SA8500 only works on DBT

[PTL-Pioneer]

Apologies - Post 73

"That would be my choice for testing this kind of problem. If not you could put a 0.1µF cap on the tip of your DMM lead, then on a low AC range compare left to right channel points along the tone control signal path (with no input, volume minimized). When you find a spot where the signal jumps up the problem is occurring between there and the prior good point."

 

[merlynski]

Post 73

If you don't have an o'scope you can measure the ac voltage only using a DMM. The cap on the probe blocks dc but passes ac. Connect left and right inputs together, then feed an audio signal into both at the same time. Follow the schematic, comparing the ac voltage at the same points, left and right channels, comparing the "good" channel to the "bad" channel. As long as the circuits have identical ac voltages at the same location along the signal path on both channels the circuits to that comparison point are working the same. As soon as you get to a comparison point where they differ you have found a clue. The failure in the bad channel will be between the last point they are the same and the first point they are different. The easiest and fastest way to do this is by starting in the middle of the signal path, halfway between the input and the output. If they match at that point the failure is after that point, if they differ the failure is before that point. Half of the circuit is eliminated as the failure area. Once you have identified which half the failure is in, split that half in half again and compare again. Each time you eliminate half of what it left, until you narrow it down to a specific component or small circuit section. Then you can look for a visible failure; measure components' values (or just replace them); clean contacts or pots; or do whatever is needed to eliminate the problem. Exact measurements along the way don't matter, as you are comparing left and right sides. This only works if you have a good and a bad channel, or know what to expect along the way. You may have to change your meter range, use a larger value cap (it is not critical as you are comparing not measuring), adjust controls or switches, etc. Hope that helped and I didn't ramble on too much.

 

[PTL-Pioneer]

Thank you so much for that detailed response, i appreciate you taking the time to write up such a detailed response. it is going to be hard to trace then in my case, as i pretty much cannot read a schematic properly. If it is only active with the tone control active, can i say with any certainty that the issue lies on the tone control board?
All capacitors (electrolytics i mean) and transistors are new on this board.
I know you have provided a legitimate way of tracing this down better so i will try and get his amp back to have a look at it again, at the least i can check out a few places and report back, thank you

 

[merlynski]

Thank you so much for that detailed response, i appreciate you taking the time to write up such a detailed response. it is going to be hard to trace then in my case, as i pretty much cannot read a schematic properly. If it is only active with the tone control active, can i say with any certainty that the issue lies on the tone control board?
All capacitors (electrolytics i mean) and transistors are new on this board.
I know you have provided a legitimate way of tracing this down better so i will try and get his amp back to have a look at it again, at the least i can check out a few places and report back, thank you

You are welcome! If you have narrowed it down to one board that technique may be less useful, as it works best for locating the circuit (or pcb) that the problem occurs in, rather than the individual component. If you check at the connections between the pcbs along the signal path it is easier when you are not good at reading schematics, as each signal path has an input and output to/from each pcb.

 

[PTL-Pioneer]

Hi all, i have my friends amp back and i am trying to figure out this buzz on the left channel which is only active with the tone control set to the on position.
With no tone control there is no buzz.
I have taken some voltages hoping to get things moving, i would really appreciate your help with this.
I can take more voltages if you point me to somewhere on the board

AWG-034
Readings all DC with reference to chassis. Tone Control ON
Pin 1 = -22.23
Pin 12 = -22.29
Pin 3 = 24.33
Pin 10 = 24.41
Q1 and Q2 Collector -10.9
Q5 and Q6 Collector 3.5 and 3.6 respectively

I know this is the hardest stuff to track down but really need to get my mate sorted out

 

[MIJ_Pioneer]

Hi all, i have my friends amp back and i am trying to figure out this buzz on the left channel which is only active with the tone control set to the on position.
With no tone control there is no buzz.
I have taken some voltages hoping to get things moving, i would really appreciate your help with this.
I can take more voltages if you point me to somewhere on the board

AWG-034
Readings all DC with reference to chassis. Tone Control ON
Pin 1 = -22.23
Pin 12 = -22.29
Pin 3 = 24.33
Pin 10 = 24.41
Q1 and Q2 Collector -10.9
Q5 and Q6 Collector 3.5 and 3.6 respectively

I know this is the hardest stuff to track down but really need to get my mate sorted out

I had a similar issue not too long ago. Not the most advanced solution but feel around the components on the tone board and feel for something loose. Just a gentle nudge. Check resistors resistance if you haven't already. In my case (amp going into protection), a resistor was loose although it looked like it was soldered in.

