Tag Archives: Micro-Tel

Micro-Tel SG-811 Signal Generator: a second unit

By luck and coincidence, I found another Micro-tel SG-811 generator on eBay, at a very reasonable price – sold as not working. Even non-working, these units are great because of the many microwave components contained: YIGs, filters, GHz-capable relais, SMA cables… and a lot of old-fashioned analog circuits.

First check – the fuse! Someone recklessly put a 10 Amp fuse in, because the smaller fuses would blow. That’s never a good idea. Most probably we will have to deal with a power supply repair.

After detail assessment – the 24 V tantalum cap is shorted, maybe this triggered a sequence of faults: the main primary transistors (MJ12002), the rectifier, and two thermistors that limit the inrush current.

Micro-tel didnt safe on screws when they designed the power supply!!

These power thermistors are hard to get – I just desoldered two similar ones from old switchmode power supplies.

A dead rectifier – easily fixed.

All the parts labeled – also replaced the 2N2222 driver transistors, and two tantalum caps that were leaking current.

The most precious parts – the RF section.

A most complicated arrangement of oscillators, switches, couplers, and so on

Some of the oscillators originally used in these units required a variable supply voltage to get stable power output, but strangely enough, the YIG oscillators fitted have built-in voltage regulators, and the supply voltage has no effect at all on their output. Still, the power supply board caused issues – end even overheated, because the voltage is set by very sensitive trimmers, and drifted above 18 Volt…

The YTO has a voltage protection diode – it was completely fried when I received the unit. Checked some good Advantek YTOs, these have 18 V 1.3 W Zeners for voltage protection.

With power back on, and the voltage at the YTOs OK, still no good output – how can it be? Some issues with the oscillator driver board that sets the current of the main coil, and without a proper magnetic field, there won’t be any oscillation.

The precision resistors, seems they were hand soldered with some bad solder (traces of corrosion, and high melting point).

First, some trouble to find the dead part – thought it is one of the opamps, LM308, replaced it with a OP02. But no luck.

So I changed it back to the old LM308, just to keep all in original state.

The bad guy… a 4051 multiplexer CMOS, these are notorious!

Another interesting assembly, the reference assy – the 1N 827 reference diodes where still very accurately set, only a few ppm of the 11.000 V, and -11.000 V!

After these repairs, and some adjustment, all is back to working condition!

Checking out the signal on a 8566B analyzer. All good!

The pulse generator, also a great feature of this unit… 1 ms pulse.

down to 1 microsecond, no problem.

… 10 microsecond pulse…

The attenuator, a really high quality HP device.

The manual has some remarkable comments – use a 2 kbyte memory, just in case a “really big program” would be needed in the future.

Still, I will do some alignment of the oscillators and filters… but that’s no big deal.

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

qbh-101

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!

Micro-Tel Precision Attenuation Measurement Receiver: an all-electronic manual for an almost all solid state device

Look at what I have here, finally, an (well, almost, 1 page missing) PDF manual for a Micro-Tel 1295 Precision Attenuation Measurement Receiver. Including all schematics…

1295 manual

…a pleasure for the eye and joy forever!

1295 a4 if module

1295 a3b6

If you have any rare manuals, let me know, much appreciated!

Micro-Tel MSR-904A Microwave Receiver: AM detector/AGC circuit fix, A3B5 assy

After fully refurbishing and fixing the MSR-904A receiver, and some months of service, another look at the AM detector. It is working fine, but at times, is seems to have some microphonic resonance and random shifts of level. Nothing that prevents use of the machine, but a small flaw worth correcting.

Locating the faulty assembly – pretty obvious when knocking at the A3B5 assembly. This takes in a sample of the 250 MHz IF signal, and performs the AM demodulation, and also generates the AGC signal.

msr-904A a3b5 assy schematic AM detector 250 MHz

These are the inner workings, the signal is first attenuated, about 20 dB, then amplified, about 25 dB, using an Avantek GPD-202.
GPDGPM gpd-202
The signal is then futher amplified via a tuned transistor amplifier.

Demodulation is performed by a zero-bias HP Schottky diode, a HSCH-3486. This was state of the art at the time, and still, there aren’t many diodes around that are any better.

hsch-3486 hsch-3206 hsch-3207

Turns out that this diode must have suffered damage earlier, and it was fixed with a drop of epoxy!

msr-904a a3b5 assy open

The HSCH-3486 are not so common any more (with the SMD package HSMS-2850 being widely used), here a comparison:

hsch-3486 sens

hsms-2850 sens

hsms-2850 hsms-2860

The HSMS-2850 and HSCH-3486 use the same technology – the HSMS-2850 is more or less a SMD version of the earlier HSCH-2486 (glass package, similar to DO-35).

