A12 input switch

[thelondonbroil]

First let me say I am having a blast building these things and learning a lot. I have a very basic knowledge of electronics so I'm happy that nothing has exploded yet! I have two A12 modules and have built both. I am using Avedis op amps. The first module seems to be working fine and sounds great with just an SM 57 in headphones. Everything checked out on that unit. The only thing I notice is a little dirt when I turn the input pot.

I was not so lucky with the second. The unit only works in 2 positions of the input pot. I A/B'd the second unit with the first unit. Both were on position 10 (since that's the one that worked best on the second) and I used the same mic, settings, etc. I noticed a distinct tonal difference between the two. The first was much more bass responsive. I'm assuming there is a short somewhere. I tried to test continuity on the input pot. Not sure if I'm doing it correctly. If you are looking at the bottom of the unit I put negative on the lone connector on the left and went down the line with the connectors on the right. The 7th connector down showed a short. I did the same with the first and no short. I tried to remove the solder on the bottom as much as possible and no luck. Maybe it's on the top of the circuit board?
Any helpful hints on how to remove this pot?
Or is there a possible short elsewhere?
Could I have damaged the op amp?

Also, when I was adjusting R19 on the second unit, I had difficulty getting it to stay on 0.00 volts. It fluctuates a little (can't remember the exact voltages but sometimes between -.005 and .000 and sometimes it would go up to .3).
Is this ok?
If so what is a safe range for it to fluctuate?
If not, what could be wrong?

Everything seems to be installed right and there are no visable shorts. I have checked it many times, but will check it again tomorrow.

Another question I have is about jumper J5, the one for the 600 ohm output. I have installed the piece, but I am confused about whether it's necessary to connect it. I've read the message board quite a bit but I can't determine whether it's recommended to leave it off or put it on. I am going into a Tascam 38 1/2" tape machine with unbalanced inputs if this is a factor.

All in all I am happy so far, if not a little frustrated by my own ignorance.
But that's why I bought these, so I could learn how they work. I definately plan on ordering more once I get these going.

Thanks for any help you can give me.

the london broil

[tpryan]

The unit only works in 2 positions of the input pot.

First off, just so everybody's on the same page, there is no input pot. You're talking about the 12 position rotary switch, SW2 on the schematic.

I A/B'd the second unit with the first unit. Both were on position 10 (since that's the one that worked best on the second) and I used the same mic, settings, etc.

So the faulty unit only works in position 10, or works best in position 10? Does it pass signal at the other positions?

I noticed a distinct tonal difference between the two. The first was much more bass responsive. I'm assuming there is a short somewhere.

Maybe, maybe not. Troubleshooting is about making measurements first and hypothesizing about possible faults second.

I tried to test continuity on the input pot. Not sure if I'm doing it correctly. If you are looking at the bottom of the unit I put negative on the lone connector on the left and went down the line with the connectors on the right. The 7th connector down showed a short. I did the same with the first and no short.

This is a 2 deck switch and each deck needs to be measured separately. With the op-amp removed and power disconnected, connect one meter probe to the socket at U1, pin 2. Connect the other meter probe to the positive lead of C3. Set your meter to measure resistances of 10K or higher. As you rotate the switch from position 1 to 12, you should see the value of R10 for the first 5 positions, then the values of R11, R12, R13, R14, R7, R8, and R9.

To test the other half of the switch, connect one meter probe to U1, pin 1 and the other probe to AGND, which you can find at U1, pin 4. At switch position 1, you'll see about 6K8, with increasing resistance up to 19K8 at position 4. At position 5, the resistance will drop abruptly to around 2K8 and stay the same until position 12.

Take these measurements and let us know what you find.

Also, when I was adjusting R19 on the second unit, I had difficulty getting it to stay on 0.00 volts. It fluctuates a little (can't remember the exact voltages but sometimes between -.005 and .000 and sometimes it would go up to .3).
Is this ok?
If so what is a safe range for it to fluctuate?

R19 should be adjusted when you first power up the board, and then after about 15 minutes. The offset voltage drifts with temperature, but you should be able to keep it below 15mV with no problems. An offset of 5mV is nothing to worry about.

Another question I have is about jumper J5, the one for the 600 ohm output. I have installed the piece, but I am confused about whether it's necessary to connect it. I've read the message board quite a bit but I can't determine whether it's recommended to leave it off or put it on. I am going into a Tascam 38 1/2" tape machine with unbalanced inputs if this is a factor.

