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AU9900 needing help, cracking/popping on the left channel

1303

1303

New Member
Hi,

I have recently acquired an AU9900, and thinking of performing a full recap on it, after all it is a more than 45 years old amp.

Besides preparing all the tools and parts for the recap, I have the following issue with the AU9900.

The amp has a random yet persistent cracking/popping sound from the left channel and the following is what I observed:

1. The cracking/popping sound started most of the time when the amp was turned on, after the speaker protection relay clicked on, and thereafter, the cracking/popping sound came in every now and then, sometimes softer while sometimes loud.

2. The cracking/popping noise comes in randomly regardless of the position of the volume control knob, even at zero volume.

3. The cracking/popping noise is present regardless of the A or B speaker terminals are used to connect to the speakers.

4. The noise disappears when the AU9900 was only used as a preamp, driving another power amp I have.

Have you ever come across a similar situation?

I am thinking, out of range bias current, arcing of leaking caps, faulty speaker protection relay, faulty power transistors on the left channel...

All inputs are welcome.
 
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1303 said:
I am thinking, out of range bias current,
Click to expand...
Very likely.
1303 said:
arcing of leaking caps,
Click to expand...
Unlikely to be actually 'arcing', but leaky capacitors, yes almost certainly.
1303 said:
faulty speaker protection relay,
Click to expand...
Very common, just caused by age, and consequent oxidisation and pitting of relay contacts.
1303 said:
faulty power transistors on the left channel...
Click to expand...
Not a certainty, if you get sound from both channels they are probably ok, You do need to replace all the 'UniWatt' transistors, with modern equivalents. These are Grey or Green bodied transistors with gold leads, and gold tabs - they are notorious for ageing very badly and definitely need to be replaced. TR6 (2SC1628) & TR7 (2SA818) + TR15 (2SC1124) & TR17 (2SA706) and their opposite numbers in the other channel of course - in the diagram you posted. Incidentally that is a 'circuit operation diagram', not exactly a 'schematic' or 'circuit diagram' - which is on page 19 of the Service Manual. :)

There are other transistors used in this unit that are notorious for noise problems - like the 2SA798 dual transistors - these need to go as well.

There are some restore threads on this model, I recommend that you read all you can find on here about this model before attempting a repair or restore.
 
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Hi, I had more or less the same thing on my AU-7700. Replaced caps, protection relay & differential transistors in the power amp, all together. Problem solved. Not sure what the exact cause was, but if you want to enjoy these amps again, problem free, tackle all the weak points from the start. I learned the hard way ;).
 
Guess I have a few more things to replace other than the capacitors and also I am sure that I would need to read as many as possible AU9900 repair/restoration threads over here.
Where do you guys go in general to order the caps and the transistors? I am in Melbourne but I guess most of the stores offer nation wide delivery anyway.
Another question I have is when you replace the power filter caps, can you source ones with the same dimensions? Or there are only some brands and models that are available in the same form factor of the original ones? Please advise.
 
That's pretty neat. Excellent work.

Anyone tried any other brands of filter caps? Also, wonder if anyone has the dimensions of the original caps available.
 
Hi, for my AU-7700 restore, I found all parts on Mouser, used Nichicon Gold tune series for power filter caps, and all other caps nichicon audio grade ones as well. For my model at least I could find perfect fits for replacing without tweaking the original chassis. Regards
 
Per the link given by Skippy (Thanks Skippy) above, the issues as to the dimensions and the screw mount fitting have become my concerns. I wonder if other members have been able to find filter caps with less tweaks.
BTW, I have not ordered from Mouser before, I would definitely hop onto Mouser's website to take a good look. Any other store suggestions?
Also, does anyone have a list of required caps for the said full recap of the AU9900? That would become handy....and save a lot of time.
 
I had one with noisy FETs in the preamp. Went around in circles a long time chasing that one.

- Pete
 
LBPete said:
I had one with noisy FETs in the preamp. Went around in circles a long time chasing that one.

- Pete
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Hi Pete, you must be thinking of another model, no FET's in the pre-amp of the AU-9900, but it does have nasty dual transistors, which do go noisy. ;)
 
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