Now, should I replace the flaky diode with a HSMS-2850? Well, after some thought, rather keep the parts as they are, for such a rare instrument. With some effort, this is what I found:

hsch-3486

Glad I didn’t have to buy at USD 20 each, that’s what some vendors are asking for.

A bit of soldering, without taking the whole assembly apart:
msr-904a a3b5 new diode soldered

Finally, a quick sensitivity and microphonic test – no issues at all.

Quite interestingly, someone must have fixed this assembly before – note the ECG opamp, 1991 datecode (it is a LM1458 replacement; for the AGC circuit).

msr-904a a3b5 assy ecg

Anodizing 7075 Alloy: Micro-Tel handles

Quite a few good tutorials exist for anodizing of aluminum, and pretty decent results can be achieved in any home shop equipped with a a sink and a few chemicals. For good results, with all the basic items mastered, the most critical item is the aluminum alloy. Generally speaking, any type of pure aluminum, and Al-Mg alloys are very much suitable for the anodization process. Zn, Si, Mn (and to some extend, Cu containing) alloys don’t work well.

The handles for the Micro-Tel MSR-904A receiver were machined from 7075 alloy, because of its strenght, and availability. 7075 has about 6% Zn, and 1.5% Cu, both of these elements are known to cause trouble when anodized. However, one can still try.

A quick, step-wise description of the process:

Step (1) – throughly clean/degrease the workpiece: first, I use methylated spirits, then, hot water and detergent, then rinse with water. Wear clean gloves.

Step (2) – etch with about 10% caustic soda. Room temperature.

eloxal naoh bath

As you can see, the part will turn black. This is typical for certain alloys.

Step (3) – use about 5% nitric acid to remove the black layer. Dip for a few minutes only. There will be some faint grey residue which needs to be brushed of mechanically (use a very clean brush – otherwise, it will contaminate the surface). Had to repeat the caustic etch twice to get a uniform and shiny surface.

eloxal hno3 bath

Step (4) – anodize. Mount the piece with heavy aluminum wire. For 7075 alloy, pure aluminum wire works. Alternatively, use thick titanium wire. Current needed is about 1.5-2.0 Amps per 100 cm2. I used 2 A, for the handle. As cathode, use a sheet of aluminum, lead, or titanium. I use just plain aluminum and it is working just fine. For the liquid, about 15-20% sulfuric acid (dilute 37% battery acid with destilled water 1:1 ratio). Keep at room temperature, cool with some ice (applied to the outside) if it heats up too much. Typical time needed is 30-60 minutes, depending on the temperature and layer thickness. Don’t let the acid heat up too much – the layer will stop growing.

eloxal oxidizing

eloxal pwr supply

Step (5) – densify by boiling in distilled water. Needs to be really boiling, not just hot!

eloxal boiling

After the first attempt – everything looked fine after step 4, but the handles turned pretty dark after densification.

eloxal handle too dark

Pretty much, a full failure.

So, etched off the oxide layer with 10% caustic soda, and repeated the process, with two modifications:

(1) Keeping the acid rather warm, about 30°C, and reduced anodizing time to 20 minutes. This will give a thinner layer.

(2) Added a bit of ammonium acetate to the water used for densification. You may also add a very small amount of acetic acid. Keeping the bath slightly acidic prevents darkening during the densification process for Zn/Cu containing alloys.

The final result:

eloxal handles final

It’s not perfectly silvery color, but a slight yellow-orange color (like lightly colored wood). And the layer is certainly not very thick. But good enough to protect the 7075 alloy from forming corrosion spots over time.

Micro-Tel MSR-904A Microwave Receiver: a wire, another 2N2905A – repair completed (!) – sensitivity test passed

After quite some heavy work with the MSR-904A, we are close to completion. The last defect, supposedly last, was an issue with the 0-9 V internal tuning voltage – could not get it to change from 0-9 V even with all adjustments suggested by the manual, and looking at the schematic – no other way to adjust. There can be drifts of some resistors over time, but all checked and very much as spot-on is they can be.

So, the issue comes from a part of the AFC circuit – there is some gain switching with a 4051 CMOS switch, and as it turns out, this was missing the -6 V supply voltage – causing a positive voltage at the output, which upset the tuning assembly, even with AFC disabled. The wire, not sure if it was broken when I received it – well, easy fix, but took me the best part of 2 hours to find it.

