Category Archives: Spectrum Analyzers

HP 8566B Spectrum Analyzer: lock and roll issues

With the 8566B main issues fixed, I carried out extensive tests, also switching it on and off many times, running it for a while. So far so good. With all the tests, discovered only one issue, unstable display for spans above 5 MHz.

The instability is difficult to see on the picture, it is a bit random, and at slow sweep, the trace becomes very wobbly and noisy. Jumps around. The common suspects are the A19, A20, and maybe A21 assemblies, these control the YTO. There are various capacitors on these boards that may fail or leak. Or some dead bits on the DAC, or similar. A bit strange that it works so well below 5 MHz span – isn’t it? Not really, because the 8566B has a different mode of operation, depending on the sweep width. Below 5 MHz, the LO stays locked all the time, above, it is only locked at the start of the sweep, and then the YTO is swept just be increasing the tuning coil current in a linear (and linearized) fashion.

After considerable study, probing, checking capacitors, including desoldering some – no success. Some more checks, solder joints fixed, finally, so occasional improvement. Touching the assemblies A19, A20, some response.
From that, suspected a contact issue with the board edge connectors, and indeed, these were not very clean. So I gave them a thorough treatment with polishing cloth, rubber erasor, alcohol. Reseated the boards, A19 and A20.

Finally, the sweep is very stable. Seems a lot of unnecessary concerns about capacitors and such, but well, finally, fixed. Screen shows a reference (stored) trace with slightly below 5 MHz, and a longer time max-hold display, slighly above 5 MHz span. Clearly, not much noise and instability. All looking good.

HP 8566B Spectrum Analyzer: Defective and fake transistors, and finally, a HP 4-404 equivalent

Recently, I got a HP 8566B at very low cost, unknown working condition of course. First, it would not power on, at least not fully. Some activity on the 8566B, but no display at the 85662A display unit.
Clearly, next step, to open it all up and do a good survey. Quickly found the issue in the 85662A power supply board. While all the other boards where good and clean, the power supply boards are directly in the airstream and showed some leg corrosion of transistors. Especially, 2n2369A transistors, and the famous 4-404 HP transistors (not that the 4-404 is anything special, but there are no replacements mentioned by HP.

Recently I found in some late 8662A boards that HP subsitute the then obsolete 4-404 with a MPS6521, a high gain NPN transistor. So I did the same and put in BC337-25 (-40 may be a closer match but not at hand here).

With these fixes, the 8566V turns on just fine, but it doesn’t turn off well. The 5.2 V rail goes high a little, and then nothing happens, no power down, when you switch the analyzer off. So measures all currents and voltages in the 5.2 V supply, and finally traced it to a defect of the main power transistor, a 2n5886 equivalent power NPN transistor in TO-3 case.

Checked it out internally, the defect is actually within the area of the bonding wire connection. So the transistor became better, non conducting, once I pulled of the bonding wires and measured directly on the die.

Quickly ordered some 2n5886 from China, a bag of 5 for less than a dollar a piece including shipping. Well, you get what you pay for. Look at the small die, the missing plate (die is directly bonded to the steel case, the bonding wires much thinner).

Doesn’t look like a genuine ON brand device, even the marking is not in accordance to the datasheet.

For comparison, the HP-Motorola part:

Tried to fix it with a 2n3055 temporarily, but it turns out there is not enough gain with that transistor. So the rail would only go up to 4 Volts.

The 2n5886 is a quite remarkable device, high current and substantial gain.

Finally, I got hold of this “Motorola” device, it may be genuine, at least it looks like solid quality. So I unstalled it for a test.

You can see it installed, a new and shiny transistor. Soldered it in generously applying solder.

Voltage is spot on without any adjustments.

Now all seems to work, except the various IF filters need a bit of alignment.

Although they are brittle, for these adjustments ceramic screwdrivers are definitely handy. Make sure to isolate other screwdrivers, easily there can be shorts when adjusting, resulting in complicated repairs of the IF signal chain.

Finally all adjusted within the toleraces, there are all too many adjustable capacitors inside this unit!!

At least, a well worthwhile repair, because the CRT is like brand new. It seems that someone had replaced it, but then put very little further use on the unit. I plan to use it together with my microwave phase noise measurement installation, for rough characterization of microwave sources prior to engaging the complicated phase noise test gear.

