Category Archives: Micro-Tel MSR-902C

Micro-Tel MSR-902C Receiver: root cause analysis, and a volt meter

Finally, some time to deal with the MSR-902C repairs. After replacing the 7401 TTL, and a 7404 TTL, the band select logic seems to work well, except two bands. This could be traced to a dead transistor on the A3A5 band control board. Still a mystery, what caused all these defects? Tracing the line going to the dead transistor (which appears to be a simple +15 V on/off switch), it only goes to one place – a circuit far inside the receiver. As it turns out, this is a hand-wired circuit, not really a circuit board, but a piece of sheet metal with various solder posts. And, on the other side, two filter. One filter mounted properly, the other tied to it with some thread. As you can see, this holds the filter in place, but it can still move around the other filter – and cause a short on the 15 V rail, including the signal coming from the transistor switch.



To avoid similar defects in the future, I put some plastic sheet around the filter, and fixed it in place with better ties.

Finally, time for some alignment of the YIG filter, by using a fairly complex setup, a microwave signal generator, a scope to test the receiver output, etc. – see below picture.


The YIG filter needs to be aligned for each band, same for the YTO band edge frequencies. This is all done on the A3B7 board. Not much adjustment needed, fortunately, only some fine tuning of the YIG preselectors.


Receiving… quite fun to operate the receiver, easy to tune over the full range of frequencies. Maybe this is what makes it so suitable for detecting microwave bugs.


Some last repair relates to the frequency display. It did work in some bands originally, not sure how the defect came about – maybe I slipped with a screwdriver, or some other mishap, or some already damaged part, I can’t tell. But now it only shows erratic values, and without a schematic, it is a tough task to fix it.


A fairly complex assembly, keep it mind, it is just a volt meter for the frequency display… so much easier nowadays…


The LED display: hand-wired with Teflon coated wires. Sure, this receiver was never intended for the layman, but for some agencies that don’t care about cost and taxpayers’ money.


After some tests and checks – the voltmeter uses a voltage to frequency/time converter, and a MIC5005 integrated timer! Quite a nice and complex chip for its age!


Two hours later – found the issue. A bad reference diode, 1n821. Unfortunately, no such diode in stock, but it is quite similar to the 1n827, only that the latter is more precise, and more expensive, and only a used part in my bin. But easy to check, just put a resistor in series, and run at about 1 mA, and check the voltage drop over the diode. All good.



Finally, reception is pretty good over all bands, no detail tests of noise levels done yet, but already now it is clear that this is pretty capable receiver, build with only the best components at a time – just the style is not quite service friendly.

Demodulators work as well, receiving 1 kHz demodulated signal, all looking pretty good and clean.


Micro-Tel MSR-902C Microwave Surveillance Receiver: A3B5 and A3B6 analysis and repair

Some progress with the MSR-902C, which is basically working, but not on all bands – see earlier post.
With no documentation on the MSR-902C available, except some data sheets, I first traced the band select signals – and they appear to be generated on the A3B5 board, which we may call, band control board. This board also converts the 0-4 V tuning voltage from the tune dial to a 0.5-9 V signal that corresponds to 1 to 18 GHz (0.5 V/GHz slope).

This is a top view of the A3B5 assembly. On the upper left, the tuning control voltage converter, left half, voltage comparators for the 1-18 GHz multi-band mode. These comparators assign the band number to the tuning voltage, when the 1-18 GHz range is used. It is disabled (even supply voltage cut) when the MSR-902C is in single-band mode (selected by band selector switch).

msr-902c a3b5 assy 60c35-2306

First observations – there are control LEDs that seem to indicate which band is currently active, but for some reason, they don’t light up on all bands. Very strange. Probed around the logic signals, and at least some signals are there, still no LED lighting up. Very suspicious. After some head-scratching, decided to probe the LEDs by supplying some external power, and poking around with a resistor. Turns out, some of the LEDs are dead. They just barely light up with 20 mA of current applied, and no sign of light at all at the current level supplied to them by the A3B5 assy.

msr-902c a3b5 led test

After repair of the defective leads, you can clearly see the difference of the new LED, vs. the old LED – some of the old ones are still working, but not very bright any more, and I’m going to replace them all, once I have this back in the main workshop with a better supply of 3 mm red LEDs.

msr-902c a3b5 led repair

Another strange observation – two of the logic chips are rather hot. What is going on there? Furtunately, these are in sockets, a 7401, and a 7404, and a quick test revealed that one gate of each of these chips is sinking current, about 200 mA. So these need to be replaced.

