Category Archives: Test Equipment Repairs

For some, it is like solving crossword puzzles: fixing defective test equipment. Preferably, mid-70 to early 90s vintage.

HP 8662A Synthesized Signal Generator

HPAK (HP Agilent Keysight) 8662A Synthesized Signal Generator: ref sum loop adjustment, output amp/peak detector repair

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Introduced in about 1980, the 8662A is a marvelous machine, it is an ultra-clean signal source, with very low close-in noise. This is the kind of oscillator used as a reference for phase noise tests, narrow channel receiver testing, and so on. Just a quick glance at it inner working, and it is clear that only the most brilliant engineers must have been working on this apparatus in those days. Sure enough, this did not come cheap, about 30 k$ in 1981, something like 75+ k$ nowadays…

Even more interesting, these machines are still in use today, and are still valued for the same reason – hardly any synthesized generator exists that has similar close-in noise.

The block diagram – essentially, there are two ultra low noise switched inductor osciallators that form the reference and output sum loops, together with with some fractional and n divider circuitry, to allow for the fine resolution. The output sections has a heterodyne band, and other bands derived by doubling or dividing the 320-640 MHz fundamental output of the output sum loop oscillator.
8662a block diagram

As with all wonders and complex machine, they sometimes stop working. This one had two issues reported, inaccurate output power (and error #10B – ALC error for some frequencies/settings), and a ref sum loop unlock condition at certain frequencies. The latter issue had been persisting for some time, but the ownder didn’t use it at the critical frequencies, the ALC loop error just recently seems to have come up, for now external reasons, apparently, during a measurement.

(1) The output sum loop A6A5, error #06. This turned out to be a drift issue, of the low noise REF oscillator. This had drifted out of pretune span, and needed just very slight adjustment of the tuning screw (under a heavy metal cover, rear panel of the instrument). Such adjustment is not mentioned in the manual, but easy enough, set the generator to 640 MHz, check that the pretune is properly adjusted (adjustment is done for -3.75 V at 320 MHz), introduce a -34 V tuning voltage signal, and adjust for a frequency a bit above 640 MHz, say, 642.5 MHz, to give the phase loop a bit of room to operate.

08662-60417 oscillator

Great care must be exercised, because the adjustment uses a copper (!) screw – red frame in the above picture, most likely, acting as a capacitor vs. a metal surface inside of the unit (don’t open up these oscillators – they are wraped in Mu metal or similar exotic material). Don’t overtighten the retaining nut, copper is pretty soft.

After that, the ref loop is stable again, even when sweeping through the full band, for some hours.

(2) The more tricky part, as it turns out, the ALC loop. This is implemented on assemblies A4A1 (which has the peak detector), and A4A7, the ALC control, this has a DAC, to set the target voltage, and a regulator, to keep the output signal at the level set by the DAC output.
8662a a4a7 probing
After some time of probing around, and substituting the input of the A4A1 assembly by a calibrated 100 MHz signal, it is clear that the A4A1 peak detector delivers insufficient voltage, and that the voltage is frequency dependent.

A quick glance at the schematic:

8662a output amp

Blue frame – the bandwidth limiting circuit, enganged at below 10 MHz – first guess: something is at fault, in this circuit. Well, unfortunately,after even more probing, and even swapping some transistor (they tested good but you never know) – the focus shifted to the part in the red box. The peak detector diode! This is really bad news, because it is part of the output amp microcircuit, part number 08662-67008. Carefully desoldered, and checked – the detector diode has 0.3 V forward, which is fine, and 0.7 V backwards – this is a far too low reverse breakdown voltage. Something must have happened with this diode, maybe, it is just age. So, I openend up the circuit (the lid can be lifted off with a knife, glued on with silver epoxy), and inspected with a microscope – no obvious defect, all kind of nicely wire-bonded parts on sapphire(?) substrate – nothing I can fix with the tools at hand.

