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-   -   Spectrum Analyser TR4172 (https://www.vintage-radio.net/forum/showthread.php?t=77436)

G0HZU_JMR 3rd Mar 2021 6:40 pm

Re: Spectrum Analyser TR4172
 
1 Attachment(s)
The square filter section will probably have a very narrow response. The idea is to have a selective filter at the first IF but to also have a lowish VSWR across a much broader bandwidth. This broadband match is desirable as this filter sits directly at the IF port of the mixer.

See below for an old simulation of a similar design I did on Genesys many years ago. It has a similar IF frequency. You can see that there are two simulation responses for the filter. One uses physical models for the printed lines and the other uses the Sonnet EM engine to do an EM analysis of the actual PCB layout. The two responses agree quite well.

The slight worry will be that if you glue the microstrip back down it might be detuned slightly away from the true IF1 frequency. However, I'd still be tempted to try gluing it as you suggested. Just use minimal glue and try and get the microstrip as flush to the PCB as possible.

Matt kd4pbs 3rd Mar 2021 7:05 pm

Re: Spectrum Analyser TR4172
 
Most excellent, sir! A huge thanks to you, Jeremy. I imagine even if I screw it up a bit, it would be better than it was. If need be, I could perhaps outboard a separate directional coupler and a sharp-tuned cavity filter - at least there's enough room in there to do so. A bodge for sure, but I simply love this old SpecAn. I don't want to retire her any time soon if I can help it.
73!
Matt

G0HZU_JMR 3rd Mar 2021 7:26 pm

Re: Spectrum Analyser TR4172
 
If you measure the gain through the whole 2.05GHz signal path in that module you should get something like 0dB +/-1dB gain at 2.05GHz as seen in my plot in post #22. I think I saw +1dB gain here for one of my analysers and slightly less on the other. If you get within a couple of dB of this I think it means the gluing was largely successful.

It's probably best to use an SMA attenuator at each cable end before you normalise the test rig to 0dB gain at 2.05GHz as this will minimise mismatch uncertainty effects. But I guess you would do this anyway (or you could use a fully corrected 2 port VNA). The IF1 amplifier drew 16mA from +15V when I did this test.

Radio Wrangler 3rd Mar 2021 9:58 pm

Re: Spectrum Analyser TR4172
 
It's quite important or getting low spurii to have all mixer ports nicely terminated at all frequencies that stuff comes out of them. It's not just the wanted frequency, it's the images, LO and RF blowing straight through etc, as well as harmonics and intermod products. The absolute last place you want these things to go is back into the mixer to cause more trouble.

So the RF lowpass on the input might be arranged to terminate the LO (and harmonics) leaking out the mixer input port, or maybe an attenuator is used to sacrifice some sensitivity for cleanliness. The LO port in posh analysers may get a circulator and load to dump anythng coming back, or maybe a beefier LO drive is used with a broadband pad.

Termination of the IF is an obvious one. Something important in high performance comms receivers as well.

David

Matt kd4pbs 3rd Mar 2021 10:51 pm

Re: Spectrum Analyser TR4172
 
3 Attachment(s)
After having to try a few more times to get this guy soldered in there, it would appear that I have gotten the mixer to once again mix. I don't have a spec an that goes high enough, but I do have a counter that does, and when applying a 2.1GHz +12dBM signal to the LO input, I see it on the output. When I switch on a 10MHz 0dBM signal to the RF in and a I get the 10MHz offset on the counter. I buttoned it up and checked the RF and LO inputs to the mixer, and it would appear that perhaps I have a blown input attenuator in the mixer... the VSWR is rather poor.
All unused ports terminated, of course.
Now to tackle the amp...

