Okay, so I plugged in a sacrificial pair of 'buds to run the test again.
This time, with the battery in either position, the P2 measurements are all 3mV or less, so that's great.
However, I still see 527mV across U3 pins 4/8 when the lone battery is in the B1 position, and even with the 'buds plugged in, it never drops. As before, when the single battery is in the B2 position, this drops by an order of magnitude to somewhere around 40mV.
Using the same 'buds, I did several turn on/turn off/unplug cycles in various power positions (one battery, both batteries, AC power, amp on, amp off, etc), and there was a very slight "click" on turn on and a small "thump" with turn off (or removing a battery).
So I *think* that we're okay, but that 527mV across those pins with the battery in B1 still has me confused/concerned. Any insights? Is anyone else experiencing this, or am I flying solo? My confusion mostly comes from the fact that it's so different in between battery positions. I wouldn't think it would make any difference, but perhaps I'm reading the circuit incorrectly.
Thanks again. I've built several amps in the past and luckily not run into any problems, so I appreciate that you guys are willing to share knowledge/troubleshooting.
This time, with the battery in either position, the P2 measurements are all 3mV or less, so that's great.
However, I still see 527mV across U3 pins 4/8 when the lone battery is in the B1 position, and even with the 'buds plugged in, it never drops. As before, when the single battery is in the B2 position, this drops by an order of magnitude to somewhere around 40mV.
Using the same 'buds, I did several turn on/turn off/unplug cycles in various power positions (one battery, both batteries, AC power, amp on, amp off, etc), and there was a very slight "click" on turn on and a small "thump" with turn off (or removing a battery).
So I *think* that we're okay, but that 527mV across those pins with the battery in B1 still has me confused/concerned. Any insights? Is anyone else experiencing this, or am I flying solo? My confusion mostly comes from the fact that it's so different in between battery positions. I wouldn't think it would make any difference, but perhaps I'm reading the circuit incorrectly.
Thanks again. I've built several amps in the past and luckily not run into any problems, so I appreciate that you guys are willing to share knowledge/troubleshooting.
I haven't built my O2 yet, but I think you're OK. I can't explain the half-volt on U3, but I would think it's mostly harmless, and should be seen across pins 4 and 8 of U1 & U4 also. My guess for the difference between B1 and B2 readings would be some bias condition of the op amp's internals. The on/off transients are normal and explained at NwAvGuy's O2 Details page.
Thanks, couch 'tater.
I knew the on/off transients were good, which is why I figured that was a good sign. In fact, these are the smallest transients I've ever heard on any amp, which was kind of my point for bringing it up.
@agdr, Thanks for your reply. I measured the ground to positive rail, when I measured it with S1 off, I got 11.82V, and with the S1 switch on, I get -0.82V, then keeping the S1 switch in the "on position", and after unplugging and replugging the adapter, I get 11.61V.
U5 and U6 aren't really 'burning hot', I could leave my fingers on the top of them for 10 secs or so, so I suppose that means it is normal?
For the negative rail and pin 1/pin 7 of U2 tests when "adaptor is plugged in, switch S1 is on, but the amp is off", I got 8.3mV for both of the tests.
@Turbon, thanks for your reply as well. I just measured the S2 shell and the via, and it seems I got an approximately 1K ohm resistance between the via and S2, I guess I'll fire up my soldering iron and fix it later on when I have time.
U5 and U6 aren't really 'burning hot', I could leave my fingers on the top of them for 10 secs or so, so I suppose that means it is normal?
For the negative rail and pin 1/pin 7 of U2 tests when "adaptor is plugged in, switch S1 is on, but the amp is off", I got 8.3mV for both of the tests.
@Turbon, thanks for your reply as well. I just measured the S2 shell and the via, and it seems I got an approximately 1K ohm resistance between the via and S2, I guess I'll fire up my soldering iron and fix it later on when I have time.
ericlaw02 - you also need that measurement between ground and the negative rail taken on R2, both with S1 off and S1 on. That one should be somewhere around -12V.
