Hi team
So I posted a few months back when I found this great looking Marantz amp online https://audiokarma.org/forums/index.php?threads/marantz-pm-550dc-sound-output-distortion.985180/
From that thread I had posted...
"The main issue is the sound output. At a low volume there is a distortion (I wanted to say crackle but it's not "crackly per say), then as I up the volume control the sound improves, it cleans up a bit but you can tell it just isn't quite right, then lifting the volume more the distortion cuts back through.
Things to note...
So I started with a full recap....
1. Changed all the elect caps in main board and phono P400 board. Nothing tested bad but replaced them as I had them out.
2. Isolated the 4 main transistors on the bottom of the heat sink and tested - all appear to be good/correct (have bought spares but not replaced)
3. Removed and checked about half of the smaller transistors on the main P700 board (always moving away from the main caps - but only to keep track). Only 1 tested bad - 2SD1111. Couldn't find this one in existence anymore so ordered 3 different types from an Australian site (they suggested as suitable replacements) and only one worked (the other didn't make the relay "click") Replaced with a KSC945
4. Removed and checked about half of the diodes (always moving away from the main caps). Nothing ever wrong so placed them back
5. Did more research and thought about the "hum"... used a phone app and it is definitely a 100hz hum. Plug in headphones with volume on zero and the sound hums in both ears. Changes a little if you select different inputs, raise the volume, adjust the equalizer, but stays at a 100hz hum.
6. Checked/retightened all screws on the body of the chassis - only found the volume pot loose to the chassis - did it make the hum quieter? Don't think so
7. Can't see any solder that is super untidy
8. Can't see any resistors that are discolored etc
9. Around the power transformer there is a bit of surface rust, but I would be surprised if that creates an issue as described above
After all that above, still if I play a Marantz tuner or turntable through, I get sound but it's still "dirty" (with the hum in behind).
I have ability to replace and test things but I'm wondering if I am getting to my limits. Not sure "yet" how to test voltages or common grounds (if I need to). So if you have some advice, some clear instructions would be appreciated. I don't yet want to take this to an electronics store for their help, and an expensive bill.
Thoughts people?
Thanks
Phil
So I posted a few months back when I found this great looking Marantz amp online https://audiokarma.org/forums/index.php?threads/marantz-pm-550dc-sound-output-distortion.985180/
From that thread I had posted...
"The main issue is the sound output. At a low volume there is a distortion (I wanted to say crackle but it's not "crackly per say), then as I up the volume control the sound improves, it cleans up a bit but you can tell it just isn't quite right, then lifting the volume more the distortion cuts back through.
Things to note...
- Using Marantz 8 ohm speakers
- Both left and right channels have the same distortion, left/right/together
- Can hear the same distortion using headphones
- Using the Phono selection
- Have tried an iPhone through the Aux channel with the same distortion
So I started with a full recap....
1. Changed all the elect caps in main board and phono P400 board. Nothing tested bad but replaced them as I had them out.
2. Isolated the 4 main transistors on the bottom of the heat sink and tested - all appear to be good/correct (have bought spares but not replaced)
3. Removed and checked about half of the smaller transistors on the main P700 board (always moving away from the main caps - but only to keep track). Only 1 tested bad - 2SD1111. Couldn't find this one in existence anymore so ordered 3 different types from an Australian site (they suggested as suitable replacements) and only one worked (the other didn't make the relay "click") Replaced with a KSC945
4. Removed and checked about half of the diodes (always moving away from the main caps). Nothing ever wrong so placed them back
5. Did more research and thought about the "hum"... used a phone app and it is definitely a 100hz hum. Plug in headphones with volume on zero and the sound hums in both ears. Changes a little if you select different inputs, raise the volume, adjust the equalizer, but stays at a 100hz hum.
6. Checked/retightened all screws on the body of the chassis - only found the volume pot loose to the chassis - did it make the hum quieter? Don't think so
7. Can't see any solder that is super untidy
8. Can't see any resistors that are discolored etc
9. Around the power transformer there is a bit of surface rust, but I would be surprised if that creates an issue as described above
After all that above, still if I play a Marantz tuner or turntable through, I get sound but it's still "dirty" (with the hum in behind).
I have ability to replace and test things but I'm wondering if I am getting to my limits. Not sure "yet" how to test voltages or common grounds (if I need to). So if you have some advice, some clear instructions would be appreciated. I don't yet want to take this to an electronics store for their help, and an expensive bill.
Thoughts people?
Thanks
Phil
