Getting a faint signal at full gain, but dirty

[Dr Bockman]

Well, I finally got my A12 all together. It tested out alright and so I have fired it up. I get a faint signal when I put the gain all the way up (input and output) but it is quite noisy as you might expect. Does anyone know where I could begin troubleshooting this? Does it need to warm up?

[Dr Bockman]

I should also mention that I had a problem getting the silver ground screw out of the bottom of the chassis, the one that you connect the ground lead from the PS02 to. I had to use pliers to get it out because it was stripped for some reason. I put it back in through the top since it wouldn't go in all the way from the bottom (I now wish I had just gone and gotten a new one). Does my previuosly stated problem sound like it could be poor grounding?

[tele_player]

doesn't sound like ground, since the preamp modules all have full power and ground connections to the power supply, and the power supply doesn't need the chassis ground to operate.

[Dr Bockman]

I still haven't fixed my problem with the faint signal on the A12 (but I got my N72 going successfully!!). I was wondering if it is necessary to have The jumper pins in J1 and J2 if I will not be using them. Also, are there any other tests I can run, such as resistance between various points, or voltage checks on transistors?

Thanks
Wil

[tele_player]

Check out the thread -
"Which jumpers with stock A12"
on this forum.

[Dr Bockman]

Well, I haven't responded to this post from mine earlier, but I will simply clarify the question I had asked.
When I was building the A12, I either lost or was short two jumper pins. Not the jumper itself, but the jumper pins that are actually soldered into the board. I realized that I wouldn't be needing to Jump J1 or J2 so I did not attempt to find 2 more, and it seemed from looking at the traces that they weren't required to be in the board if they weren't being used. Now, assuming that I have made sense here, are the pins required to be soldered in the board at J1 and J2 to make the A12 work right?

[tele_player]

If the jumpers won't be installed, the pins (aka the header) doesn't need to be installed.

[tpryan]

The only jumpers you need with the stock A12 are J3, J4, and J5. It's unlikely that the symptom you're describing has anything to do with jumpers, unless you're testing with a phantom powered mic. If the noise increases with gain, it's a good indication that the amplifier is amplifying. Check your work carefully around R2-R6, make absolutely sure you installed the right resistors there. Are you sure the input transformer is installed the right way round? Let us know what you find.

[Dr Bockman]

Well, I had a friend try and fix this problem of mine, but he couldn't really figure it out. I even plugged it in again the other day, and still the same issue. When you turn it up all the way up, you start to get a little noise like you would expect it to, but no signal.

I remember reading something about making sure there is a solder bead on either side of the board for certain components (ie ones with a trace being used on either side, I'm assuming). There are a few components where the solder has a good bead on the back side of the board but maybe doesn't quite peak its head through to the other. Any chance this could be my problem? If so, which components could this affect? Is it safe to try and melt a little solder on from the top side (the side that has the silk screen)?

[craigmorris74]

This may be totally of no help whatsoever, but when I was building my N72 kit, I had the same problem, and it turned out to be an incorrectly wired input transformer. Double check it and it's soldering.

[Dr Bockman]

Well, I gave in. I wrote the reply to my endless post and I just went ahead and sent it in today before even waiting to hear back. My tech friend who was helping me troubleshoot actually removed the input transformer to test it and said that it was ok. Watch. Watch. It will be the input transformer and I will regret not having looked twice at this one last thing. I am hoping that it is something really simple. But, also, something that will not make me feel like a poopy pile.