With the machine now ready to be put back in service and fully operational, a few hours warm-up, and all the oscillators and YIG filters fine-tuned, I slipped with the screw driver – a spark – and the 250 MHz amplifier was dead. Fortunately, no major defect, just the 2N2905A that switches on the power for the 250 MHz amp. Added a “new” one, 1984 vintage…

Finally, closed the lid, to keep my screwdrivers out, and did a quick check of sensitivity at about 1.7 GHz (because I have a really well calibrated 0.1-2.1 GHz source, a HP 8642B here already on the bench and did not want to carry around heavy microwave synthesizers…).

Here, the result, which I would call pretty much satisfactory. Doesn’t get much better unless you cool down the receiver with some liquid helium.

msr-904a sensitivity test

The machine, in all its beauty.

msr-904a completed

msr-904a completed 2

Micro-Tel MSR-904A Microwave Receiver: three bad guys, an a revised YIG filter

There are a few remaining items that need to be fixed:

(1) The frequency marker doesn’t work
(2) The AGC circuit is acting up a bit
(3) The 21.4 MHz IF chain – it is working but gain seems low
(4) The YIG driver – it has too much inherent noise when using the external input (EXT frequency mode). Added a 100 n capacitor (see earlier post) – but this interfers very much with the faster sweep rates, where full YIG driver bandwidth is needed. So I need to add a switch that takes out the 100 n cap, except when in EXT mode (not planning on any super fast sweeps in EXT mode, but definitely need low phase noise)
(5) The ‘signal strength indicator’ is working (a nice LED bargraph), but the offset can’t be adjusted properly, and it moves eratically, at times.

With the manual, the search defect (1) – marker that was so far difficult to track down has an end: the issue is one more of the CMOS switches, a 4052. Well, already the second of these switches which is bad. Maybe, I should just replace all the 4051, 4052 and 4053 switches of the units, as a matter of preventative maintenance. Well, we don’t want to fix things that ain’t broke.

Item (3) – nothing worse that disassembling the IF chain – by all admiration for this unit, the 2nd IF chain is not build for troubleshooting. But, following the instructions, first a check of the control assembly. And, quite to my surprise – a defective transistor switch, using a 2N2905 PNP transistor that provides +15 V power to one of the 24.1 MHz amplifiers – this is switched on and off depending on the BAND setting. The transistor – only had a 2N2904 around, same kind, but a bit lower gain. Well, it works – need to check stock back home in Germany when I get a chance.

These are the bad guys-
msr-904a more bad guys

Item (2) – this was “just” a matter of adjustment. Adjustment isn’t all that straightforward – and the manual doesn’t cover everything. Needed to peak the 250 MHz amp/AM detector circuit – there is a tuned amplifier inside, which has and adjustment capacitor not described in the adjustment instructions – fortunately, pretty clear from the schematic.

Item (5) – turned out to be a 1458 opamp not working properly, replaced. Pretty easy – most of the integrated circuits are in sockets. Sorry, no picture of the defective 1458 😉

Item (4) – added a good Chinese brand SIP reed relay, PAN CHANG SIP-1A05:
sip-1a05

Routed a wire with the EXT signal (low when activated) to the YIG driver board. This is now taking the 100 n cap out of the circuit, unless the EXT mode is active. Added a protection diode, to avoid voltage spikes damaging the TTL circuit driving the reed relay.

msr-904a a7b3 yig driver

msr-904a yig driver bw limit ext mode

The 100 n cap and reed relay is installed parallel to the R4 resistor – same as for the C5 capacitor, which is activated in CW mode.

Micro-Tel MSR-904A Microwave Receiver: manual found!

I had almost given up. It seemed that a full manual of a MSR-904A would be perfectly unobtainium. Well, most secrets of the MSR-904A were reverse engineered anyway, but there are some subtle details and adjustments – just a bit arbitrary to do without proper documentation and at a least a bit of insight into the thoughts of the designers of this marvelous apparatus.
One reason might be that these units were long banned from export and mostly governmentally owned. Eventually, the units reached the secondary market, but the manuals, they rest in some archives, or have long been disposed off by the agencies.

Now, to my greatest surprise – a copy of the manual has been located – in Germany. The paper copy, it seems to belong (or have belonged) to a certain H. S. in Webenheim, a place, a mere 72 miles form my German home.
Another fellow German must have scanned this, and again, another fellow, send it to me, electronically. Albeit, not for free, but, fair enough.

msr-904a manual

It is complete with all adjustments instructions, explanation of how it works, parts list, and SCHEMATICs.

Here, a block diagram of the IF chain – without the 160 MHz 2nd IF output option (by conversion of the 250 MHz 1st IF using a 410 MHz LO) installed in the unit I have for repair.

msr-904a if chain

Screening through the manual, the complexity of the thing is pretty much amazing. All without any controllers or processors – but there is at least one programmable part, a PROM that has the filter switching matrix. Come on, this could have been done with a diode matrix!

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

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!