Don't you have a frequency function on your multimeter? 60 Hz sound like a hum, 120 more a buzz.
Measure hum voltage and frequency at the outputs.
Also, measure AC between the terminals of each of the two big caps. Its best to use alligator clip leads so you don't accidentally short the 90 VDC between these terminals. This is the power supply ripple and will give some indication of the health of your main filter caps.
Hopefully restoring the input connections and ground will cure your loop issue.
Measure hum voltage and frequency at the outputs.
Also, measure AC between the terminals of each of the two big caps. Its best to use alligator clip leads so you don't accidentally short the 90 VDC between these terminals. This is the power supply ripple and will give some indication of the health of your main filter caps.
Hopefully restoring the input connections and ground will cure your loop issue.
Well, so far so good. We installed insulated RCA's and restored the signal ground connection between inputs.
The hum/buzz is now gone from the right channel...and the left channel is just as quiet. Playing music through it was a success too. No breakers were tripped either. Tomorrow I will hook it up to my Heresy's again and see if it trips the breaker like the first time I hooked it up.
Now, do I measure the AC voltage while the amp is on, but with no input? Also, if I were to replace the two big caps, what would the best option be? Thanks for all the help!
The hum/buzz is now gone from the right channel...and the left channel is just as quiet. Playing music through it was a success too. No breakers were tripped either. Tomorrow I will hook it up to my Heresy's again and see if it trips the breaker like the first time I hooked it up.
Now, do I measure the AC voltage while the amp is on, but with no input? Also, if I were to replace the two big caps, what would the best option be? Thanks for all the help!
Congrats on killing your hum.
You can measure the AC ripple both unloaded and loaded - it will give you an idea what's happening inside your amp. I really only suggested checking it as a way of isolating your hum issue. If your amp is working there really isn't a need to test ripple voltage.
You can measure the AC ripple both unloaded and loaded - it will give you an idea what's happening inside your amp. I really only suggested checking it as a way of isolating your hum issue. If your amp is working there really isn't a need to test ripple voltage.
Well, back to the drawing board! 
So far I have installed isolated RCA inputs, taken out the fan switch and tested DC offset as well as set the bias for both channels. The bias was set fairly low, around 240 mA, so I bumped it up to 350 mA per channel. The fan still works as normal with the switch out of the picture. The amp is pretty much wired as stock again besides the open air power relay.
My tests in the last post were done with old junk speakers. After confirming that the amp worked without tripping any breakers or hurting speakers, I hooked it up to my Klipsch Heresy's. Now with the efficient speakers, I can hear a buzz coming from both channels...really low in the left channel and loud enough in the right channel to hear from 2-3 feet away. I don't get any readings from the binding posts as far as AC Hz. I can knock on the side of the chassis and both channels will cut out, but then I can knock again and get them to come back in...buzz during the whole process (except when speaks cut out). Any ideas?
So far I have installed isolated RCA inputs, taken out the fan switch and tested DC offset as well as set the bias for both channels. The bias was set fairly low, around 240 mA, so I bumped it up to 350 mA per channel. The fan still works as normal with the switch out of the picture. The amp is pretty much wired as stock again besides the open air power relay.
My tests in the last post were done with old junk speakers. After confirming that the amp worked without tripping any breakers or hurting speakers, I hooked it up to my Klipsch Heresy's. Now with the efficient speakers, I can hear a buzz coming from both channels...really low in the left channel and loud enough in the right channel to hear from 2-3 feet away. I don't get any readings from the binding posts as far as AC Hz. I can knock on the side of the chassis and both channels will cut out, but then I can knock again and get them to come back in...buzz during the whole process (except when speaks cut out). Any ideas?
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The knock test says there is a loose connection or bad solder joint somewhere. Reheat your new joints - they should be shiny and smooth.
Is your hum with the amp connected to a source? Try with the inputs grounded. There could be a ground loop with your source and the amp. If you verify that the amp hums on its own, then:
What AC voltage do you get between the speaker terminals for each channel?
