Help Needed: Sansui AU6900 idle popping and hiss issue

Sebastien

Active Member
Hi Guys,

First of all any help on this is greatly appreciated.

So while i'm waiting for parts to arrive for my AU11000 i've jumped onto another project that hopefully is an easier fix.

Fault: The amp operates as normal until tone defeat is turned off. Then the amp has a quiet but erratic kind of hiss with irregular dull pops now and again.

Trouble shooting and notes that may help diagnose:

The noise isn't affected by volume and is present at 0 volume.
The noise isn't present when the power amp is separated and the amp is used as a pre amp.
DC at the speaker terminals fluctuate around 20mv on both channels.

One thing i've noted this evening while looking on board F-2599 Equalizer Circuit Board and using a CRO is that T01 (C1222 2E63A) has visible distortion that is consistent across other transistors on the board on the Collector only, but the Emitter and Base from what i can tell looks clear.

Could this indicate the culprit?

Any thoughts are greatly appreciated :)
 
The F-2599 board is the Phono input, how much signal are you putting into that board? (its sensitivity is 2.5mV) - as you may be over-driving the input causing that distortion/clipping?

Note that there are many 2SA726 and 2SC1313 transistors in the signal path, both of which are known noise culprits.
 
The F-2599 board is the Phono input, how much signal are you putting into that board? (its sensitivity is 2.5mV) - as you may be over-driving the input causing that distortion/clipping?

Thanks for the reply please bare with me as i'm pretty green.

I've replaced all the 2SA726 on this board (F-2599) with KSA992, i should have just replaced the 2SC1313 transistors while i was there as i've got some KSC1845 replacements.

The distortion/clipping is in the amplifier without any source connected, hope that helps us narrow it down :)
 
I have an Au-3900 that had hiss and popping in the left channel. I separated the pre from the power amp and it was still there. The culprit was the 2SA798 IC, the differential pair located in the front end of the power amp. The bias voltage was also jumping around with the popping noises. This is a very common problem with these amps.
 
Please post the serial number for the database. Click on the link in my signature block.

- Pete
 
Problem solved by replacing SC1313 transistors on the input board F2599!

Cheers

Please post the serial number for the database. Click on the link in my signature block.

- Pete

Will do eventually mate, i usually forget by the time i'm home :)
 
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