Hi Guys
I have a NAD 7225 on my bench for repairs, and have been having a look around all of the useful contributions on this site about the amp.
It was not working and showed the signs of burn out with R453 smoked,R431 and 447 had been hot.
So, being a new boy with amps (I'm an engineer although I'm an amp Virgin!)..I followed the advice:
415 and Q417 are the outputs, Q411 and Q413 are the drivers, Q409 is the biasing transistor, also called a Vbe multiplier. (wikipedia has a good explanation.)
Basically you need to isolate the emitters of Q415 and Q417 and insert a 0.22 or 0.33 ohm 3W resistor between the emitters and the common connection point that goes to E-401 which is an over current protection device. Also insert 4.7 ohm resistors between the drivers and the bases of Q415 and Q417.
As I said before, you'll probably find that the drivers and bias transistors are also blown, and R453 (the 180 ohm between the emitters of the drivers) gets really hot when that happens so check it.
2N2955/3055 and BD139 are easy to find, 2SB669A and 2SB649A will be harder, I found them at B&D Enterprises [/I] B&D Enterprises - electronics, projection lamps for hitachi, sony, mitsubishi, panasonic, samsung and they have the other parts as well. Buy a set for the other channel as well and do the same resistor modifications. Don't forget mica insulators for the transistors and some thermal grease.
Changed the lot. Powered up via a 60watt tungstem lamp and could hear a mains-hum from both channels.
Then I applied the mains directly and in a few seconds R 431 1/2 watt 1k8 started o smoke, so I switched off.....
This amp is really for me to play with and so it dosen't matter if I can't fix it or not but it would be rewarding to do so.!
Any advice for more experienced hobbists would be gratefully received. thanks very much.
Paul
I have a NAD 7225 on my bench for repairs, and have been having a look around all of the useful contributions on this site about the amp.
It was not working and showed the signs of burn out with R453 smoked,R431 and 447 had been hot.
So, being a new boy with amps (I'm an engineer although I'm an amp Virgin!)..I followed the advice:
415 and Q417 are the outputs, Q411 and Q413 are the drivers, Q409 is the biasing transistor, also called a Vbe multiplier. (wikipedia has a good explanation.)
Basically you need to isolate the emitters of Q415 and Q417 and insert a 0.22 or 0.33 ohm 3W resistor between the emitters and the common connection point that goes to E-401 which is an over current protection device. Also insert 4.7 ohm resistors between the drivers and the bases of Q415 and Q417.
As I said before, you'll probably find that the drivers and bias transistors are also blown, and R453 (the 180 ohm between the emitters of the drivers) gets really hot when that happens so check it.
2N2955/3055 and BD139 are easy to find, 2SB669A and 2SB649A will be harder, I found them at B&D Enterprises [/I] B&D Enterprises - electronics, projection lamps for hitachi, sony, mitsubishi, panasonic, samsung and they have the other parts as well. Buy a set for the other channel as well and do the same resistor modifications. Don't forget mica insulators for the transistors and some thermal grease.
Changed the lot. Powered up via a 60watt tungstem lamp and could hear a mains-hum from both channels.
Then I applied the mains directly and in a few seconds R 431 1/2 watt 1k8 started o smoke, so I switched off.....
This amp is really for me to play with and so it dosen't matter if I can't fix it or not but it would be rewarding to do so.!
Any advice for more experienced hobbists would be gratefully received. thanks very much.
Paul
Last edited by a moderator:
The service manual for this unit can be obtained here.
Have you done a basic diode test on the remaining original transistors? That doesn't show all kinds of defects but hard shorts or opens are detected quite reliably without having to unsolder anything.
Otherwise you'll have some poking around to do with the bulb tester back in place. The schematic gives typical voltages; note that some of them are dependent on mains voltage and thus have to be interpreted relative to whatever the supply rails are.
If R431 gets hot, the amplifier is running open loop. Basically a defect in any single transistor can cause this.
Have you done a basic diode test on the remaining original transistors? That doesn't show all kinds of defects but hard shorts or opens are detected quite reliably without having to unsolder anything.
Otherwise you'll have some poking around to do with the bulb tester back in place. The schematic gives typical voltages; note that some of them are dependent on mains voltage and thus have to be interpreted relative to whatever the supply rails are.
If R431 gets hot, the amplifier is running open loop. Basically a defect in any single transistor can cause this.
Guys
R437 between q405 and q407 smokes even running through a 60 watt lamp, in just a few seconds.
R447 and 449 have also been hot in the past and electro C427 was also blown to bits,now replaced.
I can't find anything shorted and the resistance values seem ok....
