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SA8500 only works on DBT

Long thread, sorry for being late to the party :) I did not absorb it all unless I re-read it over and over, so it is a hum issue on one channel only, with the tone on and set to one particular position?
Opens make hum that is for sure.
 
Welcome! Great to have you take time to suffer through more of my problem, really appreciate all help!

So issue is as follows - Left channel hum only when tone control activated, when it is not activated unit is dead quiet. Board has been recapped and all transistors changed.
I have pulled pretty much all resistors to test and replaced all of ceramics on the left side of the board. Waiting on the Mylar caps to come and the 4x polystyrene caps also.

I am at a total loss what the hell is causing it, i know it is very rare for mylars to go or indeed ceramics, maybe when reflowing joints i put too much heat on the poly caps, i hear the are sensitive to heat.

Maybe i should have started a new thread on this but at least its a one stop shop and was afraid of being scolded for starting another thread!
 
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Well i guess you could
1) verify the bias voltages are close to what they have printed in the big schematic, supply is +/-24V gets dropped by R60 to around +/-13V
2) 2 tone amp sections sub,main are in series, could isolate it to the sub or main, could lift one leg of C16, (+ve lead connection to the 39K & 22K,) and bypass the sub wire directly to the R38,42 with C32 +ve leg lifted

test the pots too as said above and test the switch section S10a for resistance, power off of course :)
 
rcs16 said:
Well i guess you could
1) verify the bias voltages are close to what they have printed in the big schematic, supply is +/-24V gets dropped by R60 to around +/-13V
2) 2 tone amp sections sub,main are in series, could isolate it to the sub or main, could lift one leg of C16, (+ve lead connection to the 39K & 22K,) and bypass the sub wire directly to the R38,42 with C32 +ve leg lifted

test the pots too as said above and test the switch section S10a for resistance, power off of course :)
Click to expand...
@rcs16
Verify the bias voltages are close to what they have printed in the big schematic, supply is +/-24V gets dropped by R60 to around +/-13V
On tone control board Measured R59 for voltage and it is at 13.88v and on other leg 24.28v
R60 on the right side of the board is approximately the same also

2 tone amp sections sub,main are in series, could isolate it to the sub or main, could lift one leg of C16, (+ve lead connection to the 39K & 22K,) and bypass the sub wire directly to the R38,42 with C32 +ve leg lifted
On tone board are you saying lift the positive lead of C16 and C32 and run a wire between each of the pads that now have a leg missing from the board?? Or connect the lifted legs together?
 
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The tone pots ohm out fine even on both sides nothing crazy happening.

Thank you for sticking with me and apologies again, i know i do not make it easy, i am trying my best though.

I am sticking to the left hand side of the board geographically speaking folks just to be sure i am in the correct area. I replaced the ceramics on this side of the board only, not the right hand side.
Mylars and polystrene caps are on order and will arrive in a few days hopefully.

Ohming out the pots i am probing on side of the pot (positive and then neg to middle leg. I move the pot and with each turn the resistance gets bigger or smaller depending on position.
IS that the correct way to check or do i need to repeat for both sides?
 
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When to did you testing you said you had hum in left tone circuit when enabled, was the signal prresent along with the hum or only hum?
One other thing make sure is that the grounds are sound, that you did not break any of the ground wires, attached to terminals 2,11,5,8. you could try adding new connections to those 4 ground connections, wire directly to chassis.

You can test the pots for total resistance and form each outside pin to the middle(wiper). Since you know the right is working correctly, you can compare the left against the right to see that they measure the same.
To make it simpler as a 1st test, you could leave C16 installed, lift the -ve lead of C32, jumper from C16+ to R38,42, this will bypass the sub tone amp section.
then you could do the same for the main tone, lift C48- and connect C32- to R56
make sense?
 
@rcs16 When to did you testing you said you had hum in left tone circuit when enabled, was the signal present along with the hum or only hum? Hum is present only with tone control active but when i play music it plays no problem, music and hum/buzz together
One other thing make sure is that the grounds are sound, that you did not break any of the ground wires, attached to terminals 2,11,5,8. you could try adding new connections to those 4 ground connections, wire directly to chassis I will do this now and report back immediately - Report no difference still buzzing from left channel all pints 2,5,8,11 all grounded to chassis

You can test the pots for total resistance and form each outside pin to the middle(wiper). Since you know the right is working correctly, you can compare the left against the right to see that they measure the same. Yes the pots seem to be ok then from my testing but i will repeat again per your instructions to be certain
To make it simpler as a 1st test, you could leave C16 installed, lift the -ve lead of C32, jumper from C16+ to R38,42, this will bypass the sub tone amp section. So leave -ve C32 "hanging" and run a wire from the positive of C16 to R38? OR R42? I assume it matters which side of the resistor i connect to?
then you could do the same for the main tone, lift C48- and connect C32- to R56
make sense?
 
I have re-read your posts and noticed a possible area of unchecked components, since you said:
paultlaw said:
I am sticking to the left hand side of the board geographically speaking folks just to be sure i am in the correct area. I replaced the ceramics on this side of the board only, not the right hand side.
Click to expand...

VR3-a, R23, R25 and C21 are part of the left tone circuit (20kHZ sub-tone), but are on the RIGHT hand side of the board if you are looking at the component side (top) from the front of the amp. If you have not checked them, and the long traces between them and the rest of the circuit you are missing checking some of the components in the circuit that has the hum.

The right channel 20kHz sub-tone components are connected to VR3-b close to the same 'geographical' area on the board.
The left channel components are not all on the left side of the board, nor are all the right channel components on the right side of the board. There are long traces between them along the side of the board nearest the front panel of the amp.
 
