HP 5334A calibration issue -- unbalanced signal - Page 1
Forum section: electronics / repair

author: aarons
author_rank: Regular Contributor
timestamp: July 07, 2025, 03:01:11 am
content: Hi,I was going through the calibration procedure for my HP 5334, and I got hung up on the second step.This step is adjusting the DC offset:The problem is that "0.00 0.00 A" is unattainable. Not because I hit the limits of the potentiometer, but because there appears to be a .2-.3V difference in the measurements between the positive and negative peaks. I can adjust the positive peak to 0.00, but then the negative is -0.2, and negative to 0.00, but positive is then 0.2. This happens on both channels, and also happens to any DC voltage applied to the input. I've checked all the power supply rails, and they are all within spec. The only one that is dead on is the 3V, but that is because it's the only one with an adjustment.Is anyone familiar enough with this instrument to offer some suggestions on what to check? I think the problem is before the DAC in the input amps, but I don't really understand how it is reading these peak levels.

author: wn1fju
author_rank: Frequent Contributor
timestamp: July 07, 2025, 12:51:48 pm
content: My HP 5334A does exactly the same. I can either get 0.02 0.00 or 0.00 -0.02, but never 0.00 0.00. Happens on both channels. The counter seems to work flawlessly otherwise.However, I have a second 5334A and that one attains 0.00 0.00 on both channels. Go figure.The 5334A that "misbehaves" has serial number 2350Axxxx. The one that gets 0.00 0.00 has serial number 2510Axxxx. My guess is that HP tweaked their design in the later version. In fact, the Manual Changes (Section VII) of my service manual has a somewhat mysterious sentence saying,"Values of resistors A1R84 and A1R112 were changed to prevent a false peak voltage reading when using the Read Levels function. Refer to Service Note 5334A-3."I haven't looked at the schematic to see if R84/R112 makes sense for this inability to zero the peak readings.Anybody have Service Note 5334A-3?

author: aarons
author_rank: Regular Contributor
timestamp: July 07, 2025, 05:43:56 pm
content: Mine is between your two at 2426A. I looked at the schematic and this circuit makes my head spin. Section 8-104 says "On the trigger level control voltage line, R112 and C84 form a low-pass filter to keep out digital noise from the DAC Block." However the other side of R112 depends on the state of a couple of different MUXes and the output of a DAC.Anyway, if you happen to remember next time you have your newer 5334A open, can you check the value of R112?

author: wn1fju
author_rank: Frequent Contributor
timestamp: July 07, 2025, 07:57:21 pm
content: On the newer 5334A, resistors R112 and R84 are marked violet-green-black-brown (7.5K) and measure 7.4557 and 7.4714 Kohms in circuit.On the older 5334A, resistors R112 and R84 are marked blue-brown-white-brown (6.19K) and measure 6.1724 and 6.1684 Kohms in circuit.Hmm, maybe we are onto something?I pulled out a couple of 8K resistors from my junkbox and replaced R112 and R84 with them on the older unit. Who knows what their temperature coefficient are, but a resistor is a resistor, right?And much to my amazement, the older unit, both channels, can now be adjusted to 0.00 0.00 instead of being off by 0.02 on one side. It's still a delicate adjustment and I found it beneficial to terminate the input BNC with 50 ohms (a short also works). And incidentally, on both of my 5334A's and on my 5334B, all these level readings seemed to be quantized to multiples of 0.02. Apparently, you can't get a reading like 0.01 (or any other odd last digit).I went through the sensitivity adjustment/verification to make sure the different resistors didn't screw anything up. Everything seems to work fine, but I taped the two original resistors inside the 5334A in case years from now I discover something funky and want to put them back.

author: aarons
author_rank: Regular Contributor
timestamp: July 07, 2025, 10:28:11 pm
content: I just checked mine, and they are already 7.5k (measuring 7.44 and 7.43 in circuit). I'm tempted to replace them with a slightly higher value, or even a trimmer to see the effect, but I should probably leave well enough alone for now.