If anyone would like to comment I would be grateful.
regards
R437 between q405 and q407 smokes even running through a 60 watt lamp, in just a few seconds.
R447 and 449 have also been hot in the past and electro C427 was also blown to bits,now replaced.
I can't find anything shorted and the resistance values seem ok....
If anyone would like to comment I would be grateful.
regards
Could really do with some input with this problem if anyone is out there knows about this amp please;
Symptoms now are the same as before in spite of a lot of poking around. I can't find any obvious shorted or faulty semicondutors. I've changed several caps that I thought may be suspicious but the thing just sits there taking the mick...
R437 68 ohm smokes in a few seconds after turn on (via a 60 watt lamp.) R447 1k2 1/2 watt is very discoloured,it must be getting hot and R431 1k8 also 1/2 watt is a little discoloured but not so much.
Thanks to anyone that may be able to help, please be gentle I've never worked on an amp before
Symptoms now are the same as before in spite of a lot of poking around. I can't find any obvious shorted or faulty semicondutors. I've changed several caps that I thought may be suspicious but the thing just sits there taking the mick...
R437 68 ohm smokes in a few seconds after turn on (via a 60 watt lamp.) R447 1k2 1/2 watt is very discoloured,it must be getting hot and R431 1k8 also 1/2 watt is a little discoloured but not so much.
Thanks to anyone that may be able to help, please be gentle I've never worked on an amp before
Looking at the service manual the amp part is similar to the old NAD 3020 design. They used 2955/3055's that were hometaxial instead of epitaxial, and as such they omitted the emitter resistors from the design.
If you install modern 2955/3055 devices, you need to add at least 0.33R 2W emitter resistors into the design or the amp will go into thermal runaway.
R453 smoking means both driver transistors have turned on hard. I'd check all of the transistors at this point, including the small ones like Q401, Q403, Q405.
You may find Fairchild KSA1220/KSC2690 easier to get than the 649/669 devices.
If you install modern 2955/3055 devices, you need to add at least 0.33R 2W emitter resistors into the design or the amp will go into thermal runaway.
R453 smoking means both driver transistors have turned on hard. I'd check all of the transistors at this point, including the small ones like Q401, Q403, Q405.
You may find Fairchild KSA1220/KSC2690 easier to get than the 649/669 devices.
Thanks Jaycee
I have replaced the 2955 and 3055 on both sides and included 2 watt emitter resistors as you suggest too. Also did this Basically you need to isolate the emitters of Q415 and Q417 and insert a 0.22 or 0.33 ohm 3W resistor between the emitters and the common connection point that goes to E-401 which is an over current protection device. Also insert 4.7 ohm resistors between the drivers and the bases of Q415 and Q417.Took out Q401, Q403, Q405.etc and on a meter they seem to be ok? Replaced the drivers and bias as well..There must be something staring me in the face that I can't see for the trees!
I have replaced the 2955 and 3055 on both sides and included 2 watt emitter resistors as you suggest too. Also did this Basically you need to isolate the emitters of Q415 and Q417 and insert a 0.22 or 0.33 ohm 3W resistor between the emitters and the common connection point that goes to E-401 which is an over current protection device. Also insert 4.7 ohm resistors between the drivers and the bases of Q415 and Q417.Took out Q401, Q403, Q405.etc and on a meter they seem to be ok? Replaced the drivers and bias as well..There must be something staring me in the face that I can't see for the trees!
I think that was one of my old posts that you quoted in #1...
going by a redrawn version of the schematic that I did a long time back for playing around in LTSpice (see attached pdf) because I find it much more easy to read than the factory schematic, if R437 is smoking that means Q407 is turned on hard. Possibly because Q407 is bad, or Q405 is bad, or there's a short somewhere. I'd start by pulling Q407 and checking from there.
going by a redrawn version of the schematic that I did a long time back for playing around in LTSpice (see attached pdf) because I find it much more easy to read than the factory schematic, if R437 is smoking that means Q407 is turned on hard. Possibly because Q407 is bad, or Q405 is bad, or there's a short somewhere. I'd start by pulling Q407 and checking from there.
Hi Guys,
Yes it was one of your earlier posts Mightydub, and thanks for it.!
Sesebe, the unit smokes so fast I dont have time to make any measurements. The only ones I managed to take were across R437 to compare with the other side (438)
Runing via a 60watt lamp and turning off as soon as the meter registered, I found across the smoker R437 there's dc 20 volts ish and on the other side which I assume is functional, it's really close to if not zero volts.
Yes it was one of your earlier posts Mightydub, and thanks for it.!
