A12 Problem - No Sound

[stepanek]

so i finished up my first A12 quite some time ago but haven't had any luck getting it to work. i should preface this by saying that i've built a few stompboxes, but i don't have a wealth of experience with electronics. any help would be greatly appreciated! i'll include photos & sound for description.



here's the short version:

when i power up, at first there's no sound at all. then gradually a hiss comes in mostly around 16000 Hz. you can hear it here (mp3 320kbps): http://www.mediafire.com/?4xdml01bxbk. (sorry about all the ads on this site -- it's free. maybe "Save to My Files" is easiest?)

the Gain, Trim, Phase Inverter toggle, and Phantom toggle do nothing to the sound (although when i turn on the Phantom, the 3rd LED from the front inside the chassis on the power supply blinks).



long version:

after finishing the kit, i performed steps 29-34 of the assembly instructions without problems. in step 29, i remember not being sure if i was accurately measuring resistance on the correct pins -- which pins are 1 & 2 on J7? however, i finished the rest of the steps. got all the appropriate voltage measurements before and after op-amp installed. powered on: no sound, except the hiss described above.

i took it out to retest resistance and seem to be getting proper results.

* am i correctly measuring pins 1 & 2 on J7 (view here)?
* if i reverse the + and - leads on my DMM and measure the same pins, i get no response on the DMM (view here). shouldn't that not matter? shouldn't i still get a resistance reading even if the leads are reversed? is it just a crappy DMM or am i not using it right?
* am i correctly measuring pins 2 & 3 on J7 (view here)?
* do you see any overall problems with my A12 components (top view)?
* i know it's hard to see, but how's the soldering job (back view)?
* where do i even start to look for shorts, etc? to me, it doesn't look like i have any solder bridges...

like i said, i don't know much but i'm eager to learn...and i'm itchin' to get started on my C84 & N72 kits! if i can get 'em to work. thanks again for any help.

[tpryan]

* am i correctly measuring pins 1 & 2 on J7 (view here)?

Yes. Switch your meter to the 400 ohm range.

* if i reverse the + and - leads on my DMM and measure the same pins, i get no response on the DMM (view here).

That's what you should see if there are no shorts.

* am i correctly measuring pins 2 & 3 on J7 (view here)?

Yes. Switch your meter to the 400 ohm range.

* do you see any overall problems with my A12 components (top view)?

No, but it's up to you to verify that all of the resistors are in the right locations.

* i know it's hard to see, but how's the soldering job (back view)?

It's not a disaster, but it ain't the greatest. Looks like too much solder on most of the Rs and Cs and rotary switch. The component leads aren't trimmed short enough, and the transformer leads could be neater. None of that is necessarily causing your symptom as long as the connections are electrically sound, however

* where do i even start to look for shorts, etc? to me, it doesn't look like i have any solder bridges...

If you don't find blatant shorts at step 30, apply power and measure the voltage at the op-amp socket as described in steps 31, 32 and 33. Can you adjust the voltages to +/-18V?

[stepanek]

wow -- thanks for your fast response, Tim! and thanks for looking over my work; i'll work on keeping my leads shorter and laying off the solder. re: components, i was careful to measure and label each resistor individually before installing, so those should be ok.

for this thread, i'll keep up with the photos just to make sure i'm doing things correctly.

Switch your meter to the 400 ohm range.

after switching, neither pins 1 & 2 on J7 nor pins 2 & 3 on J7 show any response on the meter.

* so, do i have a short?
* the assembly instructions read, "If you measure a steady resistance under 100 ohms, don't apply power." why does the meter give a reading on the 400K setting, but not on 400 ohm? if the resistance is very high, wouldn't the meter not be able show a measurement if it's set too low?
* with a short, how do i go about finding it? would i just measure each resistor on the back side of the board to check the solder connection?

