Hello,
I own a Harman Kardon PM640, 2X45W amplifier that I bought it back in early 80's. It still sounds very good (bass, dynamics, very smooth but not so detailed at high frequencies) and there are many problems with the buttons, tone control switches and all those mechanical parts that they are too old and causing problems to the signal path (scratches, noise, interruptions etc).
So. I am thinking to take all of these parts out off the signal path, taking the signal from the AUX RCA input, straight forward to the volume potensiometer. All other stuff will stay of the path like input selector, RIIA, tone control, balance, subsonic filter (I never use them anyway).
I am thinking of two options:
1. First Option: Keep the existent potensiometer. At the attached amplifier diagram, the two No.1 red dots, are the points where the input signal will be connected. The potensiometer is an 8-pin associated with loudness. My question here is if the 2 connection points that indicated at the diagram, are correct.
2. Second Option: Replace the potensiometer with a new one keeping loundess out. The new one it will not connected at the PCB.
My questions here are:
- Any other suggestions to improve this old amp?
Thank you in advance,
Daniel
I own a Harman Kardon PM640, 2X45W amplifier that I bought it back in early 80's. It still sounds very good (bass, dynamics, very smooth but not so detailed at high frequencies) and there are many problems with the buttons, tone control switches and all those mechanical parts that they are too old and causing problems to the signal path (scratches, noise, interruptions etc).
So. I am thinking to take all of these parts out off the signal path, taking the signal from the AUX RCA input, straight forward to the volume potensiometer. All other stuff will stay of the path like input selector, RIIA, tone control, balance, subsonic filter (I never use them anyway).
I am thinking of two options:
1. First Option: Keep the existent potensiometer. At the attached amplifier diagram, the two No.1 red dots, are the points where the input signal will be connected. The potensiometer is an 8-pin associated with loudness. My question here is if the 2 connection points that indicated at the diagram, are correct.
2. Second Option: Replace the potensiometer with a new one keeping loundess out. The new one it will not connected at the PCB.
My questions here are:
- The other two red dots (No. 2) are the points where the output of the potensiometer must connected? Or I have to connect after the C 401/402 and R 401/402
- The existent pot, is linear or logarithmic?
- The new one must be linear or logarithmic?
- Will be other problems that might caused by this modification?
- Any other suggestions to improve this old amp?
Thank you in advance,
Daniel
If you connect the AUX to points 1 you still have Balance control. VR503 is for balance. This potmeter also can crackle. It might be hard to find a new one. You can also bypass it connecting AUX to the top of VR504. But you lose Balance control.
Yes, the points 2 are to connect the output of a new potmeter. It must be log. If you bypass the Balance a value 50k will also work.
The existing pot is log and maybe even more exotic due to the loudness.
Yes, the points 2 are to connect the output of a new potmeter. It must be log. If you bypass the Balance a value 50k will also work.
The existing pot is log and maybe even more exotic due to the loudness.
Yes, i cleaned the contacts several times trhough the years.
But, even if all the contacts was OK, I would like to keep the signal path clear and simple. I almost never use all of those extra controls.
I am also thinking to replace the PS filter caps. There are 2X8000uF and I'll go for bigger ones (22000uF). The other caps at audio path looks OK (ELNA).
But, even if all the contacts was OK, I would like to keep the signal path clear and simple. I almost never use all of those extra controls.
I think I'll go with the second option. I don't need the balance control either. I'll go with a quality new potensiometer (Alps?) and I will use figh quality cables for connections.
I am also thinking to replace the PS filter caps. There are 2X8000uF and I'll go for bigger ones (22000uF). The other caps at audio path looks OK (ELNA).
Hi,there,I've had the same problem with a similar aged ITT amp.At the end I connected a little preamp with tone controls after all the controls of the org. path.This kept me away from fiddling around finding all the poti's.Greetings Frank
