ROHDE & SCHWARZ SMS2 teardown - Page 1
Forum section: electronics / testgear

author: ealex
author_rank: Frequent Contributor
timestamp: February 21, 2015, 03:18:37 pm
content: HelloThis is a quick tear down of a R&S SMS2 signal generator.I had to replace most of the electrolytic caps inside.The ones in the PSU blew up so i decided to replace all capacitors in sight.The photos are quite poor and the colors are a little off - i have some warm LED lights that look good but mess up photo cameras.I had to take the photos at F2 -> the DOF is low and there's a lot of fringing. I'll try to improve them as much as i can ( sharpen, etc )The fist photo has the instrument's front panel.The layout is quite good, all buttons well spaced out and with a nice feel.The 7 segments display is bright and big enough to read from across the room.One of the front pane screws is stuck - I could not remove it .Removing the top and bottom covers reveals some shields and the power supply on the back. Between the shielded section and the power supply there is a mechanical attenuator and the frequency doubler that pushes the range to 1GHz.All RF connections inside are made with hard coax.

author: ealex
author_rank: Frequent Contributor
timestamp: February 21, 2015, 03:27:52 pm
content: The back of the unit comes apart as one piece and contains the heavily shielded toroidal transformer, the power supply regulator board, power switch, brush-less cooling fan + driver module ( it's potted so no tear-down of it), all back-side connectors and all power related stuff - regulators, power transistors and radiators.The control board connect to the power transistors via some strange socket pins that are a pain to re-align.I had to remove all power devices and reinsert them one by one to get them back in place after removing the control board.Everything is bolted down with a ton of screws and washers - takes 15 minutes to locate and remove all screws for this section.The black caps are new - replaced after the first power up. The brown ones are the ones that got replaced this time - they look potted, some of them where discolored or crackedA third of the unit's weight is in this section, due to the power transformer and heat-sinks.

author: ealex
author_rank: Frequent Contributor
timestamp: February 21, 2015, 03:36:43 pm
content: There are 2 modules that are unshielded: the cpu board and the modulation control.The cpu board contains a Intel 8049 masked rom controller, Intel 8355 io expander, an unpopulated slot and a HEF4738 gpib controllerSome of the connectors are covered in a white residue - maybe something leaking from the plastic.The modulation control board is mostly analog, with a I8343 expander controlling a lot of analog switches that change the modulation depth, rate, etc

author: dom0
author_rank: Super Contributor
timestamp: February 21, 2015, 03:38:12 pm
content: Wow, that toroid looks like it's encased in mu-metal. And with that mask ROM there are no issues with EPROM corruption.Expensive stuff... but that's R&S!

author: ealex
author_rank: Frequent Contributor
timestamp: February 21, 2015, 03:41:04 pm
content: Removing the top-side cover of the shielded sections reveals the "motherboard" - nothing interestingAll the good stuff is on the other sideBoth RF section covers have a lot of screws, the metal pieces are somehow twisted - you need a lot of force to align them and under each aluminium shield there is a thin hard metal foil ( maybe for magnetic shielding ? )

author: dom0
author_rank: Super Contributor
timestamp: February 21, 2015, 03:42:17 pm
content: Probably to ensure good contact over the full width of the covers.

author: Hydrawerk
author_rank: Super Contributor
timestamp: February 21, 2015, 03:42:49 pm
content: How old is this unit? I like that all the ICs are placed in sockets.

author: dom0
author_rank: Super Contributor
timestamp: February 21, 2015, 03:47:38 pm
content: Datecodes imply end of 70s / early 80s (79xx and 78xx datecodes on CPU, modulation board and on some caps in the PSU)

author: babysitter
author_rank: Frequent Contributor
timestamp: February 21, 2015, 03:52:09 pm
content: Oh I like this machine, nice to see it from the inside. My ham radio mentor used it a lot to show me stuff.

author: ealex
author_rank: Frequent Contributor
timestamp: February 21, 2015, 03:57:27 pm
content: The first 3 RF modules are:100Hz oscillator- a PLL loop locked to the 10MHz main oscillator with a 25-34MHz VCO- 100-135KHz output, divided from the VCO- the cpu controls the PLL via a I8243.- the phase comparator is made from a 4046 flip-flop- all off-the shelf parts50KHz oscillator- a PLL loop with a 19.9 - 21.9 MHz VCO, locked to 10MHz reference- a PLL loop with a 20-22MHz VCO locked to the 100Hz oscillator, that controls it's VCO mixed with the first VCO- a 2-2.2MHz output divided from the 20-22MHz VCOreference oscillator- an 10MHz oven controlled crystal oscillator- a 80MHz oscillator controlled with a hard-wired PLL- the 80MHz oscillator is also controlled by the FM modulation signal.- the board outputs 40 or 80MHz from the VCO, 10MHz from the xtal oscillator and 1MHz divided from the 10MHz source.- the 80/40MHz output is sine-wave, filtered after some freq. dividers.

