Got this amp in for repair; its been repaired before by another tech back ~4 years ago. I see what happened; either the insulator strip behind the fets caused them to short, or the wrong fets were used to previously repair this amplifier. I also found an IRF9640 in the output section with 1 of three legs detached.
Question: The previous rebuild used IRF3205 in the power supply and 22 ohm gate resistors. What should I use to replace? I have more IRF3205, some IRFZ44, and very few FQP50n06 available, with several resistor values to choose.
I saw a Cadence A5+HC on the DVD using IRFZ44 and 47 ohm gate resistors.
Question: The previous rebuild used IRF3205 in the power supply and 22 ohm gate resistors. What should I use to replace? I have more IRF3205, some IRFZ44, and very few FQP50n06 available, with several resistor values to choose.
I saw a Cadence A5+HC on the DVD using IRFZ44 and 47 ohm gate resistors.
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I don't know what the original power supply transistors were in the A7 but I think it would be safe to use the 3205s.
22 ohm resistors OK? I have to replace all of them. If 47 ohm or something else would be a better option...
OK. I put one IRF3205 in the amp and it powered up, but after 5 seconds the new IRF3205 got very hot. Not sure why at this point so I'll keep on troubleshooting.
I took out the 4 rectifiers and the now (two - one per bank) IRF3205 power supply fets still get extremely hot after just a few seconds. I removed the power supply fets and checked the waveform of the fet's first leg; a clean +10v square wave. With a few fets installed, the waveform is not square, but slanted jagged starting from +10v and jagged down to about +3v on the scope.
Im thinking there might be a bad driver in the ps section of this amp unless removing the Rects did not disconnect the PS from the output section.
Im thinking there might be a bad driver in the ps section of this amp unless removing the Rects did not disconnect the PS from the output section.
I should probably already mention; but this amp probably was not in the greatest of previous repair condition for re-build this time around. None-the-less, I love working on these things no matter what as I dont <yet> do this for a living.
From the looks of it, this amp had these parts rebuilt from the past:
All PS fets,
All Output fets for both channels
Output pre/driver section from Q202 through Q207, and several resistors in between
Output pre/driver section from Q402 through Q407, and several resistors in between
Currently, R211, R206, R406, and R411 look burnt/overheated but are still measuring 3.3k.
In the PS section, previously Q15 and Q21 were replaced with A1266 transistors
R25 was replaced
Voltage regs U51 and U52 were removed/replaced.
The two chokes/coils in the center of the amp; underneith the PCB it looks like one of the coils overheated and darkened the board. Both coils are now covered in black glue (Epoxy?) In between these coils theres a 1/4 inch hole in the PCB where I can only guess a mounting screw used to live.
Alas, I recived this amp which only appeared to have a failed PS. The output section seems to be relatively in tact.
If it werent for all the privious repairs, I might not be so temped to bin this amplifier...
From the looks of it, this amp had these parts rebuilt from the past:
All PS fets,
All Output fets for both channels
Output pre/driver section from Q202 through Q207, and several resistors in between
Output pre/driver section from Q402 through Q407, and several resistors in between
Currently, R211, R206, R406, and R411 look burnt/overheated but are still measuring 3.3k.
In the PS section, previously Q15 and Q21 were replaced with A1266 transistors
R25 was replaced
Voltage regs U51 and U52 were removed/replaced.
The two chokes/coils in the center of the amp; underneith the PCB it looks like one of the coils overheated and darkened the board. Both coils are now covered in black glue (Epoxy?) In between these coils theres a 1/4 inch hole in the PCB where I can only guess a mounting screw used to live.
Alas, I recived this amp which only appeared to have a failed PS. The output section seems to be relatively in tact.
If it werent for all the privious repairs, I might not be so temped to bin this amplifier...
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I appologize to myself for my last comment. I really actually would like to learn/fix and work with this amplifier even though it is in pretty rough shape.
I removed all four main rects from the amp. Should this have divided the PS from the audio sections, and enabled me to power on the amp with isolation from the audio side?
With the rects removed the power supply fets still get instantly hot.
What transistors are for driving the power fets? Q15 and Q21?
I removed all four main rects from the amp. Should this have divided the PS from the audio sections, and enabled me to power on the amp with isolation from the audio side?
With the rects removed the power supply fets still get instantly hot.
What transistors are for driving the power fets? Q15 and Q21?
I really don't have any information on this amp so I don't know which transistors are the drivers. Follow the circuit back from the gate resistors.
With the power supply FETs out of the circuit, do you have a clean gate drive signal at the gate pads?
With the power supply FETs out of the circuit, do you have a clean gate drive signal at the gate pads?
Yes there was a clean signal on the gate pads with the fets out. With them in, it looked like a jagged saw blade on the scope.
I removed the two KTA1266 transistors from Q15 and Q21 and they are measuring up broken. For spares, I have a few possible options, but am trying out som 2N2907 transistors as I have a boat load of them in a box. The pin configuration needed to be modified but I think they might work.
http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00003088.pdf
I removed the two KTA1266 transistors from Q15 and Q21 and they are measuring up broken. For spares, I have a few possible options, but am trying out som 2N2907 transistors as I have a boat load of them in a box. The pin configuration needed to be modified but I think they might work.
http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00003088.pdf
-Nope. power supply still gets instantly hot. I think I got close thou, as one of the A1266s i removed dropped a leg right out of it's case, and the surviving one is not registering any continuity or diode check referencing Leg 1 in any direction.
I think I messed up something in the PS driver section with using a driver other than the A1266. Now, the gate pads are only getting a +5v square wave for the gate signal. Looking back to first few posts, I was seeing +10v square at the gates.
I guess I dont have a proper replacement for A1266. I have a bunch of spare parts amps and a few general purpose transistors. What is a good sub for testing? I had previously used a 2n3906 for this amp, but it was a circuit which controlled the power LED and not the PS. With 2n3906 work? What else would maybe work?
http://www.diyaudio.com/forums/car-audio/179897-alpine-mrv-100m-z48-substitute-3.html
I guess I dont have a proper replacement for A1266. I have a bunch of spare parts amps and a few general purpose transistors. What is a good sub for testing? I had previously used a 2n3906 for this amp, but it was a circuit which controlled the power LED and not the PS. With 2n3906 work? What else would maybe work?
http://www.diyaudio.com/forums/car-audio/179897-alpine-mrv-100m-z48-substitute-3.html
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I have used MPSA56s plenty of times. legs 2 and 3 have to be twisted to go into the correct holes and you have to twist the body so that the legs don't touch but other than that, they've always worked.
OK thats great cause I bought a bunch of MPSA56s a while back. With MPSA56s installed, I get a nice +10v square wave on the gates of the fets. Amp now powers up with stability; enough for me to re-install the rectifiers.
This amp is now 99.9% back together, and working audio is present. I've got to pick up some 3.3k resistors for the audio driver section though as some of the resistors look to have burnt up pretty good between this and before the first repair. It looks like the previous repairer replaced some of the output drivers but may have left these 3.3k resistors behind and in a poor state.
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