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Old 4th February 2012, 07:49 PM   #2031
jfrana is offline jfrana  United States
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Default Still strange volume control

I am hoping to pin down the problems before I order components (I would be fun to have $5 shipping for one .16 piece).

Resistance checks all seem proper.

Voltage checks are another matter.
pin 4 and pin 8 of U4 do not drop below 1v with one battery removed. Does this surely indicate broken a MOSFET (q1 or q2).

U2 AC only
p1=11.6 , p2=11.5, p3=-10, p4= -11.4, p5=-9.9, p6=-11.7, p7=11.5, p8=11.75

U2 AC w/ battery
Same as AC only

U2 battery only
p1=8.6, p2=8.4, p3=-6.8, p4=-8.6, p5=-6.8, p6=-8.6, p7=8.4, p8=8.6

(I am pretty sure I measured these values with u1, u3, u4 removed, u2 installed power on). Are the above U2 readings correct? If not, what do they indicate is damaged or improper?

Still having weird volume control (full volume, no control, on one channel and proper volume control on the other). I will try removing the pot and resoldering. Would this be due to damaged U2, Q2, Q1, U4, U3, or U1? Or is it almost certainly a solder bridge or some other non-component short?

Are there any empty holes that must be soldered (under C9 or otherwise?)

Thanks for the help and fun project.
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Old 4th February 2012, 08:33 PM   #2032
nezbleu is offline nezbleu  Canada
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Quote:
Originally Posted by jfrana View Post
Still having weird volume control (full volume, no control, on one channel and proper volume control on the other). I will try removing the pot and resoldering. Would this be due to damaged U2, Q2, Q1, U4, U3, or U1? Or is it almost certainly a solder bridge or some other non-component short?
That would not be caused by power supply issues (U2 etc). That indicates a solder bridge between wiper and input side of the pot on one channel. You should be able to track that down pretty quickly with resistance measurements. One channel will show varying resistance between the wiper (center pin) and both outside pins as the shaft turns. The other side will show constant near-zero resistance to one of the side pins (on the right facing the pot from the front, full clockwise position). I hope the bridge is on the bottom of the board so you don't have to remove the pot.
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Old 4th February 2012, 09:24 PM   #2033
agdr is offline agdr  United States
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Quote:
Originally Posted by jfrana View Post
U2 AC only
p1=11.6 , p2=11.5, p3=-10, p4= -11.4, p5=-9.9, p6=-11.7, p7=11.5, p8=11.75
Good work on making the voltage measurements. Pull U2 out also, so that ALL the chips are out, and with your headphones unplugged for all of these tests, power it up again on AC and repeat those U2 DC voltage measurements. Since U2 is out it will be easy - just measure right from the socket pins on the top of the U2 IC socket to ground (metal shell of the gain switch).

Your voltage readings on P4 and P8 say that your power supply is working OK. Those are fairly good values for the positive and negative power supply rails (a little low, but that should work itself out when the real problem is found). Same with your P4 and P8 readings on battery.

But P2... not so much. That is way off. Please double check that you are measuring a positive +11.5Vdc on P2 with respect to ground, and not a negative value (-11.5Vdc). In your previous post you listed the U2 measurement as -11.8Vdc. With U2 out you should measure somewhere around -8.7Vdc or so to ground on P2. If you still get +11.5Vdc (or -11.5Vdc), that is one of your problems. If you get that check that R5, R9 and R25 are the right values. Try reheating their solder connections even if they look OK and take a resistance reading (with the O2 AC power unplugged now) from P2 of the U2 socket to ground. Also take a look with a magnifying glass to make sure P1 and P2 are not solder bridged to each other.

Last edited by agdr; 4th February 2012 at 09:35 PM.
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Old 4th February 2012, 11:45 PM   #2034
tamu is offline tamu  Australia
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I built my second o2 and having problems getting the power to the opamps. All the measurements checks up until q1 and q2. Also I have a short on pin 4 and pin 7 of U2 - drain and source of q2. I've replaced the mosfets but the problem is still there.

