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Old 29th May 2015, 1:04 pm   #21
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Default Re: Spectrum Analyser TR4172

1906- prefix in HP-speak is either a multiple diode chip or else a matched set of individual ones.

Most of the HP diodes live on under different part numbers and in different packages on the Avago website . HP broke up into HP computers and Agilent instruments. Agilent then flogged off the old HP associates semiconductor arm and it became Avago

You can also get good mixer diodes by pulling apart old mini circuits mixers.

Some plastics and glues give off acids like HCl as they decompose, that ought to corrode most things.

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Old 31st May 2015, 2:34 pm   #22
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Default Re: Spectrum Analyser TR4172

Quote:
Some plastics and glues give off acids like HCl as they decompose, that ought to corrode most things.
I don't have much experience or knowledge about things like this but see the attached image of the insides of one of my TR4172 IF1 modules.

I can't remember which TR4172 this image came from but they were both very similar.

You can see the corrosion inside the main section and it even extends to the inner aluminium. By contrast, the input LPF section on the right still looks fairly new and shiny. So I think either the glue or the black lumps of absorber must sweat some form of catalyst for oxidation. Maybe they glued the absorber in place and immediately fitted the cover thus sealing in all the fumes from the glue as it cured? But that's just a guess.

I was expecting to see a cavity filter here but they use printed filter sections for the main IF1 filter. I measured the response on a VNA and it gave very good stopband performance.

The gain through the module was about +1dB and I measured the same on both of my TR4172 IF1 modules. I recall that the manual suggests the typical module gain is 0dB +/1dB.

The input section is a directional filter followed bu a roofing LPF, the IF1 amplifier and then the printed 2.046GHz IF filter sections. There's a load of RF absorber stuck inside the aluminium cover that fits over all this. The connections to the two SMA connectors in the main section are very fragile and had corroded to the point that one was only making a connection through touch alone. So this is one place where the module could suddenly lose gain.
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Old 18th Feb 2021, 9:25 am   #23
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Default Re: Spectrum Analyser TR4172

Thread reopened at member's request.
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Old 18th Feb 2021, 9:25 pm   #24
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Smile Re: Spectrum Analyser TR4172

Hi,

Iím new to this forum and I requested the moderator to reopen this thread because I have the same gain loss issue as described by Jeremy G0HZU. After some probing in my TR4172 I discovered that the gain loss was caused by the IF1 amplifier and after some googling I found this forum. In contradiction to the IF module of Jeremy mine looks ok and is slightly corroded. This is probably because there are no absorbers glued on the microstrip line at the output of the amplifier, see picture. Only in the cover some absorber is mounted. What looks suspicious are the film resistors. They are faded and have a strange colour. Jeremy I understood that you have modelled this IF1 amplifier stage and the biasing piggy board and I wonder if you want to share this information with me. You have simulated it with Agilent Genesys and professionally I use ADS (also Keysight). I do not know if I can import a Genesys project into ADS. If this is not possible than a schematic will also be helpful.

In my case I have observed the following: the IF module is drawing 23 mA at 15V, Vbe of the RF transistor is 0.89V, Vc_cap is 6.85V and Vce is 110 mV (much lower than in your case). Because the Rc film resistor looks suspicious it is possible that it also has a very high impedance. Can you please give me the resistor value which you have soldered on the board? You have simulated that Vce=2.5V was the optimum but at the end you have biased it at Vce=1.5 V, is this correct? Did you change something on the biasing piggy board (resistors are also faded and have a strange courol)? One more question, I assumed that the traces on the substrate board are of gold. Did you use indium solder to mount the SMD resistor to the strip line?

Sorry for all the questions but it will be very helpful and it will save me some time.

Regards, Lex
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Old 19th Feb 2021, 3:47 pm   #25
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Default Re: Spectrum Analyser TR4172

Hi Lex, I had a rummage and managed to find an old simulation of this amplifier. I think you have to first make sure the constant current biasing is operational. This uses a PNP transistor Q1 as in the circuit below.

My IF1 modules both drew about 16mA from +15V and I think the active bias fed just under 15mA into the RF amplifier BJT.

The constant current is defined by the 288R resistor and the voltage drop across it.

Constant current =(15V - VemitterQ1)/288R

Constant current = (15V- ((15V *(10000R/(10000R+4700R))) +0.65V))/288R

= (15 - ((15*0.68) + 0.65))/288 = 14.4mA

To get 2.6V at the collector the collector resistance would have to be (10.8V-2.6V)/0.0144 = approx 560R.