08662-67008

But, as luck comes along, found two of these circuits on ebay. Very mysterious. They look like old and used and ripped out of the boards (with a screwdriver, not by desoldering), and I suspected that the two might just be damaged parts – but 14 days right of return were offered, so not an issue. Even then, it doesn’t really make sense to rip the out of the board, every reasonably skilled and knowledgeable person would rather desolder them carefully, and fit a spare. However, these spare still had the through hold plated vias on the connectors, from the board. Glad the feed-throughs at the bottom of these microcircuits are so sturdy! Well, the only explanation I have – someone saw the golden parts, on some odd circuit boards, and only wanted to keep the gold, but not the boards. Fair enough.

And, this is almost the end of the story, the spares arrived within a few days, and I cleaned them up, and fitted the most “used” looking 08662-67008, and, quite to my surprise – working just fine.

The only thing that remained was the amp bias adjustment, the offset adjustement (both on A4A1) and the CW power adjustment (A4A7) – also checked the other alignments of A4A7, but they were all still fine.

In the end, still one output amp in the box, for the next 8662A, and the current one, back alive.

HP 8645A Agile Signal Generator

8645A Agile Signal Generator: when disaster strikes

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For a long time I have been looking for a reasonably prices 8645A, and finally, I found one – with the note “doesn’t power up”. Well, most likely, a defective power supply. The 8645A and some related generators (not the 8643A) use linear supplies, because these are really low noise devices, and a switching supply just doesn’t seem to do the trick. Interesting, because for the very quiet 8662A/8663A signal generators, HP was relaying on switches supplies…. maybe they just could not fit anything else.

Back to the 8645A – this is no less than a miracle, a marvelous apparatus. No idea how many manhours (man-decades) of engineering went into it. It’s complexity, and subtle detail, nothing short of a moon landing vehicle, made for an electronic test lab. They way HP designed the shielding, and implemented a rigorous low leakage approach, this alone is worth special admiration.

Even better, the unit discussed here has option 1, which is an OCXO high stability reference, and it has a build-in doubler, extending the frequency range to above 2 GHz. About USD $50k in 1990, nowadays, nobody can afford such build quality anymore.

Well, all these are good reasons for looking forward to soon doing some repairs on such kind of unit, and make it “power up again”. Well, that was the thought.

This issue: while many hours of hard work went into fabricating this thing, not more than a few seconds were spent, to consider adequate packaging, to ship a box, 80 pounds.

8645a damage

8645a damage 2

8645a damage 3

8643a damage 4

Such kind of damage, not seen before. Except for the little bit of Instapak, no other protecting foam or anything – nothing to hold it in place in the box. The result – a badly damaged front panel, broken input connector, and even the front frame, damaged beyond repair.
Just the single front end connector (which has an internal airline, gold plated) – USD 200+; the machine, it seems beyond repair.

The only good news – the seller (who did not package it himself) seems to be a very resonable person, so we will work something out. To be continued.

Micro-Tel MSR-904A Microwave Receiver

Micro-Tel MSR-904A Microwave Receiver: reducing phase noise – phase detector frequency

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Like with most PLL build, there a quite a few things that can go wrong – the result: a lot of phase noise. For the current setup, all precautions had been taken to avoid bad surprises – low noise supplies, well-proven loop filter amplifier, low noise DAC, adequate cables. And, phase lock was quickly achieved (see last post).
For more detailed analysis, both the 160 MHz and the 21.4 MHz IF signals of the MSR-904A are fed to analyzers. For the 160 MHz, to a RTL SDR stick, just for the rough picture, and the 21.4 MHz, to a 3585A analyzer. The 3585A has very low noise, ideally suited to look at phase noise, except if you are working the ultra low noise segment.

Initial finding – phase noise is down at about 60 dBc at >10 kHz offset, dropping off as expected, but the close-in noise is really bad. Close in noise often related to the phase detector, or the reference. Substituting the 10 MHz reference from the EIP545A by a really low noise HP 10 MHz OCXO didn’t change much. So to high noise level must be connected to phase detector.

