Hi All
I have an MF 3.5 amplifier and it it has a serious crackling on the right channel. It bled over to the left a bit but this was fixed by replacing the NE5532 chip on the motorised volume control.
Now it only faults on the right channel. It is a high dc voltage that jumps up and down as I rotate the volume pot. goes from the usual 20 -30mv to 190 + difficult to get it to stay put.
Measuring across the Pre amplifier O/P phonos it is about a 1.3volts.
The caps on the Volume PCB were replaced, and the ceramics tested all OK.
The fault disappears if I disconnect SK5 from the main PCB.
then I can get a 1K tone out of both channels Ok albeit a reduced volume.
So the fault appears to be around the PIC chip perhaps?
All the DC voltages from the regulators are OK and no noise on the rails.
On IC8 Pins 13/14 no noise at all.
Any suggestions would be welcome.
thanks
Phil
I have an MF 3.5 amplifier and it it has a serious crackling on the right channel. It bled over to the left a bit but this was fixed by replacing the NE5532 chip on the motorised volume control.
Now it only faults on the right channel. It is a high dc voltage that jumps up and down as I rotate the volume pot. goes from the usual 20 -30mv to 190 + difficult to get it to stay put.
Measuring across the Pre amplifier O/P phonos it is about a 1.3volts.
The caps on the Volume PCB were replaced, and the ceramics tested all OK.
The fault disappears if I disconnect SK5 from the main PCB.
then I can get a 1K tone out of both channels Ok albeit a reduced volume.
So the fault appears to be around the PIC chip perhaps?
All the DC voltages from the regulators are OK and no noise on the rails.
On IC8 Pins 13/14 no noise at all.
Any suggestions would be welcome.
thanks
Phil
It is availablea at hi fi engine. I had trouble attaching it
sorry, and thanks for the reply.
I can try again if you cannot et it.
Phil
sorry, and thanks for the reply.
I can try again if you cannot et it.
Phil
Yes, my regrets for failing to reply.
Chief suspicion falls upon the volume control. The right channel seems to be associated with IC5, pin 7. Short the selected input source for better certainty about a quiet signal path, then confirm there's no DC voltage across the pot element, i.e. SK3, pin 5. Monitor IC5, pin 7 as you rotate the volume control. Any voltage jumps are suggestive of erratic wiper resistance. If this proves to be the case, cleaning the pot might help, but you may have to replace the pot.
I may be having a blindness episode, but I can't find where IC8 Pins 13/14 go after the IC. It may not be relevant.
Good luck!
Chief suspicion falls upon the volume control. The right channel seems to be associated with IC5, pin 7. Short the selected input source for better certainty about a quiet signal path, then confirm there's no DC voltage across the pot element, i.e. SK3, pin 5. Monitor IC5, pin 7 as you rotate the volume control. Any voltage jumps are suggestive of erratic wiper resistance. If this proves to be the case, cleaning the pot might help, but you may have to replace the pot.
I may be having a blindness episode, but I can't find where IC8 Pins 13/14 go after the IC. It may not be relevant.
Good luck!
Speaker relay drive perhaps on another sheet. The ULN2003 is little more than an open collector output driver chip.
@BSST is telling you all the right things to do.
Given the design is AC coupled (so seemingly no worries or cares on using caps in the signal path) it then seems madness on the part of MF to then DC couple the wiper to the opamp input. That is the one point that is asking for trouble. The 5532 is also one of the worst opamps to go and do that with given its relatively high DC input bias currents.
Interesting comments.
Pin 13/14 go to the relay. There seems to be no measurable DC on the pot terminals either.
IC5 was replaced anyway for testing reasons it made no difference.
I will try out these suggestions on Monday. Much appreciated, seems a shame to scrap the amplifier but it is taking two much time playing about with it.
thanks
I will post next week on the results.
P
Pin 13/14 go to the relay. There seems to be no measurable DC on the pot terminals either.
IC5 was replaced anyway for testing reasons it made no difference.
I will try out these suggestions on Monday. Much appreciated, seems a shame to scrap the amplifier but it is taking two much time playing about with it.
thanks
I will post next week on the results.
P
You were correct it was indeed the ALPS pot!
Upon removing it and wiring in a standard 50K log pot the fault vanished.
Bought one it is on the way, bloody expensive!!!!
The one thing I cannot fathom is there was high offset n SK3 pin 1 as the pot is rotated, Pin 2 is the left channel.
So wanting to trace back I removed the plug from SK 3 to see if the dc was still high, well both pins then showed 3.5 volts constant?
I assume this is because the IC5 O/Puts were open to fresh air.
Oh pins 13/14 dor IC8 actually go to RLY-LH & RLY-RH on the relay perhaps?
Anyway, its fixed.
thanks for your assistence it is much appreciated.
Phil
Upon removing it and wiring in a standard 50K log pot the fault vanished.
Bought one it is on the way, bloody expensive!!!!
The one thing I cannot fathom is there was high offset n SK3 pin 1 as the pot is rotated, Pin 2 is the left channel.
So wanting to trace back I removed the plug from SK 3 to see if the dc was still high, well both pins then showed 3.5 volts constant?
I assume this is because the IC5 O/Puts were open to fresh air.
Oh pins 13/14 dor IC8 actually go to RLY-LH & RLY-RH on the relay perhaps?
Anyway, its fixed.
thanks for your assistence it is much appreciated.
Phil