Discussion in 'Exclusively Sansui' started by tder1, Apr 17, 2017.
Anyone not using minigrabber probes for work like this is begging for a disaster to happen.
I was rechecking some transistor/resistors to iron out this missing voltage on the Driver and touched something, unsure but it was deadly. I did pull all the outputs and checked the rail. All good except for 2 OP Trans. shorted and T14 and 15 on the driverboard. I know its super easy to make a mistake. I really try to be careful but it was that 1 in 100 times that got me. had to start another Post about my Filter Caps That got hurt I believe.
I blew F01.
You'll need to replace all the OP transistors on that channel, the surge created by the short circuit will have weakened the other ones. They are not particularly robust transistors.
Its a real shame you took out TR14 and 15, because they are unobtainable fast 100mhz transistors, there is no current substitute for them.
You can use MJE15033/32G, but they are not as fast a device, only 30% as fast as the Toshiba's, but they will work.
OP transistors to use are Onsemi MJ21194/93G (MJ15025/24 Motorola number).
All is not lost!!
Let us know how you get on....
Copy that Kev, I usually buy my Trans. 10 at a time cuz they are really not that expensive and I need to be sure I have what I need. I have had no problem sourcing the 968 and 2238 from Mouser but they may be Faichilds ? I have some 15032/33's here. I have more stuff than Mouser now. And have been using the the OSemi 93 and 94's with good results. IMO they rock. Still have most of the Originals and one day I'll search out Real replacements. Per Echo I will be going out today to get the mini's. It's been sort of being lazy on my part and just using what I have. Thanks for the advice guys.
You will not find those transistors, they do not exist any more, they were Toshibas. Shame they went becauseas I said in my post they were very fast 100mhz transistors, the MJE's are only 30mhz but there is no other equivalent.....
The closest modern equivalent are Onsemi MJE15033/32. Mouser will have them...
Let us know how you go...
Yeah mini grabbers are essential, but you have to careful with those using them in close quarters as well, they are not the fail safe be all and end all solution...
When the situation is difficult for example due to access, I clip on minigrabbers with the power off, I take my time and make sure they are in the right place with no chance of shorting anything out. Even if I need to make several measurements, I still power off to reposition, and use as many as 4 sets of minigrabbers, or 'croc clipped' multimeter leads (duly insulated) in various places to make sure I get the readings I need. It is laborious but reasonably foolproof.
Ok, I really appreciate you guys chiming in, I consider all the real Pros to be here. Back up and running with distortion on right as before. I am able to adjust DC with VR01. Its the test posts for Bias I have no voltage so can't adjust it with VR02. I am going to check resistors recommended and clean the posts. Kev set me straight on these transistors, perhaps they are the wrong ones? You know way more than I and I consider you a resident expert.
If you got those transistors from Mouser it must have been quite a while ago. I would regard the fact that the '968's pictured have no 'notches' in the backplate to be a warning sign for possible counterfeits.
I managed to find 2 working 968's here and they are green with a T marked on them. They have notches also and these are some I took out. Would these be good also?
I'm still looking for the original 2238's i took out. Would they have the T stamped on them for Toshiba.
Ive been thru all the suggested resistors cleaned all my TP"s just no voltage. I'll never get rid of the distortion if I cant adjust bias. It's a PITA but someone suggested I check Output Emiter Resistor to test point.
DR... I have good continuity from my test posts on 2901 to the grid resistors. Both thru the blue and grey on2903 also.Just missing voltage on 2901 somewhere ? Does that confirm your 1st concern ?
This confirms that you do have a proper connection when you are measuring the bias current and that you have no bias. Now I know how to proceed.
Please read the voltage on TR14 and TR15, B, C, E and post them here. Remember to wrap tape on the probe so only the very tip is exposed.
DR--- the voltages are not listed on any of the documentation so I don't know what they should be . 14 and 15 are the most PITA 2 to get at also. Can't believe these are correct but the meter says:
TR15 B -2.0 C -78.1 E -4.4 TR14 B -.7 C 79.3 E -.9
Check the emitter resistor on TR15, the resistor going from the Emitter to the base of the output transistor, and all the ceramic emitter resistors on the outputs.
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