Need help with Tektronix 2710 spectrum analyzer

You could check the power supply and bias amp for the YIG. But if the YIG itself is toast I think most are sealed assemblies that can't really be field-repaired.

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Regards, Joerg

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Hi Esa. First of all, let me tell you that I never seen the 2710 schematics, the things I write below are just suggestions based on intuition (maybe you had already do that).

The 26Mhz VCO you find on the center frequecy board, should enter into a mixer. Look for the LO intput level of the mixer, maybe it is too low or near the level at which the mixer starts to work (a passive mixer usually need a LO signal of 4-10dbm to work). Of course, maybe the 26MHz VCO frequency or the other frequency incoming into the mixer the LO signal, so check both the signals for the right level. Maybe it's just a "cold" solder joint.

Hope the above will help.

Good luck.

Have a great day.

Massimo

Hi Max.

I did some scans from service manual schematic and circuit description pages. Maybe these will help to get further:

I do how the frequency synthesizers work but still can't get the figure of this! For example there are no N divider at all and the description claims that the harmonics of signal called "strobe" is used instead? And this "strobe" signal is infact that ~26 MHz signal mentioned before! But there are 11 MHz VCO also... And sometimes it counts "beat" and sometimes phase offset like normal PLL. I don't really know why this is so complex - may be there wasn't dividers fast enough to count 2.11 -

3.91 GHz in the end of 1980's?

I guess that it locks to wrong harmonic of the strobe, because the frequency is always ~26 MHz off when the fault kicks in. But why this is happening only with higher frequencies - just no idea. Also, the service manual claims that with wide spans the strobe signal is not even turned on and the YIG main coil is driven with suitable current controlled by CPU. So how the frequency can be same amount off without "strobe"? Only explanation that I could guess is that the fault was on when frequency normalization was run and because of that there are wrong DAC values in some kind of "main coil current table" in the CPU.

Clearly I'm too novice with those 1980's RF circuits...

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Esa
http://www.amigazone.fi/contact

If I were you, I checked the incoming signals at the mixer HY450. If I'm right, its output (BEAT NOTE) should be either the PLL feedback signal when used in small span scans and the center frequency tuning signal in the other cases. Does Q460 work correctly?

Massimo

Yes I already did some measurements from it's output from (from J430) with oscilloscope. Signal at this point is weak, infact it can't be clearly measured with digital oscilloscope at all, execpt when it's frequency is low (couple tens of kHz) when it's amplitude raises. The frequency here changes when spectrum center frequency is tuned. The strobe signal in J460 is strong.

Infact this strobe signal is not always turned on when instument is in normal mode. It turns only momentary on when frequency correction is done between sweeps, occassionally. I guess the reason for that is keeping noise floor good (-143 dBm in this unit). But of course it's possible to turn the strobe signal continuously on with instument diagnostics menu.

When J430 is disconnedted there will be an error message "CANNOT COUNT BEAT FREQUENCY" in the screen and sweeping also stops, instrument goes useless. There are also frequency counter tests in diagnostic menus. With this function it's possible to monitor the VCO and beat frequencies real time. If J430 is not in place / no signal these counter tests will also fail. So there must be some signal on J430 because there are no error messages and instrument works perfectly below 700-1 GHz.

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Esa
http://www.amigazone.fi/contact

It should be continuously on when you use narrow spans too (below

10kHz), the reason it's not continuous on moderate spans is because it is used as reference for a while, then the correction error is sampled and hold to keep the YIG sawtooth signal centered on the desired frequency.

Do you know what's the frequency test? I mean, does the test run below or above 700MHz?

I don't agree your statement above, maybe that (in some circumstances) the mixer is underdriven by Q460 or the YIG signal is too low, in those situations the mixer may have some conversion losses above 700MHz. The fact that the defect is frequency dependant, leads me think that the problem should be into the YIG interface (limited to the HY450 mixer circuitry) or the YIG oscillator. Check the YIG level on J100 too. The YIG oscillator is ovenized. For that reason its circuitry is thermal stressed every instrument's power on/off cycle, and it's not rare to have cold solder for that.

Have a nice day.

Massimo

Max65 kirjoitti:

That could be bad. I'm quite sure that there are no spare parts available for those. Expecially HY460 which internal circuit is unknown.

Need another spectrum analyzer for that because I don't have any power meter for those frequencies... It seems that there are no adjustments for YIG level. The RF level adjustment in the local buffer seems to control only the 1st mixer injection but not HY460 input. The YIG has some internal trimmers but those are glued. I have no idea what those do and there's nothing about those in service manual. Service manual says that if YIG is defective it will be changed as a compete installation.

Is it possible to fix soldering INSIDE YIG? There is a PCB on top of YIG can, it's the 1st lo interface. I have already check solderings in it. But the YIG can itself is closed with screws. I didn't open it because I'm unsure if it will damaged if opened?? However schematics show that there is circuit inside YIG also and cold solder could be there.

