TP4 and TP9 won't stabilize

[BobCat]

<P>Hi All </P>
<P>Finished HH option 1 everything works well except TP4 swings 1 to 2 MV. and TP9 swings about 150 MV. First thing I did was unhook the coil no change then removed R5 same thing then removed IC5 and it stopped.</P>
<P>Then I did a double check of all parts and placement&nbsp;all looks good&nbsp;and swapped out IC5 IC7 IC11 and IC12 still dose the same thing. Next I hooked the coil back up and removed Q8 and Q9 TP4 and TP9 settled right down. So I replaced Q8 and Q9 with new parts with no luck.</P>
<P>The board was assembled with standard parts from the parts list.</P>
<P>Any one have an idea of what I did wrong missed or&nbsp;can try. </P>
<P>Thanks Bob</P>
<P>&nbsp;</P>

[Reg]

Hi Bobcat,

It isn't that abnormal to have quite a bit of noise even with the coil removed, especially if you are testing and looking at signals while in doors.

Now, there are different reasons you may have extra noise and a couple of items are, a noisy FET if you are set up to operate a mono coil.

The NE5534 could be noisier than normal also.

One thing you can do if you have one of the newer boards that has the jumper between TP 2 and R12 is to remove this jumper and then jumper R12 to ground on the end that goes to the jumper. This will isolate the FET as a source of the noise.

If the noise is still there, then try adding a jumper across R 13, the 1 meg resistor. This should basically eliminate the noise since it makes the gain off the preamp basically nothing. It will, however, verify the noise is coming from the preamp stage.

High value resistors such as R 13 and R 16 are both noise generators and antennas themselves. As such, they can pick up noise much like what is done by the coil.

If all your testing is done on a bench and not in the field, then I wouldn't worry at this point. If the problem is simply stray noise being picked up then this should reduce signficantly once the pc board is in a box and outside. BTW, I do recommend one shields the box at least the top and bottom to reduce noise getting in.

Try it outside and see if things get better.

Reg

[BobCat]

Hi Reg thank you for the replay
I unhooked R12 and ran a jumper to ground the noise stayed then ran a jumper across R13 and it quit. I did make some changes that did help dropped the gain on IC5 changed R13 to 220K and R12 to 270 ohms next I installed R22 150 ohms and C16 .1UF the low pass filter. The other change and I'm not sure that did any thing was to change R17 and R19 to 330K and R16 to 470K and recalibrate IC5. At this point TP4 holds real stable along with TP5 TP9 was still jumping around 70 MV.
The last of the noise was coming from IC8b more to the point the auto track switch.
Well when I finished the board the only switch I had on hand at the time has an LED inside of it after replacing the switch with a jumper the noise went with it. Outside away from the house I can adjust R31 from a slow steady click to a full scream almost the same as when R13 was unhooked.
Thanks again for the help.

Bobcat

[rajesh]

HH

hi BobCat which in these used c22 .1 uf or .47 uf .......rajesh

[BobCat]

Hi rajesh c22 is .1uf
Bobcat