HP 85662A Spectrum Analyzer Display: a quick fix of the 120 V power supply

A 85662A spectrum analyzer display for repair, the symptom (I didn’t take a picture) – a green square in the middle of the CRT -some lines are visible within, but no proper display. So, acceleration, CRT, and focus seem good – at least it seems to be a manageable fault rather. Maybe something with the XY deflection amplifiers – but why would both fail at the same time? In any case, first things first and checked the power supply. All the LEDs are on, on the power supply boards, but there is no output on the 120 VDC supply (well, some output, like 7 VDC) – the supply that is essential for the deflection system to work.

Some study of the schematic of the A1A7 assembly. Note that the voltages differ with the serial, this is a 85662-60235 part number board. Q7 is a current source that is driving the main transistor, Q8. If the voltage is trending higher, some of the Q7 current will be shunted to ground through U2.

It is a bit troublesome and dangerous to work on the life circuit (about 150 VDC at the input!). So, I did a check of all the transistors with a diode tester – and found the B-C junction of Q5 shorted. A HP part 1854-0019.

Some study of cross-reference lists, the 1854-0019 is a simple 2N2369A, found some in the basement parts storage (even a military rated and tested JANTX2369A with golden legs!).

Still, even with this fixed, no success. Further to other parts – replaced the green parts in the picture, an LM301 opamp, and another transistor, with no luck. Finally, soldered a few wires to the board and did careful checked in the circuit with power on, it can’t be helped otherwise it seems.

A few minutes later – the failure found. The Q7 current source is not giving any current, the base of Q7 is not biased properly. An open 110 kOhm resistor! It is quite rare to find defective resistors in HP equipment, but especially high value resistors running at higher voltage are prone to aging and failure, eventually.

With a simple, new resistor added, a metal film 110 kOhm, the supply is working again, and so is the 85662A.

HP 5086-7803 YIG tuned Filter and Switch: “SYTF” details and adjustment

With the repair of many spectrum analyzers, it turns out, the preselectors are usually not easily damaged, because of their self limiting characteristics, and because of the absence of active parts. The SYTF is diffient in that it has an active part, a switch.

It is already some years old, but no reason why such parts should have much aging at all.

The symptom of this unit, it is working in the low frequency region setting, LOW band out, but the high band is dead, loss in excess of 35 dB, at all frequencies, and independent of the tuning current supplied.

So my first assessment was, this unit needs replacing, and I found a replacement part online, from a US seller, not cheap, but OK, the 8561E analyzer is worth it, if it is working again after the repair. Unfortunately, several week of waiting were all for nothing – the seller shipped the wrong part and it took a while to get the money back, but finally all settled, except, we still have the defective filter.

Let’s try to investigate the nature of the defect, and open it up. Fortunately, these filters are not hermetically welded like some other YIG parts.

You can clearly see the coil, the inlet and outlets (low and high band) by rigid SMA cable (1 mm size!).

First, let’s study the switch. It is not actually switching the high band, as I originally assumed, but it is switching on and off the low band.

It is a series-shunt-series type FET switch, controlled by about -10 V negative voltage (1 kOHM vs. -15 V connection is the usual control method, floating or ground to switch off).

I could not find the exact die and model for this switch, but there are many similar models that clearly show the structure. The shunt and double series construction will provide very high isolation.

After removing carefully the gold mesh (it is only lightly glued on, I will use some tiny traces of epoxy to stick it back on), some study under the microscope.

Clearly, the spheres are misaligned! The spheres must be placed in the center of the coupling loop, to allow for RF to couple. Generally speaking, during alignment, the sphere is only turned, and then the position fixed by some epoxy – which all seems to be intact, and solidly fixed. So what has happended? I think it has to do with the mounting blocks, which are of different material compared to the based (which needs to be magnetic Fe-Nickel alloy). With frequent temperature cycling, I believe there is some migration of the mounting blocks, fractions of a micrometer every time (keep in mind, the YIG spheres are heated during operation). While we can speculate about the reason of the migration, the result is clear, and the action as well: we need to realign the spheres.

I decided not to undo the screws because the coupling loops can be easily damaged, and used a screwdriver to carefully push the mounting blocks away from the coupling section, bit by bit, under control with a microscope.

Finally managed to get all the sphere properly aligned. If you don’t know how it works, never turn the YIG spheres! These need to be aligned for thermal stability effects, not only amplitude – something which you may have trouble doing at home.

