Need Guru to look at 5534 scope shot,something is wrong.

[Brian Deese]

HI Brian,

After the solder bridge was removed was there any change in the - supply voltage? If anything were damaged it would most likely be Q3. If you apply some extra load to the supply does it compensate to keep the correct/same voltage? If the supply voltage is low this could be causing a problem with the tx transistors not turning off properly perhaps.

If you have the original coil, then it is not likely there will be a problem there.

Cheers Mick

No change in the voltages after it was removed,it looked old and I dont think it was from me.I figure the damage is done so it does not suprise me that the voltage didnt change.The tx transistors are pnp devices and their bases are pulled up to VC the pos supply voltage which is 3.48V and I believe this is the correct voltage. Which brings me to another point I should have mentioned this machine runs on 6 volts.It used 4 D cell batteries originally.I run it on AAs,and it will even run on a 9v.So to demonstrate earlier today my batt pack measured 5.7 volts and the output of the 7661 was -4.7V so you couldnt have a 5v supply on this thing,after the losses in the inverter chip etc..I know the answer is in the voltage divider connected to the non inverting input of U1A I am just to stupid to figure out what the neg supply voltage is supposed to be.

[Mechanic]

HI Brian,

After the solder bridge was removed was there any change in the - supply voltage? If anything were damaged it would most likely be Q3. If you apply some extra load to the supply does it compensate to keep the correct/same voltage? If the supply voltage is low this could be causing a problem with the tx transistors not turning off properly perhaps.

If you have the original coil, then it is not likely there will be a problem there.

Cheers Mick

[Brian Deese]

Hi Brian,

Do you have an original or home made coil on the machine?? Looks like it is under damped, but you already have a lower value damping resistor than original so decreasing the value of the damping resistor should correct the trace, but it will make it take longer for the coil to settle and will also degrade performance. I think you have a capacitance problem with the coil, the C is too high.

Also some diodes in the feedback loop of the opamp would not go astray to prevent the opamp from going into saturation. This should speed up the opamp settling time.

I do find it odd that the supplies are not matched for + and - too, perhaps Carl could shed some light on this.

Cheers Mick

Its the orig coil,may be it has too much C.I think the diodes in question limit the base voltage to safe levels and ivconic wanted to be sure we didnt have an injured transistor.Yes I am getting very suspicious of the low neg supply.Earlier today as I was cleaning and inspecting the board I believe I found a solder bridge.I say I believe it was a bridge because it took the solder mask off the trace it was stuck to when I scraped it off.This bridge would have bypassed R9 which is the base resistor for Q3 which is the transistor for the neg supply regulator.So if R9 were a short/jumper what would be damaged the lm2904 or Q3 or both?

[Mechanic]

Hi Brian,

Do you have an original or home made coil on the machine?? Looks like it is under damped, but you already have a lower value damping resistor than original so decreasing the value of the damping resistor should correct the trace, but it will make it take longer for the coil to settle and will also degrade performance. I think you have a capacitance problem with the coil, the C is too high.

Also some diodes in the feedback loop of the opamp would not go astray to prevent the opamp from going into saturation. This should speed up the opamp settling time.

I do find it odd that the supplies are not matched for + and - too, perhaps Carl could shed some light on this.

Cheers Mick

[Brian Deese]

That's why i suspected diodes and base "biasing" voltage. It is "fast" PI machine yet with bipolar transistor and not with fet. It is much more difficult to control bipolar transistor in such conditions, so any possible variations at base voltage would cause also something like "ringing" and indirectly will affect rx too.
I had similar issues long time ago with such "bipolar" design.
You actually don't need to resolder diodes to examine them; just measure voltage across them. (that's why i gave you reference 6.75V to start with)

I dont feel like I did what you told me to do,but I checked the diode with a fluke multimeter on diode test and here is the result.D16-.215v D15-.607v D12-.609v D13-.210v. should I apply more than 6.75V manually to the base and see if the diode is shunting properly? I looked at the bases on both transistors and they are at 3.48V (VC in the schematic)normally, and they get a negative going pulse to turn on,both of them look the same.Both of them have some noise on the line when the other is switched on,the noise is the highest peak and it does not go above 5Volts.

[Brian Deese]

Ah, so you actually measure the opamp feedback compensated input, and not the voltage on the coil. OK, that makes sense. Trouble with such measurement is that you actually look at the place where a voltage should be different from zero only in times the opamp is saturated, and such measurements will lead you nowhere. This is very different from non-inverting opamp stages where input follows the applied voltage. The inverting one fights back and tries hard to keep its input at zero. What you observe is just the way the opamp recovers after saturation, and it is perfectly OK the way you see it.

