HP 8903A, A5 Oscillator Board Distortion

[gdnichols31]

This is a continuation of the topic linked below. Since I repaired the CPU board I thought I would start another topic focused on the oscillator problem.

https://www.eevblog.com/forum/repair/hp-8903a-calling-all-experts-cpu-card-isnt-right/msg3108468/#msg3108468

[gdnichols31]

My HP 8903A has 5% THD on it's output. The frequency is accurate.

See the attachments,

The first shows an oscope shot of the output, doesn't look good.

The second shows a spectrum analysis;

A note about the spectrum pic, it is from the monitor output of my other HP 8903A. The monitor output has the fundamental removed and shows just the noise. I also took 32 RMS samples and averaged them using an HP 3582A.

You can plainly see the odd order harmonics. The marker is on 3,000 Hz.

I'm thinking this implies clipping somewhere in the circuit.

If you have seen this before please let me know.

Gary

[gdnichols31]

I did some probing around tonight.

I started by going thru the HP trouble shooting procedure for the oscillator board.

First I noticed that the output at the first oscillator (first attachment) is 4.70 volts, it is supposed to be 3.00 Volts RMS. The P-P voltage is 12.64V. Looking at the schematic (second Attachment) the zener diodes on the output of the U4 could be clipping the output causing the harmonics. If that were the case it would seem the voltage would be 10.2 Volts instead of the 12.64V.

So I'm thinking something is wrong in the ALC (Automatic Leveling Circuit) somewhere to make the output so high.


I found the first problem at TP3 which is the output of the first sample and hold circuit. The 3rd attachment shows the waveform at TP3. The magenta trace is TP3, the yellow trace is the gate of transistor Q15. The 4th attachment is from the service manual and shows what TP3 SHOULD like like.

See the schematic of the first  sample and hold circuit in the 5th attachment.

This a fairly simple circuit. There's not much that can go wrong. The Op amp, 2 caps, and Q15. I'm not very good and circuit analysis like this but I'm thinking Q15 is either shorted? or open? The op amp is working, I don't see how either of the caps could cause this behavior.

If anyone has any thoughts I would love to hear from you.

Gary

[thinkfat]

What's on the gate of Q15 looks like it's the sample pulse. The waveform on TP3 is undisturbed until the sample pulse shorts Q15 to actually sample the voltage. I'd say something after Q15 is shorting the signal to ground. That would be either the capacitor or the buffer opamp itself.

EDIT: check TP7, it should be a DC voltage at the level of the flat top of TP3. Check C45, it could be shorted or heavily leaking. Also check C46, it could be shorted/leaking as well.

[gdnichols31]

What's on the gate of Q15 looks like it's the sample pulse. The waveform on TP3 is undisturbed until the sample pulse shorts Q15 to actually sample the voltage. I'd say something after Q15 is shorting the signal to ground. That would be either the capacitor or the buffer opamp itself.

EDIT: check TP7, it should be a DC voltage at the level of the flat top of TP3. Check C45, it could be shorted or heavily leaking. Also check C46, it could be shorted/leaking as well.

That's a really good thought. I forgot to mention above that the voltage at TP7 is zero. Also 0 volts at the input of U14 pin 3.

I'll check C45 tonight.

Thanks

[thinkfat]

U14 is a suspect too. For this circuit to work, the input of U14 must not load the capacitor at all or it won't be able to hold the charge (that's the "hold" in sample-and-hold btw. Q15 would be the "sample" part) It will be a JFET input OpAmp or somesuch. There will be no clamping diodes on the input or any other form of protection.

PS: https://www.eevblog.com/forum/projects/op-amp-failure-modes/msg422766/#msg422766

[Runco990]

I have one of these.  Simon Spears did a repair of one with high distortion.  The capacitors on the oscillator board seem to be at fault.
Simon thought that they may absorb moisture.

[gdnichols31]

ThinkFAt You called it! C45 was shorted. 1.4 ohms.

Replaced the cap and all is good.

The first file is osc 1 TP in yellow, the 8903a output in magenta. The osc output is dead on 3.0V, I adjusted it. The output is set to 1V RMS and it's measuring 990mV RMS.

The second file is a screen shot of the spectrum. The harmonics are way down.

The frequency is off a little but I think that is just an adjustment.

Thank you very much for the help!

Now the key board needs attention.

Gary

[Tony_G]

Congrats - Good work.

Here is a link to a Groups.io post on cleaning the keys in an 8903A in case you haven't seen it yet:

https://groups.io/g/HP-Agilent-Keysight-equipment/message/96350

TonyG

[gdnichols31]

Thanks TonyG, I looked at the link.  BTW, keep the videos coming!

Since I have it working now I thought I would clean it up!

See Pic,

Gary

[gdnichols31]

TonyG,

I saw your video on the 8903B. You talked about moving the connections from the back to the front. I didn't see video for it.

Did you ever move them?

I'm thinking about moving the ones on this unit.

Gary

[Tony_G]

I looked at it and from memory, I think there are holes in the chassis for it but you'll need to drill out the faceplate. The cable should be long enough for you to reuse it for the front connections but I didn't really measure it out.

What I did, in the end, was just to add a patch panel to my cabinets and cable from the 8903B to there:



This has worked well enough for me as I primarily use the 8903B in working on signal generator modulation tests.

TonyG

[gdnichols31]

Since the oscillator is repaired and the complete instrument is working I thought i would clean it.

I'm starting another topic for that. See the Link below;

https://www.eevblog.com/forum/repair/hp-8903a-clean-up-and-moving-the-bnc-jacks-to-the-front/