Mugshots of the culprits so far….

msr-902c a3b5 dead leds

msr-902c a3b5 dead 7401

msr-902c a3b5 dead 7404

Not sure if it is very clear, but here are the connector signals of the A3B5 assy, and the description of the adjustment pots. First adjust for appropriate tuning voltages, then adjust for proper band switching in multi-band mode, monitoring the LEDs.

msr-902c a3b5 connector signals

msr-902c a3b5 adjustments

The next thing, the A3B6 assy. No apparent defect, but still needed to find out what it does, and how to adjust. It appears to be the multi-band control assy, converting the 1-18 GHz full-range tuning voltages to tuning voltages for the individual bands, by applying offset and slope corrections. The offsets/slopes are selected by CMOS multiplexers as it is custom for most of the MSR series designs. The output tuning voltage is buffered, and forwarded to the other circuits.

msr-902c a3b6 70c36-08a assy

Here, you can clearly see the order of the adjustment potentiometers. For adjustment, if may be best to first align A3B5, and then supply appropriate reference signals to the MSR-902C, or measure the LO frequency, and do the fine adjustment with the 1-18 GHz full range mode selected, and tuning through all the bands. The fine adjustments would need to be done both at the low end (for offset), and at the high end (for gain), for each band. No big deal, once you know which of the pots to turn.

msr-902c a3b6 assy1

msr-902c a3b6 schematic and adjustments

Further repair will have to wait a bit, until a few spare 7401 have been received. But all is looking pretty good.

Micro-Tel MSR-902C Microwave Surveillance Receiver: power back on – first signs of (extraterrestrial?) life

Today, a few spare MJ12002 transistors arrived. No time to lose, and put them into the power supply. Note that the new transistors are 1983 data code, whereas the Micro-Tel originals were 1988… fixing the power supply with old parts, but no reason to assume that these transistors have any issues with age. With such power supplies, I would always suggest to use a pair of transistors of the same manufacturer, rather than mixing up two very different devices. This is why both transistors were replaced, not just the defective part.

msr-902c 8322 mj12002

After this replacement, connected a 10 Ohms 25 Watts load resistor, and grounded the Interlock and ON/OFF lines. When powering up, the green AC ON light comes on, but not for too long. Look at the set of fuses sacrificed in the process:

msr-902c pwr supply rep fuses

Another set of tests – no issues found, all working fine. Something must be loading the power supply, and I can’t get any negative voltages out of it – but there must be at least one negative rail to provide -15 V to the various opamps in the receiver.

Not to long and the culprit was found – a shorted tantalum, a T310 series Kemet tantalum, directly at the – what turned out to be, -18 V output. Check out the date code. Why did Micro-Tel put a 1979, week 38 dated device, in such kind of expensive and specialized equipment (other parts suggest that this unit was made about 1989, at a price of about $40-50k – that’s about $70k in today’s dollars…).

msr-902c tantalum

Some tests show that there is a +18 V, -18 V, and a +12 V output. All are routed through feed-through capacitors. A fair bit of effort, and cost!

msr-902c pwr supply output

First test with the actual receiver connected –

msr-902c first pwr test

– connected the 1-18 GHz tuner – a bit of a cable mess.

msr-902c test setup

To test the basic functions, like, IF chain, detectors, etc, a 1.5 GHz test signal from a HP 8642B was routed to the tuner. And, to my greatest satisfaction, the MSR-902C is actually receiving!

msr-902c receiving 1.5ghz

1 kHz AM modulation…

msr-902 receiving am

… also tested the FM and AM detectors, both in sweep and fixed modes, the AFC, the IF gain, the marker – all working. Also the 8-12 GHz, and 12-18 GHz ranges, working fine. Clear signal down to -105 dBm input. So all working and pretty well tune.

msr-902c 8 to12 range

Unfortunatly, this is not the case for the 2 to 8 GHz ranges – the frequency display is not showing a reasonable value – not sure what is going on here. Maybe something with the band logic, or the signal multiplexers (see the MSR-904A repair story – these instruments are notorious for defective CMOS multiplexers).

msr-902c 2 to 8 ghz ranges defect

So far, so good – at least in some bands, we would receive satellites, or signals from other galaxies, given, there aren’t many strong sources out there, in space, and all the other solar systems, too far away!

Micro-Tel MSR-902C Microwave Surveillance Receiver: a metal box, microwave plumbing – 1 to 18 GHz tuner revealed

With no manuals available, some investigations were carried out to better understand the workings of the MSR-902C microwave tuner, which has a 1 to 18 GHz range, good noise figure, fully-fundamental mixing with 3-stage preselection over the full band. IF output is 250 MHz, so the tuner can be combined with any resonable SDR or other modern receiver, as a down-converter, offering about 40~60 MHz bandwidth, and 60 to 70 dB image rejection, and huge capacity to deal with out-of-band overload signals.

