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SA8500 only works on DBT

I am getting confused by the Service Manual, on page 44 there is a schematic of the AWG-034 pcb and the odd numbered transistors are associated with the other odd numbered components with pin 6 (right) as output, and the even numbered transistors are associated with the other even numbered components with pin 7 (left) as output, in each channel. The main/full schematic (pages 37/38) for the "GN", 220v only, type unit (that is what we are working on?) has mixed odd and even numbered transistors on each channel.
The only way I can think to keep things straight is by everybody using the same schematic section. So @paultlaw which one are you using?

paultlaw said:
Ok so far i have tested all of the following, one leg lifted. All of which are bang on to spec give or take.
R62, R32, R30, R50, R52, R48, R16, R10, R14, R18, R28, R4, R2, R6, R8, R12.

Where the hell can i go next? R23, R25, R25, R26????
Do polystyrene caps ever fail, could i have shorted one of those giving the board a quick resolder, i.e. too much heat on the styrene caps??

At a complete loss now, was really hoping the culprit would be exposed.

Open to any and all suggestions, calling all AK'ers!!!
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Check R20, R22, R24, R26, R28, R30, R32, R34, R36. Those are the resistors in the sub-tone section (between C16 and C32) that was identified as the problem area by grounding certain points in a previous post. You also said the 20kHz VR had a large effect on the buzz.

paultlaw said:
Ground the + side of C24. Result? LOUD Buzzing
Ground the - side of C26. Result? No Buzz
Click to expand...

I would concentrate in that area, check every trace for continuity between components, and the variable resistors for opens in that section.
I am also looking at the foil layout on page 43 and there is a wicked long trace, from one end of the board to the other, between the junction of C24 and R28 leading to C22. There are a number of other long traces between the components in this section as well, that run along that edge of the board.
 
merlynski said:
I am getting confused by the Service Manual, on page 44 there is a schematic of the AWG-034 pcb and the odd numbered transistors are associated with the other odd numbered components with pin 6 (right) as output, and the even numbered transistors are associated with the other even numbered components with pin 7 (left) as output, in each channel. The main/full schematic (pages 37/38) for the "GN", 220v only, type unit (that is what we are working on?) has mixed odd and even numbered transistors on each channel.
The only way I can think to keep things straight is by everybody using the same schematic section. So @paultlaw which one are you using? /QUOTE]
I am using the GN schematic - Pages 37 & 38 for full drawing.
Also using pages 43-46 for the board specific detail, as i cannot read a schematic, i am looking at the board layout drawings on those pages



merlynski said:
Check R20, R22, R24, R26, R28, R30, R32, R34, R36. Those are the resistors in the sub-tone section (between C16 and C32) that was identified as the problem area by grounding certain points in a previous post. /QUOTE] I will do these and report back, thanks
merlynski said:
You also said the 20kHz VR had a large effect on the buzz /QUOTE] Yes indeed it does, the higher the treble is the more noticeable the buzz



merlynski said:
I would concentrate in that area, check every trace for continuity between components, and the variable resistors for opens in that section.
/QUOTE] Not sure how to do that without scraping off some of the coating to expose the trace???
merlynski said:
I am also looking at the foil layout on page 43 and there is a wicked long trace, from one end of the board to the other, between the junction of C24 and R28 leading to C22. There are a number of other long traces between the components in this section as well, that run along that edge of the board.
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Ok so all of those resistors (R20, R22, R24, R26, R28, R30, R32, R34, R36) are well within spec. Unsure how to determine if the VR is ok.

I have been buzzing out all the traces and cannot find any issues there either :mad:
 
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Not sure how to do that without scraping off some of the coating to expose the trace???

Use the lowest Ω range or continuity test on your meter and touch the already exposed component lead / solder pad at each end of the trace with your meter leads and look for 0Ω, or continuity beep if your meter does that, between those points.
 
Thank you i have set meter to continuity and beeping out one end of a trace to the other by touching probes to the soldered pad. All beeps so far.