I have a test pin for the coil before the diodes also,I have measured it.I think it is in the first pictures I posted.It looks as it should in my opinion.

[Brian Deese]

That's why i suspected diodes and base "biasing" voltage. It is "fast" PI machine yet with bipolar transistor and not with fet. It is much more difficult to control bipolar transistor in such conditions, so any possible variations at base voltage would cause also something like "ringing" and indirectly will affect rx too.
I had similar issues long time ago with such "bipolar" design.
You actually don't need to resolder diodes to examine them; just measure voltage across them. (that's why i gave you reference 6.75V to start with)

I am in for the night,I will get right on that. Should I have a look at the base of the buy92's with the scope? or no?

[ivconic]

What i ment: no wonder White's did it that way.
They do know how to "milk" the juice...

[ivconic]

"...BJTs may need some more juice to drive..."

That's the spirit!

[Davor]

It is "fast" PI machine yet with bipolar transistor and not with fet.

IMHO sexy MOSFETs just make it worse. BJTs may need some more juice to drive, but they do not introduce much capacity to the deal, like MOSFETs do.

A reality check: business part of the PI signal is a beginning of the off time, and that calls for high R. MOSFETS bring low R to the table ... not very useful, and it comes with high C attached to it.
Lower R -> higher C.
Low R unimportant, high C problematic -> to hell with low R MOSFETs.

[Davor]

The yellow trace is the input to the 5534,I have the scope probe hooked on one of the overvoltage protection diodes.

Ah, so you actually measure the opamp feedback compensated input, and not the voltage on the coil. OK, that makes sense. Trouble with such measurement is that you actually look at the place where a voltage should be different from zero only in times the opamp is saturated, and such measurements will lead you nowhere. This is very different from non-inverting opamp stages where input follows the applied voltage. The inverting one fights back and tries hard to keep its input at zero. What you observe is just the way the opamp recovers after saturation, and it is perfectly OK the way you see it.

[ivconic]

I have to pull those damping resistors back out,I know it. And if you look at the above picture the yellow trace alone you can see a tiny peak cross zero.But the red trace doesnt seem to follow it in time is my complaint. I am having trouble swallowing it because the coils only measure 185uH and the tx pulse is only 50uS,its a low power machine,and it already has 330r damping resistors.I just dont see how its ringing. But something is not right obviously.

That's why i suspected diodes and base "biasing" voltage. It is "fast" PI machine yet with bipolar transistor and not with fet. It is much more difficult to control bipolar transistor in such conditions, so any possible variations at base voltage would cause also something like "ringing" and indirectly will affect rx too.
I had similar issues long time ago with such "bipolar" design.
You actually don't need to resolder diodes to examine them; just measure voltage across them. (that's why i gave you reference 6.75V to start with)

[Brian Deese]

I see nothing more than you do. But Carl's suggestion made me think that something is not "in range" there:

Carl:
"...Most likely the coil is very slightly underdamped, not enough to see it on the scope but enough to make the preamp flip. Slightly reduce the damping R by adding a resistor in parallel (perhaps 10k) and see if it goes away..."

I have to pull those damping resistors back out,I know it. And if you look at the above picture the yellow trace alone you can see a tiny peak cross zero.But the red trace doesnt seem to follow it in time is my complaint. I am having trouble swallowing it because the coils only measure 185uH and the tx pulse is only 50uS,its a low power machine,and it already has 330r damping resistors.I just dont see how its ringing. But something is not right obviously.

[Brian Deese]

OK, but how do you obtain the yellow trace? There is obviously some voltage divider in action. What kind of voltage divider is there?

Whatever you have, you'll find some kind of AC coupling.

I already like that rig. It is mostly the way I'd make a bipolar pulsed PI myself - knowing what I learned so far. With inverting opamp in a frontend and proper transistors for drives.

The yellow trace is the input to the 5534,I have the scope probe hooked on one of the overvoltage protection diodes.You mean because the yellow trace is small in amplitude? The overvoltage diodes limit the peaks to .7 volts, thats standard issue frontend correct? The red trace is the output and there is a test pin to hook the scope tp8. I was going to offer you the schematic when I saw some of your posts' but It seems you have your design figured out.I cansend the full size schematic to you,or its posted in the schematics section also.

[Davor]

Both of the scope probes are DC coupled,if thats what you mean.I only use AC coupling if I want to see ripple on the trace.

OK, but how do you obtain the yellow trace? There is obviously some voltage divider in action. What kind of voltage divider is there?

Whatever you have, you'll find some kind of AC coupling.

I already like that rig. It is mostly the way I'd make a bipolar pulsed PI myself - knowing what I learned so far. With inverting opamp in a frontend and proper transistors for drives.