Thanks!
I'll start to prepare to tear it all open one more time and see where it goes with your advice.
The reason I got concerned about the CCTUB part:
The series 47R resistor was totally fried open circuit, I couldn't read the print anymore. Learned it's 47R from pics of this area.
Now, considering that 47R fried o/c and both lil BAT series diodes measure near short, I am wondering where the current went while doing this damage. The only potential DC path I (thought) I see is to the JFET cctub? (IIRC). No idea also what the weird little 4 pin white part is connecting to the diodes, some sort of bypass, filter or integrating thing?
I didn't think current found its way to gnd or vcc rail through that?
It can! This whitish thingy is a pass-through capacitor / filter of probably several nanofarads. It should measure continuity across the end faces and high impedance from the end face to the side contacts. If there's a sufficiently high RF signal applied to the output of the SM300, the two series schottky diodes will fail (max reverse voltage 40V each) and conduct the power through the filter capacitor to ground. This will fry the 47Ohms series resistor (and probably at least deteriorate the small inductor that connects the "RF Voodoo" network
to the output circuitry of the second switch.
I attached the block diagram of the RF deck as an excerpt from Rohde&Schwarz's SM300 Service Manual - here you find the diode circuitry only shown as part of the reverse power protection scheme. The truth is, it doesn't make too much sense like this as you already pointed out. It would be much more common sense to use this signal (at least in high power mode) as an active feedback to the level control circuitry. To me it seems, the JFET is used as some kind of gain switch to change sensitivity of the protection/level control circuitry, depending on the selected power range.
I'll do the surgery once I get the little BATs and maybe I could try that apply of Vcc on that upper left feedthrough.... I wondering though, when I initially looked for a 3303 PA replacement, it seemed most of those parts can't take more than 6 V Vdd?
Finally, I'm not sure which 2 LEDs you're referring to in the current source test? You refer to 3 x 5C bipolars but I can only see 2 in pic?
You are correct, the MMICs are supposed to run at round about 6V, but to make them usable at low frequencies (down to 9kHz), the load inductor would become way too big so R&S used two constant current sources (consisting of the LEDs, a few passives and the three PNP transistors -- the one for the HMC313 pre-driver is not shown in the picture from my previous post, it's located further to the left -- and that's why the supply voltage has to be considerably higher than the bias voltage specified for the MMICs. Nevertheless, I'ld be careful supplying substantial voltages externally. When I experimented with the module, current draw became excessive after I exceeded a certain value (forgot what actually the voltage was), which may indicate a local voltage regulator becoming reverse conductive...
But anyway, this procedure helped me to understand if the driver MMIC was defective (which it was). Temporarily, I swapped in an SBB5089Z that I had available and which performed surprisingly well!
Oh, when I get at that -25 dBm crossover point in output, the output level jumps like 4 dB or so instead of ~ 1 dB. The output was low under -25 dB too, by the same amount pretty much.
I must say I don't even really understand how ALC then works here, esp. with switching the AHB3303 in & out..... how can the output level be sniffed at like -80 to -127 dBm etc?
Normally the ALC is done at full output level and then it's just a matter of step attenuators.
Unless the 2 BAT diodes that are fried are not ALC but something eise? But if so, where is the output level being sniffed/,controlled as the 4 HCM attenuators are varying the level 🤔 🙄 😳?
Totally confused. I'm completely unfamiliar with the concept level of this sig gen and the output level is controlled....
I will await your comment, and in the interim order the diodes and also the JFET I think. I'll most certainly keep in touch as to where it's going.
Thanks so much, 73.
You are 100% correct: At low output levels, the control scheme has to be modified since the level detectors at the output won't work anymore. That's probably why R&S has provided another detector right in front of the attenuator network -- as you suggested.
Since the signal level is low on all ranges on your S/G, in order to find the culprit I'ld focus on the circuitry downstream of the first AW002R2 RF switch MMIC. you may want to check the BAR63-03W PIN diode and definitely also the discrete series inductor(s). I wouldn't expect the detector branch to be able to pull down the output by that amount if the 47 Ohms resistor is intact. But you may also want to shine a light on the coupling cap at the output of the second RF switch MMIC. And -- maybe not too obvious -- also check the continuity of the PCB traces. On one of my SM300 instruments, there was a copper trace between the two discrete inductors gone completely - a single strand from a rather thick gauge litz copper wire, shoved in between the two inductors, did a very good job for me here...
If you are in doubt about the JFET, just check it with a diode tester. S and D are interchangeable on this type and you should be able to test the G-S diode without any difficulty. If this appears to be correct, there's little reason to assume it's broken. But I guess it wouldn't hurt either if you just replace it...
Good luck and I really hope you'll end up with a fully working SM300 soon! They aren't stellar performers (high level) and take ages to boot, but otherwise, low level performance isn't that bad and they offer complex modulation at a rather compact form factor. At the combined price I got my two units for (including spares) I couldn't have bought the base model of the "B brand" manufacturers new by far.
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