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Products => Test Equipment => Topic started by: wd5jfr on July 23, 2020, 03:07:20 am

Title: Rohde SM300 several issues
Post by: wd5jfr on July 23, 2020, 03:07:20 am
Just got an SM300 which passes all 10 self tests, has nice clean but low output to 3GHz and will run for several hours and then no RF output.  Running self test shows all 10 self test as Error Timeout.  Shows 481 hrs run time with 456 starts, Running right now all night about 10 hrs, see what happens. Got $600.00 in it and don't have the capability for a component fix, need to replace circuit board if they become available???  So I wonder if there's reasonable repair service out there or I should give up and sell it?
Title: Re: Rohde SM300 several issues
Post by: wd5jfr on July 23, 2020, 05:05:11 pm
After running non-stop over 13 hrs no errors.  Changed frequency setting  and did a few sweeps, still low on output.
The question: Buy, Sell, or Hold?
Any ideas appreciated.
It's kind of a nice small well designed unit easy to set-up. Now I'm wondering if there's a Rigol, Siglent, etc with similar features to look for if I can't get it repaired.
Title: Re: Rohde SM300 several issues
Post by: TurboTom on July 25, 2020, 02:27:55 pm
I also yesterday received an SM300 that I bought with the well-known defect of not being able to output (almost) any signal above -26dBm setting, paid approximately the same price.

Already fixed the PA problem but I've got to say, this piece of equipment is a complex design. One can really see the quality difference compared to a "real" budget instrument. Every critical part is double- and triple shielded with RF absorbing foam layers in between. Structural shielding parts are machined from aluminium and afterwards silver plated. Everything "smells" quality... It's no surprise that these instruments sold new twice or three times as expensive as Rigol&Siglent (R&S???  :-//) instruments nowadays.

So if the SM300 is (half-way) operational, I'ld say it's a keeper, especially considering what has to be spent on something with even basic complex modulation abilities. I'ld consider the chance to source a working spare RF assembly to be virtually non-existant, at least reasonably priced, since this is the module that usually fails on these generators.

@wd5jfr: I didn't completely understand the exact failure mode of your instrument - you say it's low on output, does that mean the level is low over the whole range, or is it (more or less) accurate below -26dBm (no modulation enabled) and drops when you select output levels above that figure? That would be a clear indication for a faulty output MMIC (read below). If it's low throughout the whole level range and shows some discontinuity at the -26 -> -25dBm threshold, it's still a strong indication for a faulty output MMIC but in conjunction with a possibly broken output RF switch (AW002R2-12). If the level is adjustable continuously over the whole range but just too low, I'ld look at the output circuitry behind the switch. Only if there are several, repeating discuntinuities, I'ld search the fault at the attenuators. The weak spot of these generators appears to be the tolerance of the output circuitry against reversely applied RF power (i.e. testing a transciever's receiving performance and accidentially hitting p.t.t...  :-BROKE).

Reaching the critical components in the RF section isn't a job for the faint of heart, there are about 100 screws of well mixed sizes to be removed in order to get access to the relevant area, several layers of conductive foam to be peeled off (possibly without damaging them) and finally the defective components have to be identified and replaced.

Since the board cannot easily be powered up when exposed and separated from the digital / ch2 AF board, real fault testing is difficult. I've been able to gradually apply the PA bias voltage to find that the constant current source for the output stage MMIC (AGB3303) maxed out already at 2.5V, so obiously this chip was faulty. I also noticed that some previous repair work had been executed on this board, probably after some catastrophic damage had been done to the final microwave switch with some pyrotechnical effects going off... At that time, the AGB3303 must have been replaced as well (as the flux residues witnessed). Since I didn't find any more defects, I went ahead and replaced the defective MMIC. But I didn't have a spare AGB3303 so I compared datasheets and took the risk to install an SBB5089Z which works surprisingly well in that application despite it's designed for constant voltage operation and not with a resistive dropper (like the original) or even a CC source as supply.

Now the generator works in all ranges almost corretly and reaches 13dBm or more but obiously, it's a db or slightly more off level. So here comes the million dollar question: Does anyone have or is aware of a calibration instruction for the SM300? I'm pretty sure with my SA and an HP power meter, I'ld be able to get the calibration done well enough to meet its spec.

I searched the web and didn't find anything...any help would be appreciated!