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Anritsu / Wiltron S331A - Service Manual / Schematic
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rg58:
Yeah, I though this as well right from the beginning and looked at it closely.

This was just some flux residue. Wiped it off, just to be sure.
rg58:
Thanks for the type of the "white pill"! You are very knowledgeable indeed, sir! Found the datasheet for it, it's an NPN transistor. Nowhere to find this SMD marking code in all my lists that correctly represent this part in this package.

If "E3" is in fact an ERA-3 with something that appears to be a dot (although it looks more like a lasered c in a circle), then the dot marks the input, opposite site is output. The "white pill" besides it marked "Bd" seems to have its base connected to the output of this MMIC. It measured good with a diode tester (2x ~0.77V), but it's still a little strange as it attenuates the signal by 22dB.

In the correct orientation of the letters "Bd" the base would be on the RIGHT, being connected to the output of the MMIC. Find this a little strange too. Could be a DC problem here, so I will do some more AC/DC probing first. Let's see if those guys are clean :)
rg58:
The VCO output looks aweful too. I tried with 2 different ground points and 2 different probes. First peak on the left are 50MHz, the marker sitting on 80MHz. Looks like a comb generator, not a VCO  :palm:

Supply voltage 11.5V clean
Tuning voltage 14.8V clean (50 MHz)

As for the rest, the transistor and the MMIC, I can't make sense of all that.

*If* I follow the signal from the other direction, everything makes sense and the signal increases, but only if I omit the pinouts of both components. Are the types correct? ERA-3 and NE021? Why would we connect a noisy MMIC infront of a low-noise transistor? All this makes no sense to me. The pinouts must be completely wrong. Although from B to C and E I can meassure 2x 0.7V.

Signal path (backwards?) looking like this  :-/O

Transistor C -> 0.75Vdc / -56.6 dBm
Transistor E -> 0.62Vdc
Transistor B -> 0.94Vdc / -32.6 dBm (Vbe only 0.32V?, diode test was ~2x 0.7V)
MMIC output -> 1.72V / -32.6 dBm
MMIC input -> 1.20V / -16.6 dBm

I have the feeling this thing is a complete mess...  :wtf:
metrologist:
This instrument uses harmonics for frequencies outside of baseband, so that transistor is used to suppress or enhance those by switching in reactance via bias circuit on its base. Those are the correct part numbers. Set start/stop freq to the the same value in baseband so it's not sweeping. That's an 800-1600M VCO.

Also, look at the traces just on the MMIC output and the two diodes connected to the 51R1 resistors to the right, I think the lower diode is baseband path, and the other path is to the left and through the inductor above the top diode, and this is also controlled by the bias circuit connected to the top side of the inductor.

I believe that the 2N2907 transistor just above the plastic mounting peg to the right of the VCO is the voltage switch to the MMIC, which goes through the 1100 and 1960 resistors.
rg58:
Thanks a lot! You were right about the VCO.

@50MHz: 1600 MHz / -28dBm - funny, this is the highest frequency that you mentioned (didn't expect THIS high, wow! needed to change SA)
@500MHz: 1000 MHz / -19dBm

Not sure how they do the mixing as these are 600MHz, but set frequency is only +450 MHz. But as you said, they probably use harmonics somewhere in the circuit and do some switching.

As for the rest, will see if I can take more measurements tomorrow  :-/O and report back to you. Seems some DC measurements from above were incorrect as the RF is effecting my multimeter.  Having a few busy days here...
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