TGSL Tuning & troubleshooting

[simonbaker]

removed 358 from socket
pin 1 -0.01V
pin 3 -5.99V
pin 5 -5.96V
pin 7 0.0

removed 393
on 393 pin 7 0V
pin 4 -6.22V
pin 8 +8V

I have traced -rail all over ... had some confusion then removed connection with TX coil ... seems that nothing is connected with - that shouldn't be ... no shorts as I can see

will post pictures od PCB now

Thanks, that tells us something -- problem is not a bad LM358 (at least not entirely).

I am wondering if your JFets TR4 and TR5 have wrong pin assignments. I think there is a difference between European and US parts, not sure. In any case, it is worth carefully checking which pins of your JFets are the S, D, and G leads, and making sure installed correctly. If improper, they could have been damaged, maybe, or at least could be passing the signal from LM393 pin 7 to capacitor C12 and charging it negative through a diode. If in ALL METAL mode, same could happen to C15.

1. Another quick test -- switch to DISC mode and check DC voltages again, and see if U103 (LM35 pin 3 is still -6V, or if it is near zero. Might as well keep LM358 out of socket for test.

Regards,

-SB

[sdesic]

There is something else we need to clarify -- whether you have a fixed DC -6 volts at the points you measured, or if there is an AC signal present.

1. Would you display again the voltage at LM393 pin 7 with your oscilloscope? And confirm the DC voltage level?

2. When you measure voltages with your voltmeter, are you using DC scale or AC scale?

3. What do you use for your ground point when measuring voltages and using o-scope probes?

It is hard to make sense of all these measurements right now -- some contradictions -- so we need to double-check these things.

Regards,-SB

I will do this later today

[sdesic]

In early part of thread you said the Tx signal was

" TX voltage on fist second is 4.7V AC ... tried on two voltmeters to be shure "

This sounds small to me

Could you post frequency of Tx signal and Peak to peak voltage.

S

I don't have freq. meter and my scope does not measure accurately (not calibrated as it is audio card scope)

[sdesic]

Earlier you said..



>3. Capacitor (C6 on some schematics) across the RX coil.
was 15nF as schematics ... replaced with 1nF and it is now 10nF (103 ceramic)


Try a Polyester cap, Ceramic isnt popular a popular coice for this location

I have replaced that ceramic cap with original one 15nF it was there only for testing

[sdesic]

One other thing -- if those DC voltages are as you say, then I would think you probably would not be able to hear any targets detected at all. Can you hear targets if pass over coils?

-SB

I have beeps that cover that sound and I don't know if sound is from detecting or not..
It is probably bad 358 IC... as it got worse than before
I have uploaded 20MB of pictures (8 of them) to wetransfer here is link to it
original size pictures (will be there for 15 days from now)
Download link

http://we.tl/ZFTHRihgAf

[sdesic]

Excellent work -- Your traces of signals of TX and LF353 pin 7 look OK -- pretty standard.

It now seems that the problem must be some connection to your negative rail somewhere that is causing these bad voltages at the LM358 inputs.

1. Try removing the LM358 chip (if in a socket), and test those DC voltages at the same pins again. Then try removing the LM393 chip also (if in a socket) and test voltages.

2. If still get -6V on those pins, probably need to check your traces for solder bridge or something like that.

3. Have you posted good photos of top and bottom of your PCB? If not, can you do that?

-SB

removed 358 from socket
pin 1 -0.01V
pin 3 -5.99V
pin 5 -5.96V
pin 7 0.0

removed 393
on 393 pin 7 0V
pin 4 -6.22V
pin 8 +8V

I have traced -rail all over ... had some confusion then removed connection with TX coil ... seems that nothing is connected with - that shouldn't be ... no shorts as I can see

will post pictures od PCB now

[simonbaker]

There is something else we need to clarify -- whether you have a fixed DC -6 volts at the points you measured, or if there is an AC signal present.

1. Would you display again the voltage at LM393 pin 7 with your oscilloscope? And confirm the DC voltage level?

2. When you measure voltages with your voltmeter, are you using DC scale or AC scale?

3. What do you use for your ground point when measuring voltages and using o-scope probes?

It is hard to make sense of all these measurements right now -- some contradictions -- so we need to double-check these things.

Regards,-SB

[golfnut]

Earlier you said..



>3. Capacitor (C6 on some schematics) across the RX coil.
was 15nF as schematics ... replaced with 1nF and it is now 10nF (103 ceramic)


Try a Polyester cap, Ceramic isnt popular a popular coice for this location

[golfnut]

In early part of thread you said the Tx signal was

" TX voltage on fist second is 4.7V AC ... tried on two voltmeters to be shure "

This sounds small to me

Could you post frequency of Tx signal and Peak to peak voltage.

S

[simonbaker]

TX signal (J1-1) and RX amplified signal (LF353 pin 7)
on 393 pin 7 -6
on 358 U103 pin 1 -6.1V
pin 3 -5.99V
pin 5 -5.96V
pin 7 -6.10

voltages are changing from -5.95 to -6.11

I have nulled coils on pin 7 of 353 ... about 60-70mV AC

One other thing -- if those DC voltages are as you say, then I would think you probably would not be able to hear any targets detected at all. Can you hear targets if pass over coils?

-SB

[simonbaker]

TX signal (J1-1) and RX amplified signal (LF353 pin 7)
on 393 pin 7 -6
on 358 U103 pin 1 -6.1V
pin 3 -5.99V
pin 5 -5.96V
pin 7 -6.10

voltages are changing from -5.95 to -6.11

I have nulled coils on pin 7 of 353 ... about 60-70mV AC

Excellent work -- Your traces of signals of TX and LF353 pin 7 look OK -- pretty standard.

It now seems that the problem must be some connection to your negative rail somewhere that is causing these bad voltages at the LM358 inputs.

1. Try removing the LM358 chip (if in a socket), and test those DC voltages at the same pins again. Then try removing the LM393 chip also (if in a socket) and test voltages.

2. If still get -6V on those pins, probably need to check your traces for solder bridge or something like that.

3. Have you posted good photos of top and bottom of your PCB? If not, can you do that?

-SB

[sdesic]

TX signal (J1-1) and RX amplified signal (LF353 pin 7)


on 393 pin 7 -6
on 358 U103 pin 1 -6.1V
pin 3 -5.99V
pin 5 -5.96V
pin 7 -6.10

voltages are changing from -5.95 to -6.11

I have nulled coils on pin 7 of 353 ... about 60-70mV AC

[simonbaker]

>When making a circuit change, keep checking those voltages that were wrong so we make sure you fix the problem.
OK, removed again C12 220nF and now measuring
on 393 U102 pin 7 -3V
on 358 U103 pin 1 is now -6V
pin 3 is now -6V
pin 5 is now -2.7V
pin 7 is now -2.7V

>I would check C12 for short circuit. Then look for solder bridges. Then suspect bad chip or part.
replaced with another but I have that -6V on pin7 I removed it ...

>I don't think a bad C11 (15pF) would drag down TR5 or the LM393 pin 7 to -6V.
OK

it is not C12 that is short circuit

First we need to see your TX signal (J1-1) and RX amplified signal (LF353 pin 7) on your oscilloscope as two traces with exact same channel settings.

It is still possible that the large voltages are normal for the RX signal you have -- if you are quite far from minimum null.

Otherwise, we will look for a short or bad part somewhere.

-SB

[simonbaker]

I could swap IC 393 and IC 358 with another on board so we know are they or not bad

Good idea -- are they in sockets?

-SB

[sdesic]

I could swap IC 393 and IC 358 with another on board so we know are they or not bad