EDIT: Cannot find any breaks in continuity. I tried to be as thorough as possible but is is not easy to do it methodically, nevertheless no signs of any damage traces, and beeps all around (continuity)
The only other thing is the mylar caps and the polystryene caps? Never thought this would be such an ordeal to find
 
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paultlaw said:
Ok so all of those resistors (R20, R22, R24, R26, R28, R30, R32, R34, R36) are well within spec. Unsure how to determine if the VR is ok.
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A VR (or potentiometer) is a resistor with a sliding (rotating) contact, or stepped changes. First step is to measure resistance across the outside legs. VR3-b is the 20kHz sub-tone VR for the left channel. You may have to lift one end of R24 and R26 to isolate it from the rest of the circuit to get a good reading. Then put one probe on the middle terminal and turn the shaft slowly and watch for the resistance to change as it rotates. Go from one end to the other. Then keep the middle probe in place and move the other probe to the other end and repeat the rotation test. It should change resistance by clicks as it rotates, this is a switch (stepped) control according to the SM.

EDIT: Correction of VR action
 
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Ok thank you, i will do that tonight after work
If it turns out the pot is fine, are the only things left to check the mylar and polystyrene caps?
 
I would check the sub-bass pot as well, because it is also part of that circuit (VR1-b). Did you try paralleling C28? Or moving the 12pF C27 to C28s place, since the other channel works ok?
When you grounded the - side of C26 the buzz quit, when you grounded the + side of c24 it got louder, right? The problem sure appears to be in that section of the circuit. I have not given up or given in to despair and will keep helping as long as you want to continue.
 
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It seems like the capacitors/transistors/resistors etc have mostly been verified as ok so....

Have you checked the soldering for dry joints lifted pads etc?

Checked that the pots are in good condition, I.e the wafers are not damaged especially seems that the the board has been worked on.

Is the tone switch in good condition and clean? If not most come apart easy enough for cleaning.

If there are wire wraps are they in good condition?

If wires have been removed for the board to be worked on have these been put back correctly and have they been checked to see if there not bridging anything?
 
merlynski said:
I would check the sub-bass pot as well, because it is also part of that circuit (VR1-b) I will do this but probably not for a few days (time constraints unfortunately. Did you try paralleling C28? I popped in small film cap very carefully by touching the two leads to C28, noise is reduced but the background buzz is still present, still quite intrusive Or moving the 12pF C27 to C28s place, since the other channel works ok? I did not do this, but i am going to replace all of the ceramic and mylar caps, will order tomorrow

When you grounded the - side of C26 the buzz quit Yes it quit, but this was with main speakers off and computer speaker signal tracer probe speaker only, when you grounded the + side of c24 it got louder, right? Correct as per the criteria main speakers off and using signal tracer The problem sure appears to be in that section of the circuit. I have not given up or given in to despair and will keep helping as long as you want to continue.
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Answers are above in the quoted message


I have not given up at all, i am just frustrated
I will continue to pursue this, if i can do fully recap a a Sony TAF670ES, a Marantz 1060 , an SX5560, an Onkyo A10 and A7090 and a bunch of SX450's i can do this with the help of audiokarma. Would never have been able to get this far without you folks
Audiokarma gave me the support and advice needed to fix up equipment, i could never have afforded or enjoyed without the help of everyone here getting them all up and running!
 
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reddish75 said:
It seems like the capacitors/transistors/resistors etc have mostly been verified as ok so....

Have you checked the soldering for dry joints lifted pads etc? Board is in good shape and all pads look good, i have literally spent hours looking at the board

Checked that the pots are in good condition, I.e the wafers are not damaged especially seems that the the board has been worked on. Pots are in great condition from what i can at least, no visual issues i can discern

Is the tone switch in good condition and clean? If not most come apart easy enough for cleaning. Have not done that, but the switch moves lovely with a nice clink and has been D5 deoxited and faderlubed multiple times. All pots and switches have been

If there are wire wraps are they in good condition? They are in great nick but moving the board all the time and removing and resoldering components may end up causing something to give

