Spectrum analyser HP8590A opt 001 - Restoration and migration to 50 ohms - Page 1
Forum section: electronics / repair

author: petercut62
author_rank: Contributor
timestamp: January 02, 2018, 03:41:41 pm
content: Hello,I've recently bought a spectrum analyser HP8590A option 001.I plan to migrate this analyseur from 75 ohms to 50 ohms. I've managed to find the 3rd converter schematics for the 50 (08590-60038 and 08590-60073) & 75 ohms (08590-60078). There is only few differences between both configurations.At first glance, the analyser works well. But when i dig a little bit, it seems lacking sensitivity. I bought a 75 ohms cord to check calibration but not receive yet. The previous owner sent me some photos of the calibration screen and calibration datas he took before sending the unit.In the 2nd col (RL-VENR) datas should have value from 3 to 2 digits at the end ; which is not the case here. Looking the calibration spectrum the third harmonic is very low and i don't know exactly why.May be a wrong calibration cable was used (50 ohms instead 75), calibration signal might be wrong or the 2 diodes in the first mixer might be damaged too (like Feedback.Loop's analyser)I bought some hardware (diodes HSMS 2802, 2812 and component for 50 ohms migration) still not delivered.Any advices are welcome.

author: slurry
author_rank: Regular Contributor
timestamp: January 02, 2018, 05:07:17 pm
content: Compared to a radio receiver/transceiver the SA can be considered "deaf", unless you have a preamplifier, either built in or external, like the HP 10855A for example.Also, with RBW at 3MHz you will not get any accuracy the readout of the frequency- nor the amplitude,narrow down on each frequency you measure and reduce RBW to at least 30kHz and things would be more accurate.

author: xwarp
author_rank: Frequent Contributor
timestamp: January 03, 2018, 01:57:00 am
content: I sent you a private message about emailing the schematics if you don't mind. I also have an 8591 that is the 75 ohm and would like to convert it to 50.

author: petercut62
author_rank: Contributor
timestamp: February 11, 2018, 03:52:42 pm
content: Hello,Little followup of the 75 to 50 ohm migration of my HP 8590A Spectrum Analyser.First after checking schematics, the differences between the 2 PCB boards are minor. 2 resistors , a calibration connector and also the calibration cable from the board to the front panel.I manage to found the calibration cable on eBay (1,65€ -$2) with both appropriate connectors on each side.After some soldering the board is ready with a 50 ohm calibration output. The other cables didn't need to be change. According the given spec the only different cable is the calibration cable from a 75 ohm unit to a 50 ohm unit.The front panel was sanded. Gaps filled with some putty , sanded again and paint with acrylic paint according to the initial panel color.After some measurement, the new calibration output was adusted to meet the -20dB specified in the service manual. Some frequency adjustments need to be done too. The signal level measurement accuracy is spot on after running all the calibration menus. But i think the diodes in the first mixer are dead. The datas are not so good at the end of the RL-VENR column of the CAL DATA. As Feedbackloop descrideb in his video. I bought some HSMS 2812 to swap the 2 diodes in the mixer.That's all for now.CheersPierre

author: xwarp
author_rank: Frequent Contributor
timestamp: February 11, 2018, 04:35:16 pm
content: I've thought about changing my 8591e from 75 to 50, but the more I thought about it, I'm going to leave it as 75 because I don't use it for critical measurements and I like the higher amplitude input capability. I don't know when the last time the unit has been calibrated and frankly speaking, the frequency measurements I've taken have been spot on with my 8568b albeit the levels are slightly different.

author: xwarp
author_rank: Frequent Contributor
timestamp: February 11, 2018, 04:37:29 pm
content: Quote from: xwarp on February 11, 2018, 04:35:16 pm