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Esa
http://www.amigazone.fi/contact/english

Hi Esa,

I don't want let you down for this, my own was just an hypothesis. The HY460 should be just a mixer and I don't believe it could be defective because it's placed in a safe area (I mean, it's not like the input mixer that could be damaged by the unknown incoming signal of the analyzer test port).

I can't figure out what trimmer you see inside the YIG, since the schematic doesn't show any there. Anyways here in Italy we say: "don't cry over spilt milk, go ahead". If it is really defective, you could try to open it and see what to do. But before it, you should establish whether it is faulty or not.

You should look the Beat Note at J430 with your oscilloscope when the analyzer shows the wrong behaviour and see if the signal is similar to the one you see at lower frequencies where it works right. Since the signal is the result of the beat of the YIG and the VCO signals, you should change the center frequency slowly and see what happen for at least 26-30MHz and compare the two behaviours. If you see differences, the faulty component/solder should be before J430, otherwise it should be after J430. This could be a good start point by my point of view, don't you?

I remember that a good friend of mine who worked for Rohde & Schwarz for a long time repaired one of that one day, I see it. The real problem was to solder the components on the alumina boards which are highly thermal dissipative, so you need an high power soldering iron and limits the time. (one other issue was don't lose the small yttrium iron garnet, I remember it was not greater than 3mm in diameter).

Yes, unluckily often the cold solders are there because of the thermal stresses I told you before.

Check for the behaviour on J430 and let me know, nothing is lose for the moment.

Massimo

Hoops I feell an idiot, in the previous message I wrote: "I can't figure out what trimmer you see inside the YIG, since the schematic doesn't show any there." Better looking to the schematic I realized that you refer to R330, R340 and R130 on the YIG bias board called A19A1 1st LO INTERFACE.

More, did you adjust the RF OUT LEVEL (R288 on the 1st LO BUFFER board? If you had to do it because the signal on J380 was to low it clould be a symptom of bad behaviour of the YIG. Anyways you could measure the voltage level on the emitter of Q190. The higher is the voltage the lower is the YIG signal incoming on the board. You should measure it below 700MHz and above it and loot for any difference. Massimo

Hi Max,

Here's a picture of HY460:

I checked that with microscope and no any visual defects. But of course that doesn't prove anything about it's condition...

That's a good idea and I planned to do that now in weekend. But like I told before the signal here is so weak that it can't be seen with my digital oscilloscope and my analog oscilloscope is too slow. So I plan to build some quick-and-easy preamp for oscilloscope - with some OP AMP! However after digging all my "treasure shelfs" it seems that I haven't got any opamps with enough bandwith. Maybe I will try one with 5 MHz bandwith (-3 dB point) it's possible that it will give some gain also with higher frequencies - or not... :-/ However the signal on the digital oscilloscope is so weak that i would need a gain of 10 or 100 or more. Maybe some video amplifier taken from scrap VGA monitor could do it...

Last time I tried to do that the signal was visible only to few hundred kHz and then the amplitude decreases rapidly. But maybe with preamp...

Yes I'll back when there's someting new.. Have to find some solution for the oscilloscope preamp..

--
Esa
http://www.amigazone.fi/contact

For now I haven't touch it at all. If I understood correctly it will only adjust the 1st mixer injection and has no effect on HY460 input?

Also I'm not sure if I lose the amplitude measurement calibration if I adjust that? Of course it's possible to recalibrate the analyzer after that, but may be it's wise to do some reference measurements before adjustment...

Have to try that also when 1st lo buffer is open - it's almost hour of disassembling, tens of screws etc. and some semi rigid cables have to replaced with coax until it's possible to test run 1st lo buffer on the table... :-) I hope that these kind of set-up or running it without it's shielding will not change it's performance too much. You can see the part of 1st lo buffer assembly on HY460 picture.

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Esa
http://www.amigazone.fi/contact

Uhmm... It looks like a signle balanced mixer, but the outcoming signal from T450 should have a great shape, it must have harmonics over the GHz limit to work with that couplers. Do you have tried to see the signal on R450 with the scope?

Never use x10 probes (if you did) since they highly reduce the bandwidth of the scope. Anyways the T450 transformer output seems to be insulated from ground and have very low impedance, so your x1 probe should be used without any problem (doens't matter where you put the ground terminal). If your scope has at least 40MHz bandwidth you should see a discrete square wave there (I guess). I wonder reading that you read a so low signal at J430 because looking to the schematic of the following stage, you can see just two opamp having low gains (about 10) connected there.

Yes, it is. I asked just to know if the YIG has fall down of some dB there.

Do not adjust any trimmer if you don't know the procedure for do it, I asked just to know if the service manual said something about it. Have a nice weekend.

Massimo