Hi,
There is another transistor(IC) , I am having problem finding replacement. I understand that this 2SA798 is an IC made up of 2 transistors. Can this component be modified.
Please assist
Thanks
Carlosraj
There is another transistor(IC) , I am having problem finding replacement. I understand that this 2SA798 is an IC made up of 2 transistors. Can this component be modified.
Please assist
Thanks
Carlosraj
Yes, you can use 2 matched small signal PNP transistor to replace it...
& glued together also it is a common emitter...
try 2SA1015 or BC560 or many other 🙂
Regards
John
& glued together also it is a common emitter...
try 2SA1015 or BC560 or many other 🙂
Regards
John
Here's a whole thread on the matter. Looks like a matched + thermally connected pair of ZTX795s is the way to go, though the trusty BC560 usually works as well. Oddly enough, these EBC pinout parts appear to be a better fit mechanically than your more typical ECB Japanese transistors (2SA970, 2/KSA992 etc.).
Fairchild KSA992 will work beautifully. I've done it by installing two like this:
(|
|)
That's seen from the top. Solder the middle legs (emitters) together in the same hole. You get:
B
C
E
C
B
As on the original 2SA798 devices. Specs will be the same or better but compatible, too.
Thermally connected shouldn't be necessary. It's normal for these input pairs to be installed relatively far away from each other. I've certainly never seen something heat shrinked together (with thermal paste [on TO-92 devices!] no less) in a commercial design.
(|
|)
That's seen from the top. Solder the middle legs (emitters) together in the same hole. You get:
B
C
E
C
B
As on the original 2SA798 devices. Specs will be the same or better but compatible, too.
Thermally connected shouldn't be necessary. It's normal for these input pairs to be installed relatively far away from each other. I've certainly never seen something heat shrinked together (with thermal paste [on TO-92 devices!] no less) in a commercial design.
Last edited:
The 2SA798 transistor is two transistors in one device. Therefor they are by nature thermally connected and the parameters are by definition matched. It is better to thermally bond them, or at least heat shrink them together. Dual transistors like this can still be found but usually in little tiny surface mount packages.
Is the 2SA1618-Y close enough to the 2SA798 to have one on the right channel and the other on the left? I ask because there's no room to use a manufactured adapter (see below) on the left channel of the amp I am fixing. I don't have the skill or good enough eyesight to make my own adapter.
An externally hosted image should be here but it was not working when we last tested it.
A standard multimeter with hfe measuring feature is enough. Even not matching is enough, since all ksa992 produced today have almost exactly the same hfe, certainly a lot closer than the two dies in any 2sa798.
Hi grazie,
Welcome is right in that the original dual transistors were not very well matched at all. You could easily do better with a pair of TO-92 transistors tied together (for temperature match. Some improvement in performance is possible with careful matching, but using the meter Hfe function should get you there. One thing to watch for. Make certain that the transistors are the same temperature when you match them, otherwise they will be far apart in gain. The beta of a transistor is extremely sensitive to temperature.
Let the new transistors sit without touching them, use tweezers to put them into the tester and take the readings down. When you have two very close, or the same number, you have a pretty good match usually.
-Chris
Welcome is right in that the original dual transistors were not very well matched at all. You could easily do better with a pair of TO-92 transistors tied together (for temperature match. Some improvement in performance is possible with careful matching, but using the meter Hfe function should get you there. One thing to watch for. Make certain that the transistors are the same temperature when you match them, otherwise they will be far apart in gain. The beta of a transistor is extremely sensitive to temperature.
Let the new transistors sit without touching them, use tweezers to put them into the tester and take the readings down. When you have two very close, or the same number, you have a pretty good match usually.
-Chris
Thanks for the further replies. Unfortunately, the Hfe function on my meter has never worked. I'll see if I can borrow one or buy another.
This method does mean drilling out the PCB though, doesn't it?
This method does mean drilling out the PCB though, doesn't it?
Okay, so make a differntial pair using sockets or header strips for the two diff pair transistors. Set one in and leave it. Replace the other of the pair with your other transistors one by one. Measure the collector voltages with your meter. Allow them to settle into whatever balance they will. The best matches will have the lowest difference between the collectors.
Repeat above for each transistor in your group, measuring the rest again through the other side. This works extremely well and you will get very good matches by doing it this way. It will take 5 or so minutes for the pair to settle down. Put foam over the pair in test and make sure they are touching each other. Try and use a similar tail current that your receiver uses. If you don't know, try running 2 to 3 mA.
-Chris
Repeat above for each transistor in your group, measuring the rest again through the other side. This works extremely well and you will get very good matches by doing it this way. It will take 5 or so minutes for the pair to settle down. Put foam over the pair in test and make sure they are touching each other. Try and use a similar tail current that your receiver uses. If you don't know, try running 2 to 3 mA.
-Chris
Never use 2N4403 transistors to sub a 2SA798 used as a differential transistor, it sounds bad. What would be the best sounding transistors to use as a 2SA798 differential sub? Preferably cheap ones so I can get lots of them for the best gain figures.