With the detector set to 1.25 MHz (:8 reference divider), there we can gain quite a few dB of noise supression, by increasing the detector frequency (within limits, doubling the detector frequency lowers the associated noise contribution by about 3 dB). And, even more, we can check out the reference doubler, which is a build-in feature of the ADF4157. With the doubler in use, it needs to be ensured that the duty cycle of the reference is close to 50%, but this is ensured by the OCXO anyway.

The ADF4157 can handle phase detector frequencies of up to 32 MHz, no issue at all with 20 MHz. The only downside – more fractional-N spurs – channel spacing for integer only dividers is now 160 MHz, rather than 10 MHz….

msr pll phase noise

msr pll phase noise averaged

Red and green traces – you can see, the PLL is completely detector noise saturated within the bandwidth.

Other traces – all with a phase detector frequency of 20 MHz – and at different charge pump currents (CPC). A CPC of 15 corresponds to a 5 mA current. This has direct impact on the phase loop cut-off frequency. There is some peaking, at 2 kHz (dark blue trace, CPC 1), and at about 7 kHz, light blue trace, CPC 10.

Comparing the yellow and magenta traces – these differ by the 10 MHz reference signal source only (yellow uses an HP 10811 OXCO, magenta uses the EIP 545A build-in reference which is pretty stable, but rather noisy). In the curent setup, both references yield very similar results – accordingly, the noise within the PLL bandwidth is dominated by the PLL cirucit itself, and the phase detector, not the reference source.

There are some mains-related spurs at 60 and 180 Hz, but these might just be due to the temporary cabling and lack of a proper case. The circuit is fully exposed, tranformers closeby, etc. For the final setup, all cables will need to be as short as possible, especially for the pretune voltage (which is about 2 MHz per Volt – 2 kHz noise for 1 mV!).

Credits go to KE5FX for the great PN.EXE phase noise measurement tool, invaluable for any such work!

Micro-Tel MSR-904A Microwave Receiver

Micro-Tel MSR-904A Microwave Receiver: phase lock test, YIG driver bandwidth modification

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Some final parts added to the MSR-904A digital interface/PLL: the actual PLL circuit (frontend), an Analog Devices ADF4157 fractional-N PLL, together with an ADF5002 8:1 prescaler. The phase detector is set at 1.25 MHz, to allow 10 MHz integer-only steps. Some experimentation with other phase detector frequencies might follow later.

Here – the schematic of the PLL frontend. The circit is wired point-to-point, sure enough, with VERY short wires, soldered using a microscoped – hope you have a steady hand. After a quick test (using the MUX output of the ADF4157), the wires and the very tiny ADF gadgets, all sealed with a few drops of epoxy.

msr pll adf5002 adf4157 schematic

On the main board, the PLL loop filter. Build around the remaining half of the already installed OPA2703 (other half used for the DAC output buffer).

msr pll loop filter

With all these parts now put together, to do some basic tests on the PLL – a Gigatronics 605 Microwave Synthesizer was connected to the MSR-904A input, and the LO sample output of the MSR-904A connected to PLL. A sample of the “LO sample” taken by a broadband -10 dB coupler is used to monitor the frequency, using an EIP 545A. The 10 MHz reference output of the EIP is used as the ADF4157 reference.

msr pll phase lock test setup
msr pll test setup 2

The MSR-904A down-converts the signal to a first 250 MHz IF (by fundamental LO), the 250 MHz IF is then mixed with 410 MHz (this can be locked to a 5 MHz signal – not locked at the moment, but the signal is very clean and stable anyway).

The 160 MHz 2nd IF is available at the rear panel, and connected to a R820T RTL SDR. This is a very handy method to monitor noise, and do some basic adjustments on the PLL. Using headphones – and the human ear as a phase noise meter… more quantitative analysis to follow.

Here, the transition from manually controlled CW mode, to PLL controlled mode.
msr-904a locked at 7250 mhz lo

A close-up:
msr-904a locked at 7250 mhz lo 2

For these tests, the LO was locked at 7.25 GHz, receiving a signal at 7.0 GHz (SDR offset set to about 160 MHz).