Sounds like you either have an intermittant short or a regulator chip that is bad. Just to confirm, where did you measure the positive rail voltage - was it at R1?
The regulators will get warm normally but not burning hot, unless something is shorted and pulling more current than normal.
During one of the times when S1 is on but the amp hasn't turned on, try touching each of the 4 ICs and see if any of them are getting very hot.
Sounds like you either have an intermittant short or a regulator chip that is bad. Just to confirm, where did you measure the positive rail voltage - was it at R1?
The regulators will get warm normally but not burning hot, unless something is shorted and pulling more current than normal.
During one of the times when S1 is on but the amp hasn't turned on, try touching each of the 4 ICs and see if any of them are getting very hot.
agdr - I got a -31V when measuring with S1 off, -28.7V with "S1 on but amp not really powered" conditions and -28.2V with "S1 on and amp working" conditions. The measurement is took with the multimeter negative touching ground (S2 shell) and multimeter positive touching the negative rail (R2). I suppose that's no where near -12V, so I guess that's the issue?
Would the S2 shell/via issue be causing the short? I've got 1K ohm resistance between the via and S2, or would it be somewhere else?
By "regulator chip going bad", do you mean the U5/U6 7812/7912 chips?
And yes, I did measure the positive rail voltage at R1.
During one of the times when S1 is on but the amp hasn't turned on, the ICs didn't really heat up much (I suppose U2 got a few Celsius degrees higher but that's nothing), and none of them are getting very hot, even U5/U6 are just mildly warm.
Would the S2 shell/via issue be causing the short? I've got 1K ohm resistance between the via and S2, or would it be somewhere else?
By "regulator chip going bad", do you mean the U5/U6 7812/7912 chips?
And yes, I did measure the positive rail voltage at R1.
During one of the times when S1 is on but the amp hasn't turned on, the ICs didn't really heat up much (I suppose U2 got a few Celsius degrees higher but that's nothing), and none of them are getting very hot, even U5/U6 are just mildly warm.
ericlaw02- That is interesting! Definitely not right. I'll have to ponder that one a bit. On an O2 I have here going from ground (gain switch S2 shell) to to R1 is +11.72V and going from ground to R2 is -11.73V which are typical numbers. That is what is coming out of the regulator chips minus about 0.22Vdc from the schottky diodes D1 and D5.
The regulator chips U5 and U6 have internal shutdown circuitry for overcurrent and overtemperature. It is possible for one of them to fail in such a way that causes that shutdown to occur when it shouldn't.
The main difference between flipping S1 on with the adaptor plugged in vs. plugging in the adaptor with S1 already on is how gradually the power rails come up. The latter case would be a more gradual rise.
Edit: what voltage of ac adaptor are you using? Just doing some quick math here -30Vdc is about what would happen using a 20VAC transformer if the negative regulator was dead shorted. 20VAC would produce about -30V at the 470uF filters, going into the negative regulator chip. Also, that issue about the via under switch S2 shorting wouldn't have any effect with this problem.
One thing to check for on the back of the board is solder bridges between the pins of either regulator chip. They can be hard to see - a magnifying glass really helps.
The regulator chips U5 and U6 have internal shutdown circuitry for overcurrent and overtemperature. It is possible for one of them to fail in such a way that causes that shutdown to occur when it shouldn't.
The main difference between flipping S1 on with the adaptor plugged in vs. plugging in the adaptor with S1 already on is how gradually the power rails come up. The latter case would be a more gradual rise.
Edit: what voltage of ac adaptor are you using? Just doing some quick math here -30Vdc is about what would happen using a 20VAC transformer if the negative regulator was dead shorted. 20VAC would produce about -30V at the 470uF filters, going into the negative regulator chip. Also, that issue about the via under switch S2 shorting wouldn't have any effect with this problem.
One thing to check for on the back of the board is solder bridges between the pins of either regulator chip. They can be hard to see - a magnifying glass really helps.
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Is there an acceptable value for the resistance measured between S2 and the via? I also measured about 1k ohms, but I have been using my amp without noticing any channel balance or distortion problems - it only came to my attention as I've only just got round to casing it.
