Tag Archives: repair

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

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.

Micro-Tel MSR-904A: some basic repairs

Quick initial assessment, these are some of the items that will need attention:

(1) Exterior. Need to fabricate instrument feet, re-paint the panels, handles are missing – either need to get spares, or fabricate replacement handles (can only be done back at the main workshop in Germany, lacking machine tools here).

(2) The ground leakage -need to check the power supply. Hope it is not the transformer or other critical part.

(3) Power cable. Absolutely non-standard! Uses a BENDIX connector, 3 pin, type PT02E8-3P-027.
msr-904a mains connector bendix PT02E8-3P
Interestingly enough, found a suitable cable, especially made for the MSR-904, on xbay, Army surplus! PN: SC-D-627094-5FT NSN:5995-00-165-3806, the guy has more then 10 pieces – seems the Army was really worried to run out of cables for their MSR-904s.

(4) The frequency display works but doesn’t show the right frequencies.

(5) The F2 adjustment (upper sweep stop frequency in F1-F2 mode) doesn’t work.

(6) Figuring out the major adjustment pots – this is all documentation I have:
msr-904a documentation

(7) Figuring out the pinout of the “Monitor” port (intended to connect a storage scope, I might connect a digitizer), and of the “Remote” port – the remote control signals (TTL).

(8) Figuring out the external frequency control and phase lock voltage requirements.

(9) Drafting a block diagram of the RF deck and IF chain, just to better understand the inner workings, and to see, which parts-components Micro-Tel used.

Now, on item (2).

The power supply –
msr-904a triple shielded power supply
It’s held in place, and held together, by a cup full of screws. And, it has a layer of what is presumably Mu metal (high magnetically shielding sheet metal), to keep the 50-60 Hz in the transformer.

The filter, at the input, it is a sight in itself. Not sure how much it would cost to fabricate a custom aluminum case of this size, and to manually assembly it these days. Parts value alone, over 250 USD.
msr-904a mains filter
Well, and as it turns out, exactly these parts are leaky. SPI filters, 51-321-610, still available, after being around for 30+ years, at Mouser and elsewhere – 119.64 USD each, 18 pcs minimum order….

The spec data:
SCI 51-321-610 hermetic RFI filter

After a quick thought – I will give these parts a miss. With all the shielding, transformers and wires, we can do without hermetic feed-through filters – keeping in mind that also the bottom and top lid of the unit have ventillation holes.

So, filters removed, and wires re-connected… and, quite to my satisfaction, no ground leakage any more – not even a few microamps.

Before putting it back together – quick check of the power supply – all seems to be working fine now, and well adjusted.

Item (4) – complicated. Took me quite some hours. The frequency meter is acutally a voltage meter, and this is controlled by the tuning voltage, and a complex digital circuit spread over some hard to reach board. After searching around – it’s just a defective CMOS multiplexer switch, setting the gain of one of the voltage conditioning stages (which are needed to handle the various bands). It’s and CD4051, standard item, no problem. Put in a good one, from another part of the circuit that is not criticial at this point, and ordered a few spares, just USD 1.75 for 3 pcs, including shipment, from Macau.

msr-904a a few more broken parts

On item (5) – the sweep circuit is pretty similar to the Micro-Tel SG-811, and for the SG-811, I have the schematics around.
It uses a dual 10 turn potentiometer. 10k.
msr-904a helipot 8106 defective

Helipot 6108 series, a type common to high-grade analog-control instruments. Seems that one of the stages (the stage that controls the sweep range) is defective – the hybrid resistance material used for the pot (these don’t use wire, because they are made for high resolution applications) is open at the “cold” end – sweeper is always at full scale.

As these are all fully sealed units, no way to repair – found an exact replacement second-hand, for a reasonable charge. For the time being- changed the wires: the F2 display (controlled by the second stage of the pot, which is still working), has been disabled, and the wires changed so that the acutal sweep range is now controllable – so I can do all adjustments, just don’t get a display for the F2 frequency.

