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SA8500 only works on DBT

Yup, that was pin 12, not 17, bad label.
I need to mull this over a bit, strange there is a buzz/hum on C32 and C48, but not between them on C40. upload_2019-12-20_20-22-57.gif
Does turning the Tone control pots affect the buzz/hum at those locations, C32, C48 ? If so which one and how much?
 
C48 buzz gets louder when the 20khz treble pot is turned up full.
Same with C32 but a very minor increase in buzz volume.
 
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Have you replaced C28 and/or C44 (12pF)?
Try paralleling them, 1 at a time, with another small cap and see if buzz changes or goes away.
 
No did not replace those, and i do not have any values that small unfortunately, will i get some ordered and check that out?
I could pull them out and test them in my cheap chinese component tester??
 
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paultlaw said:
No did not replace those, and i do not have any values that small unfortunately, will i get some ordered and check that out?
Click to expand...
Try the smallest one you have, just tack solder it to the bottom of the board temporarily. It looks like one of the stages in that area is oscillating. If the 20k tone control affects it the most it is not 60 or 120 hz ac line noise (or 50, 100 hz if you are not in the Americas), but a higher frequency. Does it sound like a higher frequency whine? Some DMMs have a frequency counter setting, does yours?
What did you use to replace Q5 and Q7?
 
Yes actually there is a HZ setting on my DMM, obviously never used it. How do i use it? Do i need to put neg to chassis and probe?
Used KSC1845FTA for those. Q1 Q2 replaced with KSA992 all the rest of the transistors on the board got the KSC1845FTA
 
paultlaw said:
Yes actually there is a HZ setting on my DMM, obviously never used it. How do i use it? Do i need to put neg to chassis and probe?
Used KSC1845FTA for those. Q1 Q2 replaced with KSA992 all the rest of the transistors on the board got the KSC1845FTA
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Yup, black to chassis/ground, probe to measurement point, should read in Hz or kHz, they don't usually go very high.
 
My multimeter just reads 0 Hz, not sure it works properly, cheap chinese meter! Tried pin 7 and pin 12 with unit on same result 0hz unfortunately
 
paultlaw said:
My multimeter just reads 0 Hz, not sure it works properly, cheap chinese meter! Tried pin 7 and pin 12 with unit on same result 0hz unfortunately
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Pin 12 is power supply (dc) 0Hz is good! Pin 7 is output, with tone off should read 0Hz. Touch it to C48 and hold a bit . . .
 
Apologies, but not sure where that is, can i try on side of the main filter caps??
Probed power line in sits at 50hz but jumps about very close to that, bloody strange!
 
paultlaw said:
Apologies, but not sure where that is, can i try on side of the main filter caps??
Probed power line in sits at 50hz but jumps about very close to that, bloody strange!
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Well, at least it reads your ac line frequency. What does the hum/buzz sound like from the computer speakers?
 
Hard to describe it, like when i pull the a 3.5mm to rca jack from my amp with volume on and touch the jack, loud buzzing. I could easily record the sound if i could upload a sound file?
 
paultlaw said:
https://drive.google.com/open?id=1pEIcR3dWGosLl0sCuOmJcS4-VI-uX7vl

This link should work for anyone wanting to listen, left speaker, turn controls on, then at approx 7 seconds i turn tone off. If site rules have been broken i will kill the link and please accept my apologies
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Thanks for that sound. That sounds more like an open ground noise than an oscillation, unless it is harmonics/noise from an oscillation above the audio range. Check the ground connections to your AWG-034 and the ground path (traces) on the pcb.
 
I have pushed and prodded everywhere on the board but to no avail, I have moved the wiring around trying to see if it resulted in any noise change. So will prob need good guidance on where I need to look exactly unfortunately. Again guys thanks for your patience would really love to sort this for my mate
 
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