Where do I find this trans?

[Jonny Hotnuts]

I found that I have a bad output transistor (and the resistor to the transistor is also bad) but could not find anything else wrong with the amp.

How likely is it that if I was to replace the transistor and the resistor that it will not go into protect mode (will go into protect as soon as it’s powered up with or without speaker load). Is it common that one trans will go bad in a group of many or is there likely another problem I have not found?

Here is a pic of the trans. I would like to replace it, where can I find one?

 

[grjr]

Wow, that's an expensive transistor You can get it at mouser.com, don't know how likely that just one transistor is bad but sometimes you do get lucky. It's recommended to replace all transistors connected in parallel with the bad transistor for reliability, others connected with it may be damaged internally but test ok with a multimeter.

 

[Jonny Hotnuts]

The markings on the bad unit are:

T420
MJE (I know this means Motorola plastic case)
4353



I checked mouser and found this:

http://www.mouser.com/search/refine.aspx?Ntt=863-MJE4353G


They do not sell Motorola but have a transistors, this one is made by ON Semiconductor. Being very green to this sort of thing I was wondering if you could tell me if the listed transistor will work.

The one from mouser is MJE 4353G (I don’t know what the G value means).

Thanks for your help.....yes they are not cheap but the amps original MSRP was 1K. To me it is worth the 50$ if I can fix it and the journey of doing it myself

 

[Perry Babin]

The G indicates that it's lead free. That transistor will work.

Grjr is correct, you need to replace all that are in parallel.

On-semi is Motorola's discrete semiconductor division.

 

[jol50]

If you test this one and it is shorted one way or another, then cut it out and test the rest....if they test good (and are) it should fire up and run. If this is the only + or - powered transistor on that channel it will sound nasty without it. Otherwise if the others work it should idle/play at low output. Once in a while they test good but are not. Then you can check your dc offsets and other stuff if it runs (and see what happens without trashing new parts). However the other parallel ones that still work can be damaged from the stress, since when the one blew they had the whole load. If its just you and you don't care if it blows again you can replace one, as it might let go again so it is not recommened.

And here I thought PB meant lead free .

 

[Jonny Hotnuts]

Because I *know the resistor is bad, could that give me a false bad on the transistor?

 

[jol50]

I don't think so if I get your question right, but I am not the mad amp scientist yet either. Check one of the others beyond the resistor, but if you mean the base/gate resistor I have not seen that. Unless it is linked with another without much separation but usually they are not. (unless you have a problem in the differential amps or feedback circuit/etc., the few I have seen did not show up that way.)

If you have one shorted it shows best tested from the base or gate to the other legs. Then I just chop the other two off; the output and rail voltage legs (if they show short it could be any one of the transistors in parallel you have to guess). Then you should see the ohms go way up on the others if that is the bad one....then you can run that way it if the rest look ok as long as the bad one is not shorted to the heatsink or touching anything/etc., or just cut all three legs off. I only leave it hanging there sometimes so I know what I need to order.

If the transistor is bad, it usually *does* give a false resistor reading if resistor is bad or not. However if the transistor shorted the resistor it can damage it and change its spec under use...and they are so cheap might as well put them in. They will not run equal if those resistors are not equal, and again the overworked one will fail sooner.