Well, and after all this, the unit is at least basically working, responding to controls, and not triggering any fuses. To move things further, setting it up with a few GHz range synthesizers, and an EIP 545A counter, for some first tests with RF.

msr-904a adjustments

To be continued…

HPAK (HP Agilent now Keysight) 8782B Vector Signal Generator: an easy fix

The 8782B is a quite powerful generator to simulate digital signals, not considering the power in dBm, but the support of all the common digital modulation schemes, BPSK, QPSK, 8PSK, 16/64/256 QAM, at exeptional signal integrity.
Frequency range is 0 to 250 MHz, actually, it works to 370 MHz – but this instrument is mainly intended to generate complex IF signals, which can then be used to test the IF chain of a receiver, or, you just add a mixer, another generator for the LO, and a filter, if needed, to get the signal up into the GHz region.

This unit arrived in a HUGE box, it just fits the car!

All frequencies are derived from a HP 10811E ovenized oscillator, which is held by a shock-absorbing mount, and is extremely stable (better than 0.5 ppb per day!), and low phase noise, resulting in below -125 dBc at 1 kHz from the carrier, and below -130 dBc further out.

The symptoms:

(1) The right LCD display is missing partial digits. Seems to be related to the mounting/contact of the LCD with the board.
8782b display defect

(2) Output is 30 dB down, but otherwise working just fine.
Quick look at the output amplifier – a real marvel of RF engineering.
8782b a10a2 output amp assy

(3) It’s in good condition, but will benefit from a bit of de-dusting and cleaning.

The repair:

(1) After taking off the front panel, the LCD is a separate assembly, seems to be made by HP (which test equipment company would nowadays still make their own LCD displays?), and easy to remove. And, the defect, readily found. One of the side bars holding the assembly together and pressing the glass display to the control board, to make contact via an elastic strip, it had slipped off. The root cause – somebody must have used nail clippers to fabricate a piece of acrylic that is used to protect the LCD surface – maybe it was broken or scratched. And with the uneven edges, it caused tension on the display unit, pushing off the side bar.
8782b display protector
Well, Let’s just straighten out things and put it back together. No surprise, the LCD is working again just great.

(2) The level control. According to the description of the sender, the output is about 30 dB down. Most likely, something with the attenuator, 08780-60093 = 33322GC, 110 dB, 10 dB step. So, the attenuator, easily removed, and the non-attenuated signal directly fed to the dated but very trustworthy HPAK 8565A spectrum analyzer available here at the bench. Now there is plenty of signal, but it isn’t accuarate at all, even relative steps, like 1 dB steps, give variable response, anything from 0.5 to 2 dB steps… The level spec of the 8782B isn’t all that great, but I know from earlier work with such units, the performance is much better than spec, and 1 dB step, should be 1 dB. Turns out, just a little pitfall, the 8782B has a calibration routine, also for the level, and somehow, this unit seems not to have been calibrated for a while. Well, easy enough, just pushed the “CAL” button, and after a few minutes, you bet, the levels are spot-on, and, within 0.1 dB or so.
Just a quick fix on the attenuator control circuit, and, the level is back.

(3) For cleaning, don’t use anything too harsh on these instruments, they use different plastics and pains compared to earlier HP models. I get good results with about 20-25% isopropylic alcohol (1 part of the alcohol, 3 parts of distilled water).

Some quick tests – all self tests passed OK – and here, a BPSK signal:

8782b bpsk test

Level accuracy, all well within spec (measured with a Micro-Tel 1295):
8782b level accuracy test

And, with the 8782B the boring times in the lab are over, Thank’s to the build-in game – everyone will be looking at you, working hard, pushing the buttons!
8782b special function 600 game

Harrison HP 6202B Power Supply – revisited

After doing a quick repair yesterday – a sleepless night, because of the out of spec ripple. Well, not quite that bad. But typically, all HP instruments hold up to standard, it would be a big surprise if this one can’t be brought back into spec.