If wires have been removed for the board to be worked on have these been put back correctly and have they been checked to see if there not bridging anything? No the wirewraps are intact except those that were soldered prior to my work on the unit
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Answers are above in the quoted message


Reddish75, that onkyo A10 really is a savage unit, im delighted with it! Its my favorite along with the A7090!
 
i dont expect it will make a difference but easy and free it may have been tried already . put the covers on the unit and try again .
and check for loose nuts on the controls if they fix to the chassis .
just taking guesses now to rule out things it likely isnt .
 
paultlaw said:
Answers are above in the quoted message


I have not given up at all, i am just frustrated
I will continue to pursue this, if i can do fully recap a a Sony TAF670ES, a Marantz 1060 , an SX5560, an Onkyo A10 and A7090 and a bunch of SX450's i can do this with the help of audiokarma. Would never have been able to get this far without you folks
Audiokarma gave me the support and advice needed to fix up equipment, i could never have afforded or enjoyed without the help of everyone here getting them all up and running!
Click to expand...
The main speakers and power amp are after the point where the signal tracer is connected (downstream), so it makes no difference whether they are on or off.
C28 is negative feedback to reduce the bandwidth of the transistor circuit. If paralleling it did not kill the buzz replacing it will not help.
I would like to try grounding a few more spots when you get ready to continue. I'll be around :)
 
paultlaw said:
Reddish75, that onkyo A10 really is a savage unit.
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Glad you got it going and you like it.

Back to the Pioneer, I see that Q6 was mentioned to be checked for orientation was this checked? If not I check this and possibly replace to see if that's leaky.

Also, check continuity of the ground wires Pins 5 and 8 (not sure which will be on the faulty channel) meaning trace these wires to where the other end connects to.

I've seen power supply mentioned as a possible source of the fault, this is not the case as it's common to both channels.

Quickest and dirtiest way of checking the poly caps is to swap them over and see if the buzz moves channel.

You said that you have cleaned the defeat switch? I did a test with a very similar switch taken apart before cleaning and oxidation looked at, which was pretty bad. Firstly it was reassembled then cleaned with deoxit D5 as recommended. I then disassembled the switch, the contacts were still oxidised, the only way to clean this properly I found was clean with isopropyl alcohol first then clean with brasso, reassemble then use something like fader lube

Last thing look at would be the pots, no other way to check or clean these properly other than remove them clean them up and swap the wafers over to see if the fault moves
 
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reddish75 said:
Glad you got it going and you like it.

Back to the Pioneer, I see that Q6 was mentioned to be checked for orientation was this checked? If not I check this and possibly replace to see if that's leaky.

Also, check continuity of the ground wires Pins 5 and 8 (not sure which will be on the faulty channel) meaning trace these wires to where the other end connects to.

I've seen power supply mentioned as a possible source of the fault, this is not the case as it's common to both channels.

Quickest and dirtiest way of checking the poly caps is to swap them over and see if the buzz moves channel.

You said that you have cleaned the defeat switch? I did a test with a very similar switch taken apart before cleaning and oxidation looked at, which was pretty bad. Firstly it was reassembled then cleaned with deoxit D5 as recommended. I then disassembled the switch, the contacts were still oxidised, the only way to clean this properly I found was clean with isopropyl alcohol first then clean with brasso, reassemble then use something like fader lube

Last thing look at would be the pots, no other way to check or clean these properly other than remove them clean them up and swap the wafers over to see if the fault moves
Click to expand...

I took apart the tone defeat switch to make sure, and man was it clean, all the contacts are in great shape, like new nearly if you can believe it, reassembled again.
Am going to order the ceramic, mylar and polystyrene caps and replace them all. Farnell has the ceramics but not the polystyrenes or mylar i am having difficulty finding,except on ebay, so will need to search about to figure it all out, otherwise its cheaper to buy another tone board altogether
 
You're doin it!

I'd you ever need to test the switch you can put it in the position you want and bridge the leads on the other end. That will eliminate the switch contacts.
 
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