Hameg - Rohde Schwarz HZ109 probe problem - Page 1
Forum section: electronics / testgear

author: borjam
author_rank: Unknown
timestamp: August 08, 2017, 07:54:25 am
content: Hello,I have an HZ109 differential probe that seems to have a problem. It's way too noisy and subject to some apparently internally generated interference.Is it possible to obtain a service manual in order to check its calibration? Or what's the best course of action to have it serviced? It was purchased from Farnell (in Spain).Thanks!

author: Rich@RohdeScopesUSA
author_rank: Frequent Contributor
timestamp: August 08, 2017, 01:32:13 pm
content: Hi borjam,Unfortunately, there are no user-serviceable pieces on the HZ109:https://cdn.rohde-schwarz.com/pws/dl_downloads/dl_common_library/dl_manuals/gb_1/h/hz1xx/HZ109_Englisch.pdfYou'll need to send it in for service if it is not working properly. While I'm not in Europe, I'd start with Farnell and go from there.-Rich

author: mk_
author_rank: Regular Contributor
timestamp: August 08, 2017, 01:59:13 pm
content: Quote from: borjam on August 08, 2017, 07:54:25 amHameg used to buy their probes from Testec, at least the passives one. So - take a look at testec and compare.Anyway. there is no black magic inside a 35V/40MHz-diffprobe, so open it, make pictures and post them here.how to calibrate a diffprobe can be found on saphire-website - sure it will not work on your probe but to see how you can verify your probe it could helphth

author: borjam
author_rank: Unknown
timestamp: August 08, 2017, 02:30:35 pm
content: Quote from: mk_ on August 08, 2017, 01:59:13 pmThank you. I believe these ones were sold also as "Elditest" or "General Elektronik".There are four trimmers inside (visible when replacing the battery). Maybe I'll investigate and, if unsuccessful, just sent it tocalibrate.QuoteThank you, that website is really useful. So the four trimmers might be adjustments for X1 and X10, low frequency CMRR and high frequency compensation. Interesting.

author: Hydron
author_rank: Super Contributor
timestamp: August 08, 2017, 04:24:05 pm
content: None of the trimmers are likely to fix any noise issues unfortunately.As for the timmers, there are likely 3 associated with the input divider (high freq CMRR/compensation capacitor trimmer x2, low freq CMRR resistor trimmer x1).The other might be offset. I'm surprised there aren't two (one for each range), unless there is one or more extra adjustments hidden under a shield.

author: borjam
author_rank: Unknown
timestamp: August 08, 2017, 09:14:30 pm
content: I think I have managed to do it.Opening the probe, with the probe cover, there are (left edge)- Two variable condensers in series with the differential terminals. HF compensation and, of course, if imbalanced, more common mode noise, especially 50 Hz mains.- A trimmer potentiometer between them. Turns out ot be a DC balance between them. Again, adjusting it reduced noise.Moving a bit more to the right, two offset potentiometers (same distance from the left edge) adjusting the offsets for the x1 and x10 adjustments. It was very tricky (despite being multiturn pots I had to be really gentle with the screwdriver) but I managed to center the trace within +- 2mV (measured with a multimeter).The rightmost pot is the gain adjustment.I'll use it for a while examining more or less known signals and report back just in case has the same problem.

author: borjam
author_rank: Unknown
timestamp: August 15, 2017, 12:22:59 pm
content: Updated description, with photoFor the record, this is what I've done.Following @mk_ 's suggestion I had a look at Sapphire's website. I found that the controls could resemble those in the SI-9001 probe.http://www.sapphire.com.tw/nsi9001cp.htmFollowing the instructions I managed to adjust it better. I am not sure about the offset controls (1 and 2) since each one of them affected the other one. But I found a position showing a minimum offset for both modes (x1 and x10).The CMRR adjustments are tricky. Especially, be careful with the instructions for low frequency CMRR, as this probe is specified for a maximum of 100 V between one of the inputs and earth. That means you can't use the mains outlet for that. Don't connect the inputs to the mains outlet.I found a solution. I have a crappy "walmart" 12 V DC mains adapter that shows around 80 V to earth in the output. It's not dangerous (using the low Z measurement with a Fluke 289 the voltage goes to 0, so the available current is really negligible) but it was useful for this application. It helped to bring the low frequency CMRR down.I don't think I've achieved a super lab quality accuracy but it's more than enough for my purposes, checking serial line communications with long wires (40 m) and avoiding undesired connections to earth.