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ampaddict

Sansui AU-4900 protection problem

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Posted (edited)

Hey folks,

 

I'd be grateful for a little guidance in pinpointing a voltage problem with my AU-4900 please. 

 

Having read dozens of forums here on SNA and on AK, I've tried all of the known weak points (I think). It seems my issue is not uncommon so I hope I'm just missing something simple. I'm especially thankful for the knowledge shared by @skippy124  - I thoroughly enjoy your work John!

 

I'm handy with a soldering iron and have all the common test equipment and my trusty DBT.

 

Problem: Amp won't come out of protection mode due to high voltages on Left output devices.

 

Initially, I was able to adjust the VRs to get the bias set correctly but then protection kicked in.

 

As the voltages to the Right amp section seem good and the PS section is sending the correct voltages out, my first thoughts was that this is a protection circuit issue. I've basically replaced all components in the protection circuit, to no avail.  I've also replaced all of the transistors on the tone board and 90% on the amp board and all diodes on the amp board bar the Zener (on order). Replacing components has not fixed the issue, nor made is worse.

 

I've attached a circuit diagram of the protection circuit with some voltages (via a DBT).

 

Here are the components I've replaced:

 

Amp Board

 

D01, 02 -> 1N4148

D605, 607-615 -> 1N4007G

TR601, 602 (SDC1364) -> KSC1945CGTA

TR15, 16 (2SC711) -> KSC945CGTA

C602 -> 1uF 63V WIMA MKS2

C604 -> 470uF 25V Panasonic FM

C605 -> 100uF 50V Panasonic FM

 

TR601, 602, 602 (2SC1364) -> KSC945CGTA

TR03, 04 (2SC1951) unchanged and tried swapping from a working 4900

TR05, 06 (2SC711) unchanged and tried swapping from a working 4900

TR07, 08 (2SD438) -> KSC2690AYS

TR09, 10 (2SB560) -> KSA1220AYS

TR11, 12 (2SD188A) unchanged

TR13, 14 (2SB545A) unchanged

IC1, 2 (dual 2SA798) -> paired KSA992FTA with matching Hfe

 

Fusable resistors R37,38,39,40

 

I've also swapped out the 4 germanium diodes from a working 4900.

 

Tone Board

 

All (2SA726) -> KSA992FTA

All (2SC1313) -> KSC1845FTA

 

Voltages on output devices:

 

TR14 E= -29.5mV, B= -629mV, C= -29.6V

TR13 E= 24.75V, B= 24.75V, C= -29.62V

TR12 E= 20mV, B= 600mV, C= 29.68V

TR11 E= 24.78V, B= 24.95V, C= 29.68V

 

I've even tried removing the output devices but that has no effect on the Left voltages. 

 

Any other measurements that would be helpful for diagnosis, please shout.

 

Thanks in advance.

 

Brad

 

Protection CCT.JPG

Edited by ampaddict

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Posted (edited)

Check R35, 6.8ohms, these are known to stress out/appear somewhat chalky. Replace with higher wattage unit. Diode test TR09(amp powered off) and also measure Vdc at TR09 b and e to GND/chassis.

Edited by mbz
b and e to GND

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Posted (edited)

thanks Mike.

 

R35 checked spot on 6.8. I don't have a replacement to hand but I do have a 6.2ohm .5W carbon. 

 

TR09 diode tested fine when removed and here are the gnd Vdc readings:

E = 23.98V - 24.15V fluctuating

C = 23.95V - 24.55V fluctuating

B = -29.35V stable

 

I'm recapping as I go, so perhaps the fluctuation is the caps settling in?

 

 

Edited by ampaddict

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42 minutes ago, ampaddict said:

TR09 diode tested fine when removed and here are the gnd Vdc readings:

E = 23.98V - 24.15V fluctuating

C = 23.95V - 24.55V fluctuating

B = -29.35V stable

Double check those voltages. TR09c is connected to the negative B3 "rail", nominally -35Vdc. c=e points to a short but probably a sucker bet. You can always compare with the working channel TR10. TR09b should be about -1.5Vdc

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Posted (edited)
49 minutes ago, ampaddict said:

I'm recapping as I go, so perhaps the fluctuation is the caps settling in?

Nah... failed silicon...

 

Also measure TR03 and TR05 b,c,e to GND voltages, trying to find a "normal" voltage then work forward...

Edited by mbz
TR03, TR05

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Double checked and same. Using a DBT so may explain the lower voltages?

 

TR10 looks a little more normal e= -.063V b= -1.2V c= -29.35V

TR03 e= -29.33V b= -29.35V c= 24.85V

TR05 e= 24.90V b= 24.96V c= 25.05V

 

 

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TR03 looks suspect Vbe, it's not conduction which stuffs up TR05 Vbe and also TR07b, no current flow/volt drop across R15, R17. You could diode test TR03 but I'd replace it. SM has it as 2SC1951, is that what's installed. Will do a search for subs...

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AK suggests ZTX694B as a good replacement for 2SC1951. KSC1815, KSC1845 would probably melt, you could try them as a test but prefer to chase the ZTX...

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Posted (edited)

I swapped out the 2SC1951 with same from a working unit as I thought they looked sus. I have ZTX694B on order from E14. There's something else causing all these discretes to appear shorted I think. Pulled the Zener ZD601 (EQA01-13R) and diode tested OL one direction and 0.67V the other - that seems ok.

Edited by ampaddict

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12 minutes ago, ampaddict said:

There's something else causing all these discretes to appear shorted I think.

That would leave IC1, correct pinout on KSA992's? Check emitter voltage

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pickle me Grandma! I swapped out IC1 and 2 from the working unit and all is fine. You were right, it must have been my poor pairing of the KSA992's. I'll go back and redo the KSA992 pairs carefully, as the 2SA798's in the ICs are noise prone. Thanks for all you help Mike, you're a legend!

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to confirm I stuffed up one of the KSA992 pairs, I had the bases connected, instead of the emitters. Thanks again!

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