I am getting a basic bench set up. Just got a good power supply, already have a good DMM, iron.
I am finally ready to try and diagnose the amp problem ... gain, muting circuit device, or other.
Question: I run Ubuntu, and want to do a USB oscilloscope with Ubuntu/Linux support. Any suggestions?
Thanks, I know old thread and slow progress but it is happening.
I am finally ready to try and diagnose the amp problem ... gain, muting circuit device, or other.
Question: I run Ubuntu, and want to do a USB oscilloscope with Ubuntu/Linux support. Any suggestions?
Thanks, I know old thread and slow progress but it is happening.
Thanks Perry for following up so quickly on this thread, I appreciate it.
Do you mean why do I want an oscope, or why do I want a USB scope in particular?
Assuming the former, I thought the latter would be economical. I admit I don't have a tangible need for either right now as far as I know.
It does seem to be a little bit of a minefield. If I feel like I need a scope, maybe I should just do it right and get a real one - thanks in advance for any further suggestions.
I am going to try to see if I can service the gain pot that is in the amp now to correct the problem tomorrow. I suspect this won't work (it has already been looked at, cleaned, etc.), then I will update and reference your post above. Something seems to be wrong with the input stage, only one channel.
Thank you again!
Do you mean why do I want an oscope, or why do I want a USB scope in particular?
Assuming the former, I thought the latter would be economical. I admit I don't have a tangible need for either right now as far as I know.
It does seem to be a little bit of a minefield. If I feel like I need a scope, maybe I should just do it right and get a real one - thanks in advance for any further suggestions.
I am going to try to see if I can service the gain pot that is in the amp now to correct the problem tomorrow. I suspect this won't work (it has already been looked at, cleaned, etc.), then I will update and reference your post above. Something seems to be wrong with the input stage, only one channel.
Thank you again!
Many scopes (USB) have menus that require multiple steps to make changes. Standard scopes give direct access so one step to make changes. I'd prefer to have a cheap 20mHz analog scope with knobs and switches than a fancy scope with menus.
Also, the digital scopes typically give you notchy waveforms, even when the signal is a smooth curve.
It's personal preference.
Also, the digital scopes typically give you notchy waveforms, even when the signal is a smooth curve.
It's personal preference.
I retested, the right channel responds to gain adjustment, the left does not.
I probed back toward the inputs, and found this is the case several places in the input stages.
I tried cleaning the gain, no change.
With the amp off, I measured the resistance between the outer leads of both gangs of the gain pot while adjusting the dial. One varies from low resistance to ~45K, while the other remains fixed at about 50K (it is a 50K, 2 gang pot). This would seem to indicate that the second gang measured above is probably bad, although it is still in the board.
So it looks like the board needs to come out, and perhaps the first thing to do is to remove the suspect gain and verify it isn't working (?).
Thank you
I probed back toward the inputs, and found this is the case several places in the input stages.
I tried cleaning the gain, no change.
With the amp off, I measured the resistance between the outer leads of both gangs of the gain pot while adjusting the dial. One varies from low resistance to ~45K, while the other remains fixed at about 50K (it is a 50K, 2 gang pot). This would seem to indicate that the second gang measured above is probably bad, although it is still in the board.
So it looks like the board needs to come out, and perhaps the first thing to do is to remove the suspect gain and verify it isn't working (?).
Thank you
rear gang (suspect):
center to left: stays at 55K
center to right: stays at 0.1
front gang :
center to left: varies ~45K to 26
center to right: stays at 0.1
I think the center to right staying put on both is possibly due to external circuit.
The discrepancy suggests a problem, whether or not it is external to the pot - not sure yet.
It looks like the pot may have been changed out, or removed and reinstalled. This amp went through two techs, one did nearly all the caps, but left the gain scratchy. The second took care of the scratchy part, but left the amp as it is now.
center to left: stays at 55K
center to right: stays at 0.1
front gang :
center to left: varies ~45K to 26
center to right: stays at 0.1
I think the center to right staying put on both is possibly due to external circuit.
The discrepancy suggests a problem, whether or not it is external to the pot - not sure yet.
It looks like the pot may have been changed out, or removed and reinstalled. This amp went through two techs, one did nearly all the caps, but left the gain scratchy. The second took care of the scratchy part, but left the amp as it is now.
I don't know why I didn't think of this earlier, but I have a working 250 just sitting there, so I repeated the pot measurements.
Both gangs on it produced measurements similar to those for the "front gang" above.
The suspect pot does look like the correct part, but probably was either removed and reinstalled, or replaced.
I can supply pictures.
Both gangs on it produced measurements similar to those for the "front gang" above.
The suspect pot does look like the correct part, but probably was either removed and reinstalled, or replaced.
I can supply pictures.
I see, very good. It seems like the board needs to come out either way, then I can get a better idea of my options/where to go next.
I took the board out (just left the rails in place for now at least), managed to remove the gain, and cleared the contacts so a new gain can be installed.
I measured the gain after removal, and now both gangs measure the same ... it seems to be working.
I destroyed the old gain during removal, so it will have to be replaced. It seems to be Audio (log) taper, 50K - that is if I understand the taper convention correctly. When I measure resistance between the left (CCW side pin) and the center pin and rotate the dial CW, at half dial you get about 10% pot resistance, then it increases much more quickly as you rotate CW fully. Is this correct?
From here I need to continue to troubleshoot - any help would be appreciated. I don't mind removing the "chopper" JFET to test it, maybe an op-amp is out, I don't know what it would be. It looked to me like the faulty channel was showing no response to gain adjustment very early on, suggesting that first op-amp chip might be bad?
I measured the gain after removal, and now both gangs measure the same ... it seems to be working.
I destroyed the old gain during removal, so it will have to be replaced. It seems to be Audio (log) taper, 50K - that is if I understand the taper convention correctly. When I measure resistance between the left (CCW side pin) and the center pin and rotate the dial CW, at half dial you get about 10% pot resistance, then it increases much more quickly as you rotate CW fully. Is this correct?
From here I need to continue to troubleshoot - any help would be appreciated. I don't mind removing the "chopper" JFET to test it, maybe an op-amp is out, I don't know what it would be. It looked to me like the faulty channel was showing no response to gain adjustment very early on, suggesting that first op-amp chip might be bad?
I ordered:
2 x Bourns PDB182-K220K-503A
The pin rows are about 1 mm too close together, but will do I think.
With the amp cold, both channels measure ~6.4 M from left to center pin of (now absent) gain. I wonder if there was just some bad connection issue with the old gain.
I probably better wait until the new gains arrive, then install and test.
2 x Bourns PDB182-K220K-503A
The pin rows are about 1 mm too close together, but will do I think.
With the amp cold, both channels measure ~6.4 M from left to center pin of (now absent) gain. I wonder if there was just some bad connection issue with the old gain.
I probably better wait until the new gains arrive, then install and test.
I mean the rails are stuck to the tops of the devices, I just unscrewed the rails and board, and lifted the board with rails still on the devices right out.
Ahhh sorry about that.
The non-responding channel was always at low gain, at or nearly at minimum.
I may have those, let me check.
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