Re: Spectrum Analyser TR4172
I thought an update on what I found with my TR might be of interest as my repairs have been very slow....
1. Hairline crack in the trace in the attenuator (just after the AC/DC coupling selection). This acted as a capacitor at high frequency, but caused much extra attenuation at low and I didn't actually find it until I put the unit on the VNA which produced a very capacitative S21. DC measurement confirmed the problem. Soldered a piece of wire over the break under the microscope.
2. PLL fault for 101/100MHz PLL caused by two problems. Someone had "undone the screws" in the xtal harmonic multiplier (alignment using another SA cured this) and there was a leaky NP electrolytic in the loop filter. Unfortunately the manual and the boards I have don't match - for example my VCO modules are 3 pin devices with the tuning and power combined, but the KO4BB manual shows 4 pin devices also mine uses a MC4044 PFD whereas manual shows JK flipflops and an AND gate. As a result the alignment instructions don't work (it is necessary to know what voltage to apply with the loop opened before tuning). Mostly working but would like to know the correct alignment procedure. There seem more operating manuals than service. If I were to guess from the sequence of parts numbers I'd say my board is earlier.
3. 2.046GHz filter that doesn't work in the IF interface. As suspected the gold traces had lifted leaving the resistive material below (must have happened long before it came into my hands). I replaced by a piece of fibreglass board with a 50 ohm microstrip for now and have redesigned a 4 pole bandpass filter onto RO3210 which needs fabricating (with proper photo-etching not dubious toner transfer).
4. Bias network for 1st IF amp not working. Added external SMD resistors and replaced bias transistor until correct quiescent current.
With these done it self-calibrates and works fine (although I am sure IM performance is impaired by the "special" filter). It's a very pleasant SA to use and feels fairly modern. I haven't tried the impedance mode yet.
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