Matt kd4pbs 3rd Mar 2021 11:23 pm

Re: Spectrum Analyser TR4172
 
3 Attachment(s)
Well, not so much luck with the amp.
I went ahead and looked at the input impedance, and as expected, she's a bit off. At least I'll know if it gets better after I attempt a repair.
Just for kicks I measured the LPF response.
Then on to check the gain. I see about -45dBM through it without power, and that drops several dB when applying 15V. Current draw is only 1.085mA. Checking voltages on the transistor, and I see my 15V on the green dot in my picture, but when I place the probe onto the red dot, the voltage starts at around 300mV and drops to around 160mV within just a couple of seconds.
I'm done for the day... time for some grub in my pie hole.
Thanks again for the tips, guys... I'll get this thing going one way or another.
Regards-
-Matt

G0HZU_JMR 4th Mar 2021 12:20 am

Re: Spectrum Analyser TR4172
 
1 Attachment(s)
I can add some performance info about the mixer if that helps.

The 2HI performance and IP3 performance of this mixer across HF and into VHF was very special in its day. When integrated into the analyser the analyser boasted an input IP3 of +22dBm with 0dB attenuation at the front panel. Obviously there is also a permanent 3dB attenuator there but this is still really good performance from a mixer. The 2HI performance was even better. A good spectrum analyser will manage a 2HI of +50dBm down at HF/VHF. Most barely manage +30dBm. This high level mixer with its 8 diodes was at least 20dB better than the classic HP8568B in this 2HI test.

I can remember testing the mixer in isolation when my TR4172 was failing at IF1. I recall it managed an input IP3 of +25dBm. I think the conversion loss was quite poor because of the input attenuator and the RC compensation. I think it was something like 13.5dB loss through the mixer at HF/VHF. The mixer performs best with at least +13dBm LO drive and the distortion figures improve as the drive level is increased to +20dBm.

Note I wouldn't advise running the analyser if you ever remove the mixer block or disconnect the LO cable because this will leave the large (expensive + unobtanium) LO drive module unterminated and this might stress it.

It does look like your IF1 amplifier block is faulty...

I had a rummage and found a plot of the input LPF in my first TR4172. See the plot below. This shows just under 2dB loss at 1800MHz. There is very little in the way of IF rejection in this filter and that is why I think there is also a stub filter.

G0HZU_JMR 4th Mar 2021 12:37 am

Re: Spectrum Analyser TR4172
 
One other thing, I keep meaning to mention that it seems odd to see that big chunky brown axial resistor on the LO port of the mixer. There will be +22dBm across 2GHz to 4GHz here and it seems odd to have that large type of shunt component fitted here. It isn't shown in the manual and I'm quite sure it isn't there in either of my TR4172 analysers.

What value is the resistance? It might be there to stabilise the LO port impedance but it seems an odd package choice. However, Takeda Riken (Advantest) do seem to fit some odd components in odd ways.

Matt kd4pbs 4th Mar 2021 12:58 am

Re: Spectrum Analyser TR4172
 
Quote:

Originally Posted by G0HZU_JMR (Post 1348925)
I can add some performance info about the mixer if that helps.

The 2HI performance and IP3 performance of this mixer across HF and into VHF was very special in its day. When integrated into the analyser the analyser boasted an input IP3 of +22dBm with 0dB attenuation at the front panel. Obviously there is also a permanent 3dB attenuator there but this is still really good performance from a mixer. The 2HI performance was even better. A good spectrum analyser will manage a 2HI of +50dBm down at HF/VHF. Most barely manage +30dBm. This high level mixer with its 8 diodes was at least 20dB better than the classic HP8568B in this 2HI test.

I can remember testing the mixer in isolation when my TR4172 was failing at IF1. I recall it managed an input IP3 of +25dBm. I think the conversion loss was quite poor because of the input attenuator and the RC compensation. I think it was something like 13.5dB loss through the mixer at HF/VHF. The mixer performs best with at least +13dBm LO drive and the distortion figures improve as the drive level is increased to +20dBm.

I have read about this SpecAn's legendary sensitivity, image rejection, and noise figure. These are all parts of the reason why I really don't want to retire her, although if I do, her guts will go to those needing to maintain another.

Quote:

Note I wouldn't advise running the analyser if you ever remove the mixer block or disconnect the LO cable because this will leave the large (expensive + unobtanium) LO drive module unterminated and this might stress it.
Duly noted. I was worried that the YIG was falling in output at first due to my prior experiences with the High Priced equipment - namely my 8753D, but thankfully that turned out to be well within spec. I'd hate to think of the bodgerry I'd need to come up with or the expense I'd have to incur to rig a different one or have this one repaired.

