is there a difference between the
2N3055AG and the 2N2055G ? the specifications show identical max voltage limitsVcb/Vce/Veb and max collector current 15A just wondered if you know what it might be😕
I wouldn't know without researching it but would suspect it could be something like one being lead (as in Pb) free.
Having just looked quickly it seems not to be that. Possibly 'Automotive' rated meaning it is certified to meet some standard. It's not going to make any difference for your amp, the 2N3055 is an ancient device and I would bet the modern parts are quite different in a lot of ways... but none of that should make any difference for your application.
MJ15015 is a modern higher VCEO version. It is the same type of device as ON’s version of the 2N3055. Similar weaknesses - hFE is on the low side and variable.
You probably mean MOSFETs, and not trivially.. you may as well build a new amplifier if you are going to do that.
Given that the design is an old one and that it has worked OK for decades means I wouldn't look to hard for alternatives. Failure of the output transistors is almost certainly not the original cause of the fault but a consequence of an issue elsewhere around the driver and VAS stages.
All things are possible given enough thought but it is not a straightforward swap.
Updating an old Rotel RA820A to use HEXFETS's amongst other mods 🙂
VAS 😉 the voltage amplifier stage. Those two interconnected transistors before the drivers in the NAD.
Distortion In Power Amplifiers
Scroll to the bottom of the page to see fig 33 showing the main parts of an amplifier.
Distortion In Power Amplifiers
Scroll to the bottom of the page to see fig 33 showing the main parts of an amplifier.
so i am only getting 0.4v at R459 and 0.35v at R460
All of the voltages are even on both sides now.
Ive checked all the joints and reflowed a couple that looked a bit suspect, but all said and done the only thing left to do i guess is unplug the bulb tester and turn it on?
Those voltages sound OK as far as it goes...
The important voltages to check next are the supplies (just to be certain they are up close to the correct value) and then we check the DC offset which is the voltage across the speaker terminals. It should be near to zero.
Checking supplies is always the first thing to measure... always, each and every time. Never assume they are correct and present until proven to be so.
If that is OK you can test the amp with a speaker but leave the bulb connected for now.
If you have replaced transistors then the quiescent current should be checked and that is done by measuring the voltage across R461. R461 is shown as being shorted out on the diagram... you have to remove the short and make sure the 1 ohm is fitted as shown.
The voltage across that resistor tells us the current (by calculation). The voltage will be very small, a few millivolts.
That test is done without a speaker attached and would be done twice, once using the bulb and then when we are happy it is OK we test again without the bulb.
The important voltages to check next are the supplies (just to be certain they are up close to the correct value) and then we check the DC offset which is the voltage across the speaker terminals. It should be near to zero.
Checking supplies is always the first thing to measure... always, each and every time. Never assume they are correct and present until proven to be so.
If that is OK you can test the amp with a speaker but leave the bulb connected for now.
If you have replaced transistors then the quiescent current should be checked and that is done by measuring the voltage across R461. R461 is shown as being shorted out on the diagram... you have to remove the short and make sure the 1 ohm is fitted as shown.
The voltage across that resistor tells us the current (by calculation). The voltage will be very small, a few millivolts.
That test is done without a speaker attached and would be done twice, once using the bulb and then when we are happy it is OK we test again without the bulb.
so i guess you are talking about checking the centre voltage and idling current?
i do the centre voltages as a matter of course, it was one of the first things i learnt to do on these.
centre voltages were -0.020v (L) and -0.019v (R) which are within the +/- 30mv tolerance
Idling current i havent done before but i have measured RX1 and RX2
0.007V across RX1 buT across RX2 it was 0.035v, just outside 30mv limit, so i changed the resistor to a higher value one (i pressumed this is what it meant by adjust) and it is now 0.021v so they are both in limits now,i didnt have one high enough to bring it lower.
as a note, i noticed the short was missing across R462(the channel i didnt changed the parts on)
resoldered shorts
so tested still connected to lamp-this was ok
testing without lamp tester.
now i am using it with speakers and all seems fine🙂
i do the centre voltages as a matter of course, it was one of the first things i learnt to do on these.
centre voltages were -0.020v (L) and -0.019v (R) which are within the +/- 30mv tolerance
Idling current i havent done before but i have measured RX1 and RX2
0.007V across RX1 buT across RX2 it was 0.035v, just outside 30mv limit, so i changed the resistor to a higher value one (i pressumed this is what it meant by adjust) and it is now 0.021v so they are both in limits now,i didnt have one high enough to bring it lower.
as a note, i noticed the short was missing across R462(the channel i didnt changed the parts on)
resoldered shorts
so tested still connected to lamp-this was ok
testing without lamp tester.
now i am using it with speakers and all seems fine🙂
That all sounds promising (that it is working) 
RX1 and RX2 are values to adjust to get the correct bias current but you measure the voltage across R461 and R462 to determine the current.
So 0.035v across the 1 ohm gives I=V/R which is 0.035/1 giving 35 milliamps. That sounds reasonable for such an amp.
0.007 is a current of just 7 milliamps which is pretty low. That means distortion might be higher than it should be but a lower current is also safer for the amp.
RX1 and RX2 are values to adjust to get the correct bias current but you measure the voltage across R461 and R462 to determine the current.
So 0.035v across the 1 ohm gives I=V/R which is 0.035/1 giving 35 milliamps. That sounds reasonable for such an amp.
0.007 is a current of just 7 milliamps which is pretty low. That means distortion might be higher than it should be but a lower current is also safer for the amp.