Bought a used 7250PE on ebay with the typical “powers on, but not tested” disclaimer.
On initial power up noticed hiss from right channel and similar lower volume hiss on left. This was present at zero volume. All the input sources worked. Turned up some music and the hiss faded into background. In quiet passages though, you could definitely hear the right channel hiss across the room. First step was deoxit on the main controls. No love.
I pulled the preamp jumpers on the back and still had hiss, so that narrowed it to power amplifier. I went on a long search for a bad transistor or resistor using freeze spray, heat from a soldering tip, tapping the PCB with the eraser end of a pencil, plus DVM checks of components in-circuit. I don’t have an oscilloscope. Anyway, as I went along eliminating certain parts of the circuit (I do have the service manual), I kept doing internet searches. Eventually I found an old post here where someone identified two trim pots out of spec in a 7250PE with bigger problems than mine. Sure enough, they were way out of spec on mine too, and that ended up being the problem.
I found three references on-line to these same trim pots, all failing in the same manner. I am #4 on the list, and I suspect every 7250PE out there has this problem. The pots in question are R618 and R668, both 200 ohm trimmers used for the “minimum distortion adjustment” described in the service manual. Others have reported the resistance of their failed trimmers as high as 1.5M ohm. Mine were 200K on the left and 500K on the right. Out of circuit both my trimmers had tight rivets. When I measured resistance from the rivet to the resistive surface only 1 mm away it was sky high. So I suspect there is galvanic corrosion between the resistive surface and the rivet. In the power amp, NAD uses three trimmers per channel. The two affected ones are physically smaller than the other four (used for idle and centering) and have 5 mm spacing between legs. Idle and centering cannot be adjusted properly with this distortion trimmer resistance so high. My understanding is high resistance in this trimmer causes the switching transistor to utilize the high voltage rail at all times.
I replaced these trimmers with new 200 ohm units. I had pictures of the old ones, and just set the new ones to the same clock position as the bad ones. Others on-line simply substituted 100 ohm ¼ watt resistors. This is a non-critical adjustment that technically requires a distortion meter to adjust perfectly.
All the larger type trimmers showed resistance within spec.
With new trimmers I was able to easily set idle and centering. Hiss noise was gone. I had the slightest hum (sounded like 60 Hz) in both channels, and decided to be anal and start checking the electrolytics. Starting with the power supply I marked the PCB for my check points with power off. With power on I did not get very far, as I quickly realized the hum was positional, and went away when the amp was on its side. The circuits are obviously very sensitive to ambient fields. With the case back on— no hum!
I did measure some DC voltages while I was at it, and will post those here, as they are not listed on the schematic. These were measured with the impedance switch set to 8 ohms. I did not measure AC voltage since I had already figured out the hum was positional.
C865, C866 +76.7 VDC and -76.7 VDC (80V caps)
C862, C863 +46.2 and – 46.2 (50V caps)
C851, C852 +35 and -35 (35V caps)
This amp is from about 1986. I also have an NAD 7125 (ca. 1983) and that only uses the larger size trimmers. My suspicion is that any NAD product from this era that uses these same small trimmers will have problems. Possibly, these smaller ones are only used in the 7250PE.
Since this receiver is now working fine to my ears, I decided to live with it for a while before considering anything like a recap. I have been all over this thing looking for bulging caps or signs of leakage several times. And, yes, I am aware that the operating voltages on those filter caps are very close to (or at) the limits of their ratings.
On initial power up noticed hiss from right channel and similar lower volume hiss on left. This was present at zero volume. All the input sources worked. Turned up some music and the hiss faded into background. In quiet passages though, you could definitely hear the right channel hiss across the room. First step was deoxit on the main controls. No love.
I pulled the preamp jumpers on the back and still had hiss, so that narrowed it to power amplifier. I went on a long search for a bad transistor or resistor using freeze spray, heat from a soldering tip, tapping the PCB with the eraser end of a pencil, plus DVM checks of components in-circuit. I don’t have an oscilloscope. Anyway, as I went along eliminating certain parts of the circuit (I do have the service manual), I kept doing internet searches. Eventually I found an old post here where someone identified two trim pots out of spec in a 7250PE with bigger problems than mine. Sure enough, they were way out of spec on mine too, and that ended up being the problem.
I found three references on-line to these same trim pots, all failing in the same manner. I am #4 on the list, and I suspect every 7250PE out there has this problem. The pots in question are R618 and R668, both 200 ohm trimmers used for the “minimum distortion adjustment” described in the service manual. Others have reported the resistance of their failed trimmers as high as 1.5M ohm. Mine were 200K on the left and 500K on the right. Out of circuit both my trimmers had tight rivets. When I measured resistance from the rivet to the resistive surface only 1 mm away it was sky high. So I suspect there is galvanic corrosion between the resistive surface and the rivet. In the power amp, NAD uses three trimmers per channel. The two affected ones are physically smaller than the other four (used for idle and centering) and have 5 mm spacing between legs. Idle and centering cannot be adjusted properly with this distortion trimmer resistance so high. My understanding is high resistance in this trimmer causes the switching transistor to utilize the high voltage rail at all times.
I replaced these trimmers with new 200 ohm units. I had pictures of the old ones, and just set the new ones to the same clock position as the bad ones. Others on-line simply substituted 100 ohm ¼ watt resistors. This is a non-critical adjustment that technically requires a distortion meter to adjust perfectly.
All the larger type trimmers showed resistance within spec.
With new trimmers I was able to easily set idle and centering. Hiss noise was gone. I had the slightest hum (sounded like 60 Hz) in both channels, and decided to be anal and start checking the electrolytics. Starting with the power supply I marked the PCB for my check points with power off. With power on I did not get very far, as I quickly realized the hum was positional, and went away when the amp was on its side. The circuits are obviously very sensitive to ambient fields. With the case back on— no hum!
I did measure some DC voltages while I was at it, and will post those here, as they are not listed on the schematic. These were measured with the impedance switch set to 8 ohms. I did not measure AC voltage since I had already figured out the hum was positional.
C865, C866 +76.7 VDC and -76.7 VDC (80V caps)
C862, C863 +46.2 and – 46.2 (50V caps)
C851, C852 +35 and -35 (35V caps)
This amp is from about 1986. I also have an NAD 7125 (ca. 1983) and that only uses the larger size trimmers. My suspicion is that any NAD product from this era that uses these same small trimmers will have problems. Possibly, these smaller ones are only used in the 7250PE.
Since this receiver is now working fine to my ears, I decided to live with it for a while before considering anything like a recap. I have been all over this thing looking for bulging caps or signs of leakage several times. And, yes, I am aware that the operating voltages on those filter caps are very close to (or at) the limits of their ratings.
Thanks for the post. Perfect timing. I have a 7225PE that measures 17mv on right output and 1100mv on the left and the left trimpoot is at its limit. I am trying to trace the problem and had not considered the trimpots could be an issue. Worth investigating even though I dont hear a lot of hiss.
Cheers
Cheers