What else to do when output transistors go bad?

W.T. Holt

Active Member
I have replaced output transistors in many different receivers and usually they function just fine afterward. Some continue to repeatedly blow the transistors. Besides merely replacing the transistors, what other steps and measures should be taken to ensure an amplifier is running properly and wont experience repeat issues with blown transistors?

Occasionally I have found that an emitter resistor associated with the bad transistor has also faulted but I believe that is a result of the transistor malfunction, not a cause. Please correct me if this is not the case.

Bias is an issue that is a bit foggy to me and can certainly cause trouble for the outputs. From reading I have done, this measurements seem to be inherent to a specific transistor pair and cant really be altered unless the manufacturer built in an adjustment circuit. In general is there a way to ensure your amps bias is set properly in lieu of a trimpot or worse yet, a handy service manual?

All thoughts and input welcome, just trying to open a conversation on a fairly common issue in hopes of educating myself and others.

Thanks!
 
Bias is an issue that is a bit foggy to me and can certainly cause trouble for the outputs. From reading I have done, this measurements seem to be inherent to a specific transistor pair and cant really be altered unless the manufacturer built in an adjustment circuit. In general is there a way to ensure your amps bias is set properly in lieu of a trimpot or worse yet, a handy service manual?

Thanks!
You can always check bias by measuring voltage across an emitter resistor.
 
For complementary emitter follower output stage ( NPN emitter follower on top and PNP emitter follower on the bottom type), the optimal bias is set to 26mV drop across ONE of the emitter resistor. This is called "Oliver's" condition. BUT I am not sure all manufacturers follow this.

Emitter resistor usually is not the cause, burn resistor usually result in the bad transistor.

I can't think of an easy way to test. It usually have to take all the big output transistors out of the closed loop to test. It is very hard to generalize. I guess DBT is the best way.
 
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There are plenty of threads on AK describing how to build a DBT and how to use it.

You are basically wiring a 100 W light bulb in series with the unit being tested. If no problems it is bright for a second then gets dim. If it stays bright something is shorted. The bulb limits the current and usually prevents further damage.
 
You should never replace outputs without thoroughly checking all upstream DC coupled transistors. Then only power up using a DBT until confident that all is OK.

By upstream I imagine you mean specifically on the amp channel with the bad transistors. Could transistors from other channels or in the preamp be causing issues as well?
 
Yes, specifically on the channel with bad outputs. Keep going back thru the drivers and preamp until you get to a capacitor in series with the signal path.
 
I suspect that we should always remind people that they need to use incandescent bulbs in their DBT. CFL bulbs will not work for this application.
 
First, I do a thorough search of the 'net to see if a problem is common and if maybe there's a known design flaw. Has anybody else trod this path before? Then, as said above, work your way back, checking drivers and other parts carefully. Sometimes these can be a really tough nut to crack. I've had transistors that were perfectly good, but started to break down at a lower voltage than they should. Amp worked great until near full output, then bang. The only good way to find those is with a curve tracer, which few have. Replacement is the only other course of action, but since OEM devices are no longer made, you have to put on your design engineer's hat, whether it fits or not, and understand what the circuit is doing, if it's not doing the right things with new transistors. Bias issues are the most common, where the trimmer range won't get you where you need to be, but you can also get oscillation and other issues.
 
I wrote on another thread but no body was really talking about the issue anymore or something i need help im working on my Technics SA-700 i have the service manual so please dont send me a link for it the issue i thought was a blown channel on the REMOTE side it was not what happens when i put a speaker fuse in on that side and try hooking up a driver it sucks the woofer to the magnet and hums loudly did this with new power transistors in it and funny part is it dose this with no transistors in that channel at all! Im stumped im guess is that its the Power cap for that channel? But i def not sure by any means i would love someone to chime in mon this i love this amp and would really like to get that other channel going please help you can also email me at budswilla@gmail.com
 
I wrote on another thread but no body was really talking about the issue anymore or something i need help im working on my Technics SA-700 i have the service manual so please dont send me a link for it the issue i thought was a blown channel on the REMOTE side it was not what happens when i put a speaker fuse in on that side and try hooking up a driver it sucks the woofer to the magnet and hums loudly did this with new power transistors in it and funny part is it dose this with no transistors in that channel at all! Im stumped im guess is that its the Power cap for that channel? But i def not sure by any means i would love someone to chime in mon this i love this amp and would really like to get that other channel going please help you can also email me at budswilla@gmail.com
Start your own thread and space out the info a little, kinda hard to understand. Don't hook up any drivers to a broken amp, just check it with a DMM.....sounds like it's spitting out pure DC.
 
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