Note – same as for the Micro-Tel 1295, and the SG-811 – the YIG driver has a bandwidth limit (by a 100 uF Tantalum capacitor – and a 499 k resistor!) that is controlled by a reed relais on the YIG driver. This doesn’t allow low phase noise operation, even with the best PLL. Well, 100 uF is a bit too much. Therefore, a 100 n capacitor was added – this is enough to suppress most of the noise of the YIG driver stage, and still the circuit remains fast enough for full band sweeps at moderate scan rates. Might modify this later, by adding a bit of logic that adds the 100 n capacitor only when the external frequency control is active, but disconnects it during full band sweep, etc.
msr-904a YIG driver board

Micro-Tel MSR-904A Microwave Receiver

Micro-Tel MSR-904A Interface/PLL: low noise power supplies for PLL and pretune circuits

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Recently, some pretune DAC and microcontroller circuitry has been build, see earlier post. This is now basically functional, however, we need to confirm that is is really working as desired. Never trust any circuits just build – especially when it comes to “unpredictable” aspects like noise. The parts used, they will most likely perform up to their specification, but there can be all kinds of hidden issues that will later on lead to lengthy troubleshooting of phase noise or spur issues.
From experience, power supply related noise is one of the most severe and troublesome item, if not taken care of at an early stage of design, or prototype construction.

Many articles exist on how to characterize power supply noise, especially at very low levels. This is not really what we need here, because we are talking about a real-world circuit that will later work with a mains power supply, in a reasonably well shielded case. So, our standard will be the lowest noise analyzer I have around here, an HPAK (HP Agilent, now: Keysight) 3585A Spectrum analyzer. This has pretty low noise anyway, down to -137 dBm for a 3 Hz bandwidth.

msr pll 3585a noise floor

The only downside of the 3585A, it is about 80 pounds – you will need a sturdy bench and a strong assistant to lift it.
msr pll 3585a analyzer
As a side note: The instrument on my bench, it has an interesting sticker, formerly owned by ST (STMicroelectronics, formerly known as SGS-Thomson as printed on the cal label). ST does a pretty massive amount of R&D in the field of semiconductors, and has a long-standing history of inventions. Well, fair enough, I got this instrument in bad shape, seems to have passed through many hands since ST time, but it is now fully repaired and calibrated, providing great service.

Step (1) – analysis of the circuits already build. Just some 0-25 kHz spectra.

Noise floor, probe grounded at AGND.
msr pll agnd floor 25k

317 regulator output (11.4 V)
msr pll 317 noise 25k

– well, much worse than expected! More than 30 dB above the noise level!!

Well, after scratching my head for a while – and doing some measurements around the not-too-complicated 317 circuit, one 22 uF cap was added, to the adjustment input. Ideally, for best frequency response, use a low ESR cap, with wide response, like a tantalum or multilayer ceramic. I could not be bothered, just used a plain electrolytic.

Improved schematic:
msr pll power supply updated
(red frame shows additional cap)

The result:
msr pll 317 noise 22 uf ref bypass 25k
A 20 dB improvement, fair enough!

Step (2) – PLL low noise power supplies (2x 3.1 V)

The PLL (an ADF4157 fractional-N synthesizer with ADF5002 prescaler) requires a +3.1 V power supply (2.7 to 3.3 V for the ADF4157, 3.0 to 3.6 for the ADF5002 – so I decided on 3.1 V for both devices). Also, we need a charge pump supply, for the ADF4157. This can be up to 5.5 V, but for simplicity of design, and to follow earlier (rather commercial) designs I did fully using 3.3 V technology, another independent supply is required, for 3.1 V.

These supplies need to very low noise, supply line noise will end up at the charge pump output, increasing phase noise. Glitches on the PLL supply lines can cause all kinds of issues, even the reliability of the circuit might be compromised (miscounting of the prescaler, etc.).

Quite a few more recent parts exist to provide about 3 V regulated output (see TI, Analog, LT), but these devices are non too widespread, and not much better, if not even worse than a trusty old part: the LM723 (aka µA723) regulator. This has a low noise reference build in, and should provide much better performance than any 3-pin regulator.

