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

[Brian Deese]

I was having a look at an old pi machine I have that doesnt work very good,using my new scope I saw what looks like ringing on the preamp output.This machine has 330 ohm damping resistor so I didnt think it was ringing.I check the coil signal before the limiting diodes and it looks good,no ringing.I check at the peamp input and it looks good smooth tx pulse,flyback,and tailout to zero volts.But the output of the preamp shows tx pulse,flyback,cross zero opposite polarity,then tailout to zero.Let me post a picture for you.The yellow is the input,the red is the output,second pic yellow is coil signal before diodes,no ringing.

[Brian Deese]

Yeah, thats much better,I resized the pictures.sorry for the medium quality,I use a good camera next time.What the hell is up with this amp?

[bklein]

How about you just try putting a trimpot for the feedback resistor and you may get a better understanding of what is happening.

[Carl-NC]

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.

[Brian Deese]

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.

Ok I put 10k resistor across the existing 330r resistor(in parallel) and the results are-the negative going pulse in question gets slightly wider about 1uS and the curve gets a little longer,very slight but if you touch the 10k intermittently you can see it change.The schematic calls for 390r and I am using 330r because I didnt have 390r.

[porkluvr]

Does your design call for maybe a 3pf (or more) feedback capacitance across the 5534 i/o, or maybe 3pF (or more) across pins 5~8?

You've thrown us a bone, what about showing us the whole front end?

[Brian Deese]

Does your design call for maybe a 3pf (or more) feedback capacitance across the 5534 i/o, or maybe 3pF (or more) across pins 5~8?

You've thrown us a bone, what about showing us the whole front end?

Ok, here it is.

[Brian Deese]

Is it normal for the positive and negative supplies to be different levels,last night I measured vc at 3.48v and ve at 2.82v if I recall.The other machines I have use 5v+/- supplies.If you change the hor division on the scope to see 3 cycles you can see the difference in the output of the amp.

[ivconic]

Have you checked D12&D13 and D15&D16?

[porkluvr]

Is it normal for the positive and negative supplies to be different levels,last night I measured vc at 3.48v and ve at 2.82v if I recall.The other machines I have use 5v+/- supplies.If you change the hor division on the scope to see 3 cycles you can see the difference in the output of the amp.

That may be another problem.

It's impossible to determine the timebase and amplitude/division from your photo.

I was going to tell you that your scope shot really looked OK to me considering the high gain; slow recovery would be normal... but then I can't understand why your output would be FOLLOWING the input instead of inverting it and being a mirror image. So maybe I just better shut up instead.

[bklein]

Yeah what he said...
Looks like you have dynamic damping on the coil.
Is this a homemade implementation or a commercial one? (fixing or making?)
Remove the damping portions (upper/lower) temporarily (lift resistors or diodes). See if you get what you'd expect.
Make sure offset is correct (play with it, or center with no input signal).
Make sure zero V on one side of schematic is connected to the zero V on the other.
+/- voltage seems very low - do you have that schematic? I'd expect higher.
Maybe something elsewhere is loading down your supplies.

[Brian Deese]

That may be another problem.

It's impossible to determine the timebase and amplitude/division from your photo.

I was going to tell you that your scope shot really looked OK to me considering the high gain; slow recovery would be normal... but then I can't understand why your output would be FOLLOWING the input instead of inverting it and being a mirror image. So maybe I just better shut up instead.

I can clear that up for you,I inverted the red channel on the scope because it was easier for me to see.I didnt tell you the coil was connected to the inverting input on the amp so I didnt think it would matter.I better get some better pics while I have your guys attention,I really want to get some service out of this machine.The thing uses linear voltage regs made up of lm2904 as a comparator(I think) and discreet transistors.I have not posted the whole schematic because I was trying to keep it simple,the schematic is large and over complicated for me anyway, and I have to scratch my head when I look at it.

[Brian Deese]

Yeah what he said...
Looks like you have dynamic damping on the coil.
Is this a homemade implementation or a commercial one? (fixing or making?)
Remove the damping portions (upper/lower) temporarily (lift resistors or diodes). See if you get what you'd expect.
Make sure offset is correct (play with it, or center with no input signal).
Make sure zero V on one side of schematic is connected to the zero V on the other.
+/- voltage seems very low - do you have that schematic? I'd expect higher.
Maybe something elsewhere is loading down your supplies.

I dont know what dynamic damping is bklein
It is a whites af-108 and I am trying to repair it.
I have centered the offset.
I have played with the offset in the past,I have got more sens with it offset from zero,but I dont remember the value.As of now its set at zero though.
I do have the schematic,I will post it.It uses linear regs made of discreet components.

[Brian Deese]

Have you checked D12&D13 and D15&D16?

No I have not,can I test them in circuit?

[Brian Deese]

Here are some better screen shots,of three cycles,and one up close. I think you can see the vert/hor divisions in the pics.Yellow is input to amp and red is output. Notice the difference in the pos and neg output of the amp due to the lower VE than VC. No inverted channels this time,enjoy.