HP 334A calibration problem

[mpegjohn]

Hi,
Having replaced the meter movement on an HP-334A distortion analyzer and changed all the electrolytics in both the PSU and the A2 PCB I decided to do some rough calibration, to see if I could get the thing in a reasonable state.
I managed to do the checks on the voltmeter, even on the 300uV range. I was so happy.
Now I moved to the Sensitivity switch calibration. section 5-30 in the manual.

I set my 334A sensitivity controls as stated to position 5 and the voltmeter to 10mV.
Set my oscillator to 400Hz and adjusted the amplitude until the meter read 9mV. The actual input is around 34mV according to my DVM. (is that to be expected?).
I then changed the frequency to 100KHz and the meter went hard over to full scale. I tried adjusting S2C5 underneath as it says but it does nothing.
I tried 1KHz and the same happens, the meter hits the end stop. Back to 400Hz and all is fine.

I am at a loss as to what to look at. Any help will be much appreciated.

Best regards,
John.

[mpegjohn]

OK,
I have done some more tests on this problem.
If I measure the output of the impedance convertor on a scope and change the input frequency from 400Hz, 1KHz and 100KHz on different sensitivity settings the output from the convertor looks pretty flat across the ranges. So it is not the input attenuator.
So it must be something to do with the actual voltmeter, although I have gone through the cal procedure, so it should be flat.

Any help please!!

Best regards,
John

[factory]

My 333A is a long way off getting repaired, the casing for the panel meter is completely destroyed, so I'm not familiar with using these.

One thing I note from the manual, is the selected parts list, one of these is A2C30, hopefully you didn't replace that one, the value is selected, also it's a very reliable 150D tantalum type.

David

[mpegjohn]

Originally Posted by factory

My 333A is a long way off getting repaired, the casing for the panel meter is completely destroyed, so I'm not familiar with using these.

One thing I note from the manual, is the selected parts list, one of these is A2C30, hopefully you didn't replace that one, the value is selected, also it's a very reliable 150D tantalum type.

David

Hi David,
I have a spare panel meter, that sort of works now, but is very sticky. Would you be interested?

A3C30 was not fitted on my board. I replaced all electrolytics with pretty much the equivalent 30D types from Mouser. I also replaced the 1.8uF Tant with a similar one, again from Mouser.

John

[factory]

Strange it's not fitted.
Only other thing I can suggest, is to double check everything that was replaced.

This is the panel meter from my 333A, not sure what impact would have caused this much damage without breaking the glass. The movement might be OK, I should scan the scale plate in case it gets worse. I've had to clean the flakes that get stuck in the movement from meters before. I've been looking for some time for a donor with three terminals on the back, nothing else seems to use this type.



I should really check the condition of the photo diodes first, as I read the legs get eaten away from the rotting foam, in another thread. It's an early 1970s one, has all the original caps & mostly uses carbon comp resistors, these were made from the 1960s all the way into the 1980s, they had replaced most of the carbon resistors with metal film by then.



David

[mpegjohn]

Hi all,
Happy days, I figured it out.
Don't do this calibration with the high pass filter switched in.

My word that took some diagnosing. I left the xthing for a couple of days and then a light bulb moment happened.

It doesn't say that in the manual, but that is what it was.
I have managed to calibrate the whole thing now.

I have also replaced all the electrolytics/ Tanty's on the A6 PCB. So only one PCB left to do now.

John

[factory]

Good to hear you've figured it out, I would not have guessed that, I've had another look at the manual and there is this note of some settings to use (unless a test specifies otherwise), it's repeated just before the adjustment section.



A note of the differences between the various models, the 331/2A doesn't have the high pass filter and my 333A doesn't have the AM detector.



David

[mpegjohn]

Originally Posted by factory

Good to hear you've figured it out, I would not have guessed that, I've had another look at the manual and there is this note of some settings to use (unless a test specifies otherwise), it's repeated just before the adjustment section.

Attachment 295305

A note of the differences between the various models, the 331/2A doesn't have the high pass filter and my 333A doesn't have the AM detector.

Attachment 295306

David

Hi David,
Yes, "always read the manual". I never do, waste half my time because of that!!
It should teach me a lesson, but it won't.

John

[Diabolical Artificer]

Have you checked the LDR's & lamps in the auto circuit? The foam in the box turns to mush & eats into the legs of the lamps.

Andy.

[factory]

He might have done that, there was a mention of the lamps, in the recent LDR wanted post.

David