Sansui AU-D11 II: To Recap or Not to Recap....

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Working on a Sansui AU-D11 II whose right channel offset was causing the amplifier to go into protect when the speakers were connected.

I completed the DC offset procedure for this amplifier, including the preamp and got good results. On AUX input. Fine. Now I move onto the phono input and it's ugly. The phono section oscillates at 725KHz, 1450KHz, etc., with RF bubbles appearing on top of the audio waveform. Turning up the volume puts the amp into protect.

Furthermore, I found that engaging the tone controls causes the amp to become an RF oscillator, even with the volume at zero, causing the line power consumption to go north of 350W.

My thinking goes along the lines that maybe the capacitors have become dried out and the ESR, particularly at RF frequencies, has gone way out of spec.

Recapping the phono board is going to involve some risky disassembly. The plastic pushrods that operate switches are on this board and to remove the board, it is necessary to pry off the plastic tabs. If they are brittle from age, they will break off. I'm giving this some thought before I jump down that rabbit hole.

Another thing that's making me doubt the capacitor issue is that placing my portable o'scope across a number of caps in the amplifier yielded no sign of RF signal. I would expect to see RF across the dried out cap. If not the caps, what could be making the amplifier so unstable?

The bit of research I've done seems to concur with the failing capacitors theory, at least after all DC offset adjustments have been done. Before I start breaking plastic pushrods to disassemble that section, what's the odds of it being bad caps?

Disclosure: This amp had been to another shop prior to mine, and it worked for one evening after "repair" at the other shop.
 
Yes, the amplifier is oscillating.

If I disconnect the load from the right channel, the left channel works as it should. This is with input to the phono section.

If I send signal to the AUX input, the amp is relatively stable with both channels into the load. And yes, the loads are independent and floating, in light of the bridge design of the floating amp outputs.

I recapped the easily accessed parts of the main board and changed the two large caps on the phono board. No change in performance.

More interesting is that if I engage the tone controls when inputting the test signal at the phono stage, the oscillation goes from a bubble to full power oscillation which then trips the protection.
 
The last time I switched on this amplifier, I heard a pop and saw a puff of smoke come out. The NPN output transistor is shorted. It took out the .27 ohm 7W ceramic wirewound resistor (can't find these anywhere for sale) and the pre driver 2SC2705 is shorted. Not sure which caused which, but whatever intermittent problem there was just went full failure.
 
Aud11II problem

So did you find a solution to the issue with this amp. I own one and is working fine. And i was wondering if you could contact "hyperion" on audiokarma sansui forums .
He has done some work on aud11II.
And you could clear up the problems.
Regards . Shaharyar
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.