After all the alignment, a quick test setup, with a current source for the main coil, and another supply for the heater and switch connection. Note that the current source is set in parallel with a capacitor (22 uF) to allow for stable regulation with the strongly inductive load.

The insertion loss test – done by checking at several frequencies, using the lines of a good comb generator.

The insertion loss, in my simple setup, it is about 4-6 dB for a 3 stage filter, not bad. I don’t know the original performance spec, but it is definitely in the typical range of such filters, and good enough for a spectrum analyzer in any case (in the worst case, we will loose 1-2 dB of sensitivity). Maybe I will eventually find a new filter at a reasonable price, to check it – it could also have moved spheres.

The tuning current is very linear, I don’t expect any issues with using this part in the analyzer (the tuning characteristics can be programmed and stored in the EEPROM of the analyzer, to control its DAC appropriately, also, we will need to recalibrate the flatness).

HP 8561E Spectrum Analyzer: 100/300/600 MHz system fixed!

Progress with the HP 8561E – the 100 MHz to 600 MHz system, driving the 2nd converter, and other sub-system including the 300 MHz output.

Initially, it looked like a failure of the 600 MHz doubler, so I decided to open up the RF case of the A15 assembly, and to go to component level troubleshooting.

According to the block diagram, the 8561E first uses a trippler to convert the 100 MHz VCO output to 300 MHz, and then a doubler to convert the 300 MHz to 600 MHz. Note that there are various revisions of this board, not all use the same frequency multiplication scheme.

Very soon it became clear the the double is not getting enough 300 MHz power to work. So, to check it, I injected a 300 MHz signal after opening the signal chain after the tripler (there is an attenuator, just desoldered the middle resistor of the PI configuration attenuator, and checked all the components around this area)

Only about 2 dBm are needed to drive the tripler, there is an amp stage in front of the doubler. A quick test – the doubler is working just fine!

So, probably a fault in the trippler? At least, there is 300 MHz present. What is going on? Let’s go one step further back – removed the tripler transistor, marked “Hb” which is a NE85635 transistor.

Let’s drive the 300 MHz circuit from an external generator. This is running at quite high power – about 18 dBm!

With the injected signal, all is working fine! So, the 300 MHz transistor probably failed? By luck and coincidence, I found a spare 2sc3603 transistor, marked “Oq”, and soldered it in.
Surprisingly, the old transistor, once desoldered, tested just fine. And, to confirm this, the 8561E still not working!

The 300 MHz system can be conveniently monitor by checking the 300 MHz cal output with another (working) spectrum analyzer.

Well, we need to go back one more step – to the 100 MHz amp.
The tricky fact – the 100 MHz system is working, but after some careful measurements and calculations (I don’t have a precise active probe here), the power at the output of the 100 MHz amp is clearly low. This needs to deliver well over 10 dBm of power, otherwise, all the following systems won’t work properly.

Fortunately, the 100 MHz amp is a fairly common part, a MSA-0505 gain block.

These MSA-0505 are used in many HP circuits, just took one from an old board:

The A15 board, still with the 100 MHz gain block, and the tripler transistor replaced…

A last step – replaced the Oq transistor, with the old/original HP part (300 MHz circuit).

Finally, some tests – the 8561E (at least the low band up to 3 GHz is working again, and the CAL output is in spec (0.02 dB difference to another calibrated 300 MHz source!).

HP 8561E Spectrum Analyzer: 600 MHz troubles

Some investigations into the 600 MHz generation of the 8561E. The target is clear, we need to get a clean 0 dBm 600 MHz to the 2nd converter to make it work (the converter itself is running at 3600 MHz). The source, it is on the A15 board, which is one of the more complex circuits that have been designed by some clever engineers at HP. And, there are multiple versions of this board to make things a bit more complicated.

The 600 MHz system is located in the corner close to the J701 connector (which goes to the 2nd converter). It works by first locking a 100 MHz source to the 10 MHz reference, and then multiplying it to 300 MHz (which also drives other circuits, and the calibration output – which is working just fine), and then doubling it to 600 MHz followed by some amplification. Only output J701 is used, the other output, no use in the 8561E (and also this has low output).

The suspicious section, what could be wrong. Most suspicious, the output amp, because there is 600 MHz present, just not enough signal power. So this amp could be blown, and absorbing energy rather than adding RF energy.

It is a quite common amplifier, MSA-0386 which is a 12 dB silicon bipolar amp (labelled A-03 on the package). Ordered a few pieces, less than 1 EUR each, and these are useful parts anyway.

