Hello,
after modding my Musical Fidelity X-T100 I connected it for 2 minutes without heatsinking the SAP15N and P transistors (Darlington output stage as described in the Sanken datasheet)
and they burnt.
At least, that is what I think: they were very hot after the "smokey" event and the 4A fuses blew.
I modded the amp especially in the preamp stage that seems to work fine.
The only items that I changed in the power stage are the rectifier bridge (rebuilt with 8A stealth diodes K-K DC+ / A-A DC-) and two capacitors.
When I replace the fuses and turn of the amp, the amp lights on and the fuses do NOT blow, but there is no sound at the speakers.
Any idea and suggestion on how to proceed in trouble shooting? I think that I have to replace the transistors (need to check whether B-C-E are shorted), but if they were shorted the fuses would have to blow, no?
Any help and advise is greatly appreicated.
Patrick
after modding my Musical Fidelity X-T100 I connected it for 2 minutes without heatsinking the SAP15N and P transistors (Darlington output stage as described in the Sanken datasheet)
and they burnt.
At least, that is what I think: they were very hot after the "smokey" event and the 4A fuses blew.
I modded the amp especially in the preamp stage that seems to work fine.
The only items that I changed in the power stage are the rectifier bridge (rebuilt with 8A stealth diodes K-K DC+ / A-A DC-) and two capacitors.
When I replace the fuses and turn of the amp, the amp lights on and the fuses do NOT blow, but there is no sound at the speakers.
Any idea and suggestion on how to proceed in trouble shooting? I think that I have to replace the transistors (need to check whether B-C-E are shorted), but if they were shorted the fuses would have to blow, no?
Any help and advise is greatly appreicated.
Patrick
Hello and thanks,
I am assuming the outputs are dead.
I re-checked the fuses and they blow, connecting another speaker to the amp does not produce any sound.
For what concerns my main speakers, they should be OK as I have an impedance mathcing transformer between them and the amp but yes, that one might be cooked.
What I wonder: can it really take so little time for a pair of transistors to heat up so much?
Thanks
Patrick
I am assuming the outputs are dead.
I re-checked the fuses and they blow, connecting another speaker to the amp does not produce any sound.
For what concerns my main speakers, they should be OK as I have an impedance mathcing transformer between them and the amp but yes, that one might be cooked.
What I wonder: can it really take so little time for a pair of transistors to heat up so much?
Thanks
Patrick
p.heimpold said:
the SAP16N, SAP16P are now unobtainium -- here's the Sanken replacement from Profusion:
http://www.profusionplc.com/pro/gex/prodGen.html?prdtyp=power darlington
I thought that these transistors had built in over-temp compensation -- there is a diode string in the P and N devices which change the bias current with temperature -- at least that's how they are supposed to work. I think that there might be other things going on.
FWIW, I haven't used the replacement devices yet.
Hello Jackinnj,
my understanding is that the current sensing is meant to regulate "the temperature dependency of the idling current".
I understand your concern that something else might be going on.
I also would like to ensure this.
For what concerns the Power Supply of the output stage, I only rebuilt the rectifier bridge with 4 diodes. C-C to DC+, A-A to DC-.
Any suggestion on how to measure it is working fine?
I unsoldered the two PNP transistors and it becomes apparent that:
1) one has a nice crack
2) one has a "vulcan", looks like a burnt part inside the transistor
I measured resistance across B / C / E and they look OK (value pops up shortly and then gets back to zero) except for C-E on transistor 2), there seems to be a permanent short.
Measuring tension between the legs shows that BE on transistor 2) is about 970mV whereas the others are around 890mV.
My guess: P Transistors are damaged.
Will check the N Transistors now.
Bets regards
Patrick
my understanding is that the current sensing is meant to regulate "the temperature dependency of the idling current".
I understand your concern that something else might be going on.
I also would like to ensure this.
For what concerns the Power Supply of the output stage, I only rebuilt the rectifier bridge with 4 diodes. C-C to DC+, A-A to DC-.
Any suggestion on how to measure it is working fine?
I unsoldered the two PNP transistors and it becomes apparent that:
1) one has a nice crack
2) one has a "vulcan", looks like a burnt part inside the transistor
I measured resistance across B / C / E and they look OK (value pops up shortly and then gets back to zero) except for C-E on transistor 2), there seems to be a permanent short.
Measuring tension between the legs shows that BE on transistor 2) is about 970mV whereas the others are around 890mV.
My guess: P Transistors are damaged.
Will check the N Transistors now.
Bets regards
Patrick
Hello again,
I received my replacement transistors today and replaced the old ones.
Well, everything seems to work fine now, thanks everybody for the help and moral support.
I was lucky enough to get pairs of the same batch, nevertheless
I still need a clue on how to regulate them.
There is a small variable resistor between leg 2 of the NPN and leg 4 of the PNP transistor.
According to the application not from Sanken, this restor is used to regulate the idling current.
When I read the note, I am a little bit lost in understandign what I should exactly do (what to measure and to which value to settle).
Maybe sombeody knows or has time to read page two of the Application Note
Thank you very much in advance!
Patrick
I received my replacement transistors today and replaced the old ones.
Well, everything seems to work fine now, thanks everybody for the help and moral support.
I was lucky enough to get pairs of the same batch, nevertheless
I still need a clue on how to regulate them.
There is a small variable resistor between leg 2 of the NPN and leg 4 of the PNP transistor.
According to the application not from Sanken, this restor is used to regulate the idling current.
When I read the note, I am a little bit lost in understandign what I should exactly do (what to measure and to which value to settle).
Maybe sombeody knows or has time to read page two of the Application Note
Thank you very much in advance!
Patrick
You short the input of the amplifier, then measure the voltage from Pin S ("sense") to E ("Emitter) -- 40 milliamps times 220 milliohms is 8.8 milliVolts -- so adjust the 200 ohm resistor until this value is attained.
by the way, one of the problems with the Sanken's is that the emitter resistor can blow out if you don't properly heatsink the devices (or have the bias current set too high). If you blow out this resistor, however, you can just insert a 220 milliOhm, 3 watt device in its place.
by the way, one of the problems with the Sanken's is that the emitter resistor can blow out if you don't properly heatsink the devices (or have the bias current set too high). If you blow out this resistor, however, you can just insert a 220 milliOhm, 3 watt device in its place.
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