Sesebe, the unit smokes so fast I dont have time to make any measurements. The only ones I managed to take were across R437 to compare with the other side (438)
Runing via a 60watt lamp and turning off as soon as the meter registered, I found across the smoker R437 there's dc 20 volts ish and on the other side which I assume is functional, it's really close to if not zero volts.
..R437 is smoking that means Q407 is turned on hard. Possibly because Q407 is bad, or Q405 is bad, or there's a short somewhere. I'd start by pulling Q407 and checking from there.
Hi its me again..
I've pulled Q407 it's ok, Q405 is also ok,Q403 and Q401 are good to, according to my meter,I removed them to check.R447 still gets well hot enough to discolour
If I take out Q403 the smoke stops at R437, could this be "on" all the time somehow? Any others suggestions would be welcome please, I'm completely lost with this now and also kind of developing a keen interest to solve it! ?
thanks very much
Hi its me again..
I've pulled Q407 it's ok, Q405 is also ok,Q403 and Q401 are good to, according to my meter,I removed them to check.R447 still gets well hot enough to discolourIf I take out Q403 the smoke stops at R437, could this be "on" all the time somehow? Any others suggestions would be welcome please, I'm completely lost with this now and also kind of developing a keen interest to solve it! ?
thanks very much
Here's the deductive process you have to follow:
1) R37 is getting hot from too much current is going through it.
2) The only paths for current through R437 are Q405 and/or Q407. Is one of them shorted?
3) If both Q405 and Q407 are OK, then something must be turning Q405 on, thereby turning on Q407. Or there is some fault at the base of Q407.
4) What is the voltage at the base of Q405 (with the amp input grounded)? It should be about 38.5V. Anything much lower than that will turn on Q405 more, which will turn on Q407 more, which will mean too much current through R437. If this is the case then you have to work backwards to find the reason.
5) What is the voltage at the base of Q407 (with the amp input grounded)? This should be low. Assuming 0.7V Vbe for all transistors, then with the input grounded and the output at 0V, adding up the Vbes for Q415, Q411, and Q407, the base of Q407 should be about 2.1V, more or less depending on how the bias is set and the actual Vbes of the transistors. If it is significantly higher, then Q407 will be turned on hard, meaning the output will have a high DC level on it. Again, you have to work backwards to find out the reason.
6) Also you should check R447 and R449.
1) R37 is getting hot from too much current is going through it.
2) The only paths for current through R437 are Q405 and/or Q407. Is one of them shorted?
3) If both Q405 and Q407 are OK, then something must be turning Q405 on, thereby turning on Q407. Or there is some fault at the base of Q407.
4) What is the voltage at the base of Q405 (with the amp input grounded)? It should be about 38.5V. Anything much lower than that will turn on Q405 more, which will turn on Q407 more, which will mean too much current through R437. If this is the case then you have to work backwards to find the reason.
5) What is the voltage at the base of Q407 (with the amp input grounded)? This should be low. Assuming 0.7V Vbe for all transistors, then with the input grounded and the output at 0V, adding up the Vbes for Q415, Q411, and Q407, the base of Q407 should be about 2.1V, more or less depending on how the bias is set and the actual Vbes of the transistors. If it is significantly higher, then Q407 will be turned on hard, meaning the output will have a high DC level on it. Again, you have to work backwards to find out the reason.
6) Also you should check R447 and R449.
Thank you mightydub, I'll take a look as you suggest. really appreciate you taking the time with this! Can I ask :
Ground the amp input with respect to 0 volts I presume?
For voltage measurements run the amp at 240v,? So it's ok to just let the smoker R437 burnt to open circuit ...again...How will that influence the measurements if at all please?
Sorry but I'll pick it up as we go along, you deduction explanation is really helpful as I have zero amp experience, I've been digital for a while now
thanks
Paul
Ground the amp input with respect to 0 volts I presume?
For voltage measurements run the amp at 240v,? So it's ok to just let the smoker R437 burnt to open circuit ...again...How will that influence the measurements if at all please?
Sorry but I'll pick it up as we go along, you deduction explanation is really helpful as I have zero amp experience, I've been digital for a while now
thanks
Paul
Ground the amp's input to its signal ground. You might want to get an RCA plug and rig it up so that the tip is shorted to the ring - very useful for this.
If you pull Q403 and that stops the thing blowing up, you can keep the amp running and measure the voltage points on the good channel. Then measure on the bad. That should give you a clue.
It might be worth just shotgun replacing Q401/403/405 as they are only cheap. It can happen that a part passes a diode check but fails in circuit.
