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SA8500 only works on DBT

Touch the signal tracer to each of the pads that C32 connects to normally (leave C32 unconnected) and see if the buzz is there on either one, please.
 
It's been really hard to tell, but I think they both are buzzing, my volumes maxed on tracer, so I'm going back on what I said and saying there is buzz on - c26 and on +c32. I have moved everything into the quietest room in the house so I can listen to it a bit better And I have to touch a finger on chassis too when using the probe to reduce noise into the tracer, which makes it awkward also.
Nothing sound wise from the lifted cap negative pad

Sorry for the jumping about
 
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This question might have been asked before but that replacement resistor has that value been checked with the good channel to see if correct? As component value changes aren't always documented in the manuals
 
reddish75 said:
This question might have been asked before but that replacement resistor has that value been checked with the good channel to see if correct? As component value changes aren't always documented in the manuals
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Yeah I tested it and broke one of the leads so replaced it with one I had in my box
 
paultlaw said:
It's been really hard to tell, but I think they both are buzzing, my volumes maxed on tracer, so I'm going back on what I said and saying there is buzz on - c26 and on +c32. I have moved everything into the quietest room in the house so I can listen to it a bit better And I have to touch a finger on chassis too when using the probe to reduce noise into the tracer, which makes it awkward also.
Nothing sound wise from the lifted cap negative pad

Sorry for the jumping about
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There should be a ground/common lead on the computer speakers input that should be connected to the chassis to reduce environmental electrical noise when using the signal tracer.
Is the buzz much lower when C32 is lifted and comes back louder when it is reconnected (try with a clip lead/jumper to make it easier to connect & disconnect)?
 
merlynski said:
There should be a ground/common lead on the computer speakers input that should be connected to the chassis to reduce environmental electrical noise when using the signal tracer.
Is the buzz much lower when C32 is lifted and comes back louder when it is reconnected (try with a clip lead/jumper to make it easier to connect & disconnect)?
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I have the ground of the tracer clamped to chassis while probing. Reconnected c32 and retested and volume of buzz is the exactly the same on both c26 and c32
 
So, if I understand correctly the buzz at C26 is the same volume whether C32 is connected or disconnected?
 
merlynski said:
Leave C32 disconnected for now.
The buzz, then, is in the circuit between C16 and C32?
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I am not sure if i am honest, given my level of understanding of these things, i was hoping you could tell me.
Given the testing results though you are most likely correct. If that the case, is there a list of components that can be checked or replaced?

Component list to check/replace for vetting (i know some have already been checked):
R24, R26,R28, R20, R22, R30, R32, R34, R36
C18, C20, C22, C14
 
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I am learning your level of understanding along with the troubleshooting process, but never felt you were being dishonest.

Looking at the GN schematic on pages 37/38 of the SM the components involved in that stage are the ones you listed plus C24, C26, C28, C30 and Q5. Also the Sub Tone VRs: 'BASS 50 Hz' and 'Treble 20kHz'. And the circuit board traces associated with them.
C14 is outside the area of concern as it feeds the tone bypass signal from before C16, (with tone off), which does not have the buzz.
The pcb picture in post 135 has a lot of white residue on the bottom. I would scrub that off with some alcohol or acetone and a stiff brush so the bottom is completely clean. Look closely for solder bridges and/or cracked traces. Look closely at the top (component) side for any solder flakes, wire clippings or anything else that may be causing a short. Look under the VRs closely. It looks to me like that 'Sub Tone' stage is oscillating, but anyway, the problem is right in that area.
 
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:dunno:R24, R26,R28, R20, R22, R30, R32, R34, R36 (R20, R22, R24, R26, R28, R30, R32, R34, R36 all ok)
C18 Good C20 Good, C22 Good, C24 Good, C26 Good, C28 Good, C30 Good and Q5 Good :dunno:

Also the Sub Tone VRs: 'BASS 50 Hz' and 'Treble 20kHz'. They ohm out good
And the circuit board traces associated with them. I have cleaned the board again, deoxited all the VR's again. And beeping out the traces has yielded me no fails or dodgy results

Im at a loss to be honest now at this stage, but i really want to thank you all especially @merlynski for sticking with me.
I might just buy my mate a donor board
 
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late thought but just wondered if you tried with covers back on the unit ? i know its a stab in the dark but ...
 
Last stab in the dark really as you say that all components check out, inspect the wiring very carefully for any insulation that could have been melted accidentally with the soldering iron or any 'signal' carrying wires possibly touching ground wires especially those wires where the left, right signal and ground are all together.

Also of any grounds connect to the chassis try using serrated washers between screw, o'ring and chassis.
 
petehall347 said:
late thought but just wondered if you tried with covers back on the unit ? i know its a stab in the dark but ...
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Hi Pete, that definitely does not work for this either,unfortunately, i mean with tone control to off position and all covers removed the unit is dead quiet.
 
reddish75 said:
Last stab in the dark really as you say that all components check out, inspect the wiring very carefully for any insulation that could have been melted accidentally with the soldering iron or any 'signal' carrying wires possibly touching ground wires especially those wires where the left, right signal and ground are all together.

Also of any grounds connect to the chassis try using serrated washers between screw, o'ring and chassis.
Click to expand...

Thanks Reddish75, i will have another in depth look at the wiring this evening.

Been staying up late so i am already tired after work when i am looking at these things.

Ill take some pictures of the wiring later and ill post them, thank you
 
Anyone have a source for one of these boards? there is just one on ebay but absolutely mental price when you total shipping and import charges to ireland
 
Replaced all of the green mylars on the board as my order came in for those, unfortunately no change.
I am thinking of ordering a full set of resistors for the board certainly cheaper than buying a new board. Let me know if you think this is a poor choice, maybe a fresh board would be better.

I inspected the wiring last night for quite a while and could find no physical signs of any damage.
Folks this is driving me crazy
 
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