I'm sure a more seasoned veteran will be in here soon.

 

[PTL-Pioneer]

thanks for the reply, i have been poking about a bit trying to see if i locate a dodgy component or wire, everything seems pretty solid. When i get back from work i will pull the board out for a better look and hopefully something will turn up. I made up that DC blocking probe using an old RCA cable, seems to work fine too, i am assuming as i am only comparing one channel to the other it is working. I will do more of that probing later also

 

[PTL-Pioneer]

I have changed out all of the transistors again for new ones on the tone control board, and the capacitors, still no joy. Still buzzing from left channel with tone controls engaged. Nearing defeat here folks, do not know really what else to do.

 

[merlynski]

Do you have a signal tracer, or another amp you could feed the dc blocked signal to?
Even a pair of powered computer speakers to make a signal tracer?

 

[PTL-Pioneer]

I have a pair of computer speakers I could use. Do I need to feed a signal into the amp and use the probe to listen on the computer speakers? Probe to be a 3.5mm audio cable chopped up, ground to amp chassis and the signal cable using film cap to block dc?

 

[merlynski]

I have a pair of computer speakers I could use. Do I need to feed a signal into the amp and use the probe to listen on the computer speakers? Probe to be a 3.5mm audio cable chopped up, ground to amp chassis and the signal cable using film cap to block dc?

If the buzz happens without an input do it that way, without input.
Your description of the cable is spot on!
With the buzz present at the output (on the main speakers I presume), touch (or clip lead) the probe (cap) to AWG-034 pin 7, adjust the comp speaker volume and listen to tell if the buzz is present there. If so touch (move) the probe to the - side of C48 on the AWG-034 and listen for the buzz. If it is present keep the probe on the cap - of C48 and switch the tone control off. Does the buzz stay or disappear? The circuit may be oscillating when the tone switch is on.

 

[PTL-Pioneer]

thanks for the reply, much appreciated as always - this close to the holiday period i might grab an oscilloscope if i can find one cheap. As well as making the signal tracer. I think it has gone past time i learned how to do this kind of troubleshooting, i imagine i have frustrated many of the veteran users on AK multiple times at this point. Shotgunning everything all the time is a waste. And i should be enjoying figuring out what is wrong not dreading it!

 

[merlynski]

thanks for the reply, much appreciated as always - this close to the holiday period i might grab an oscilloscope if i can find one cheap. As well as making the signal tracer. I think it has gone past time i learned how to do this kind of troubleshooting, i imagine i have frustrated many of the veteran users on AK multiple times at this point. Shotgunning everything all the time is a waste. And i should be enjoying figuring out what is wrong not dreading it!

An oscilloscope is actually less useful than you might think. A good schematic is my favorite tool! Learn Kirchhoff's laws, They are the other side of Ohm's law. https://www.thoughtco.com/kirchhoffs-laws-for-current-and-voltage-2698910
The challenge of troubleshooting, and the feeling from a successful repair overcomes the dread pretty quickly.
If it ever worked in the past you can fix it . Persevere and never admit defeat, there are plenty of AKers who will help you if follow directions and communicate clearly.

 

[PTL-Pioneer]

Ok probe is made, computer speakers sorted. Probing pin 7 with tone control on, hum present.
Move probe to - of C48 hum present with tone control on. Hum Present with tone control off at - C48,

I will await further instruction before moving ahead.

 

[merlynski]

Ok probe is made, computer speakers sorted. Probing pin 7 with tone control on, hum present.
Move probe to - of C48 hum present with tone control on. Hum Present with tone control off at - C48,

I will await further instruction before moving ahead.

OK, good info!
Probe the - side of C16, should be no hum . . . ?
Probe the + side of C24, hum there?