Other faults could relate to the input amplifier, but would we then get 600 MHz at all? The diodes – these are not likely to fail. The power supply decoupling – which has several capacitors and tantalums – may be faulty, but in general, the A15 boards don’t suffer from bad tantalums.

Well, first we need to get some small Torx bit, to get into this assembly…

HP 8561E Spectrum Analyzer, 30 Hz to 6.5 GHz: some consequences of a past disaster

Even today, 6.5 GHz bandwidth spectrum analyzers don’t come easy, you are looking at 6+ kEUR for even the most basic Rigol or similar low cost brand unit (which won’t be very useful to design VCOs or other precision gear), and all the high quality low SSB noise instrumentation, it is well outside the budget of the common hobbyist or even small company in this frequency range. This may be one of the reasons why the earlier HP and similar name brand equipment still is in high demand, and good working units still have their market, even being 30 years old. Some claim that with age, the performance might degrade, but as per my own tests and experience, this is not the case. These YTOs and YTFs and mixer still work like at the first day. Well, only if they work at all…

Here, we are dealing with a unit described as non-working, in good general condition, but not showing any signal, except, noise. The price was right, there doesn’t seem to be a big demand for defective spectrum analyzers in Japan – I was the only bidder.

Soon after the auction, a large box arrived, and the 8561E powered on just fine, with a good display (well, after some cleaning of the CRT and filter). Checked with the 300 MHz calibration signal – no response.
Further to check, it is quite handy to use a comb generator. This one has good output well into the high GHz range, at 100 MHz spacing.

The result – no response of the 8561E in the low band, and some minor response in the upper band (the 8561E uses 1st harmonic mixing, but two bands, one below 2.9 GHz utilizing a second converter, and one above 2.9 GHz, directly mixing down to 310.7 MHz first IF).

That’s how the comb gen output should look like on the analyzer (below, tests with a different – working – analyzer): all equi-desitant peaks, 100 MHz spaced, and somewhere between -10 and 0 dBm.

Next, I checked the LO, and it is osciallating and it is phase locked, and the power level is right, at least at the LO output. That’s a relieve – at least the YTO is working and locked, and no complicated PLL troubleshooting.

Well, what can we do further, we need to take the thing apart (at least, I have the manuals and even the component level information package, CLIP), starting to probe the signal chain. The most easily accessible part is the mixer (all bias/switch voltages checked OK), let’s take it out and see if it works – feeding a test signal directly to the mixer, and taking the output to a working analyzer.

Low band – looks fine.

High band – working, but we are missing 25 dB signal strength! Something is not right.

Let’s have a look inside – nothing obvious like a broken bond wire, or burn mark found – but also hard to find without a microscope.

Reparing the mixer itself, no option here, this a marvel of engineering, and you need a larger workshop than mine to fix all these tiny pieces of silicon, mounted with gold on axis-oriented saphire substrate…

This may become a more serious repair, over several weeks to get spare parts in, so best to keep all parts and screws well arranged, I keep them in plastic bags, so I can check later if all made it back to the instrument.

Now, with the mixer situation clear – it has a defect in the high band, lets turn to the incomming path, from the main connector, to the mixer input.
Low band is fine, but high band again – weak signal. What can we do? We can at least isolate the fault. Turns out, the attenuator is working just fine, but trouble with the SYTF. HP introduced this combination of a solid state switch and a filter with 8561E, the 8561B still had a separate switch (which would be easier to fix, compared to a fancy switch-YTF combination.

Next, let’s check the IF processing path – just feeding a 310.7 MHz signal to the analyzer, past the mixer and 2nd converter – all working fine. Great!

Next, let’s check the 2nd converter that is needed for the low band using a 3.9107 GHz 1st IF. It is not converting, and no 3.6 GHz signal at the test port (the LO of the 2nd converter is generated by multiplication of a 600 MHz signal). Further check shows that the 600 MHz signal is low, about -20 dBm (should be about 0 dBm). Still the 600 MHz is phase locked, and the reference output (300 MHz, but derived from the same circuit), is working, so probably something with the 600 MHz output stage.

