Help

[KingJL]

I have noise on the output of the pre-amp when R37 is connected to sync IC1. I have rechecked all components, solder connections, etc. I am pulling my hair out. Is it possible that it is the 7660 itself? I hate to pull it since this is a SMT board. I would like to do that as a last resort (Not only that, but I presently have no spare SOIC version). When R37 is connected there is a drop on IC2 pin 3 from 8.73 to 7.82. The output of IC1 pin 2 is no longer nice and square. There is a rounding of the leading edge. With R37 disconnected, the effect does not seem to be there at the same level. It could still be affecting but at a freq that is no longer syncronized. The does appear to be a small squarewave signal at the charge pump freq on the pre-amp output with R37 disconnected, but the amplitude is much lower (square wave signal < 10mv).

Settings:
Freq set to 800pps, PW set at 40. (Increasiing freq reduces slightly the waveform noise at the pre-amp.)
Battery Voltage: 10.5
Coil not connected R5 not installed.

Scope settings:
Horizontal sweep set at .5ms/Div
Vert set at 50mv/Div (pre-amp output); 2V/Div (ic1-pin2)

See pics in attached zip.

I would really like to solve this!

Thanks in advance, all help welcomed.


See pics.

[Carl-NC]

As I recall, the 7660 sync is rather sensitive to the values of the series R & C on the sync pin, so double-check your parts.

[KingJL]

Pre-amp w/R37 out

Here is a picture of the pre-amp out w/o R37 sync to 7660. Scope same settings (50 mv/Div, .5 ms/Div).

Carl,
Double checked parts R37-10k, c27-1000pf. With the other board, everything is OK.

[1843]

Hi KingJL,

When you sync the 7660, its frequency becomes too low.
To solving the noise problem, you should use large capacitors (eg 470uF) instead of C3 and C4.
Try this, I'm sure the your problem will solve.

Regards,
1843

[Reg]

I suspect 8143 is correct.

Take a look at the battery voltage with the scope to see if you have a lot of noise on it. If you do, you may need to clean that up also by using a better battery source.

Then take a look at the +5V out to see if it is nice an solid 5 volts with no droops or other quirks.

I suspect you will see one or both of these not looking right and looking much like what you see at the output of the preamp.

The 7660 is there to create the +5V. Syncing this chip does slow it down which will have an impact on the +5V output.

You can use larger capacitors as 1843 suggested or use better ones such as use tantalums. They are a little more expensive but can be worth it.

Reg

[KingJL]

follow-up

Carl, 1843, Reg,

Thanks for the feedback. I have checked and rechecked all parts and connections. I have one circuit that operates normally (no/minimum noise) and one that shows the varition that occurs every other clock cycle. This also shows up on the +5 line (< 1 mv). The battery line is clean.

The noise/distortion generated at the pre-amp is negligable with the sync line (R37) disconnected. I am going to finish the build and see if there are any problems running non-synced. If so then I will re-visit the pump circuit. I originally suspected C3 and had replaced it. The next step (if needed) will be to increase C3,C4,C5.

Thanks all. I wanted your input to see if I had overlooked anything.