Giga-tronics 8541C - "A UNCALIBRATED" - Page 1
Forum section: electronics / testgear

author: rplabs
author_rank: Contributor
timestamp: August 11, 2024, 12:03:15 pm
content: Hello,I have a Giga-tronics 8541C and a 80320 sensor. Everything was fine until I said to reset the unit to "default settings" so I went to Service Menu and hit the CLEAR ALL MEMORY option.After this I got the "A UNCALIBRATED" on the display so I connected the sensor to the 50MHz calibrator connector and hit the CAL/ZERO button but after around 3 seconds it returns to the same "A UNCALIBRATED" text on the display.I looked in the Service manual and it says the the CLEAR ALL MEMORY option will erase also the the amplitude calibration data performed by the Power Sweep Calibrator.I opened the power meter and I checked the voltages on the A1 board and +5V is spot on at +5.00V, -15V is at -15.02V and the +15V is at +14.97VI measured the 50MHz out and it is around 49.7MHz but the service manual says it should be between 49 and 51MHz. Also it says the power output is only good for 15 seconds and after it starts to drift.How to restore the power meter to a working state? Do you have any tips? Am I doing something wrong?Many thanks!

author: squadchannel
author_rank: Frequent Contributor
timestamp: August 11, 2024, 02:13:25 pm
content: It is possible that you have deleted important calibration data.If so, it would need to be recalibrated by a repair service. It will also cost money.However, it should not normally be possible to delete it intentionally.

author: rplabs
author_rank: Contributor
timestamp: August 11, 2024, 02:32:47 pm
content: Quote from: squadchannel on August 11, 2024, 02:13:25 pmI was thinking the same after I read the service manual but that would be very strange to allow this feature.I saw that in the same service menu I can set the calibrator coefficient % but it needs to be calculated after measurement with an HP 432A and a 478A thermistor mount.I ran more tests from the "Service Menu", maybe some can check their 8451C if it has similar values?Sensor 80320A is connected via cable to the power meter and also to the calibrator connector.Sensor "A" DVM -> 1114 mV Range 3Under "Hardware Tests" there is a test called "5 VOLTS" and it shows 38xx (xx is changing between 17 and 21). I measured all 5V testpoints on both A1 and A2 boards and they are OK. Does this test means I have somewhere 3.800V ?

author: Fungus
author_rank: Super Contributor
timestamp: August 11, 2024, 07:11:56 pm
content: Quote from: rplabs on August 11, 2024, 02:32:47 pmNo.All readings are done by comparing the incoming voltage with a precision internal reference. You then multiply the ratio of those two by a scale factor to get the number displayed on screen.That "scale factor" is the calibration info that's been lost.

author: rplabs
author_rank: Contributor
timestamp: August 13, 2024, 12:36:48 am
content: Quote from: Fungus on August 11, 2024, 07:11:56 pmDo you know how to add the "scale factor" for the 8541C ? Or how is it calibrated in the cal lab?

author: rplabs
author_rank: Contributor
timestamp: August 19, 2024, 08:52:21 am
content: Does anyone know how to add the "scale factor" for the 8541C? I am stuck with this power meter and I would like to fix it.Does anyone know the procedure to calibrate it? Can it be done directly from the keypad or do I need some "special" software?Thanks!

author: rplabs
author_rank: Contributor
timestamp: August 22, 2024, 09:02:21 pm
content: UPDATE: Power Meter doesn't have any issue. It looks like the feature that I used is to clear user memory and it doesn't affect the calibration.Here is the application note:http://spanawave.com/store/catalog/PDF/AN-GT128A-8540_instrument_memory.pdfI bought another sensor and the calibration and zeroing works fine with the new sensor.What is strange is that the initial 80320A sensor that worked is now not working. I noticed that the 3.5mm connector doesn't screw correctly to the adapter that I used for the Type-N 50MHz calibration connector compared to the 80304A. I don't have the original Type-N to 3.5mm 50MHz adapter and I used a Rosenberger precision Type-N to 3.5mm adapter for testing.When I connect the 80304A sensor the threads on the Rosenberger adapter are not visible when sensor is fully screwed in but for the old 80320A there are a few threads visible. Visually both look the same. I don't understand what is the issue.Does anyone know if it is possible to disassemble the Giga-tronics power sensor 3.5mm connectors?