This is the rough scheme, leaving out all ordinary electronics in the case, just the microwave parts (note that there is another SMA attenuator in the feed line of the splitter, coming from the 8-18 GHz YTO, not shown in the sketch).

tuner1to18 scheme

Essentially, there are two inputs. One covering 1 to 12 GHz, and another one, covering 12 to 18 GHz. The 8 to 18 GHz YTO is used for both bands, and PIN switches are used throughout to route the signals.
The IF goes through a 300 MHz low-pass and a +13 dB monolithic amplifier.

Note that there are some different/earlier versions of the MSR-902 and maybe also MSR-902C which use a slightly different configuration, with a LO doubler. Maybe the could not get proper 8-18 GHz YTOs at the time, at any resonable cost, and had to resort to another topology (using a doubler) for this reason. However, I have never seen any of the earlier tuners, and can only report what I heared about them, with documentation on these units being almost completely absent.

tuner1to18 case

tuner1to18 view1

tuner1to18 view2

tuner1to18 view3

tuner1to18 view4

tuner1to18 view5

For some of the key devices, see references below. Glad not to show list prices, as these would quickly add up to USD 10 or 20k, for all these microwave parts. Not to mention that these are all US made, most advanced and highest grade components of their kind. Datecodes are from the late 80s, mostly 1988, but still today, there aren’t much other options around to build a tuner of this kind. Maybe there just aren’t enough entities around that can afford such device nowadays, and software and digital signal processing certainly have contributed that todays devices can achieve perfect results even with less expensive, heavy, and energy-consuming parts. Still it is very instructive to study the design of this tuner. It even has a LO sample output, and with some effort, all the YTOs could be phase locked with relative ease (using GeSi dividers, etc).

anaren 70119


narda 4016d-10

narda 4202b-10

anaren 42040

pin switch american microwave corp SW-2181-3

american microwave corp sw-218-2

avantek av-7104

norsal dbmb-2-18

Micro-Tel MSR-902C Microwave Surveillance Receiver: a very intriguing, 60 pound briefcase

A few days a ago, a most intriguing briefcase arrived, brown color, looking like the late 70s… Samsonite. It is heavy! Really heavy!!

msr-902c briefcase

Inside – a fully equipped MSR-902C receiver, including all cables (which are rare, and extemely expensive to fabricate, because they use special military connectors). This receiver can more or less receive any signal, down to very low levels, and comes in 3 modules, the actual receiver, a 1-18 GHz tuner, and a 18-26 GHz tuner. Other tuners and harmonic mixers were also available from Micro-Tel, but most likely, not many of these have ever been sold.

msr-902c view1

A brief description of the MSR-902, which is very close to the 902C:

msr-902 description

Unfortunately, there is very little literature or even manuals on the MSR-902C, no instructions, no schematic – fortunately, is shares some circuits with the MSR-904A, and 1295 Micro-Tel receiver, and it is an all-discrete construction, with a lot of wires and circuit boards, so it is repairable, even without schematic (just taking 10x longer….). Should you have a manual, or any other related documentation for the MSR-902C,

Inside of the main receiver (the tuners have not yet been touched), a most amazing combination of wires, switches, boards, and so on. All hand-soldered in Maryland, USA.

msr-902c wires

msr-902c wires2

msr-902c wires3

It is a marvel of engineering, but, currently, not in working order. It blows the fuse, as soon as it is connected to mains power. Something wrong with the power supply. After removing a cup full of screws, here it is.

msr-902c pwr supply

Strongly shielded by a thin magnetic shield, all nicely machined and assembled. Now all has to come apart for repair.

msr-902c mag shield

The internals of the power supply, a good number of boards and parts. The power supply can either work from AC mains, or from 12 VDC. The 12 VDC section appears to be find.

msr-902c side view

msr-902c top view

After some tests, found the first suspect item, a full short on one of the MJ12002 transistor that drive the primary of the switchmode power supply converter.

msr-902c dead mj12002

msr-902c transistor short

It a quite old-fashined part, but could still find 3 pieces, USD 5 each. Not cheap, but OK.

msr-902c pwr transistor mj12002

Once the transistor had been removed, time for some checks of the drive circuit. This circuit is based on an MC3420 switchmode controller.

msr-902c pwr supply disassembled

As you can see, the switch mode regulator is working, just no drive transistors around that could actually drive the transformer. But will be only a matter of days.

msr-902c pwr supply drive signal

For those interested, here are the specifications (of the very closely related MSR-902).

msr-902 specifications

More to come – stay tuned!