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Alan Bain 2nd Jun 2022 10:56 am

Re: Spectrum Analyser TR4172
By some micacle I saw the capacitor which had pinged out of the black blob on the board. It is connected between base and ground of the "bias" transistor. I was able to solder leads on and measure on a bridge 10nF in a 0805 package. Leaving this here in case of use to anyone in future.

Matt kd4pbs 3rd Jun 2022 1:32 pm

Re: Spectrum Analyser TR4172
Good find! For sure, far less than I thought it would be.
I do hope you get this going, Alan. It's a wonderful SpecAn... when it works. Super low noise, great filtering, and a high resolution, even by today's standards. What's not to love? Oh yeah... the age-related issues... I forgot ;)

Alan Bain 22nd Jun 2022 8:17 am

Re: Spectrum Analyser TR4172
Yes, replacing smelly electrolytics in the RF section is a fairly slow process. Almost back able to do some actual RF testing!

Alan Bain 25th Jun 2022 8:05 pm

Re: Spectrum Analyser TR4172
Back together and starting to test to find what I forgot to connect - running into a few manual discrepancies; manual makes reference of a T.G CNTR button but when I look where that should be I find TUNED AMP. Is this just a new name for the same thing (as in count the actual signal not just calculated centre + offset)?

Alan Bain 26th Jun 2022 7:16 pm

Re: Spectrum Analyser TR4172
2 Attachment(s)
Here is the repaired first IF bias board (after and before pictures!). I have to admit that it was tricky. The board material conducts heat away very effectively and the gold layer appears to be pure gold (so no nickel in the alloy) and doesn't wet with solder. I found I had to use a powerful iron to be able to work quickly. The resistive layer underneath does wet with difficulty. I was very reluctant to use anything corrosive to clean the surfaces given the comments in this thread about glue cure by products damaging trace adhesion. New parts deliberately not straight to avoid the damaged trace sections. One trace to decoupling 10n cap replaced by a wire (single strand of 7/0.2 hookup wire).

Afterwards now drawing 13mA instead of 22mA+ and voltages look sane. Time for more analyser testing.

Alan Bain 27th Jun 2022 2:43 pm

Re: Spectrum Analyser TR4172
After testing further I've noticed that the directional coupler is terminated in 50ohm on input side but measures about 600ohms looking back from C1. While I don't have any values any design for such a coupler that I have seen has had both ports terminated equally. Fortunately I have some precision 0402 50ohm terminating resistors, but does anyone have any ideas of a flux that allows the gold to be wetted?

Matt kd4pbs 15th Jul 2022 3:30 pm

Re: Spectrum Analyser TR4172
Can't help you there, Alan. I noticed the same thing with the insanely difficult to solder properties of the substrate, and with my iron at a heat setting that would overcome the thermal conduction from the board, it would melt the traces.
I think I do remember having the same issue that you see... the directional coupler/filter not being equally terminated.
Ultimately, after bodging the SA enough that I got it working and calibrated, after a few weeks I noticed it went deaf again. Likely a leg of the mixer corroded again. I have since pulled it from the shelf and set it in the morgue awaiting either the day it gets carted to the dump or I find the required parts to make it right.

Alan Bain 16th Jul 2022 7:09 pm

Re: Spectrum Analyser TR4172
1 Attachment(s)
So having mended the IF amp, remaining deafness hard to trace without an SA which covers 2-3GHz. Thanks to a kind loan, the deafness traced to one of the stripline 2.046GHz bandpass filter sections which is an approximately 60db attenuator. Careful inspection shows one section of stripline has no gold so no great surprise and I should have noticed sooner! For the moment I have put in a piece of 50ohm semi-rigid coax to bypass this section and with this bodge I can get it to calibrate.

I vaguely recollect doing the J-admittance inverter calculations to design one of these filters for a Chebychev response and I have a quick response measurement from the other so that shouldn't be so hard although this looks like an Alumina substrate around 0.72mm thick ... don't think the cheap Chinese PCB people do Alumina, or Rogers 3010! And I also seem to remember some iterative length adjustment was needed to the lines....

Attachment 261070

Alan Bain 23rd Sep 2022 6:39 pm

Re: Spectrum Analyser TR4172
1 Attachment(s)
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.

G0HZU_JMR 23rd Sep 2022 9:45 pm

Re: Spectrum Analyser TR4172
Good work! My TR4172 also suffered a microstrip failure in the attenuator section.

If you do experiment with the impedance mode, turn down the CRT brightness first. Otherwise, it can draw a pin sharp stationary dot on the smith chart that can be frighteningly bright. However, I've not burnt the CRT on mine so maybe it is fairly immune to burn. The calibration process for the impedance mode is quite clunky and time consuming but on the few occasions I tried to use it, the performance was much better than I was expecting. However, I think a modern nanovna is going to be easier to use and will usually give much better performance for impedance measurements.

I spent many years using this TR4172 at work, so I grew quite fond of it. When the chance came to save it from disposal, I bought it from the company for the cost of a small charitable donation. The strongest points of the TR4172 were the second harmonic intercept and the LO phase noise at 100kHz offset. The mixer IP3 was impressive too at about 21dBm across HF and into VHF.

I mainly used it for VHF and UHF synthesiser design back in the early 1990s and also for distortion measurements. The KE5FX plotter software can work with it via GPIB but it is very fiddly to set up on a TR4172 that has the options that mine has. It's not an issue with the KE5FX SW, I think it depends if the TR4172 has the enhanced GPIB capability fitted. No-one at work could ever get this TR4172 to plot with an old school HP plotter but it isn't faulty as such.

Alan Bain 25th Sep 2022 8:10 pm

Re: Spectrum Analyser TR4172
It seems the CRTs do burn; mine has a very pronounced image of the rectangular graticule even when switched off!

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