AU 9900 Tone & Filter crackle

[rockhound]

AU 9900 plays ok when Tone &Filer is disengaged or Filter is engaged.

Turning the knob to Tone & Filter I get full power constant crackling noise in one channel.

I cleaned Pre/Main switch and Filter &Tone switch,
will check ground on tone board.
Can use some help what to try next.

 

[Tom B]

crackling only when you turn or after you stop as well?

Noise while turning a control or operating a switch can be dirty connection, or it can be as the result of DC voltage appearing on the control or switch. This can be checked with a multimeter. Anything above 10-20mv DC can cause a problem. This problem is usually caused by a leaky electrolytic cap usually a low value like 3.3uf, 2.2uf, 1uf, 4.7uf or 10uf. If you find DC on a switch or control, going back in the circuit to the first electrolytic coupling cap you find connected to the contact with DC volts on it.
If the issue is just background crackling on one side, not something from turning a tone control, or switching a switch, it is usually a noisy preamp transistor. Use of a signal traced is the best way to locate the bad one.

 

[rockhound]

It is a constant full volume, constant noise with the Filter &Tone knob set to Tone

 

[Tom B]

It is a constant full volume, constant noise with the Filter &Tone knob set to Tone

I would suspect preamp transistors but it could be anyone of those on the F-2574, F-2575, or the F-2576. More likely the 2SA798f dual transistors that you will probably need to replace with 2 individual transistors like on-semi KSA992fa

Sometimes you can use heat and cold to troubleshoot. Briefly hit the questionable preamp with circuit chiller spray. A very short burst. If the noise changes (goes away or get a lot worse) this is suspect. If you have a suspect, then try to warm it by having the tip of your soldering iron close to the transistor but not touching. If the symptomes change again like higher or lower, I would say it should be replaced. You can usually locate the channel you want to work on easily by briefly touching connections on the board till you get a noise out of the channel with the crackling. The transistors on that side of the board would then be suspect, or just touch the leads of thos preamp transistors with your finger to locate the ones for that channel.

 

[rockhound]

Yes
A shot of "cooler spray" near TR03 does effect the noise on F-2574

 

[Tom B]

Yes
A shot of "cooler spray" near TR03 does effect the noise on F-2574

It could be that but I was more suspecting the fist 2 set up as a differential amp. They are referred to as "IC01" and not as transistors, although they are just 2 transistors internal.
Make sure to hit the IC1 with the chiller as well.
KSC1845's usually work well as lo noise hi gain transistors and can be used to replace npn preamps transistors.

 

[rockhound]

I should backup and say I was wrong The noise isn't constant it fluctuates in intensity.
Making me think the spray was doing something pretty much anywhere I sprayed it.


IC02 on F-2576 seems to be the one that makes the most noise when hit with a cold shot of spray though

 

[71R/T]

I would suspect preamp transistors but it could be anyone of those on the F-2574, F-2575, or the F-2576. More likely the 2SA798f dual transistors that you will probably need to replace with 2 individual transistors like on-semi KSA992fa

Sometimes you can use heat and cold to troubleshoot. Briefly hit the questionable preamp with circuit chiller spray. A very short burst. If the noise changes (goes away or get a lot worse) this is suspect. If you have a suspect, then try to warm it by having the tip of your soldering iron close to the transistor but not touching. If the symptomes change again like higher or lower, I would say it should be replaced. You can usually locate the channel you want to work on easily by briefly touching connections on the board till you get a noise out of the channel with the crackling. The transistors on that side of the board would then be suspect, or just touch the leads of thos preamp transistors with your finger to locate the ones for that channel.

Be careful with those 2SA798f transistors. I just had an AU-9900 on the bench that had a bad one in the tone control treble circuit that was causing a >2 Mhz oscillation that was going right into the power amp section. Not a good thing as the outputs in the right channel were toast.

 

[rockhound]

.... swapping out the 2SA798f dual transistors.

If it was just a swap I could do it. but being that there are pieces cobbled together to make a sub, it's beyond my knowledge I thought there was a link or a sticky to using the 2 transistors for the swap but I might be wrong or thinking of a different fix, solder two center (emitters) together for BCECB using 2 KSA992fa ? Am I even close ?

 

[Tom B]

You are right on. the leads of the transistors are so small, you should bee able feed both emitters through the same hole on the board. (pin 3 the middle pin. Are you familiar with identifying b e and c of those small transistors?typically they are BCE or ECB. So one transistor will be facing one direction and the other the oppisite to allow both E to be at the middle pin.

 

[rockhound]

Re-reading the thread I noticed some new info above regarding DC current on the switch.
I have no scope to trace, but can measure for the presence of more than 10-20mv DC at the switch.

 

[Tom B]

DC on a contact will cause a Pop when activated, but usually doesn't create continuous crackle.

 

[rockhound]

Well that's is good news

 

[rockhound]

I know not to "shotgun" new parts before fixing the initial problem
But
Are there any other weak points on these tone / filter boards so when I order I can include them
I would assume all the TR's but IDK so I ask
hate to order 4 22cent parts for 7.95 shipping

 

[rockhound]

Wow fun job getting those boards out !
Just to be clear i want to replace ....
F-2574 IC01, IC02
F-2575 IC01, IC02
F-2576 IC01, IC02
with
KSA992fa paired together

 

[rockhound]

I am having trouble finding KSA992fa on Mouser or Digikey
They have KSA992fata
Is there another place to buy them?

 

[slimecity]

I have used whatever grade 992 to replace these dual trannies, with good results.

I would also poke the board with somethign non conductive like a chopstick in several places to ensure its not a bad solder connection causing this. The 9900 has a lot of "pass through" soldered wire connections, whish can cause troubles with intermittent contact.

 

[rockhound]

Will send out order tomorrow for a dozen or two KSA992xx and try to match them in pairs.
The amp is disassembled already.
When I get the parts replaced, If I still have the crackling symptoms, ill poke it to check for bad solder.
Thankyou

 

[Tom B]

I would suggest replacing the 2 on the first board you identified, and look to fix the problem first. Then you can go to the others. You may also want to replace the small electrolytic coupling caps on each of those 3 boards as well. I usually use Nichicon fine gold, and get them from Mouser. Look at mouser pricing and see where ordering 10 makes more sense than less. They usuaslly give good quantity discounts.

 

[rockhound]

Ok Will replace the one bad ones first.
The Ksa992FATA I was looking at weren't shipping until Mar.19th, so I will order KSA992FBTA
and some caps today.

I have no problem replacing the Caps on those Boards but I do have a question.
If I never use the Tone Filter Bass Mid Treble or any of those features (I never do)
If defeated are they out of the circuit ? and can wait to be replaced at my leisure.