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AU-719 transistor substitute.

TupolevFan

TupolevFan

Super Member
Hello everyone. Friend of mine asked me to repair his AU-719 which was blowing a fuse.
On inspection 2SC2581/2SA1106 are blown in one of the channels. Driver 2SB537 shorted.
So i decided to replace transistors as following:
2SC2581/2SA1106 -> 2SC4468/2SA1695 ( I do have them)
2SB537/2SD382 ->MJE15030/MJE15031
Now i want to replace pre-driver transistors as well:
2SC2071/2SA939 can be replaces with KSA1381/2SC3503 or KSA1220/KSC2690 I can not decide which one to choose for pre driver. Any advice ? Ph i also have TTA004/TTC004

Thanks a lot
 
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The reason i am asking is that i think KSA1381/2SC3503 would be closer match for 2SC2071/2SA939 in terms of Ic. It seems to me that KSA1220/KSC2690 would be a bit of overkill here. However data sheet states that minimal gain for A939 is 150 in reality i measure 125. Same for C2071 (around 137) for TTA/TTC004 i measure 200 Same i measure fo A1220/C2690. Uce max here is not that important as the rail here just +- 63V. Somehow i am leaning towards Toshiba TTA/TTC004. Well i think i know reason for output's failure. The output transistors were very loose and thermal paste is dry. This is in both channels and yet catastrophic failure happened just in one i see signs of overheating in the other (little board to which output transistors attached is darkening.
 
One more thing regarding MV103 and MV12. I read conflicting reports about preventive replacement of those. So what is the consensus ? Replace or not replace?
 
I leave the MV103's alone - I've never seen one fail (as yet) - same goes for the MV12's as well never seen one fail and have never come across a noisy one
 
Another thing here is that owner tried to blow the fuse 3 times and in the last attempt he put 10 Amps fuse. I am just wondering if i should replace rectifier diodes because i saw cases when those fail later because of the stress like this....
 
Are you talking of the main common Anode and Cathode metal framed rectifiers? - if so, do the individual rectifiers test OK? - If any have blown, I think that there is provision on the PCB for individual rectifiers (you will need to confirm this)
 
AUD101 said:
Are you talking of the main common Anode and Cathode metal framed rectifiers? - if so, do the individual rectifiers test OK? - If any have blown, I think that there is provision on the PCB for individual rectifiers (you will need to confirm this)
Click to expand...
Well so far they test OK. But obviously they were stressed. I just have not decided if i should take preventive action. I Have an idea how I would handle that. The main question if this necessary or should i leave it alone for now.
 
Hyperion said:
If they test OK I would definitely leave them alone, those rectifiers are as 'tough as old boots' and are very difficult to blow in normal circumstances.
Click to expand...
John you set up my mind :))) I was hesitant to change it just hope that owners fuse experimentation did not do any damage to it.
 
TupolevFan said:
Well so far they test OK. But obviously they were stressed. I just have not decided if i should take preventive action. I Have an idea how I would handle that. The main question if this necessary or should i leave it alone for now.
Click to expand...
As John says, The SS-5, SS-5R are very tough diodes. I have never seen any of them blow, even under stress.
 
OK here is an update. I am basically done with full rebuild of this amp. Here what was done on driver board:
C03 Black flag capacitor was replaced with Kemet Ceramic low ESL G0G capacitors 22pF
TR10,TR14 replaced with Toshiba TTA004B
TR12,TR13 replaced with TTC004b
TR11 just in case was replaced with KSC945C
TR15 -MJE15030
TR16 - MJE15031
TR701,TR702 replaced with 2SC4468
TR703,TR704 replaced with 2SA1695
C16,C17 replaced with 1000uFX80V low ESR
C05,C08 replaced with Wima 4.7uF x 50V film capacitors and c06,c07 removed because of that.
C09 replaced with 68uF X 63V low ESR
C01 replaced with 150 uF X 63V low ESR.
ZD01 with 27 V 1W Zenner
ZD02 with 2% 0.5W 13V Zenner
VR01 with new multi turn 100 Ohm 0.5W trimmer
V02 with new multi turn 500Ohm 0.5W trimmer
All resistors 1/2 W like R12,R10,R36, R42, R43, R46, R45 were replaced with same Value metal film of Vishay RN65,RN70 series 1/2W.
R03 replaced with high quality 3W Wirewound resistor.
C24 replaced with Kemet 1uF x 100V Electrolytic
All 0.25W resistors with value 1k and above in a signal path replaced with metal film Vishay RN60 series.
Oh forgot to mention Ceramic Aluminium Oxide plates were used as thermal pads for outputs and drivers

Replaced ALL 4 main 10000uF x 71V caps with Cornell Dubilier 383 series 105C capacitors 12000uF x 100V. Used adapter for wires recommended by Echowars.

Power Supply ( F3101)
TR606 replaced with MJE15030
TR613 replaced with MJE15031
TR610,TR611 replaced with gain matched KSA1013YTA
TR608,609 replaced with gain matched KSC2383YTA
TR612 replaced with KSC2383YTA
TR607 replaced with KSA1013YTA
ZD602 with 2% 0.5W 13V zenner
C607,C608 with 1500uF x 80V
C615,C614 with 470uF x 63V. Thanks @Hyperion i installed C615 correctly and did not blow it since markings were incorrect.
C609 replaced with 220uF x 50V
C612,C613 replaced with 1uFx 100V.
Replaced bridge rectifier D609 by a new one 400V 3A.
R634,R635 replaced with RN65 Series 1/2W
protection part of the board replaced just electrolytics mostly of the same value just higher max Voltage. Except for C601 which was replaced with 100uF x 63V.

Tone control board. (F3098)

TR01, TR03 replaced with gain matched Toshiba 2SC3324BL
TR02,TR04 replaced with gain matched Toshiba 2SC3324BL
TR05,TR06 just for the sake of it replaced with KSA992FB
High value resistors (1k and higher) replaced with high quality metal films.
C05,C06 upgraded to PP film capacitors.
C17,C19 and C18,C20 replaced with single Kemet 5,6 x 50V film capacitors and C22,C21 removed.
C25,26,27,28 replaced with 68 uF x 63V
C11,C12 replaced with Wima 10 uF x 50V film capacitors. C09,C10 removed. This required some manipulation with surrounding resistors in order to fit Wimas.

phono stage to be continued....

Amp passed bulb test. then DC offset and bias was adjusted as per SM - no issues here.
Power test 8Ohm @900HZ produced impressive 115 - 120 W for each channel just before clipping.
 
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