If you're driving 600 ohm loads, disconnect the jumper. Otherwise, connect it.

[thelondonbroil]

Thanks for your tip. Here are the results of my test.

I reconnected everything in my second unit to see if signal was passing and now I get no signal at any position of SW2.

I disconnected power and the op amp.

I Connected one meter probe to socket at U1, pin 2 and connected other to positive lead of C3:

In positions 1-5 of SW2 I found the resistance of R10, but it took about a minute to increase from 6k5

Here are the results from positions 6-12:
6- 2K83
7- checks out as R12
8- checks out as R13
9- checks out as R14
10- checks out as R7
11- checks out as R8
12- checks out as R9

I then connected one meter probe to U1, pin 1 and the other to U1, pin 4:

I get nothing in positions 1 and 2
Position 3 reads 16k97
Position 4 reads 20k
I get nothing for positions 5-11 and position 12 reads 2K8

Antoher disturbing thing is I reconnected the first unit which initially worked fine in all positions. I get sold signal in positions 1-10 and 13, but now I get no signal in position 11 of SW2.

I did the same test as above and everything checked out for the first half of the switch. When I connected one probe to U1, pin 1 and the other to U1, pin 4. Everything checks out except in position 11 of SW2. In this position I get nothing.

thelondonbroil

[tpryan]

When you say "I get nothing", does that mean an open or a short?

Do you have a way to post some close-up photos of the bottom of the board, under the rotary switch? It sounds like you have two faulty switches, two damaged PC boards, or improper soldering. I could certainly be wrong, but I suspect the latter.

[thelondonbroil]

Sorry, by "nothing" I mean open.

I am on the road for work so I will try to post pictures as soon as I can.

[thelondonbroil]

I went back and rechecked my solder points. You were right, poor soldering job. I redid it and now the measurements check out. Only I think I may have damaged my SW2 in the process. I don't want to assume anything, but I get a weak signal in every position of the switch, unless I apply a little pressure and then I get full signal, but I have to hold the switch just so.
Could this still be a product of a bad solder job, or is it possible that I may have damaged the switch or pc board in the process of repairing my mistakes. How should I check for this?
I will investigate further when I have a chance. I am at least a little encouraged that the measurements are correct and I am able to get some sound out of it.
Sorry to bother you with newbie questions, but I am learning quite a bit by doing this and am not discouraged. Thanks.

thelondonbroil

[thelondonbroil]

I'll still post some pics as soon as I get to my digital camera.
thelondonbroil

[tpryan]

I went back and rechecked my solder points. You were right, poor soldering job. I redid it and now the measurements check out. Only I think I may have damaged my SW2 in the process. I don't want to assume anything, but I get a weak signal in every position of the switch, unless I apply a little pressure and then I get full signal, but I have to hold the switch just so.

Symptoms like this are almost always the result of loose connections. In this case, I still suspect the soldering on the switch. What you may have done is apply a blob of solder to the switch pins while failing to heat the PCB pads sufficiently. This could create a joint that looks OK at first glance, but really isn't. Go back over each connection and keep the iron as close to parallel with the board as possible. Make sure the tip makes contact with both the switch pin and the PCB pad simultaneously. Heat the joint for a full second, apply the smallest amount of new solder, remove the solder, then remove the iron.

Let us know what happens.

[Category 5]

Depending on how much solder is already on hte joints, it may be beneficial to use solder wick to remove the existing solder, and carefully redo the joints after clean up. Just a thought.

Shane

[thelondonbroil]

Thank you guys for the advice, I'll try it and let you know.
thelondonbroil

[thelondonbroil]

Well, I keep getting just about there with this unit and then I mess it up. I finally got the unit to work, but compared to the first unit the output level was lower. (position 9 on SW 2 of the 2nd unit was about the same level as position 7 on the first) I rechecked R19 and it fluctuated between 0 and -.042, which I understood not to be a problem. I then checked my soldering job for the 15th time and in the process of testing with a mic I broke CONN 2. I need to order another one. Can I do so from you? Pac Rad here in LA doesn't sell xlr connectors for circuit boards. Let me know how I should go about this.

I don't know why I'm having so much trouble with this one, the first one went well and I've been using it for vocals the last few days. Sounds great, and I can't wait to try it on drums and guitar.

Thanks
thelondonbroil