The schematic – main DC input, and 3.1 V low noise power supply section:
msr pll low noise power supply 3 volt

That’t the little board, during test:
msr pll test setup

And here, we have the results – all tests now using 1.2 KHz stop frequency (not much going on at higher frequencies), 10 Hz resolution bandwidth, 3 Hz video bandwidth, and, using the noise measurement function of the 3585A – this directly measures and calculates the noise level, at a given frequency, for a 1 Hz bandwidth. Very handy for conversion to nV/sqrt-Hz (nV divided by square-root Hertz is a common way of expressing power supply noise).

Noise floor:
msr pll noise floor 1k2 grounded at agnd

The 317 output (11.4 V) – supply of the pretune DAC circuit and amplifiers, and for the PLL active loop filter
msr pll 317 noise 1k2

The 7805 output (5 V) – digital supply, DAC supply
msr pll 7805 noise 1k2

The 723 output (3.1 V) – Vdd section
msr pll 732 vdd 1k2

The 723 output (3.1 V) – Vp section (charge pump supply)
msr pll 732 vp 1k2

A converter worksheet, to relate the dBm numbers, to nV/sqrt-Hz (calculation also has provisions to convert from other bandwidth – not considering a few extra dB to account for the averaging nature of the detector, etc. – we rely on the 1 Hz normalized value of the 3585A anyway, just in case you need to convert from other BW, please keep dectector response related offset correction, if the task requires such levels of accuracy).
power supply noise calc 3585a

Converted values
-141 dBm – about 20 nV/sqrt-Hz (Vp supply) – very close to noise floor of the setup, the LM723 still seems good enough!
-122 dBm – about 180 nV/sqrt-Hz (5 V, 11.4 V supply)

Also, quick look at the DAC pretune output – at the OPA2703 scaling amplifier output:

msr pll dac amp vtune output 1k2
Virtually, below noise floor.

Note: there are some litte contributions at 60 Hz/180 Hz from mains. These are due to the test setup/signals picked-up by the test cables – don’t seem to originate from the circuit itself.

Micro-Tel MSR-904A Microwave Receiver

Micro-Tel MSR-904A Remote Interface: pretune DAC, precision reference, and some auxilliary circuit

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Not a very exciting circuit today, but definitely, a very important one: the pretune DAC for the MSR-904A. This DAC will drive the 0..10 V input of the MSR-904A, to set the frequency for a given band. The frequency needs to be set to about 1 MHz or better, and the DAC needs to be virtually free of noise – any noise will be converted to phase noise, and cause a lot of hazzle for the PLL circuit to be added later.

Rather than a dual supply, the intention is to use a single +18 V supply for the whole remote control circuit. Therefore, we need a few linear regulators, to derive the +5 V for the digital circuits, including the ATmega8-16, and a positive voltage of about 12 V, for the analog circuit. The output driver for the pretune (0..10 V) uses an OPA2703 rail-to-rail opamp. So, I decided on a 11.4 V positive supply, for convenience of resistor values available – 270 Ohm, and 2k2, for a LM317 regulator.

The DAC, a Texas instrument DAC8831. A highly linear device – 1 LSB of INL error. Low noise, low power. The DAC is connected to a +5 V precision reference, a MAX6350. This is a pretty stable and low noise reference, very much recommended for 16 bit converters.

Well, not much more to say, here is the schematic:

msr-904a interface pretune circuit and power supply

And, a quick glance at the board:

msr-904a interface pretune and digital control

There is some space left on the board – for an ADC (to monitor signal strength), and for the PLL power supply (needs 3x 3 V, UA723 – for low noise), and a 10 MHz/5 MHz distribution circuit.

Micro-Tel MSR-904A Microwave Receiver

Micro-Tel MSR-904A Microwave Receiver: remote control (digital interface)

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The MSR-904A has a remote control interface, to control most of the front panel settings by TTL level signals – operation mode, band, filters, IF attenuator, detector. All in all, 22 signal lines are needed.
The circuit will also need provisions for latter addition of the PLL filter and PLL control – just a few digital lines. All will be controlled by a single USB interface.