I chose to run the collector of Q2 at about 2.5V so I assume I added a resistor here to achieve this. I can't remember if I cut away the old failed printed resistor or not. I just used ordinary 60/40 solder but that was because it was all I had.

Your circuit values for the printed resistors may be different if your circuit draws 23mA. Also, these resistances may change over time. I can't be certain but I think the resistances shown below were measured from the real board and I think both boards were the same apart from the Q2 collector resistor that had failed.
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Old 19th Feb 2021, 4:13 pm   #26
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Default Re: Spectrum Analyser TR4172

I had a quick play with the simulator and tried to come up with a failure mode that fits your symptoms below.

23 mA at 15V, Vbe of the RF transistor is 0.89V, Vc_cap is 6.85V and Vce is 110 mV

If there is a significant breakdown leakage path (maybe even a short) between collector and emitter of the active bias PNP transistor Q2 the circuit will draw 21mA and will give the following readings:

21 mA at 15V, Vbe of the RF transistor 0.9V, Vc_cap is 9V and Vce is 140 mV

This matches fairly close and a lot depends on how badly the transistor has failed. I think pretty much any SOT23 PNP will work fine here as a replacement for Q2 if your circuit has failed in this way.

Could the blob of gunk that sits over Q2 (shown in your image) have caused this collector emitter leakage path?
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Old 19th Feb 2021, 8:15 pm   #27
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Thumbs up Re: Spectrum Analyser TR4172

Hi Jeremy,

Thank you for the quick response. The schematic is very helpful and now I better understand what is going on. I agree that probably something is wrong on the piggy bias board. I did not mention it but VTP2 is much lower in my case, 7.31 V which is divided from the 15V and loaded by Q1 (PNP). I think I will first disconnect the biasing from the RF transistor Q2 and check all resistors values. The resistors on the biasing piggy board are most suspicious. Based on that outcome I will try to remove the blob on Q1. Tomorrow busy so this will be Sunday. Thanks so far.

Best regards, Lex
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Old 19th Feb 2021, 10:31 pm   #28
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Default Re: Spectrum Analyser TR4172

Glad to be of help! The TR4172 is a very good spectrum analyser so I hope I can help you get it working again.

I earlier spotted some notes in the simulation workspace and this has the following info:

Quote:

Second TR4172 notes and differences

15V supply current 15.8mA

R5 280R constant current resistor (was 288R)
R2 (collector) 445R 4.13V (was damaged)

Vbase RF = 0.80V
VTP1 4.13V
VTP2 10.20V
VTP3 1.06V
VTP4 10.79V
VTP5 0.80V
These look to be genuine voltages measured of the real circuit of the second TR4172. The collector resistor of the RF BJT on the first
one had been so badly damaged/dissolved by the glue it was almost eaten away.

However, I'm a bit confused by my notes above vs what I wrote in an earlier post about the second TR4172. It looks like the second one had a 445R resistor here although I don't know if I put this 445R resistor there or not.
I do recall measuring 1.5V here on the second TR4172 but it was 5 years ago!
It looks like it was measuring 4.13V here just before I put it back together.

I think it would be really useful if you could measure all the resistor values on your board. There's a good chance they are all healthy and it would be nice to know the correct values and the correct test point voltages.
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Old 21st Feb 2021, 7:32 pm   #29
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Default Re: Spectrum Analyser TR4172

Hi Jeremy, I think that you are right that something is wrong with the PNP. I have measured the following resistor values:
R1 = 2k1, R2 = 391, R3 = 9k96, R4 = 3k28, R5 = 316

I have disconnected the RF transistor but if the PNP is broken some values can be incorrect. I did not remeasure the voltages but this is wat I have measured the last time:
VTP1 = 110 mV
VTP2 = 7.31 V
VTP3 = 6.85 V
VTP4 = 7.94 V
VTP5 = 0.89 V

Note, VTP2 is not what you expect based on the voltage divide ratio. Based on the measured resistor values and voltages the current through the RF transistor is about 20 mA. It is a pity that they used the black blob on the PNP. It will be hard to remove it without damaging R3 and R4. What do you think?

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Old 21st Feb 2021, 9:25 pm   #30
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Default Re: Spectrum Analyser TR4172

It's hard to tell at the moment but maybe the B-E and C-E could be faulty in the PNP transistor.