Check all of the physical connections to the chassis.
With a dual channel hum, check the AC ripple on the main filter caps.
Although it may seem counter intuitive, breaking one of the connections from the RCA jacks to the star ground may help. As DJK pointed out, on your older boards, there is a signal to power ground connection on the board. The grounding scheme in the published manual (for newer boards) is likely different from what you need to keep yours from humming.
Is your hum with the amp connected to a source? Try with the inputs grounded. There could be a ground loop with your source and the amp. If you verify that the amp hums on its own, then:
What AC voltage do you get between the speaker terminals for each channel?
Check all of the physical connections to the chassis.
With a dual channel hum, check the AC ripple on the main filter caps.
Although it may seem counter intuitive, breaking one of the connections from the RCA jacks to the star ground may help. As DJK pointed out, on your older boards, there is a signal to power ground connection on the board. The grounding scheme in the published manual (for newer boards) is likely different from what you need to keep yours from humming.
Re-heated new solder joints. I get hum with or without a source.
What do you mean by "try with the inputs grounded"...didn't I just spend all that time grounding them?
VAC @ binding post with no speakers hooked up = 0.3 VAC, both channels.
Do I just go measure from + to - on each big cap to measure ripple?
It's becoming a PITA to remove the amp module every time I need to check the inputs. Do I have to screw the amp module back down before powering up the amp to do checks? Theres no easy way to get at the bottom of this amp. with the flimsy faceplate.
What do you mean by "try with the inputs grounded"...didn't I just spend all that time grounding them?
VAC @ binding post with no speakers hooked up = 0.3 VAC, both channels.
Do I just go measure from + to - on each big cap to measure ripple?
It's becoming a PITA to remove the amp module every time I need to check the inputs. Do I have to screw the amp module back down before powering up the amp to do checks? Theres no easy way to get at the bottom of this amp. with the flimsy faceplate.
inputs grounded means input hot connected to ground - you can do this with a cheap RCA patch cord, just clip it with a few inches of lead, strip the outer jacket off, push back the shield, strip the core and connect the core wire to the shield and plug it in to your inputs. No need for dis-assembly at all.
If you have a few clip leads you can also clip from the RCA hot to ground or the spot where the lead reaches the board to ground.
That's a heck of a hum or - that you didn't hear it with your first speakers. That tells me something changed between the first speaker test and now. Again, tighten all the mechanicals.
That your meter cannot lock onto the frequency at that voltage suggests that it it made up of a number of frequencies or that it is too high a frequency for the meter. What does it sound like? A nice comfortable hum or is it a bit buzzy? Do you know anyone with an oscilloscope?
Yes, PSU ripple is the AC voltage from one side of each cap to the other. Since they have one terminal connected you can clip your DMM common lead there and touch either cap's hot terminal. Let' start troubleshooting with PSU ripple. Measure and report back and we can decide where to go from there.
If you have a few clip leads you can also clip from the RCA hot to ground or the spot where the lead reaches the board to ground.
That's a heck of a hum or - that you didn't hear it with your first speakers. That tells me something changed between the first speaker test and now. Again, tighten all the mechanicals.
That your meter cannot lock onto the frequency at that voltage suggests that it it made up of a number of frequencies or that it is too high a frequency for the meter. What does it sound like? A nice comfortable hum or is it a bit buzzy? Do you know anyone with an oscilloscope?
Yes, PSU ripple is the AC voltage from one side of each cap to the other. Since they have one terminal connected you can clip your DMM common lead there and touch either cap's hot terminal. Let' start troubleshooting with PSU ripple. Measure and report back and we can decide where to go from there.
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So, do I clip the RCA off, connect core to shield, then put the RCA back on? Or do I just shove the bare wire in there?
Where is the RCA "hot?" I don't know where you mean to clip to.
The noise is a buzz.