First, we need a cable – the MSR-904A uses a DSUB-37 connector, but not all pins are used – so an adapter cable was fabricated to convert this to a much more common (and available) DSUB-25:
msr-904a remote control cable
– quite a few wires!

msr-904a remote interface wiring and register layout

The digital control is implemented by a set of three 74LS164 shift registers, serial in, 8 bit out. These registers are very fast, can be set in a few microseconds. The three registers are named 1-Q0 (LSB of register 1) to 3-Q7 (MSB of register 3).
The micro is an ATmega8-16PU, running at 16 MHz – this has plenty of power to handle the USB interface, the digital control, and later, the PLL loop. There is also a standard 10 pin ISP header, not shown in the schematic.

msr-904a digital interface schematic

The circuit – build on a perfboard. No plans to fabricate a PCB, I don’t anticipate a big demand for MSR-904A remote control units, but still it should last many years. This is why a proper FR4 perfboard with plated-through holes is used.

msr-904a digital interface

Micro-Tel MSR-904A Microwave Receiver

Micro-Tel MSR-904A Microwave Receiver: a broken trace, a replaced pot, and an escape proof guarantee

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First of all, mystery solved – the remote enable input for the IF attenuator. After quite painful tracing of wires and disassembly of the logic boards, a broken trace! Just a little crack, but big enough to block the electrons’ flow. A little bit of solder, and then, suddenly, the MSR-904A’s IF attenuator can be remotely controlled.

Today, a also the dual pot for the F2 adjustment arrived – fitted, also this, working again.

But most interesting, a little box, with a label not seen before:

msr-904a mains cable label

The part has a NSN number, original unit price was no less then 71.38 USD!

Enclosed – the most special mains power cable I have ever seen:

msr-904a mains cable
-it was well packaged in a multilayer heat-sealed bag.

And, a label, which will be kept – Federal Prison Industries: Escape Proof Guarantee
unicor federal prison industries escape proof guarantee

Micro-Tel MSR-904A Microwave Receiver

Micro-Tel MSR-904A Microwave Receiver: crosshair, imagesetting film printing, remote control input

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Some more progress related to the MSR-904A:

(1) The crosshair. The old one is badly damaged.
msr-904a crosshair

Sure, I could just use a laser or inkjet printer, but with a laser, the resolution is not good-doesn’t look sharp enough. With inkjet, I doubt it would be permanent enough, and also there, the printout is never sharp enough. So I decided to go for the solution that also Micro-Tel used, so-called imagesetting film. Back home in Germany, not problem, but here – first I need to find a source. Turns out, not too difficult, about 20 USD for 8 pcs of crosshair (minimum order fee), a company located in New Jersey, not around the corner, but not far. 2400×2400 dpi, acetate/emulsion film.

With a film printing service identified, we need to get the digital data, of the crosshair. With the human eye being a pretty precise tool to determine even small differences, and aiming for perfection with the replica – first, determined the line width and distances with a measurement microscope. Such a little microscope is extremely handy, I use it all the time for inspecting circuit boards, etc.

msr-904a crosshair measurement microscope

msr-904a crosshair measurement
one of the big divisions: 0.275 mm (26 pt at 2400 dpi) wide. Line is about 0.125 (12 pt at 2400 dpi) wide.

Here – a reconstruction drawing:
msr_crosshair

The new prints should be in the mail tomorrow!

(2) The remote control input: a 37 pin connector! Fortunately, not all wires are connected (x in the draft).

msr-904a remote control input

Controlling the function, bands and IF bandwidth, and detector characteristics (log-lin), all fairly straightforward. All of these inputs seem to have pull-up resistors, so grounding them works fine to switch. This is quite hand for control via optocoupler – no external voltage required.

The IF attenuator – traced the lines to the control board, it is a BCD control input, 2×4 bit. Unfortunately, I can’t get it to switch… the ‘enable’ signal doesn’t seem to reach the control board – more effort will be required to trace this last line! – Solved: a broken trace on one of the logic boards!