Quote:

It does look like your IF1 amplifier block is faulty...

I had a rummage and found a plot of the input LPF in my first TR4172. See the plot below. This shows just under 2dB loss at 1800MHz. There is very little in the way of IF rejection in this filter and that is why I think there is also a stub filter.
That is very helpful; I need to verify that the poor response I see is not due to some shady adaptors; I calibrated the VNA to the N connectors, and this includes the N to SMA adaptors I used to gointa and getouta.

Matt kd4pbs 4th Mar 2021 1:06 am

Re: Spectrum Analyser TR4172
 
Quote:

Originally Posted by G0HZU_JMR (Post 1348931)
One other thing, I keep meaning to mention that it seems odd to see that big chunky brown axial resistor on the LO port of the mixer. There will be +22dBm across 2GHz to 4GHz here and it seems odd to have that large type of shunt component fitted here. It isn't shown in the manual and I'm quite sure it isn't there in either of my TR4172 analysers.

What value is the resistance? It might be there to stabilise the LO port impedance but it seems an odd package choice. However, Takeda Riken (Advantest) do seem to fit some odd components in odd ways.

It's a 1K 1/4 watt resistor. The ultrasonic bath handily removed the rust, grunge, and the paint.
The point at which it fastens is key... I imagine it's at a node along the stripline that is ideal for that complex impedance. I'm figuring it kills some parasitic oscillations or tunes the line a bit sharper to help narrow things down, but I'm just assuming... when it comes to MMIC tech, striplines, and all of these microwave kinds of craziness, it's all FM to me.

Matt kd4pbs 4th Mar 2021 1:14 am

Re: Spectrum Analyser TR4172
 
1 Attachment(s)
Oh, and remember, this is evidently a very early model. I really need to look at some component date codes to see when it was probably made. I see the big blob of black epoxy on the amp board that wasn't on yours, and there are things that are in the Advantest manual that this doesn't do or doesn't have.
One of the most interesting things is what I found on the YIG driver. Check out this bit of modification... It appears to be done by an official repair shop, if not by TR themselves. I bet neither of yours have this...
Attachment 228219

Matt kd4pbs 4th Mar 2021 4:32 pm

Re: Spectrum Analyser TR4172
 
I was able to procure a couple of replacement 2SC2150 transistors (I believe in having spares) just in case this one is dead. For sure, the black spot on the case is a bit suspicious, and it doesn't appear to be dirt. They're coming from your side of the pond, Jeremy, so it will take a little while.
I also have a request in for getting a couple of ring diode mixer replacements from a company in the states... hopefully they will let me have them. It is almost like the mixer is some guarded top secret contraband key piece of some cold war tech or something... I had to answer lots of questions about who I am and what I need the diode for. Hopefully they will be able to sell me just a couple, in case the attempted repair doesn't go so well.
In the meantime, I will be measuring what I can on the little piggyback board. I sure do hope it isn't the regulator transistor under that epoxy.
Thanks, everyone!
Regards,
Matt

G0HZU_JMR 4th Mar 2021 4:44 pm

Re: Spectrum Analyser TR4172
 
I agree, I haven't seen anything like the strange little piggy board in the YIG driver circuit on my analysers. I think my main TR4172 was made in 1987. I think the earliest versions of this analyser date back to about 1981.

Can you double check if the 3dB attenuator is OK? The easiest check is to just use a DMM and measure the resistances around it at DC. I think it is a pi attenuator so it should measure just under 150R to ground on either side of the attenuator.

It should measure about 17R across its input and output as there will be a series 17.6R resistor here. I think it would take a lot of RF power to totally damage it so maybe it is OK and there is a poor connection somewhere that makes it look bad. If it measures OK with a DMM it will probably be OK at RF as well because it will use very good resistors inside that package.