If you don't find blatant shorts at step 30, apply power and measure the voltage at the op-amp socket as described in steps 31, 32 and 33. Can you adjust the voltages to +/-18V?

given the above, i'm a little hesitant to plug it in and test now. however, i did previously complete steps 31-33 without problems: in step 32 my readings were around + and - 14V as written, in step 33 i adjusted R25 & R29 to 18V without problems, and in step 34 i adjusted R19 so that the op-amp pins read 0V both right when installed and after 15minutes of use. if the voltages had been off, i would've stopped completely and posted to the forum then.

i'll definitely redo the readings if you think it's ok to turn on the power. if there was a short i didn't discover before, have i fried anything by turning on the power?

thanks again for any advice!

[tpryan]

If you can adjust the op-amp voltages to +/-18V and get +48V at both ends of R33 and R34 (with phantom switched on), you should be fine.

[stepanek]

i can adjust the op-amp voltages to +/- 18V (U1 pins 4 & 3; U1 pins 4 & 5)

R33 and R34 don't return any voltages with phantom on or off (e.g. R33). i tried it both with the op-amp removed and installed. no luck.

[tpryan]

R33 and R34 don't return any voltages with phantom on or off (e.g. R33).

Do you measure +48V at J7, pin 5? How about R32 (with switch ON)?

[stepanek]

Do you measure +48V at J7, pin 5?

not quite sure how to measure voltage at J7, pin 5. how should i place my + & - probes? i used just one probe on J7 pin 5, with the other probe not touching anything...not sure if that's correct. i tried both positive and negative probes, and the DMM did not register any voltage, with the +48 switched on or off.

How about R32 (with switch ON)?

R32 did not show any voltages with +48 switched on or off.

[dkatz42]

You always have to measure voltage between two points. If only one point is specified, put the positive probe there, and connect the negative probe to ground (pins 2, 4, or 6 on the power supply connector, if memory serves.)

[stepanek]

thanks dkatz42!

J7 with + probe on pin 5 & - probe on pin 2 reads +48.5V (here)

still nothing at R32 with phantom power switched on or off.

[tpryan]

What voltage do you measure at pin 3 of SW1? It's the outermost pin, the one easiest to probe.

still nothing at R32 with phantom power switched on or off.

You're measuring with respect to J7, pin 2, right? You won't measure a voltage drop directly across R32 until you connect a phantom-powered mic.

[stepanek]

What voltage do you measure at pin 3 of SW1?

+48.5V with phantom switch off; +48.5V with phantom switched on.

How about R32 (with switch ON)?...You're measuring with respect to J7, pin 2, right?

slowly, slowly i learn. i measured both sides of R32, R33, & R34 with respect to J7 pin 2 and with a phantom-powered mic connected. i got slightly different results on each side of the resistors, so i designated below:

R32

* transformer side = +48.5V
* non-transformer side = +48.2V

R33

* transformer side = +41.3V
* non-transformer side = +48.2V

R34

* transformer side = +41.3V
* non-transformer side = +48.2V

[tpryan]

OK, all the voltages seem fine so far. What voltage do you measure at pin 6 (output) of the op-amp? Does the op-amp get noticeably warm? Does one of the voltage regulators get warmer than the other?

[stepanek]

What voltage do you measure at pin 6 (output) of the op-amp?

measuring with my + probe on pin 6 of the op-amp and - probe on pin 2 of J7, i got around +8V right after i turned it on. however, within a minute or two it dropped and fluctuated from between about +4.3V and +4.7V.

crap! and so i measured the op-amp with my + probe on pin 6 and - probe on pin 4 (like it says in step 33), and similar to above (i'm guessing that op-amp pin 4 is a ground?), my reading now fluctuates between +4.3V and +4.7V. i'm positive that when i first completed the kit i followed step 33 and adjusted R19 so this voltage dropped to 0V ...

should i adjust R19 again?

Does the op-amp get noticeably warm?

yes. pretty hot.

Does one of the voltage regulators get warmer than the other?

kinda hard to tell, but i would say with some certainty that U2 gets warmer - quite hot occasionally.

[tpryan]

Sounds like a bad SC25. E-mail sales@seventhcircleaudio.com for details on how to return it.