First build - no left channel, muffled right channel [resolved]

[ibzrg1570]

Hello all,

I completed building my stock Crack but the amp only plays audio out of the right channel and sounds a bit muffled with no treble or bass extension. Also, when I turn off the amp with audio still playing, I can hear the left channel briefly coming in after the right channel fades out. I've checked that my connections are in the correct locations and passed all resistance tests.

The tips section in the manual suggests that a muffled sound indicates insufficient voltage, and I'm indeed seeing some low values at terminals 5, A1, and B4 - all measured 57 volts vs the expected 75-90 volts at 5 and 90 volts at A1/B4. I'm also getting 79 volts at terminals A6 and B1 vs the expected 90 volts, but that falls within the 10-15% error threshold specified in the manual.

So far I've tried retesting my resistors at terminals 1/2, 4/5, and at the output jack, measuring the resistance through the ground path per Grainger49's stickied post, as well as resoldering the tip on the output jack. All the numbers check out and no difference in the sound. FAQ #12 addresses the single channel issue, so I understand that there could be numerous potential causes for these problems. This is my first time assembling electronics, and my knowledge of circuits is extremely limited, so I would appreciate any ideas on where to start my investigation.

[Paul Birkeland]

I would imagine that you just have a flaky solder joint that is causing your problem.  I would simply reheat all of your joints to be sure that the solder flows well and makes good connections.

-PB

[ibzrg1570]

I would imagine that you just have a flaky solder joint that is causing your problem.  I would simply reheat all of your joints to be sure that the solder flows well and makes good connections.

-PB

Thanks for the reply. I gave this a shot and reheated all the joints that I could safely reach without damaging the wires around them, but I'm getting similar voltage readings and the sound is unchanged. One thing that I should have noted in my original post - when doing a resistance check at terminal 13 my reading starts at over 900K ohms and starts falling slowly. Could this be related?

[Paul Birkeland]

What resistance measurements do you get at the RCA jacks? 

[ibzrg1570]

Left 100k and right 106k

[Paul Birkeland]

Can you download a 60Hz tone any play it back through your Crack with your computer and/or your phone?  If so, with no headphones attached, play the 60Hz tone through your Crack with the volume pot all the way up.  Set your meter to AC volts on the 2V scale.  Ground the black probe, then measure the AC voltage on the upper and lower middle lugs of the volume pot.

-PB

[ibzrg1570]

I'm getting 0V on the upper middle lug and 2.8V on the lower middle lug, same results when the 60Hz tone is not playing.

[Paul Birkeland]

Be sure you're on AC volts, not DC volts.  If you get 2.8V DC on the output lug of your pot, that merits investigation as well.

-PB

[ibzrg1570]

That was AC, I'm getting 1.6V DC on the lower lug, 0V on the upper.

[Paul Birkeland]

That was AC, I'm getting 1.6V DC on the lower lug, 0V on the upper.

OK, well, you should have 0V on both lugs.  The upper lug goes to A7, and A8 has an LED that sits at 1.57 volts, so either A7 and A8 are touching, or there's enough debris in there to cause a short between those pins.

-PB

[ibzrg1570]

It looks pretty clear between A7 and A8, so I'm not sure that's the issue. Have a look for yourself, maybe I'm missing something.

[Paul Birkeland]

You might try reheating the solder ball on the center lug of the 9 pin socket.  You want to heat it enough that it flows out an flattens.  (Same with other joints in the amp).

-PB

[ibzrg1570]

Paul,

I moved some wires around so I could reach the solder ball on the center lug and it works now! Thanks so much for all your help, it's been a long journey and I'm excited to finally enjoy the Crack.