Remember we had localized the source area to around C24 and C26, as grounding the input to the left tone circuit (- of C16) did not kill the hum.

paultlaw said:
Is that to be done while -C48 is being probed by signal trace? Sorry just wanna make sure, i am assuming so.

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

I see no point in changing parts in other areas of the circuit. If it was me I would concentrate on looking in that area.
 
Ok so i think i may yet again have made a mistake.
I have switched the speakers selector to off when performing this test tone control activated.
Now when i use the signal tracer (powered computer speaker probe) and touch the +side of C24 - NO BUZZ. When i probe the negative side of C26 BUZZ
??? Have i been doing something wrong here? Should i have my speaker selector to a speaker or off?

I do apologize folks, again! Hopefully this time will get it right. Speakers selector off, tone control engaged. all negative legs of caps tested using signal tracer.

C24 - No
C16 - No
C26 - Buzz
C14 - No
C42 - Buzz
C32 - Buzz
C40 - No
C48 - Buzz
 
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paultlaw said:
Ok so i think i may yet again have made a mistake.
I have switched the speakers selector to off when performing this test tone control activated.
Now when i use the signal tracer (powered computer speaker probe) and touch the +side of C24 - NO BUZZ. When i probe the negative side of C26 BUZZ
??? Have i been doing something wrong here? Should i have my speaker selector to a speaker or off?

I do apologize folks
Click to expand...
The amps speaker selector should make no difference because it is way downstream from the tone controls, except you hear the buzz from the speakers.
Remember you had the signal tracer clipped to C48 - and individually grounded the those points on the two caps (see posts 126 through 129), and the buzz at C48 changed.
Touching the signal tracer to those same points confirms to me the problem is in that area:

Buzz present on C26 with signal tracer, but buzz goes away at C48 when that point is grounded, that point is part of the buzz signal path.
No buzz on C24+ with signal tracer, but grounding that point made it louder at C48, that tells me it may be a part of the signal path of the buzz.

I think the problem has to be right in that area . . .
 
paultlaw said:
Ok so i think i may yet again have made a mistake.
I have switched the speakers selector to off when performing this test tone control activated.
Now when i use the signal tracer (powered computer speaker probe) and touch the +side of C24 - NO BUZZ. When i probe the negative side of C26 BUZZ
??? Have i been doing something wrong here? Should i have my speaker selector to a speaker or off?

I do apologize folks, again! Hopefully this time will get it right. Speakers selector off, tone control engaged. all negative legs of caps tested using signal tracer.

C24 - No
C16 - No
C26 - Buzz
C14 - No
C42 - Buzz
C32 - Buzz
C40 - No
C48 - Buzz
Click to expand...

OK, no buzz on C16 means the buzz is NOT coming into the tone circuit from before it (upstream). It is the input to the tone circuit.
C24: No buzz on C24: that is a very low impedance point for ac signal because it feeds the base of the transistor (Q5 on page 37/38 of SM, GN schematic) so the signal tracer will not reproduce it, it is too small a voltage signal (buzz).
C26: Buzz, probably coming from Q5 collector as Q5 is a voltage gain amp circuit.
C14: Same explanation as C16, also C14 is the tone control bypass signal path when tone switch is "OFF."
C40: Same explanation as C24.
C42 and C32 are both downstream from the buzz source so will, of course, have buzz but are not part of the source UNLESS C50 is open/bad and the +13v supply line is part of the feedback path. Is there a buzz on the + of C50 (100µF, 16V)?
C48: output of tone circuit, buzz always there.

P.S. When I directed you to ground certain points during troubleshooting those points were carefully chosen to NOT destroy any components, just to kill the signal. Randomly grounding the wrong parts of the circuit will probably cause magic smoke to escape.
 
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merlynski said:
OK, no buzz on C16 means the buzz is NOT coming into the tone circuit from before it (upstream). It is the input to the tone circuit.
No buzz on C24: that is a very low impedance point for ac signal because it feeds the base of the transistor (Q5 on page 37/38 of SM, GN schematic) so the signal tracer will not reproduce it, it is too small a voltage signal (buzz).
C26: Buzz, probably coming from Q5 collector as Q5 is a voltage gain amp circuit.
C14: Same explanation as C16, also C14 is the tone control bypass signal path when tone switch is "OFF."
C40: Same explanation as C24.
C42 and C32 are both downstream from the buzz source so will, of course, have buzz but are not part of the source UNLESS C50 is open/bad and the +13v supply line is part of the feedback path. Is there a buzz on the + of C50 (100µF, 16V)?
C48: output of tone circuit, buzz always there.

P.S. When I directed you to ground certain points during troubleshooting those points were carefully chosen to NOT destroy any components, just to kill the signal. Randomly grounding the wrong parts of the circuit will probably cause magic smoke to escape.
Click to expand...

Is there a buzz on the + of C50 (100µF, 16V) - No Buzz
 
paultlaw said:
View attachment 1719891
Caps with buzzing highlighted on schematic
Click to expand...
That is not the main schematic on pages 37/38 of the SM, It is the board schematic from page 44, note the component designations are different between the two. That had me confused earlier and I have used component designations from pages 37/38 since I noticed it and asked which you were using. This is an error in the SM. I have noticed Pioneer usually puts the left channel above the right channel on schematics but on page 44 the left channel is the lower one and the right channel is the upper one.
 
paultlaw said:
Is there a buzz on the + of C50 (100µF, 16V) - No Buzz
Click to expand...
OK, that tells me the source is in the area between C16 (no buzz) and C32 (buzz). Unsolder one end of C32 and leave it pulled up, then power up and see if the buzz is still present at the - of C26. Try to isolate the 'bad' amp stage.
 
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