I'm guessing that the blob is some form of hard resin. If so then I think other people on the forum will be able to offer their experience here. I can only suggest a few obvious techniques (scalpel or Dremel) that will probably damage the resistors.

If the resistors do get damaged then they could be cut or drilled away and they could be replaced with SMD versions in something like 1206 or 0805 package. I suspect the PNP will be a simple BJT. The M6 code suggests that the 2SA812 is a possibility and this version of the 2SA812 is in the gain group 200-400.
Something like a BC856B or BC857B would probably be fine here.
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Old 23rd Feb 2021, 8:22 pm   #31
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Default Re: Spectrum Analyser TR4172

Thanks, I have ordered the BC857B and in the meantime I will try to remove the blob. Actually I do not understand why they have used the blob unless it is a bare die bonded on the board. We will see whatís inside. Question, do you want to share your model of the 2SC5010 with me than I can do some simulations myself in combination with the BC857B. You can send me a private mail if you want to.
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Old 23rd Feb 2021, 9:23 pm   #32
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Default Re: Spectrum Analyser TR4172

Sadly, the 2SC5010 model (along with many others) is embedded into a huge common non-linear library file within Genesys so it can't easily be understood or extracted.

I'm not sure how accurate the model is. I can't remember if I removed the SMD caps and measured them or if I reverse engineered what they should be by using the simulator but I got the results below.

Gain 9.9dB at 2.05GHz
Noise Figure 1.9dB at 2.05GHz
OIP3 +23dBm at 2.05GHz
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Old 24th Feb 2021, 5:12 pm   #33
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Default Re: Spectrum Analyser TR4172

I did a simulation myself with the BC857B and some NPN and I concluded that it must be the B-C transition. This also explains the low VTP2 of 7.3 V which I have measured. I will keep you informed.

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Old 2nd Mar 2021, 8:01 pm   #34
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Default Re: Spectrum Analyser TR4172

First post, so please bear with me if I write something ignorant...

It is apparently TR4172 ailment season. I acquired a TakedaRiken branded TR4172 a few years back for a song at a local hamfest (anyone remember those? [dang covid-19!]). I think it was $150US plus the herniated disc it took to carry both units to the car. I fully expected it to not work well, and indeed it was about 20dB deaf on input 1 and 14dB on input 2. Enough deafness that it would not pass the auto calibration routine. Still, it worked good enough for the light duty work I needed, and was still rather linear across it's operating range. Fast forward to this week, and while checking the stability of the timebase, leaving the SpecAn on overnight, I notice that the signal I had fed into it was around 20 dB less than it was the day before. Hmm... not fun! After muddling through the test flowcharts I finally decided to simply check the 1st mixer, remembering that more often than not, this is where deafness occurs. Sure enough, I discovered what G0HZU had found; lots of corrosion inside the mixer. In fact, the diode ring has pretty much crumbled apart without much chance of reviving it. The leads have basically eroded to nothing and it appears that even the package has deteriorated. Now I am stuck to find a replacement mixer or diode, and I should probably also open the IF interface to look for issues, and/or also lend a hand if anyone needs any info or measurements.
I want to give a huge thanks to everyone on this thread - I've learned something from it, and at least have a part number to investigate for the ring mixer.
On that note, has anyone ever considered or done a replacement swap with a more commonly available mixer? Would it be advisable?
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Old 3rd Mar 2021, 3:47 pm   #35
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Default Re: Spectrum Analyser TR4172

And the obligatory thousand words each...

Fortunately, after a bath in the ultrasonic cleaner for a while I found that things in the mixer weren't quite as bad as I first thought. It appears that the mixer has just enough leads left to facilitate some solder surgery, so I may be in luck there. Pictures for that to come...
Unfortunately, the interface module was in pretty poor shape. They appear to have used a ceramic substrate with gold plated copper or pure gold foil. Whatever it is, one can see the lifted trace in the coupler section. This was likely the cause of the initial loss of sensitivity. I'm debating on the method to affix this, and the best I can come up with is using a sharp needle to apply a very thin layer of cyanoacrylate to the substrate and an artist's brush to lightly press down on the trace, similar to applying gold leaf.
Anyone have any better ideas?
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Old 3rd Mar 2021, 4:07 pm   #36
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Default Re: Spectrum Analyser TR4172

Hi Matt
Are you sure it is the first mixer you are referring to? This has 8 diodes as per the image from the manual below. On my TR4172 analyser this mixer is in a flat pill package with 4 gold legs. It looks fairly robust to me and I'm not sure how it would crumble apart.