Any ideas where to check first? I'll re check the resistances again and see if I can find something unusual.
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Old 5th February 2012, 12:03 AM   #2035
agdr is offline agdr  United States
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Originally Posted by tamu View Post
Also I have a short on pin 4 and pin 7 of U2 - drain and source of q2. I've replaced the mosfets but the problem is still there.
Remove U2 from the socket, and with the batteries out and the AC unplugged, measure the resistance between pins 4 to 7 of the U2 socket again and see if you get close to zero ohms (shorted).

If you were measuring voltages, in circuit, rather than resistances then pins 4 and 7 will look like a short when that part of U2 is "on". There is a pulldown transistor in there between those pins. If you were measuring resistances (with U2 installed) when you found the 4-7 short, and you had the batteries out and the AC unplugged, and that short goes away when you measure again with U2 out, then your U2 chip is bad.

Last edited by agdr; 5th February 2012 at 12:06 AM.
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Old 5th February 2012, 12:07 AM   #2036
tamu is offline tamu  Australia
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Quote:
Originally Posted by agdr View Post
Remove U2 from the socket, and with the batteries out and the AC unplugged, measure the resistance between pins 4 to 7 of the U2 socket again and see if you get close to zero ohms (shorted).

If you were measuring voltages, in circuit, rather than resistances then pins 4 and 7 will look like a short when that part of U2 is "on". There is a pulldown transistor in there between those pins. If you were measuring resistances (with U2 installed) when you found the 4-7 short, and you had the batteries out and the AC unplugged, and the short goes away when you measure again with U2 out, then your U2 chip is bad.
I was using the continuity buzzer. just checked it with resistance and without the u2 installed. I get 0 ohms..
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Old 5th February 2012, 12:12 AM   #2037
agdr is offline agdr  United States
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Quote:
Originally Posted by tamu View Post
I was using the continuity buzzer. just checked it with resistance and without the u2 installed. I get 0 ohms..
Since you have already replaced Q2 that just leaves C21. Try unsoldering it and see if the short goes away. That would be fairly rare for the cap to be shorted, but it does happen.
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Old 5th February 2012, 12:24 AM   #2038
tamu is offline tamu  Australia
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Originally Posted by agdr View Post
Since you have already replaced Q2 that just leaves C21. Try unsoldering it and see if the short goes away. That would be fairly rare for the cap to be shorted, but it does happen.
nope cap is ok too. its still showing a short somewhere
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Old 5th February 2012, 12:40 AM   #2039
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Hi guys,
been reading part of this big thread, but was wondering where to get the PCB and shematic.
Thanks
Tony
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Old 5th February 2012, 12:42 AM   #2040
agdr is offline agdr  United States
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Quote:
Originally Posted by tamu View Post
nope cap is ok too. its still showing a short somewhere
Luckily the traces from pin 7 of U2 don't go very many places. With the battery part of the board at the bottom, one trace on the bottom of the PCB from pin 7 goes to the bottom of R25, to the left of the U2 socket. Measure across R25. That one is a huge 1.5 megohm resistor. As long as you read something large like that it is OK and that one isn't the short.

Next the other trace from pin 7 on the PCB bottom goes out the right side of the socket to C21 and R8. Same thing with R8 - that one is a 270K and should measure something up there across it rather than short. If that one is also OK that just leaves the mosfet (again) or a solder bridge to one of those two traces on the bottom of the PCB.

If neither R25 or R8 read a short, then just brute force it. There are only those 4 things on that trace hooked to U2 pin 7. You already have C21 out. Unsolder the bottom of R25 and the bottom leg of R8. Then you are down to just the mosfet or a solder bridge if 4 and 7 still read shorted.

One more thing to double check is that the mosfet is turned the right way, with the metal back toward the top of the board.

Last edited by agdr; 5th February 2012 at 12:48 AM.
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