Matt kd4pbs 4th Mar 2021 7:20 pm

Re: Spectrum Analyser TR4172
 
Will do. I'll check that this evening. I do seem to remember checking it at DC the other day and I had some very high numbers, in the 10s of K, so this is part of why I think it's toast. If so, I'll just straight through that guy and put an external SMA attenuator in there.
Incidentally, I did some photoshop-fu using my pic and your pic of that piggy back board, and your schematic to come up with an annotated picture of what's what.
Hopefully this will help people troubleshoot this little board...
https://drive.google.com/file/d/1Amv...ew?usp=sharing

Matt kd4pbs 5th Mar 2021 12:49 am

Re: Spectrum Analyser TR4172
 
I am still getting used to the 34401 DMM I recently purchased... the pad is fine. I was reading it wrong is all.
The two shunts measure 147.44Ω and 147.9Ω, and the series reads 16.325Ω. Edit: Plugging those numbers into an attenuator calculator makes a poor 50Ω attenuator with 4+ dB of attenuation. I might just yank that out anyway and go the external route.

I checked around the amp circuit a bit more and it appears that the amplifier transistor is indeed kaput; no reading between base and collector. Good thing I have a couple on the way.
I'm also guessing that the reason it went North is due to something going terribly wrong under the epoxy lump. I'd think that I should still see some Vbe due to the base-emitter junction looking okay with respect to the voltage drop reading. More to follow when I learn it :)

Matt kd4pbs 5th Mar 2021 1:39 am

Re: Spectrum Analyser TR4172
 
Something I was contemplating, Lex... If you haven't tried to remove the epoxy yet, it might be possible to remove the epoxy by heating it up with a heat gun. It might take some time, but controlling the heat to below the melting point of the solder would probably soften it enough to remove without damaging anything.
Regards,
Matt

G0HZU_JMR 5th Mar 2021 11:30 am

Re: Spectrum Analyser TR4172
 
Quote:

Originally Posted by Matt kd4pbs (Post 1349332)
I am still getting used to the 34401 DMM I recently purchased... the pad is fine. I was reading it wrong is all.
The two shunts measure 147.44Ω and 147.9Ω, and the series reads 16.325Ω. Edit: Plugging those numbers into an attenuator calculator makes a poor 50Ω attenuator with 4+ dB of attenuation. I might just yank that out anyway and go the external route.

The attenuator readings look to be spot on. Don't forget that when it is in circuit and you measure one side of the attenuator you will be reading 290R in parallel with (17+292) because the resistors are linked via the ground connection. This means each shunt connection will work out to be just under 150R when measured in circuit.

I think your 3dB attenuator is healthy.

Matt kd4pbs 5th Mar 2021 2:23 pm

Re: Spectrum Analyser TR4172
 
Indeed. Oops! A trap for young players ;)
Thanks!

Matt kd4pbs 5th Mar 2021 3:33 pm

Re: Spectrum Analyser TR4172
 
1 Attachment(s)
I love it when a problem reveals itself...
In probing out the PNP transistor, having to apply a slight amount of pressure to the capacitor in the lower left, the epoxy blob popped loose revealing the largest problem.

Attachment 228324

I'm surprised it even worked at all before this!
At least it's all DC here, and easy to reproduce.
Lex, I'd be willing to wager your issue is similar.

G0HZU_JMR 5th Mar 2021 3:56 pm

Re: Spectrum Analyser TR4172
 
Wow, that looks nasty! I agree that Lex is likely to see the same problem and it should be easy and cheap to repair this. I was hoping that your circuit was going to prove to be intact so we could check what the correct resistances should be for the IF1 bias board.

However, both of my TR4172 IF1 modules did not have the epoxy blob and the printed resistances were very similar on both apart from the R2 collector resistor of the 2SC5010. This had been eaten away by the absorber glue that sat over it on my first TR4172.

When I repaired my first TR4172 I had no idea what resistance should be here for R2 and I had to resort to a simulation in Genesys to try and find the best value for gain vs noise figure and signal handling. I think I chose to put 560R here for R2 although it may be that a typical (factory) resistance here will be somewhere around 400R.


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