I have a second hand Marantz PM230 amplifier that I'm trying to repair. It's an 80s era amp and uses a voltage amplifier IC in the output stage. My knowledge of analogue electronics is amateur at best but I'm keen to learn more and have made repairing the amp into a hobby project.
It worked perfectly for a while but is now silent on both output channels. When changing inputs on the amp there is a popping sound (but only if you haven't changed for a while) but no other sound output even at high volume.
I read about the amp online and the STK3042IIA voltage amplifier IC and power amplifier transistors seem to be the usual suspects. I've checked all of the power amp transistors with a multimeter in circuit and have replaced the STK IC to no avail.
I've visually inspected the top and bottom of the main board for bad solder joints, bulging caps or heat damage but cannot see anything obviously bad. I checked the AC voltage outputs from the transformer and get 27V for J808 and J809, but 0V for J807 which appears to come from the centre of the transformer on the schematic - does this sound right? Is it meant to be some kind of ground?
I have also checked the DC voltage rails on the main board and these are ±37V (schematic says ±33.3V).
I have verified with an oscilloscope and 440Hz sine input signal that the CD/AUX input path including volume and balance controls seem okay. The input selector buttons and LEDs all work properly.
I moved to the flat & tone amps which are the next stages (see page 7 of the manual linked below). There is no voltage on either left or right pins of the tone amp board output (WE04 pins 1 & 2 on schematic) which leads me to suspect a fault with it. I've also cleaned the controls out with switch cleaner.
I then noticed that someone had soldered a jumper wire between pin 4 of QE02 and pin 2 of WE03 on the bottom of the tone amp board (which I believe is ground?). I am not sure why they would do that - perhaps to bypass a faulty component? I've checked all of the resistors on the tone amp board in circuit and all look okay. I tried removing the jumper wire to see what would happen (still silence, nothing dramatic). I have checked the inputs from both channels to QE01 (flat amp) at pins 3 and 5 and they are okay. However I don't really understand how the tone amp board circuit works.
I would be grateful if anyone has any ideas of how I can proceed with troubleshooting please.
Links:
PM230 mainboard (high res): https://amoss.me/pm230/pm230-mainboard.JPG
PE00 Jumper wire (now removed): https://amoss.me/pm230/pe00-jumper.jpeg
PM230 Schematic (stitched together from manual pages): https://amoss.me/pm230/schematic.jpg
PM230 Service Manual: https://amoss.me/pm230/hfe_marantz_pm230_service_en.pdf
QE01/NJM4560 Dual Opamp Manual: https://amoss.me/pm230/NJM4560_E.pdf
QE02/NJM4558 Dual Opamp Manual: https://amoss.me/pm230/NJM4558_NJM4559_E-218630.pdf
PS: I have the luxury of having another PM230 which I bought as faulty - but which may be useful for parts or comparing readings.
It worked perfectly for a while but is now silent on both output channels. When changing inputs on the amp there is a popping sound (but only if you haven't changed for a while) but no other sound output even at high volume.
I read about the amp online and the STK3042IIA voltage amplifier IC and power amplifier transistors seem to be the usual suspects. I've checked all of the power amp transistors with a multimeter in circuit and have replaced the STK IC to no avail.
I've visually inspected the top and bottom of the main board for bad solder joints, bulging caps or heat damage but cannot see anything obviously bad. I checked the AC voltage outputs from the transformer and get 27V for J808 and J809, but 0V for J807 which appears to come from the centre of the transformer on the schematic - does this sound right? Is it meant to be some kind of ground?
I have also checked the DC voltage rails on the main board and these are ±37V (schematic says ±33.3V).
I have verified with an oscilloscope and 440Hz sine input signal that the CD/AUX input path including volume and balance controls seem okay. The input selector buttons and LEDs all work properly.
I moved to the flat & tone amps which are the next stages (see page 7 of the manual linked below). There is no voltage on either left or right pins of the tone amp board output (WE04 pins 1 & 2 on schematic) which leads me to suspect a fault with it. I've also cleaned the controls out with switch cleaner.
I then noticed that someone had soldered a jumper wire between pin 4 of QE02 and pin 2 of WE03 on the bottom of the tone amp board (which I believe is ground?). I am not sure why they would do that - perhaps to bypass a faulty component? I've checked all of the resistors on the tone amp board in circuit and all look okay. I tried removing the jumper wire to see what would happen (still silence, nothing dramatic). I have checked the inputs from both channels to QE01 (flat amp) at pins 3 and 5 and they are okay. However I don't really understand how the tone amp board circuit works.
I would be grateful if anyone has any ideas of how I can proceed with troubleshooting please.
Links:
PM230 mainboard (high res): https://amoss.me/pm230/pm230-mainboard.JPG
PE00 Jumper wire (now removed): https://amoss.me/pm230/pe00-jumper.jpeg
PM230 Schematic (stitched together from manual pages): https://amoss.me/pm230/schematic.jpg
PM230 Service Manual: https://amoss.me/pm230/hfe_marantz_pm230_service_en.pdf
QE01/NJM4560 Dual Opamp Manual: https://amoss.me/pm230/NJM4560_E.pdf
QE02/NJM4558 Dual Opamp Manual: https://amoss.me/pm230/NJM4558_NJM4559_E-218630.pdf
PS: I have the luxury of having another PM230 which I bought as faulty - but which may be useful for parts or comparing readings.
