HP 8561A calibration constants - Page 1
Forum section: electronics / rf-microwave

author: Mrt12
author_rank: Regular Contributor
timestamp: February 12, 2019, 09:28:07 pm
content: Hi Guys,I got a very old HP 8561A spectrum analyzer. I was not able to find a service manual for it, there is only a manual for the 856xE and EC, and I don't know the differences to the A version...Anyway, I have read somewhere that this unit has its calibration constants stored in a RAM with backup battery. Since the instrument is old, I would like to view the values of those calibration constants to make a backup, while I order a backup battery.The instrument is quite new to me, so does someone know how I can view the calibration constants?And, even more important: if the battery fails, what calibrations do I lose, and is it possible to restore them? I have quite a bit of equipment here like signal generators up to 26.5 GHz and spectrum analyzers and stuff, but I don't know whether the calibration of the 8561A would need e.g. some special test software and so on.And, is there a service manual somewhere? I think I also need to do some repair, because after power-up, it shows ERROR 334, LO AMPLITUDE. Interestingly the error goes away when PRESET is depressed, but again appears for certain span/center frequency settings.

author: scopeman
author_rank: Frequent Contributor
timestamp: March 03, 2019, 12:30:01 am
content: MRT!2,IIRC the 859X series uses SRAM and a backup battery to store the calibration constants. i believe the later 859X models also have super capacitor to hold the cal ram data while the battery is changed.The 856X series stores the calibration constants in EEPROM and uses the battery backed ram to store things like trace data and the like so you should not use any calibration data,IIRC the only difference between the one you have is that the band coverage so there may be some differences in that regard but the AN/USM-489 manual should get you by.The info below is taken from the Military AN/USM-489 TM 11-6625-3250-40 Manual which is the HP8662A of the series.The A2 Board contains the following:"RAM (circuit S). Four of the eight RAM IC’s are battery-backed. The battery-backed RAMstores trace information (two Display Memory RAMs) and AN/USM-489A state information(two program RAMs). A total of eight traces and ten states may be stored. Typical batterylife is ten years with the lithium battery and two years with the optional silver-oxide battery.Trace and state information may be retained for up to 30 minutes with a dead battery andpower turned off. This is due to the RAM’s very low data retention current.STATE storage is in two of the six Program RAMs and TRACE storage is in the two DisplayRAMs. With low battery voltage, it is normal for STATES and TRACES to be retained if thepower is off for less than one minute. If the power is left off for more than thirty minuteswith low battery voltage, the stored STATES and TRACES will be lost.EEPROM (circuit U). The EEPROM stores important amplitude-related correction data. Thisincludes data for mixer-bias DACs, YTF Slope and Offset DACs, RF Gain DACs (flatnesscorrection), and a Preselector Peak DAC. The AN/USM-489A serial number, model number,and installed options are also stored in EEROM."You can find the complete set of AN/USM-489 set of manuals here:http://radionerds.com/index.php/AN~USM-489As far as service is concerned I would suggest replacing all of the electrolytic capacitors in the supply. If you contact me off list I probably have some power supply service info but I will need to know the HP part number for the power supply used in your actual unit.Best regards and good luck,SamW3OHM