I'm still going to desolder the switch and bend the metal bit sticking out near the via, just to ensure reliability, but is my amp OK as-is?
I'm still going to desolder the switch and bend the metal bit sticking out near the via, just to ensure reliability, but is my amp OK as-is?
ericlaw02 - so thinking about it a bit, my recommendation would be to get the power supply section measuring correctly with S1 in the off position (an no batteries installed) first. Probably best to not turn on S1 anymore until you can get the power supply voltages measuring OK since -30V on the negative rail and 11.7v on the positive rail would give you 41.7V across the chips. They are only rated for 36V max.
For that matter, even once you get it fixed, since the chips are cheap I would recommend buying another set of the 4 NJM ICs and replacing the existing. The ones in there now may not last and/or may not perform as well as they could if they have been exposed to 40V.
As for the power supply, first check that the orientations of the diodes d3, d4, d1 and d5 and caps C2-C5 all match the pictures on the PCB. Then make sure U5 and U6 in the right places with the right orientations. U6 is the negative LM7912, closest to the edge of the board, with the metal side facing the board edge. Then check the whole power supply section for solder bridges or bad solder joints. I see on the data sheet that the input and output pins of the 7912 are right next to each other - that would be easy to solder bridge.
Then with S1 off try measuring ground to the side of diode D3 with the band and you should get around +30Vdc if your adaptor is 20VAC. Then measure ground to the non-banded side of D4 and that one should give around -30Vdc. If those are OK and the outputs of either regulator is wrong, replace that regulator chip.
I'm out of town the next few days but all these measurements will help RocketScientist with diagnosing. Good luck!
For that matter, even once you get it fixed, since the chips are cheap I would recommend buying another set of the 4 NJM ICs and replacing the existing. The ones in there now may not last and/or may not perform as well as they could if they have been exposed to 40V.
As for the power supply, first check that the orientations of the diodes d3, d4, d1 and d5 and caps C2-C5 all match the pictures on the PCB. Then make sure U5 and U6 in the right places with the right orientations. U6 is the negative LM7912, closest to the edge of the board, with the metal side facing the board edge. Then check the whole power supply section for solder bridges or bad solder joints. I see on the data sheet that the input and output pins of the 7912 are right next to each other - that would be easy to solder bridge.
Then with S1 off try measuring ground to the side of diode D3 with the band and you should get around +30Vdc if your adaptor is 20VAC. Then measure ground to the non-banded side of D4 and that one should give around -30Vdc. If those are OK and the outputs of either regulator is wrong, replace that regulator chip.
I'm out of town the next few days but all these measurements will help RocketScientist with diagnosing. Good luck!
Just an FYI for those of you really new to this (or to those of you not paying attention, like me)....
When performing the first "after build" test for voltage, use the VDC setting on your DMM, *not* the VAC.
I couldn't figure out why my reading was 50.2 volts. Then I realized...
Same here! After about 15 minutes of measurements, I realized I was testing with the VAC setting. Everything checked out OK after that, and the amp played beautifully into my K701s until I decided to test it with the board inserted into the B2 case. Now the amp won't turn on. I checked all clearances and made sure no solder points were touching the chassis before I slid the board in the channels, and the minute I pushed the power switch, the amp went dead. If it makes a difference, that was using battery power, not the power brick.
Any ideas on what could have happened? What components should I be checking, and voltages (or lack thereof) should I be looking for?
Does it happen on both Gain settings? If it is better on one vs the other, check that the pair of gain resistors are the same value.
I did before inserting the board in the case. The board is out of the case now and the amp still won't turn on. I'm almost certain I fried something, so the question now is, what should I be checking, and what component(s) to replace?
How low of a volume setting? Just before bottoming out the volume I also have imbalance, but that's normal with volume pots.
Question : Anyone use the sockets for solderless gain change? If so, how do you keep the resistors from coming out?
At the time, I just reheated the pads to move the switch a little bit, I guess it was enough to lift it off that via because that fixed the problem.