Hello,
this Pioneer SA-9500 amplifier belongs to a friend of mine.
About a year ago he asked me to clean it up and recap if necessary.
OK, did it. - replaced lytics (Wima film caps into signal path, otherwise Panasonic FM/FR), cleaned all switches (disassembled the rocker type ones).
According to RMAA meausrements (from speaker terminals) improvement was noticeable.
In fact, it became too god since now the owner says he can hear hum in the speakers and earphones.
It has been on and off my desk intermittently for several months (no hurry with this) and I have sorted out the earthing wiriing, replaced plain signal wires with shielded ones, bypassed the input buffer, bypassed also on Tone off postition the first stages of control amplifier.
So the signal path from input sockets to power amplifier input is completely passive in Tone OFF mode.
Bought also a Tektronics analogue scope (to join the digital ones I have).
Now to the hum - it seems to be the mains ferquency (50Hz), on RMAA it shows like this:
Looking at the scope (on speaker terminals:
- when power amplifier input is grounded (Power amp input switch is set to "separated") everything looks ok and the owner does not hear hum:
- when power amplifier is connected to the input signal path wiring (and the RCA input socket is grounded) then with voume regulator at minimum position things start to show up:
- when volume knob is turned to max position then it besomes more clear:
Next step would be "walking the signal path" with the scope and grounding it at every following stage but can anybody suggest what could be causing this kind of noise?
this Pioneer SA-9500 amplifier belongs to a friend of mine.
About a year ago he asked me to clean it up and recap if necessary.
OK, did it. - replaced lytics (Wima film caps into signal path, otherwise Panasonic FM/FR), cleaned all switches (disassembled the rocker type ones).
According to RMAA meausrements (from speaker terminals) improvement was noticeable.
In fact, it became too god since now the owner says he can hear hum in the speakers and earphones.
It has been on and off my desk intermittently for several months (no hurry with this) and I have sorted out the earthing wiriing, replaced plain signal wires with shielded ones, bypassed the input buffer, bypassed also on Tone off postition the first stages of control amplifier.
So the signal path from input sockets to power amplifier input is completely passive in Tone OFF mode.
Bought also a Tektronics analogue scope (to join the digital ones I have).
Now to the hum - it seems to be the mains ferquency (50Hz), on RMAA it shows like this:
Looking at the scope (on speaker terminals:
- when power amplifier input is grounded (Power amp input switch is set to "separated") everything looks ok and the owner does not hear hum:
- when power amplifier is connected to the input signal path wiring (and the RCA input socket is grounded) then with voume regulator at minimum position things start to show up:
- when volume knob is turned to max position then it besomes more clear:
Next step would be "walking the signal path" with the scope and grounding it at every following stage but can anybody suggest what could be causing this kind of noise?
I'm in the process of recapping a SA-9900 and will look at the scope pictures in a few days to compare.
These are very well made amps, and I wouldn't change grounding 'at random', risking ground loops.
Hugo
These are very well made amps, and I wouldn't change grounding 'at random', risking ground loops.
Hugo
I did not change it "at random", some sense was applied 😉
The initial cabling was quite messy, probably with more ground loops than its current state.
But I took lots of photos so it can be restored if necessary.
Practical question - when a two channel scope has the ground of one probe (on amplifier output) connected to the amplifier ground then does the ground of the other probe necessarily have to be connected to the amplifier too?
Two screenshots from output B, channel A. Inputs shorted, first one with volume at -70dB, second one with volume at 0dB.
A tiny amount of noise can be seen at 0dB.
The high frequency display is irrelevant as it is dirt hanging in the air in the house. (PC's, smartphone, Wifi, I guess)
Hugo
A tiny amount of noise can be seen at 0dB.
The high frequency display is irrelevant as it is dirt hanging in the air in the house. (PC's, smartphone, Wifi, I guess)
Hugo
I'll have a look tomorrow.
I measured with the ground lead of the probe to the negative speaker output and the probe to the positive output.
Hugo
I measured with the ground lead of the probe to the negative speaker output and the probe to the positive output.
Hugo
Regarding scope probes earthing - the scope has an auxiliary earthing point located on front panel between the channels.
Probably worth testing out its physical effect on the readout.
Channels inside the scope are well physically separated but using one channel ground for the other channel may have some impact:
:
Probably worth testing out its physical effect on the readout.
Channels inside the scope are well physically separated but using one channel ground for the other channel may have some impact:
There are some mixed answers here explaining the use of that ground point:
https://electronics.stackexchange.c...the-ground-port-on-an-analog-oscilloscope-for
The Tektronix manual might have a more accurate explanation.
Hugo
https://electronics.stackexchange.c...the-ground-port-on-an-analog-oscilloscope-for
The Tektronix manual might have a more accurate explanation.
Hugo
Measurement again taken with shorted inputs.
FFT from 0Hz to 1kHz, one can see a small hum up to +/- 73Hz but it is also visible with disconnected probe.
No hum at all from the amp, not on the scope and not with headphones, even with volume at 0dB.
Note that the scale is in mdB.
Hugo
FFT from 0Hz to 1kHz, one can see a small hum up to +/- 73Hz but it is also visible with disconnected probe.
No hum at all from the amp, not on the scope and not with headphones, even with volume at 0dB.
Note that the scale is in mdB.
Hugo
Are your scope pictures of the B+ and B- (+ and - 52V) and also the + and - 48V rails after the regulator clean?
Hugo
Hugo
I check grounds with a good DMM. I clip one lead to a star ground. That could be the middle point between the big caps or the centertaps on the transformers in this schematic. Then I probe all other grounds to see if the resistance isn't too big, taking the resistance of the leads of the meter into account.
I consider less than 0.2 - 0.3 ohm as good in most circuits.
Hugo
I consider less than 0.2 - 0.3 ohm as good in most circuits.
Hugo
This meter can zero the values with the REL-delta push button.
It's also important to give it some time when measuring. Low ohmic values tend to take time to stabilise.
Hugo
It's also important to give it some time when measuring. Low ohmic values tend to take time to stabilise.
Hugo
Watching with interest....
I recently performed the same type of restoration on a Pioneer SA-8500 with the addition of changing a few known to be problematic/noisy transistors. The background is very quiet now which has uncovered a slight hum when my ear is close to the speaker. Mine is the 120V mains version with a hardwired two conductor power cord.
I recently performed the same type of restoration on a Pioneer SA-8500 with the addition of changing a few known to be problematic/noisy transistors. The background is very quiet now which has uncovered a slight hum when my ear is close to the speaker. Mine is the 120V mains version with a hardwired two conductor power cord.