Hi there,
If you've seen my other thread about transceiver testing, you might have gotten the impression I wanted to avoid blowing out my spectrum analyser. It would seem my fears were somewhat unfounded, as the previous owner has already beaten me to it!
I'm intending this to be something between a repair log and a Q&A - so if you can help me, or think I've made a howler of a mistake in my logic, please shout out!
First problem I noticed was that the tracking generator output was 10dB low - at 0dBm out, I was seeing -12dBm (connection was a 1m TE Connectivity BNC cable and two BNC-to-N adapters). The 1dB steps were gone, all I had were the 5dB steps. The analyser's been like that since I got it, and until I looked at the service manual I thought it was yet another software quirk (if I remember correctly, there are a few other things you can set in increments the analyser hardware can't actually handle).
A quick check of the AGC level on the TG revealed it was pinned at -13V; the AGC is negative-acting (it feeds four PIN series diodes), so 0V is "turn the bugger off!" and -6V-ish is "Och, ah' cannae give ye any more, Cap'n!". Negative thirteen is definitely a sign of brokenness. At this point the service manual pointed me at the AGC Control board (BLC-015680). Well, it turns out this board was fine. All the opamps were behaving sanely, the L-inputs (1dB-increment output power control) were switching properly, and there was a signal on the DET input which changed when I shorted the AGC pin to ground at the feedthrough (shorting the AGC pin is in the service manual test procedure, worry not!). Of course, the service manual doggedly insists that the AGC control board has to be at fault, the rest of the TG is infallible...
So it's probably fair to assume the detector, 4GHz oscillator, mixer and AGC control are fine - the middle two being an absolute twat to test, requiring a 4GHz-capable power meter and/or spectrum analyser!
The rest of the circuit can be (partially) tested with an oscilloscope -- simply set the analyser to a centre frequency of 30MHz, zero span, and turn the TG on. Past the mixer, you won't see anything faster than 30MHz.
The 4GHz oscillator output is the thing that the "fine gain" control (AGC Control board) adjusts via the AGC signal. This is fed into a mixer (the other input is the local oscillator), the output of which is fed into the output amplifier. Which is where we continue the story!
The troubleshooting flowchart indicates that there are five amplification stages between the mixer and the low-pass filter (a funky gold-on-ceramic thing which looks like an interestingly shaped PCB trace and positively exclaims "I am black magic you will never understand"
).
If you'd like to follow along at home, I've attached the schematic for the power amplifier.
First step, "Thou shalt check the rails". 15V, -15V and 12V are right where they're supposed to be.
The first stage is U1, a MiniCircuits MAR-7. This provides 4dB of gain. Its cagemate U2 is the same part, and provides 18dB of gain.
This feeds into Q1/Q2, a combination of a 2SK878 AlGaAs/GaAs HEMT FET and a 2SA1162 PNP which should give 9dB of gain.
Q3/Q4 is more of the same, but with a 2SK571 GaAs MES-type FET and the same 2SA1162 PNP tuned for the same 9dB of gain.
Finally we have Q5 and Q6, a "900075" GaAs power FET and a 2SA1162. This has minus 7dB of gain (an attenuator?) and finally feeds into the funky ceramic microwave low-pass filter.
That gives us a total gain of 33dB leading up to the LPF. So let's trace the signal through.
30MHz centre, zero span, TG on, 0dBm. Go.
At "IN", we have 127mV of signal according to the DSO, in the form of a somewhat distorted sine wave (01_IN.PNG). After passing through U2 and C1, this becomes a respectable 219mV (02_U2in.PNG). A quick calculation pins that down at 4.7dB gain. Looks like that's working.
U1 is more of the same. Our 219mV becomes 1.8V at U1's output (03_U1out.PNG). There's our 18dB! By the time it's been through R3, C3 and C4, it's down at 530mV.
At the Q1/L3 junction (Q1/Q2 amplifier "output"), we see 2.91V (04_Q1out.PNG). Against our 1.8V that's only 4.1dB gain (we're expecting 9dB). Against 530mV, we're looking at 14dB, which is a bit much. This is starting to get interesting...
At the Q3/L7 junction (Q3/Q4 amplifier "output"), we have 3.34V (05_Q3out.PNG), but our sine wave has cast off its spectral purity and become something quite distorted! The positive peak clips at about +1.5V and folds back to about +1V. Oh dear.
By the time we've got to the output of the power amplifier (Q5/Q6), we've got something that bears a striking resemblance to a cardiac sinus wave (06_Q5out.PNG), and there just isn't any point in measuring the amplitude (it's about 543mV).
What I can't tell is, are the transistors acting up because something further down the line is loading them down?
What should my next step be? Start desoldering capacitors to isolate sections and re-measure? That seems a little dicey - the PCB is an alumina-base (from what I've been told) thing with gold tracks. Heating and reheating the thing seems likely to cause damage, and tricky to repair damage at that.
Does anyone have any suggestions?
Thanks,
Phil.