Good evening, with this post I wanted to share my journey on restoring and then modding a broken MPA1004 amplifier (16 mosfets, 2SK1058/2SJ162) that I got for spare parts at the price of 40 euros.
It was FILTHY. Full of hair, dust, it was probably running in a basement somewhere. Also all the mount brackets for the mosfets were missing, capacitors bulging, lid missing as well.
I had to literally wash the board with degreaser and water (after removing most of the components) and then in isopropyl alcohol.
(the board in the picture on the right has different color because I also did the procedure on two more of the same amps, but it's equivalent)
Then I took the heatsink and drilled and threaded all the 16 holes for the mosfets to have better thermal contact (and because I had no brackets)
Mounted the components, new capacitors, replaced all the driver transistors because they were out of specs and rusted, replaced the op amps
Could not find any NE5517 so I improvised making an adapter from the SMD LM13700. Then replaced all the potentiometers because they were heavily oxidized.
The amplifier is now perfectly working, I used the Hitachi Application Note as a reference for setting the bias current because the circuit is more or less the same. I was lucky none of the mosfets were broken.
This amplifier is currently in use in my living room connected to the Onkyo TX-SR505E 7.1 Audio receiver to power this giant one meter wide subwoofer I built here and have movie nights with friends and family.
It was FILTHY. Full of hair, dust, it was probably running in a basement somewhere. Also all the mount brackets for the mosfets were missing, capacitors bulging, lid missing as well.
I had to literally wash the board with degreaser and water (after removing most of the components) and then in isopropyl alcohol.
(the board in the picture on the right has different color because I also did the procedure on two more of the same amps, but it's equivalent)
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Then I took the heatsink and drilled and threaded all the 16 holes for the mosfets to have better thermal contact (and because I had no brackets)
Mounted the components, new capacitors, replaced all the driver transistors because they were out of specs and rusted, replaced the op amps
Could not find any NE5517 so I improvised making an adapter from the SMD LM13700. Then replaced all the potentiometers because they were heavily oxidized.
The amplifier is now perfectly working, I used the Hitachi Application Note as a reference for setting the bias current because the circuit is more or less the same. I was lucky none of the mosfets were broken.
This amplifier is currently in use in my living room connected to the Onkyo TX-SR505E 7.1 Audio receiver to power this giant one meter wide subwoofer I built here and have movie nights with friends and family.
For the modding... First of all I didn’t like the long power cable sticking out of the back, I wanted something more “transportation friendly”. So I sawed a rectangular hole on the back and mounted an IEC C14 connector.
I took the opportunity then to drill two holes on the front panel next to the volume potentiometers to mount two VU meters to have a feedback on the output signal produced.
The problem now was that the old label did not match anymore with the holes and during the drilling step I had to remove it and make a new one.
Used the program Adobe InDesign to make a more modern layout positioning the text, volume scale and model number in the right position, then printed the label on adhesive PVC, attached it to the front panel and carved the holes using a blade.
Of course the unit was also missing a lid so I made one cutting some 4mm thick plexiglass. After mounting the VU meters the result is this:
It turned out much better than expected and I am quite happy with it.
I took the opportunity then to drill two holes on the front panel next to the volume potentiometers to mount two VU meters to have a feedback on the output signal produced.
The problem now was that the old label did not match anymore with the holes and during the drilling step I had to remove it and make a new one.
Used the program Adobe InDesign to make a more modern layout positioning the text, volume scale and model number in the right position, then printed the label on adhesive PVC, attached it to the front panel and carved the holes using a blade.
Of course the unit was also missing a lid so I made one cutting some 4mm thick plexiglass. After mounting the VU meters the result is this:
It turned out much better than expected and I am quite happy with it.
Super work here Mimiragon! 
These LatFet amps are worthy for restoring. Did you play the amp driving full range speakers before relegated to subwoofer duty?
Hopefully your neighbors are far enough away their dishes won’t rattle!
Enjoy!
Edit: Well, I found your sub build thread, that answers the neighbor concern
These LatFet amps are worthy for restoring. Did you play the amp driving full range speakers before relegated to subwoofer duty?
Hopefully your neighbors are far enough away their dishes won’t rattle!
Enjoy!
Edit: Well, I found your sub build thread, that answers the neighbor concern
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Thank you so much! 🙏
Yes I tried them, they do work great as you said, I had to replace the fans to "appreciate the silence" in idle or all I could hear was some windy noise 😆
Overall I had about 6 of these amps to repair (3 were the 8 mosfets version and 3 the 16) and they are all working properly now.
Two went back to some DJ to make loud noises but the others have found a proper home and are connected to tube preamps and fancier equipment, people reported great results from them.
Yes I tried them, they do work great as you said, I had to replace the fans to "appreciate the silence" in idle or all I could hear was some windy noise 😆
Overall I had about 6 of these amps to repair (3 were the 8 mosfets version and 3 the 16) and they are all working properly now.
Two went back to some DJ to make loud noises but the others have found a proper home and are connected to tube preamps and fancier equipment, people reported great results from them.
I didn't find a single broken mosfet in any of them. The majority of the problems came from the gunk under the boards, broken solder joints, close to explosion capacitors or the protection circuit failing and staying always on.
There is also a tiny 1uF ceramic capacitor in the protection circuit that should charge a few seconds after turning the amp on and trigger the relay, but being it almost 30 years old, in all of the units the capacitor had to be replaced.
There is also a tiny 1uF ceramic capacitor in the protection circuit that should charge a few seconds after turning the amp on and trigger the relay, but being it almost 30 years old, in all of the units the capacitor had to be replaced.
The output resistors were close to the fans and received some direct airflow. The air must have been really humid when it previously was because they were completely rusted and falling apart. Replacing them was necessary as well.
Added a second NTC in series with the first one, the spike caused by the 1800VA transformer turning on was so high that my other equipment would turn off
Added a second NTC in series with the first one, the spike caused by the 1800VA transformer turning on was so high that my other equipment would turn off
I went to a print shop in my town, they managed to squeeze the file in between two bigger labels so In the end they gave it to me for free.
Not sure what kind of printer it is but it's similar to a CMYK Xerox laser printer. The ink is fused on the surface of the plastic, so you need to avoid scratching it with anything sharp.
Not sure what kind of printer it is but it's similar to a CMYK Xerox laser printer. The ink is fused on the surface of the plastic, so you need to avoid scratching it with anything sharp.