However, if this mixer has failed then there are a few things to bear in mind:

The mixer contains 8 diodes and the local oscillator drive power to this mixer is typically +22dBm. This will make it difficult to find a drop in replacement if the original part cannot be purchased at a reasonable price anywhere.

In the diagram below I've offered a temporary bodge solution using Avago HSMS-2822 or HSMS-282C diode pairs. This 'might' work and it might cope with the +22dBm LO drive level. Otherwise you might have to fit an SMA 3dB or 6dB attenuator inline with the semi-rigid cable from the LO amplifier module.

The Avago HSMS-2822 (SOT-23) and HSMS-282C (smaller SOT-323) diode pairs are obsolete but they are cheap ($1 each?) and you might still be able to buy them on Ebay. I doubt this fudged mixer will work very well (especially at 1800MHz) and it won't have very good balance either so some aspects of the analyser performance will be compromised. I'd expect it to be better than a broken mixer if you really just want a cheap repair that restores some basic functionality.

You could opt to buy a quad diode package as this will work better at higher frequency but this is unlikely to cope with more than +10dBm LO power.

Edit:
I've changed the part number to the 15V version of the diode.
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Old 3rd Mar 2021, 5:52 pm   #37
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Default Re: Spectrum Analyser TR4172

Hi, Jeremy.
Yes, it is unfortunately the 1st mixer. I thought the package had crumbled, but thankfully I was mistaken. It was just REAL dirty and cruddy (see the pic). I also had never seen up close a package quite like this and did not realize the black area was part of the package. It was only the legs that crumbled. One would think that gold plated copper would hold up to just about anything corrosive, but here we are...
I think I may have been able to save the mixer though. It's currently soaking in some denatured alcohol to remove the flux, but see the attached picture taken before the flux removal. There were basically no legs left after 35 years or so exposed to whatever evils kill these devices. All that I was able to solder to was whatever was left of the thin leads sticking out of the package. It isn't pretty by any stretch of the imagination. Hopefully I'll see some response through it on my 8753, and hopefully it will last if it works.
As for the IF amp block, I'm about to attack it and see what I can do with it. That will be another very intricate repair, but I can't see any other way to do it other than some glue.
What's worse is that there are very thin film resistor elements at the ends of the coupler traces. Thankfully the trace has not become detached from the resistor. Worst case there is that I'd have to solder in a SMT device if I break that bond. I'm not sure though... this is all new territory for me. I'm more used to large scale RF stuff... TV and radio transmitters in the several tens or hundreds of kilowatt range. T&M equipment repair is something that I've just recently delved into out of necessity, and I don't have much experience with microwave equipment on this small of a scale... usually it is a matter of swapping modules, not repairing them.
Thanks for the ideas and advice.
Regards,
Matt
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Old 3rd Mar 2021, 6:03 pm   #38
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Default Re: Spectrum Analyser TR4172

Wow, I just saw your post #35. It wasn't visible whilst I typed the stuff above. I can see the state of the first mixer now. I can't explain what has caused this unless there is something in the mixer box that caused the corrosion. This might be due to some sort of chemical reaction to fumes from RF absorber or glue and this might be hidden underneath.

Your directional filter also looks to be peeling away from the PCB in the IF1 module. This doesn't look good. The PCB traces on this board are very fragile.
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Old 3rd Mar 2021, 6:09 pm   #39
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Default Re: Spectrum Analyser TR4172

In the worst case, it would be possible to remove and scrap the square directional filter section shown in 20210302_173808.jpg and make a copy as this looks to be a separate PCB. There isn't anything special there and it just has to have the right dimensions to suit the alternative PCB material Er and thickness. Make sure you never touch the other printed BPF sections though. This 2.05GHz BPF would be much harder to replicate.
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Old 3rd Mar 2021, 6:25 pm   #40
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Default Re: Spectrum Analyser TR4172

I'm still on the probationary period of having to have the posts approved by a moderator.
Yeah, if I can't glue this stuff, then that would be the next step, and I do have the equipment to fab a board if need be. I had not thought of that; thanks!
I concur that these issues are likely the glue they used to glue the absorber material to the covers. It is amazing how it even tarnished the silver plated brass plate in the amp section. I'm still on the fence as to whether I should bother removing the absorber and re-gluing with something else, or to take the chance that perhaps all that could have out-gassed already has. Perhaps I'll just leave it and check in a couple of years.
73,
Matt
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