One of my A12's has lower gain than the other.

[packagebear]

Hi,

I just noticed that one of my A12's has lower output than it's twin. It's been a while since I built them and I just noticed. However it could have been off since I built it(It's my second rack which doesn't get as much use)

I've built 6 A12's that are all fine so I must have missed something.

When they're set at say #6 on the gain switch one is about (3 positions) 15 dB down from the other one. This is the same with dynamic and condenser mics as well.

IN any case if anyone could point me in the right direction to troubleshoot this I'd appreciate it.

Thanks in advance

Kris

[tpryan]

Verify that R23 (10K) and R24 (trim control) both measure 10K. Make sure you have connected the output transformer correctly. Re-flow the solder joints at the input transformer. Let us know what you find.

[packagebear]

I'm kind of confused by the resistors.

R23 is blank, and R24 is in the middle of the board.

the output transformer is correct and I reflowed the input but nothing has changed.

I'm not sure exactly which revision board i have, maybe that is the confusion.
right behind R16 it says 0306 AC-02 94v-0


The "good" A12 i have next to it has the same layout.. nothing at R23 R24 in the middle of the board.

Please let me know what else to check out.

thanks
kris

[packagebear]

bump

[tpryan]

On rev 1.4 those resistors are numbered R15 and R16.

[packagebear]

Ok,

I got 9.84 k on R15, and 7.68K at R16. I did this with the module out of the rack not connected to the Power supply.

Both of those values took a little bit of time to ramp up.

thanks
kris

[packagebear]

bump...
I'm coming back from tour in a few days and have a session I'd like to use this one on.. Any suggestions???

thanks
kris

[tpryan]

The quickest thing at this point is to drive it with a test signal and make some AC voltage measurements. Having another module handy for comparison will make it even easier. You'll need something that can generate a steady tone around 1K and a decent DMM.

[packagebear]

Ok,

I drove 1KHZ into each A12... one came out 28.5 volts AC the other one came out 5.6 Volts AC..

I measured the positive to pin 2 and the negative to pin 3 of each module.
I'm not sure if that is the correct way to do it though.

This was with the gains maxed on both units.

Hope this info helps. Please let me know what to try next.

thanks
kris

[tpryan]

Try the same test with the gain set at step 6 and also at minimum gain. Make sure the trim control is fully CW.

[packagebear]

Tested with trim fully at CW

Pre #1 @ gain 6 .4 volts @ gain 1 0 volts

Pre#2 @ gain 6 3.6 volts @ gain 1 .1 volts

I dont have the best dmm so these are with the meter set to 200 VAC. Not sure how much that'll affect what I;ve come up with.

I'm going to check the other A12 I built that are in a different rack shortly and see what they read as well.

Thanks again

kris

[tpryan]

How about the voltages at the input transformer? Measure between pins 1 and 4 and also between pins 5 and 8. You'll definitely have to click up a few notches on the meter to get a better measurement.

[packagebear]

on the good one

1+4 .209
5+8 2.053


on the "bad" one

1+4 .245
5+8 2.171


these measurements were taken with the gain @ 6 and the trim CW.

thanks
kris

[tpryan]

OK, now we're getting somewhere. What you're telling us is that the input transformer is OK and the gain error is probably present at all switch settings. If you're absolutely sure that R15 and R16 are correct, check C7, R20 and R21. Also, what happens if you swap op-amps?

[packagebear]

I double checked R15 and R16 as well as C7, R21 and R20 *they're all the correct values.

I swapped the op amps and took voltage measurements on pins 2+3 on the output xlrs. AT gain 6, trim CW w/ a 1KHZ test tone. .

I got .830 VAC on the "bad module" and 2.727 VAC on the "good module" this is with the opamps swapped.


Anything else I should check out?

thanks again
Kris