors.
 
I have another couple of amps and receivers that need maintenance so I guess I could work out a parts list to get me up to the farnell threshold. It means waiting a while, which is a pity, but I really don't feel like wasting €20 on shipping for a small bag of transistors.

I remembered I could do with a better wire stripper - order sent :)

The thread linked below says the 4401 will work too. Be aware the pinout is different...

Hi,
Another alternative for the 2SC871 is either a BC549 or BC550 (both are low noise transistors). The only issue with these is that the lead pin outs will be different, so you would need to take this in to account when mounting them to the PCB.

Thanks for the really helpful advice, both of you.
I ended up buying a few dozen of both BC549 and 4401. Silly, I know, but since they were both a few cents per unit, buying both was easier than choosing one :) Plus it's good to build up an inventory. I'll flip a coin once I get them.
I got Fairchild - seemed to be considered decent.

I'll report back in a few days...
 
did it work

hi there,

i know this thread is a bit older now but i have the exact same problem (although the crackling seems a bit more severe on mine)... did you manage to fix it? im not going to attempt a fix but will take it somewhere and wondering if the cause is worth fixing.

cheers,

LuKe
 
Sorry man, been away from email for a while.
yes, I fixed it - as far as I can tell there were at least two components causing the crackling, a capacitor (I replaced all electrolytic capacitors) and the transistor mentioned in later posts. The amp sounds much better than before - the low in in particular is has more presence and definition. Even for a beginner like me the whole process (tho somewhat time consuming) was pretty straightforward and interesting.
 
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