I have a buddy with an O-scope and signal generator, but they're in his storage unit and would take a few days to get up and running to test the amp on.
I can handle measuring the PSU ripple for now. I'll have to run and get some clips and post back soon. I'd still like to figure out this RCA thing too.
Where is the RCA "hot?" I don't know where you mean to clip to.
The noise is a buzz.
I have a buddy with an O-scope and signal generator, but they're in his storage unit and would take a few days to get up and running to test the amp on.
I can handle measuring the PSU ripple for now. I'll have to run and get some clips and post back soon. I'd still like to figure out this RCA thing too.
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The core of the RCA is hot - that's the signal. The jacks probably have a solder lug under the nut and a central terminal. Use a test lead to connect those together or the central terminal to ground somewhere convenient.
Leave a few inches of wire connected to the RCA and plug the RCA plug into the input after you twist the core and shield together. You can also just plug a cable in and plug the other end of the cable together (with one end in your left hand and the pin facing right, put theme together with the other pin pointing left, center pins next to each other and push together just enough that they stick - doesn't even cost a cable.)
Buzzy output may mean that you have an oscillation or that PSU ripple is high. Once your friend digs out the scope, look at the waveform on the output of the amp. That will tell you shape and frequency. A 120 Hz sawtooth shaped wave says PSU ripple, but you probably already will know if that's the issue when you measure PSU ripple tonight. Report the ripple voltage on each cap at idle.
Leave a few inches of wire connected to the RCA and plug the RCA plug into the input after you twist the core and shield together. You can also just plug a cable in and plug the other end of the cable together (with one end in your left hand and the pin facing right, put theme together with the other pin pointing left, center pins next to each other and push together just enough that they stick - doesn't even cost a cable.)
Buzzy output may mean that you have an oscillation or that PSU ripple is high. Once your friend digs out the scope, look at the waveform on the output of the amp. That will tell you shape and frequency. A 120 Hz sawtooth shaped wave says PSU ripple, but you probably already will know if that's the issue when you measure PSU ripple tonight. Report the ripple voltage on each cap at idle.
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Ok, I created a mono RCA shorting plug. I found when it's plugged in to the right channel (the noisy channel), it does nothing...and when I plug it into the left channel (less noisy channel), it makes it sound like the right channel. I checked every screw in this thing.
When I push on solder connections at the top of the speaker relay with my meter test probe, I can get the sound to go in and out. Maybe the speaker relay is bad? The solder joints look fine to me on it.
When I push on solder connections at the top of the speaker relay with my meter test probe, I can get the sound to go in and out. Maybe the speaker relay is bad? The solder joints look fine to me on it.
AC ripple looks good - no worries under a volt range at idle.
Reheat the connections to the speaker relay. Cracks in solder joints too small to be seen can cause issues. Any help?
What do you mean by the noisy channel? In post 46 you said that you measured the same hum voltage on each. Do you still get .3V on both?
Pull the rail fuses on one channel. Does the remaining channel hum? Shut down and wait a few minutes. Pull those fuses and replace the first ones. Hum or quiet?
If single channel operation is quiet and dual channels hum, you probably have a ground loop, which can be a royal pain to track down and eliminate.
Reheat the connections to the speaker relay. Cracks in solder joints too small to be seen can cause issues. Any help?
What do you mean by the noisy channel? In post 46 you said that you measured the same hum voltage on each. Do you still get .3V on both?
Pull the rail fuses on one channel. Does the remaining channel hum? Shut down and wait a few minutes. Pull those fuses and replace the first ones. Hum or quiet?
If single channel operation is quiet and dual channels hum, you probably have a ground loop, which can be a royal pain to track down and eliminate.
The noisy channel is the right channel, which had the hum in the first place, but now it's a louder buzz. I don't get any AC Hz readings, and VAC at the binding posts is 0.3 VAC left, and 0.4 VAC right.
I pulled the rail fuses on the left channel and got a quiet right channel. When I pulled the right channel fuses, the speaker relay wouldn't kick on.
