A12 Issue: Low gain and only high frequencies

[trevor]

I have built two each of the N72, C84, J99 and A12 modules. The second A12 was the fourth module I built and the only one to exhibit significant issues other than a slight gain difference between my two J99 modules.

I have searched the forums and found similar issues, but, none of the suggested remedies solved the problem. I have re-flowed all joints more than once with no improvement. The calibration of the pre appears spot on compared to the first working A12. I am wondering if there may be an issue with the input transformer.

With reworking the joints, the construction looks pretty shoddy, but, I have followed the same practices for all builds with excellent results. What are some suggestions for solving this issue? Is there a way to specifically check the transformers for proper operation?

I am not a tech, just someone with enough understanding to get into trouble. Any guidance or suggestions would be greatly appreciated.

I can post audio examples if that will be helpful.

Thank you for your time,


Trevor Brown

[trevor]

I should have specified that the pre at issue is rev 1.4 and the SC25 appears to be fine as I have swapped it with that of my working A12 and it sounds fantastic.

All components on the defective pre appear correct and voltages measure as they should.

[tpryan]

What resistance do you measure at T1 (input transformer) between

pins 1 and 4
pins 5 and 8
pins 1 and 6
pins 5 and 6
pins 6 and 7

Let us know what you find.

[trevor]

Viewing the back of the board, and going by years old memory, I am using the upper right pin as #1 and #5 as the upper left pin. Assuming this is the correct pin identification, I measure the following:

pins 1 and 4 = 76.3k
pins 5 and 8 = 36.7
pins 1 and 6 = No reading
pins 5 and 6 = No reading
pins 6 and 7 = No reading

Please let me know if I have incorrectly identified the pins. I suspect I should have readings for the last three measurements.

Thank you,

Trevor

[tpryan]

You're misidentifying the pins, but that looks like an open secondary. You can see transformer data here: http://cinemag.biz/mic_input/CMMI-8-PCA.pdf

Make sure you probe the actual pins and not the PCB pads.

[trevor]

Thank you for the clarification. Cinimag label is up near the outside edge of the board. This is the same orientation as the functioning A12 I have.

This would put pin 1 at the lower left and pin 8 at the lower right if when looking at the back of the board with T1 away from me at the top of the board. Based on that assumption, I read the following:


pins 1 and 4 = 36.8 ohms
pins 5 and 8 = 77k
pins 1 and 6 = 3.47k
pins 5 and 6 = 229.57k (rises from about 219k then slows. Difficult to read)
pins 6 and 7 = 0.3 ohms


Please let me know if I am again in error and I will update as soon as I can.

Thanks again for your time and assistance,


trevor

[tpryan]

Yep, that looks like an open secondary winding. We'll need to get the transformer back, or the entire board if that's easier for you. Send an e-mail to rma@seventhcircleaudio.com for return instructions.

[DaveR]

Hey guys,

Just finished my first A12 and although steps 31-35 all check out fine, I get a very low signal (even with gain up to notch 12). I found this post and tried to troubleshoot the problem and see what my resistances were across the pins Tim asked Trevor to take readings of. Here are mine:

Pins 1-4: 36.6 ohms
Pins 5-8: 2.913 k ohms
Pins 1-6: 30 Mega ohms
Pins 5-6: 0.3 ohms
Pins 6-7: 0.1 ohms

Let me know if these sound like something I did wrong (I went back over all the soldered connections and things look good) or if it could be a component failure.

[tpryan]

Looks like you may have pin 5 shorted to the transformer case, possibly with a solder blob on the top side of the board.

[DaveR]

Well, you were right about a solder blob on pin 5, the only problem is how to get at it. All I have with me here is solder wick and I've tried to get as much up off the pin as possible, measured and got the same reading of 2.913 kohms on pins 5&8 and .3 ohms on pins 5&6.

I'm going to stop by radio shack on the ride home from work and pick up a solder sucker and try to lift the component out of the board.

Just out of curiosity, what are the readings I am supposed to be getting across 5&8 and 5&6?

Is there an easier way to get at the solder between the board and the transformer or is lifting the component the only way to be sure to get it all? I'll try to suck up the solder only on pin 5 first, of course, but with the paper underneath the component and solder apparently touching the component, I'd imagine the paper is going to be compromised too.

Thanks for all the help in advance, Tim. Besides my little mess-up, this kit was a blast to build and I can't wait to get started on the other 3... just want to get this one working first!

Thanks again!

[DaveR]

I got the solder sucker and was able to remove most of the solder from around the pins. I was able, with great trouble, to lift the cinemag up off the board, but not all the way. From what I could see there was no blob of solder on the component side of pin 5 or any other pins for that matter. The paper isolator was able to shift freely along the pins. So I eased the component back into the board and resoldered.

The problem still persists, so I guess it has to be somewhere else. Resistance readings were the same except now I don't get a value for pins 1&6.

Thanks in advance for the help.

[DaveR]

I would like to first and for most apologize for my own stupidity.

Both A12s work great. Tested well, now I'm gearing up for a session so now I'm excited.

Turns out my I/O is set up in a less than intuitive way in that the "Mic/Line" switch does not actually switch between "Mic/Line"... go figure. Well, alls well that ends well.

Thanks for trying to help Tim! Next up, N72s!