 

[PTL-Pioneer]

Ok brilliant, i forgot to add that pin 6 is dead quiet with tone control on/off.
Will do that probing when i get home, just wondering though, why you selected C16 and C24 as the next probe points?
I was think C40 next, although i am not entirely sure why!!
Just trying to understand things a bit better, are you jumping ahead to the next section to just narrow the area of interest quicker?
Thanks for all of you help, feel free to give me a list of places to probe and i will do it all tonight

 

[merlynski]

Ok brilliant, i forgot to add that pin 6 is dead quiet with tone control on/off.
Will do that probing when i get home, just wondering though, why you selected C16 and C24 as the next probe points?
I was think C40 next, although i am not entirely sure why!!
Just trying to understand things a bit better, are you jumping ahead to the next section to just narrow the area of interest quicker?
Thanks for all of you help, feel free to give me a list of places to probe and i will do it all tonight

Pin 6 is the right channel output of the AWG-034 and it was the left channel with the hum, correct?
There are 3 stages (sections) on the each channel of the AWG-034, for the left it is Q2 & Q4, the input stage, which is common to the tone on or tone off switch position, so I bypassed it; the second stage is between C16 and C32, and includes Q5 for gain; the third stage is between C32 and C48 (tone section output cap), and includes Q7 for gain. C24 is dc blocking and signal feed from the second stage tone shaping components to the second stage gain transistor. C40 is dc blocking and signal feed from the third stage tone shaping components to the third stage gain transistor. Since the hum was present on C48 - we already know there is hum in the third stage.
I am stepping through the stages to separate and narrow down the stage that is the problem, as you suspected. Once the bad stage is identified we will look closer at it.
Leave the Tone switch off for now.
So the list is:
stage 2 input, C16 - : ?
stage 2 middle, C24 - : ?
stage 2 out/stage 3 in, C32 - : ?
stage 3 middle, C40 - : probably has hum but ?
stage 3 output, C48 - : already known hum

 

[PTL-Pioneer]

Hi There
So here we go with the buzz results, thank you again

Pin 6 is the right channel output of the AWG-034 and it was the left channel with the hum, correct? YES

Leave the Tone switch off for now. No sound at all really through tracer unless switch is in on position?? All probing performed with tone control on.
So the list is:
stage 2 input, C16 - : No Buzz
stage 2 middle, C24 - : No Buzz
stage 2 out/stage 3 in, C32 - : BUZZ
stage 3 middle, C40 - : No Buzz
stage 3 output, C48 - : already known hum

I am assuming we have some problem but definitely no buzz at C40 but definitely buzzing at C48

 

[merlynski]

Probing pin 7 with tone control on, hum present.
Move probe to - of C48 hum present with tone control on. Hum Present with tone control off at - C48,

From previous post the buzz/hum was present at C48 - with tone switch on or off, is this correct or not correct?

Please repeat the list with tone control off:
stage 2 input, C16 - :
stage 2 middle, C24 - :
stage 2 out/stage 3 in, C32 - :
stage 3 middle, C40 - :

Starting to look like a power supply problem, since there is no buzz/hum at C24 and C40.

Probe the + side of C50, do you have a buzz/hum there? Measure dc volts there also, sb +13vdc.
Probe the - side of C52, do you have a buzz/hum there? Measure dc volts there also, sb -13vdc.
Probe AWG-034 pin 10, do you have a buzz/hum there? Measure dc volts there also, sb +24vdc.
Probe AWG-034 pin 17, do you have a buzz/hum there? Measure dc volts there also, sb -24vdc.

EDIT: more?

 

[PTL-Pioneer]

From previous post the buzz/hum was present at C48 - with tone switch on or off, is this correct or not correct? This is correct Hum Present with tone switch in both on and off position

Please repeat the list with tone control off: Tone Control to off position
stage 2 input, C16 - :No Buzz
stage 2 middle, C24 - : No Buzz
stage 2 out/stage 3 in, C32 - : BUZZ
stage 3 middle, C40 - : No Buzz

Probe the + side of C50, do you have a buzz/hum there? No Buzz Measure dc volts there also, sb +13vdc.13.8
Probe the - side of C52, do you have a buzz/hum there? No Buzz Measure dc volts there also, sb -13vdc. -11.7
Probe AWG-034 pin 10, do you have a buzz/hum there? No Buzz Measure dc volts there also, sb +24vdc. 24.27V
Probe AWG-034 pin 17, do you have a buzz/hum there? No pin 17 on this board, only 12 pins. No Buzz on Pin 12 Measure dc volts there also, sb -24vdc. Pin 12 = -22.27


I hope i am doing this ok for you, thanks for your patience

Pin 31 of AWS085 - No Buzz