I pulled the rail fuses on the left channel and got a quiet right channel. When I pulled the right channel fuses, the speaker relay wouldn't kick on.
The PC-9 board gets its power from the right channel, so the relay won't kick on with just the left channel fuses in place You can measure the hum voltage of the left channel between pin 7 of the relay (with W and R wires attached) and ground (the black binding post). Quiet single channel operation pretty much confirms an internal ground loop.
Have you tried to reheat the connections to the wire between the big caps? Any cold/cracked joint there could cause a problem. That's a mass of wire that needs a powerful iron to make a good joint. I wouldn't hesitate to use a 100W gun there.
If that fails, try clipping one of the leads from the RCA jack ground to the board. The connection to the ground between the jacks provides that signal ground, and the resistor DJK mentioned provides the signal ground connection for that channel. Clip as as close to the RCA as possible to allow reconnection if needed.
Push those twisted wires from the PC9 to the bottom of the module as far away from the signal leads as possible.
Anyone else have any ideas?
Have you tried to reheat the connections to the wire between the big caps? Any cold/cracked joint there could cause a problem. That's a mass of wire that needs a powerful iron to make a good joint. I wouldn't hesitate to use a 100W gun there.
If that fails, try clipping one of the leads from the RCA jack ground to the board. The connection to the ground between the jacks provides that signal ground, and the resistor DJK mentioned provides the signal ground connection for that channel. Clip as as close to the RCA as possible to allow reconnection if needed.
Push those twisted wires from the PC9 to the bottom of the module as far away from the signal leads as possible.
Anyone else have any ideas?
I tried re-heating the joint between the big caps, but I only have a 15w iron and it wouldn't heat up the solder that is there. I did however reheat the joints on the top of the speaker relay. Nothing changed.
I clipped the ground from the left RCA to the signal ground. The buzz stayed the same.
I clipped the ground from the left RCA to the signal ground. The buzz stayed the same.
My guess is the issue is there in that massive central ground - probably worth buying a cheap gun to try heating it. IIRC, initially it was only the right channel humming. Then you heated it a bit with a low power iron, maybe enough to soften up part of the joint and now you have hum in both channels.
Hopefully one of the grounding experts will chime in.
Hopefully one of the grounding experts will chime in.
No, in post #44, I said that we put in new RCA's, checked for proper DC offset and set the bias for both channels to 350 mA...then upon hooking the amp back up, there was a buzz/hum in both channels. The right is much louder though. It was noisy before I did anything to the existing solder joints.
It was a single channel until we got the amp running "normally". I'm guessing since the bias was low and we turned it up, you can now hear the hum in the left channel. I can get the hum to go away in the left channel with the RCA shorting plug, but not with the right.
Now, this morning, I clipped the ground wire going to the left RCA from the signal ground. Nothing changed. Did I clip the wrong wire? What wire are you talking about? I don't understand your last sentence.
Now, this morning, I clipped the ground wire going to the left RCA from the signal ground. Nothing changed. Did I clip the wrong wire? What wire are you talking about? I don't understand your last sentence.
When you measure the resistance from the left black binding post to the left RCA jack sleeve you should get 0R5 (half an ohm) and 0 from right RCA to right black post. While you're at it, check that is 0 resistance between the two black binding posts.
Changing the bias as you did should not affect the level of hum at all. It will be interesting to see a screen shot of your output when your buddy gets his scope out.
The clip you did was to try to isolate the two signal grounds. Either 0R5 doesn't provide enough isolation or there is something else going on. Chasing ground loops often seems like an exercise in futility.
Changing the bias as you did should not affect the level of hum at all. It will be interesting to see a screen shot of your output when your buddy gets his scope out.
The clip you did was to try to isolate the two signal grounds. Either 0R5 doesn't provide enough isolation or there is something else going on. Chasing ground loops often seems like an exercise in futility.
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