Low Gain?

[bradb]

I recently finished my 3rd A12, used it in a session yesterday and found that the gain is much much less than my other 2 A12s.

Its not the opamp, I've swapped them between good and bad channels and found that its the card itself.

I've checked my input resistor network, R15 = 10k, the input switch for solder bridges. I did find that the resistors R2 - R6 measured a bit differently between good and bad. Bias is at +/- 18V. Input impedance (Between Pins 2 & 3) on bad and good A12s is roughly 32 Ohms.

When I measured at the empty pins at C2, on the bad A12 I got 2.7k, on both of the good A12s, I got 3.02k . Close, but could be something...

Any other tests to suggest?

Could I have a bad input tranny?

I noticed that the older a12 kits had the output transformer wiring that was sequentially matched to the PCB, the transformer in the new kit isn't sequential, I still just matched up colors....

thanks

[bradb]

I found tim's advice from a post:

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.

My "bad" A12 exhibits these measurements exactly, no problems there!


so... hmmm whats next to check?

[tpryan]

With R16 fully clockwise, what resistance do you measure between U1, pin 6 and U1, pin 2? Are you sure that R1 is 97R6 and not 97K6?

[bradb]

Hi Tim,

R16 Full CW, between U1 pins 2-6: resistance fluctuated between 6k and 20.7k. it rises to 20k and then drops down to 6k, over and over.

This same action happens on my "good" A12.


R1 =9K73 on both good and bad A12s.... did you make a typo? or am i all screwed up. I can measure these exactly becase the cap is in series with it.

[bradb]

Here are some more data points,

On all 3 cards, I have: Opamps out, R16 Max CW and SW2 Max CW.:

1.
On both "good" A12s, output resistance between pins 2 and 3 is 17 Ohms.

on the "bad" A12, I get 35 Ohms. This transformer looks different than the other, older ones. BUT the part number matches the BOM, CMOQ-2S.

2.
On both "good" A12s, between brown and red on the output transformer, I get 9.2 ohms

on the "bad" A12, I get 18.6 ohms between red and brown

3.
On both "good" A12s, between grn and blu, I get 9.1 Ohms

on the "bad" A12 between grn and blu I get 18 Ohms

4.
Now here is something really crazy, I'm measuring at J6, where I have nothing connected on ANY of my A12s..

On my "good" A12s, I get 9.1 Ohms

On my "bad" I get 18.6 ohms.

I've triple checked my output wiring, its correct. I just checked it again.

As i mentioned the transformer leads aren't matched to the screen printing anymore, could cinemag have changed their wire labeling?


EDIT:

I just unsoldered the output transformer on the "Bad" A12, between all pairs, Gray Vio, Blue Green, Yel Orange, Red Brown, I get about 18.X Ohms.

[bradb]

Any ideas guys? Tim?