Tag Archives: MSR-904A

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:


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

Micro-Tel MSR-904A Microwave Receiver: a set of new handles

For a while I have been looking for a set of of spare handles for the MSR-904A, but to no avail; the unit was missing the handles – had some rough rack-mounting fixtures. Fortunately, most of the Micro-Tel equipment uses the same kind of handle, so at least I know how they should look like.

handle micro-tel

These handles are actually of a very nice design, and don’t look too complicated to fabricate. So I thought I would give it a try and machine a set of spares.

For the material I selected a piece of 7075 alloy T651 temper (fully hardened, stress relieved). This is a quite strong alloy, and it needs to be because the MSR-904A is heavy, and the full weight rests on handles if it is set down on the floor/carried around, etc.

How to machine such handles – well, it is best done using a CNC mill, and luckily, I have one in the basement, even if it is just a small machine.
fkm349vl mill

It is a FKM349VL table-top mill, not a very heavy machine, but pretty capable if used correctly. This is not for heavy loads and throughput-optimized toolpaths, but it can yield very usable accuracy, and the surface finish is very nice, provided that good tools are used. Typically, I get parts that are reproducible to within 0.01 mm, and absolute accuracy typically better than 0.03 mm, depending on how the machining goes.

handle intermediate pieces

Starting from a plate, first, the opening of the handle was machines, and chamfered. The original handle uses a radius chamfer, but well, I only have tooling for 45° chamfers around, fair enough.

Then, the holes were drilled, and countersunk, followed by machining of the left and right (short) edge.

A bit more tricky, the piece where the handle is mounted to the case. This is fairly thin, and some excess metal was left to allow easy clamping of the piece. Sure, there are other ways of achieving the same result, but it saves a lot of time if the vice doesn’t need to be re-adjusted, and if everything can be done in two or three clamp positions.

As one of the last steps, the excess metal is taken off.

handle removing excess

All in all, considerably more effort than I thought:

(1) Cut the plate to approximate size; machine one long side flat

(2) Machine handle cut-out

(3) Machine chamfer (upper side)

(4) Machine left and right edge (short sides)

(5) Spot drill, drill, and countersink holes

(6) Turn around – align

(7) Machine other chamfer

(8) Machine mounting piece

(9) Mount set of 4 pcs upside in vice

(10) Cut-off excess

(11) Cut recess of mount piece, left and right

(12) Machine front chamfer, 2x

(13) Deburr, finish

handles final

These were the tools used –

handle tools

And, not to forget, EMC2 with the Axis interface. Thank You EMC2 (LinuxCNC) team for providing such great software, free of charge!

handle axis

I might still anodize the handles – but first need to do some tests with 7075 alloy (never anodized this alloy before, and don’t want to waste the handles!).

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:

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!

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

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 Remote Interface: pretune DAC, precision reference, and some auxilliary circuit

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: remote control (digital interface)

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: a broken trace, a replaced pot, and an escape proof guarantee

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