Some of the pins, despite having wires attached to them, remain unidentified – or might be reserved for options not implemented. Except for the IF attenuator control enable signal, all functions needed have been identified anyway.

msr-904a remote control input pinout

(3) The remote frequency control input – analog voltage: quick check with a frequency counter connected to the LO sample, and a DC voltage supplied – it is a 0 to 10 V input.
Scaling of the input voltage can be adjusted on the A6B2 board, R56 is for the mid-range adjustement (offset – supply 5 V and adjust for mid-band frequency), R68 is the gain adjustment (set at 0 V, and adjust for lower band limit; check setting at 10 V – should be at the high end of the band – and it really is).

Micro-Tel MSR-904A Microwave Receiver

Micro-Tel MSR-904A Microwave Receiver: some progress

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With the basics done (power supply, potentiometer), a few hours were spent to get everything tuned up.
And, quite amazingly, it is receiving:

msr-904a first sign of activity
– notice the dirt, and the sticker residues. Also the crosshair (which is printed on a piece of plastic foil), will be replaced.

Finally, the exterior. The front panel, easy enough, all brushed and cleaned with diluted isopropyl alcohol.

The top, bottom and side panels with the sticky green stuff – all the old junk (“paint”) has been removed, my soakin the panels in methylated sprits, sanding, solvent cleaning, sanding. Then, a layer of aluminum primer (self-etching automotive primer). Followed by a light sanding, and a layer of ‘Hunters Green” alkyd paint. After 10 hours of air drying, final curing at about 150-170 deg C, for a bit over an hour.

All in all, quite an effort. The result –
msr-904a panels - newly painted

Missing items – one fastener like this – no idea where to find, seems like a part from the aerospace industry.
micro-tel quick release fasteners
Should you have any of these around, even of somewhat different length, or if you know a source, please let me know!

msr-904a receiving at 8.1 ghz
Sweeping test around 8.1 GHz – with the refurbished panels installed.

Two more handles are still needed – either need to get spare handles from a parts units (which may be impossible to find), or ship the MSR-904A with 2 handles only, and provide the remaining two later, once I had a chance to fabricate them back home in Germany. At least, I have the exact dimensions measured, just a matter of some CNC milling.

msr-904a receiving AM modulated signal at 8.1 ghz
-this is a test using a 8.1 GHz AM modulated signal, with about 1 kHz modulation frequency. Carefully checked the IF chain (different chains are used, depending on filter setting) – the MSR-904A uses 250 MHz, 160 MHz (by mixing the 250 MHz IF with 410 MHz, from a low noise LO), and 21.4 MHz (for the 100 kHz BW setting).
All seems to be functional.

With the receiver now basically functioning – some weekness of the AFC circuit alignment, and the frequency control was noticed. Therefore, some more effort was spent on the frequency control and AFC circuits, and the tuning indicator circuits. Really tough without any instructions or schematic.
There are some nice indicators on the front panel, LED bar graph displays – one for signal strength, and one for tuning.

These displays, now, in working condition and properly adjusted, are great fun to use. They are extremely responsive – nothing to compare with the time lag and sluggish nature of a modern SDR.

After several hours – here, receiving at 6.1 GHz, with 1 MHz bandwidth, and the AFC keeping the frequency, counteracting artificial drift:
msr-904a receiving at 6.15 GHz with AFC active

Monitoring of the AFC control is by looking at the IF frequency, 250 MHz (on the EIP 545A counter), derived from the (non-phaselocked) MSR-904A LO frequency, minus the RF input frequency (from the Gigatronics 605 Microwave Synthesizer; the EIP 545A is locked to the 10 MHz signal from the 605).
This setup allows me to check for any drift of the MSR-904A IF chain (and AFC, if activated), to 1 Hz resolution.
Had it running now for several hours, no issue, signal stays perfectly tuned.

The only remaining item, internally, is the alignment of the cross-band assembly – still lacking one CD4051 multiplexer circuit – which is on its way. A quick check with a CD4051 taken from another board showed that there is no defect, the board just needs some alignment of the band-to-band transition points. The crossband assembly allows a full 0.5-18 GHz sweep, with automatic band selection.