HP 8958A and 8956A Teardown and Request - Page 1
Forum section: electronics / testgear

author: skennedy
author_rank: Contributor
timestamp: February 17, 2016, 04:58:58 am
content: After a late night ebay browsing session, I recently purchased a lot of test equipment containing a few interesting bits of equipment including a HP 8958A and HP8956A. Both seem to be pretty impressive bits of equipment although it has been pretty hard to find any real details other than vague datasheets.The 8958A RF Test System Interface arrived not working. The power supply was faulty with the +5V rail not being present at all. HP had used a Power-One CP131-A supply to supply all of the rails in this unit. A pretty nicely designed unit and after a bit of debugging found out that CR3, a Varo R711A dual common anode diode was open. Replacement with two 1N4004 brought back the unit to life. The problem now is that I have no documentation on how to make use of all of the RF switching a processing inside this unit.The 8956A Cellular Radio Interface arrived working but it appeared not to be complete. Opening up the unit it looks like a few RF cables had been removed but the RF switch remained and a large item in the front left had also been removed. The front panel works fine with relays clicking as I select the different settings but I have no documentation.Hopefully I will upload some photos when I get some time this weekend to show you the insides. Both units have quite a lot of useful RF components and lots and lots of cables but I would love to get these units working and hopefully being controlled via their GPIB interface.Does anyone have a copy of a user and/or service manual for these units? There appears to be one on ebay but the shipping alone is more than the cost of both unitsThanks,Simon

author: skennedy
author_rank: Contributor
timestamp: March 12, 2016, 06:18:08 am
content: I have been pretty busy recently but here are some photos of the inside of the HP 8958A. This unit has lots of RF connectors and adapters worth a fair bit more than I payed for the unit but it would be nice to put it to some use. Hopefully someone will be able to help me out! I'm looking for some documentation that describes the functions, a informative datasheet, manual or service manual would be amazing!Front Panel, retro HP and as such shows a similar design to the HP 8568 and 8566 spectrum analysers which are a similar age. Two D-SUB connectors along the bottom look interesting as are the two RF connectorsRear Panel, dominated by lots of RF, GPIB and external power supply connectors. Interestingly there are three fuses in addition to the mains input fuse.With the top off we can see the two large shielded sections, a front panel board and lots of rigid coax.Mini-circuits ZFSC-2-5 RF Splitter, $60 by it selfBack panel N-connectors are bulkhead N to SMA connectorsLeft hand compartment contains the off the shelf power supply providing + and - 15V and +5V, a large really and fan.Right hand compartment contains 8 cards in the card cage.

author: bitseeker
author_rank: Super Contributor
timestamp: March 12, 2016, 07:09:38 am
content: Thanks for coming back and updating. It looks pretty good inside. Unfortunately, I don't have any service manuals for these.Hopefully, your late-night eBay adventures will result in more teardown threads.

author: skennedy
author_rank: Contributor
timestamp: March 12, 2016, 08:12:27 am
content: Lets have a look at the eight boards in the card cage. I suspect it will be pretty difficult to tell exactly what each does but I will give it a shot.First Up is A3. Looks like a lot of 74 logic and a GPIB transceiver (1820-2424 is a MC3447P3). Probably the GPIB boardA4, Looks likely to be the main processor board (1820-2624 is a Motorola 8609) with two mask roms and one EEPROMA5, also looks like the GPIB processor board. The Intel P8921a is a GPIB talker/Listener. No idea what the 1LJ6-0001 is.A7, looks to be peripheral interface board with two Motorola MC6821 peripheral interface adapters. Lots of other 74 logic here as wellA8, is an analog board with lots of passives, op-amps, analog switches and a voltage controlled oscillator.A9, probably the DTMF decoder boardIt contains a MT8860XE DTMF Decoder and MT8865XC DTMF Filter as well as passives and op-ampsA11, contains reed relays and a few op-ampsA12, contains an analog devices DAC08EQ DAC, ULN2803A Darlington array, an op-amp and 74 logic.

author: PA0PBZ
author_rank: Super Contributor
timestamp: March 12, 2016, 10:08:21 am
content: The RF content is a bit disappointing, it really only switches the RF path from the front to the back of the unit it seems.According to a leaflet it "controls the audio and dc paths between the test instrumentation and the radio" so that makes sense.