GQ preamp query.......

[Anonymous]

Hi,
Not being very familiar with op amps, I'm a little confused about what's going on with my Goldquest. I've been fiddling with what I believe to be the DC offset trimmer on the preamp +ve input. When I ground the -ve input, and take a squiz at the output while I rotate the trimmer through the full range, there's a point where a sawtooth appears, increases in amplitude, builds to the point of clipping, momentarily squares off and then disappears again.
It has a frequency of 50kHz, so I think it must be related to the master clock generator. I tried decoupling this with a monolithic, but it was still present.
I suspect this signal will also be appearing on the -ve input so the CMR of the amp should take care of it. If my assumption is correct, should I adjust the trimmer for maximum amplitude of this signal? At this point there is still plenty of head, and foot room, also performance doesn't appear to be affected either way.
Any explanations humbly accepted.
Cheers Kev.

 

[Anonymous]

Hi Kev,
I'm afraid you cannot ground the -ve input of the opamp otherwise it loses the feedback that determines the gain and becomes a highly sensitive comparator. A few microvolts will cause it to flip from one state to the other and the offset trimmer will do exactly what you describe. You have to set the offset trimmer without altering anything else, with the coil connected and TX running. Set to 0V baseline after the +ve and -ve transmitter transients have died away.
Eric.

 

[Anonymous]

Thanks Eric,
I really stuffed that one up, it's so obvious, why didn't I see it. Certainly a little knowledge is a dangerous thing.
You must laugh sometimes at the silly things people like me do <img src="/metal/html/biggrin.gif" border=0 width=15 height=15 alt="">
Cheers
Kev.

 

[Anonymous]

Hi Kev,
We all learn by making mistakes. I'm certainly still learning that way.
By the way, the offset trimmer goes to pin 1, which is further down the opamp circuit than the input. This is a throwback from the previous opamp which was a 709. Works OK though.
Eric.