author: ealex
author_rank: Frequent Contributor
timestamp: February 21, 2015, 04:13:57 pm
content: following those modules:the "converter" board- 380MHz VCO locked to the 1MHz source, hard-wired PLL- a mixer with the 40/80Mhz signal from the reference board- band-pass filters and diode switches for 300,340,420 and 460MHz mixer productsglobal PLL board that receives all reference signals and controls the main oscillatorit receives the 2-2.2Mhz reference oscillator, a 20-60MHz divided signal from the main oscillator and a lot of control signals from the CPU board. It outputs a 0-26V tuning voltagemain oscillator board- 260-380MHz VCO- 380-520MHz VCO- diode switches controlled from the CPU board- mixer with the 300-340 or 420-460MHz oscillator- 260-520MHz and 20-60Mhz outputs

author: ealex
author_rank: Frequent Contributor
timestamp: February 21, 2015, 04:15:02 pm
content: the last 2 board arefine attenuator- pin diode attenuator- switched filters and signal paths for the different output ranges- amplitude modulationoutput amplifier- main amplifier- AGC block- multiple signal paths, band-dependent

author: ealex
author_rank: Frequent Contributor
timestamp: February 21, 2015, 04:37:51 pm
content: The unit is generating it's output range by mixing the 260-520MHz VCO's with the fixed frequency ones.The range from 520 to 1040MHz is covered from a frequency doubler module inserted after the output amplifier.The fine step attenuator is paired with a mechanical one that's using some custom made attenuator pads and custom relays.This unit also got the output protection module, that forces the mechanical attenuator open when power is fed back into the output.Most of the parts are off-the-shelf - normal op-amps, digital logic, etc. There are some hybrid amplifier modules that are no longer produced, but the documentation provides useful information to locate replacement ( gain, freq. range, etc)The main PLL module got some ECL chips that act as a HF divider - also no longer in production, but they should be replaceable.Most of the chips and transistors are mounted in sockets. Everything is heavily shielded with double-shields - thick aluminium outer shield and a thin / elastic metal interior shield. Most of the shields are clip-on, some of them covering the entire board. The converter board and output amplifier have an extra set of screws - over the filter section and output transistor heat-sinks.Keeping this unit running for the next 100 years should be simpleYou can find high resolution schematics, scanned from the user manual at the following link:https://drive.google.com/folderview?id=0B0CB5A9yU03XbnRSa0xVZ04yaXc&usp=sharingYou can also find these document on BAMA - last time i checked the .djvu files where in good order, the .pdf fiels have some problems.http://bama.edebris.com/manuals/rohdeschwarz/sms2/

author: Tom D
author_rank: Contributor
timestamp: February 21, 2015, 04:49:15 pm
content: It's actually an SMS, not an SMS2. I have two of these myself, although both of mine don't have the 1,040 MHz option; they only go from 0.1 MHz to 520 MHz, as they were both Australian ex-military. These were ordered by the army here with the high stability ovenized oscillator and RF output overload protection.It also looks like the overload protection option was added later on your unit (the small PCB the RF output is connected to in your picture of the bottom view) - if it was factory built with this option, it would not have the "max. 5V" printed on the front panel next to RF output.My units also have a different microprocessor board which puts the unit through a self-test and remembers the last settings at powerup the machine was at before the previous shutdown. This board has a lithium backup battery and allows you to store and recall up to 40 setups, as well as keying in the GPIB address, and also allows the machine to go down to 0.1 MHz rather than 0.4 MHz.I rather like these SMS units myself, they're very easy to repair if a problem develops, as most of the chips in them are common IC's.Tom

author: tinkera123
author_rank: Newbie
timestamp: October 30, 2019, 05:54:29 am
content: If anyone on this site has experience in pulling one of these Signal Generators apart ..... and is willing to assist me in doing so, via email, please post a response or contact me via my email address (which I believe is visible).A friend rescued one of these for me and it was okay for a short time, but has developed a fault ... the +28v Regulator. Now I have to pull it apart.I would appreciate your advice before I fall into a 'rookie' mistake.Cheers, Ian