Let’s review. (1) a dead mixer, (2) a dead SYTF, (3) a probably working 2nd converter (at least as much as I can tell now, with feeding a good 600 MHz signal), (4) working LO system, (5) working IF and other processing system, (6) good CRT and mechanical structure. And, looking at the noise specifications, and the total package of properties – is it worth the repair? Well, used units in mixed state go for 2 kEUR, good calibrated working units for 4-5 kEUR, so if we can find the spare parts (a mixer and a SYTF) for a few hundert EUR, it will still be a very economically reasonable repair. Fortunately, I was able to locate a “working and guaranteed” mixer, and a spare 2nd converter (just in case) for a good price. Only the SYTF, a bit more expensive, but it is guaranteed working as well. Now, we need to wait for these parts to ship from the US and China, to Japan.

I wonder what has happend in the past, what kind of disaster? Usually, the mixer is not easily damaged, because of the limiting characteristics of the YTF. And the 600 MHz fail looks completely unrelated. But who knows the sequence of failure, maybe the 600 MHz failed long ago, and the analyzer was only used in the high bad, well, until someone connected it to a high voltage generator.

A few other things to consider – the lithium battery. It is still good. Usually I only change them once defective, or in case I sell an instrument.

The fan, a Papst Multifan 8312 (working, but a bit noisy) is still available, but I will only replace it after fixing all the RF chain.

Another bonus – the 8561E also came with a working mass memory module. Mass refers to a massive 250 kilobytes in this context!

HP 8659B Spectrum Analyzer: mostly, the known issues

Mostly, the well-known issues for this 8569B: a bad fan, a bad 5.2 V supply capacitor (see the 5.2 V rail ripple below!), some issues with the display adjustment, and a bad control assembly with contact broken off. The control assembly, interestingly enough, somebody else had fixed some part of it before, from the handwriting, an American.
Still some minor issues with the Z axis control (brightness control), but this will be fixed soon, and then the analyzer will be thoroughly tested and will find good use again.

HP 8569A Spectrum Analyzer: A shaky fix

Coming back to an earlier post, 8569A CRT DONOR NEEDED, I was desperately looking for a new CRT for an old 8569A.

After some hints of a kind reader of this blog, I decided to give the box another try, and removed the CRT. There was nothing wrong with the grounding, or the Aquadaq coverage, but at least, there was a sign of defect – a glass chip rattling loose inside the CRT. Turned the CRT with the face to the ground and gave it a few good hits. Reconnected everything (with carefully inserting the CRT back in the box). There result – all working great again. Seems that a piece of glass got stuck in CRT deflection unit, and caused the distortion of the screen. Appears to be a pretty rare defect, but at least, the CRT should be good for now (unless you sit the 8569A on the back feet so that the glas piece can get stuck again in the deflection unit).

Still, some more trouble with this unit:

(1) Rebuild the switch assembly and reference level encoders, about 3 hours of work, and a lot of tiny M1.2 screws, etc.

(2) Some start-up issues: the 8569A would sometimes only start when the “reset” button is pushed. Checked the EPROMs – and, one of them corrupted. Replaced the whole set, just to be sure. These are all TMS2516 EPROMs (let me know if you need the good binary images).

8569a differences

(3) Still more tedious work, replacing the mains filter… many wires… and the mains filter X2 cap.

dav

Now, some quick tests: sensitivity test of all bands, and at 18 GHz, all to specification!

dav

Also checked the power reference, with the best two powder meters at hand, all reading to at least 0.1 dB with no adjustments.

sdr

This repair is done, but please, don’t put this 8569A on its rear feet!

HP 8569A Spectrum Analyzer: Any organ (ehh, CRT) donors out there

This 8569A came from the US, purchased by a local HAM operator, and in need of repair. Some switches don’t move nicely, so it seems liked a regular repair job of a 8569A.

First issue, 5 minutes after turn-on, where not the switches, but a blown mains capacitor (aging X2 grade capacitor). Well, this is easy to fix.

With the instrument powered on, it quickly became evident that the there is some distortion of the display, in particular, in the upper section of the CRT. This is not a common fault, and hard to explain other than by a broken deflection system of the CRT.

To be absolutely sure that no other assemblies are causing this defect, the X-Y deflection signals were coupled to a working 8569A, and indeed, no distortion visible.

The CRT display of a working 8569A – with the X-Y signals of routed from the unit that shows the distortions:

Sure, the display is not quite synced (also because of the extension leads with alligator clips). How to proceed? Well, this will need a 8569A CRT for repair, from a donor unit. Let’s hope, at least for the HAM friend that one will come around soon.