It might be worth checking if R411/R415 are ok and D403 too. Maybe the pot R411 has gone open.
If you pull Q403 and that stops the thing blowing up, you can keep the amp running and measure the voltage points on the good channel. Then measure on the bad. That should give you a clue.
It might be worth just shotgun replacing Q401/403/405 as they are only cheap. It can happen that a part passes a diode check but fails in circuit.
It might be worth checking if R411/R415 are ok and D403 too. Maybe the pot R411 has gone open.
Hi,
Check the temperature of R451, R427, and other resistors with bulb on line. You can remove the transistors: Q411 and Q413 for these tests until you find the solutions.
Check for shortcut acros C425, C419, C423, and shortcut from colector of Q409 to heat spreader.
You can check the shortcut of all final tranzistors pins to heat spreader
Check the temperature of R451, R427, and other resistors with bulb on line. You can remove the transistors: Q411 and Q413 for these tests until you find the solutions.
Check for shortcut acros C425, C419, C423, and shortcut from colector of Q409 to heat spreader.
You can check the shortcut of all final tranzistors pins to heat spreader
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...so I grounded the main amp as per suggestion. Then pulled Q401. This stops the thing burning R437.
With the bulb still in series (only a very dim glow now with Q401 out) I have had a poke around measurement taken WRT amp ground terminal on back of unit.
R437 (68 ohm) dc 36.6v, similar both sides.
R447 and 449 measure ok.
Q406 has 36v on base.
Q409 has38v base, emitter 32v collector 38 volts The other side of the amp BD139 has about 0.5v on all legs
Q405 has 36 .6 volts on its base....
Q413 has C32.9 B 32.9 E22volts
Pot R411 is ok. R415 also measures ok.
D 403 seems fine.
So far the only way I've found to stop the smoke from R437 is by pulling 401
It's driving me mad now.......
With the bulb still in series (only a very dim glow now with Q401 out) I have had a poke around measurement taken WRT amp ground terminal on back of unit.
R437 (68 ohm) dc 36.6v, similar both sides.
R447 and 449 measure ok.
Q406 has 36v on base.
Q409 has38v base, emitter 32v collector 38 volts The other side of the amp BD139 has about 0.5v on all legs
Q405 has 36 .6 volts on its base....
Q413 has C32.9 B 32.9 E22volts
Pot R411 is ok. R415 also measures ok.
D 403 seems fine.
So far the only way I've found to stop the smoke from R437 is by pulling 401
It's driving me mad now.......
Jaycee has a good suggestion - just replace Q401, Q403, and Q405 as they are cheap (pennies each).
Earlier you said that C427 had blown up and R431 was getting hot - if C427 shorted, even briefly, then it is possible that the Vce(max) of Q401 was exceeded, and it has failed. Possibly it damaged Q403 and Q405 in the process. Replacing all of them makes sense and it's cheap.
Also compare voltages between circuit points on the good vs. bad channels so you can figure out where the likely problem is.
And if you don't understand the basics of transistors - find a good reference and read up. It will make all of this stuff that seems like voodoo start to make sense.
Earlier you said that C427 had blown up and R431 was getting hot - if C427 shorted, even briefly, then it is possible that the Vce(max) of Q401 was exceeded, and it has failed. Possibly it damaged Q403 and Q405 in the process. Replacing all of them makes sense and it's cheap.
Also compare voltages between circuit points on the good vs. bad channels so you can figure out where the likely problem is.
And if you don't understand the basics of transistors - find a good reference and read up. It will make all of this stuff that seems like voodoo start to make sense.
Taking a closer look at your measurements - the measurements at Q409 and Q413 are way off. Compare to the voltages noted on the schematic.
When you replaced the outputs (Q415, Q417) and the bias spreader (Q409) you may have shorted one of the cases to the heatsink. The case on the TO-3 package is the collector - if you didn't save and re-use the insulators for the case and mounting screws, then you likely have a short to the heatsink. On the BD139 (at least the ones in my parts box) there's a metal slug that is connected to the collector - you need to have that one insulated from the heatsink too.
Check for a short between the collectors of Q409, Q415, Q417, and the heatsink.
When you replaced the outputs (Q415, Q417) and the bias spreader (Q409) you may have shorted one of the cases to the heatsink. The case on the TO-3 package is the collector - if you didn't save and re-use the insulators for the case and mounting screws, then you likely have a short to the heatsink. On the BD139 (at least the ones in my parts box) there's a metal slug that is connected to the collector - you need to have that one insulated from the heatsink too.
Check for a short between the collectors of Q409, Q415, Q417, and the heatsink.
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