First, desoldered all of the capacitors (they used a lot of solder!), and checked them – surprisingly enough, they still work perfectly fine, all 1967 vintage.
Next item, the recifier diodes – some might be leaky. Therefore, just replaced them with “modern” 1N4004. But this didn’t cure the ripple.

Next item, the 6202B has a reference circuit, and, the output had 10 mV noise – ripple. That’s far too much, and did not fully feed through to the output.
What caused this issue? – the defective capacitor (see earlier post) is the filter cap for the reference. Seems the replacement cap doesn’t give the same filtering – just added a bigger capacitor in parallel.

The result – at about 50% load (2 mV per DIV, sorry, don’t have a better scope here)
6202B improved noise

About 0.8 mVpp, estimated 0.25-0.3 mVrms. Amazing. Fully working again.

3562A repair: 32 kbyte of bad EPROM data….

Using the little AVRmega32L board, and the various plugs and cables, the two ROM boards (one good, one bad) were read, and all images of the ROMs compared. And, finally, the 6th EPROM of the lower byte – U106, only reads 0x00. That’s not how it is supposed to be. Also, after leaving the board switched on, U106 is warming up, much more than the others. So it is definitely at fault.

3562a U106 BAD EPROM

After some careful desoldering, the culprit was extracted. Cross checked the analysis with a regular EPROM programmer, and in fact, it is not working at all. Well, well. Now, just need to get a 27256 or 27c256, and this will fix the 3562A, and it can go back into service (imagine, this unit is still commercially used). Fair enough.

3562A Dynamic Signal Analyzer: reading the ROMs

I happen to have a 3562A for repair, which had a defective power supply, and after fixing this, it still doesn’t work – traced the error to the ROM board. It is really a coincidence that I own a more or less identical unit, which is working. Therefore, after checking all the supply rails to make sure nothing is going to damage it, I swapped out the ROM board with the working unit, and, there we go, it does the trick.

Now, how to find the defect? – first, a quick check of all the address logic, to no avail.

So the defect must be located in one of the ROMs. Have a look, the board has 2×18 pcs, Intel P27256, 32 kByte each:
3562a rom board 03562-66503 rev b

These chips are representing 589814 words of data (16 bit bus): a massive 1.179648 Megabytes, holding the program for the 3562a.

To get it powered up, first surprise, the thing draws nearly 2 Amps, about 55 mA each. Checked with the datasheet – and in fact that’s what these little heaters need.

Checking with the schematic, seems that this is an earlier version of the board, Revision B. That’s a pitty, because Rev C is much more common, and ROM images would be available off the web. Judging from the datecodes, manufactured in 1986. Still, a great instrument, low noise, build in source, and easy to use.

The circit of the board is really nothing special, a few decoders, and the memory bank, with some bus drivers.
03562-66503 address decoder

The plan is as follows:

(1) Read out the bad board (reading data from this first, just in case I accidentially damage something).
(2) Read the working board.
(3) Compare the EPROM images from both boards.
(4) Replace any defective EPROM(s). The ones installed are single time programmable, plastic case, but not an issue at all to replace them with regular UV-erasable EPROMs, if needed.

Desoldering all the 36 EPROMs – absolutely no option with the tools I have around here. With a few wires, and a ATmega32L board (JY-MCU AVR minium board V 1.4, always handy to have a few of them around), it was just a matter on an hour to get everything set up, not the fastest way, but the data will be clocked out byte by byte…

reading from the 3562a rom board 03562-66503 rev b

Now it is just a matter of time, for the defective chip to show up.

Leveler calibration PROM, now: EPROM

Fortunately, the manual of the Wavetek has some detail on the leveler correction PROM. Essentially, it is fed with 8 bits representing the frequency, 255 (0xff) for 7.0 GHz, 0 (0x00) for 12.4 GHz. For each of the 256 steps, it has a correction byte stored in a Texas Instruments TBP28L22N 256×8 PROM. This is programmed at the factory, to match each individual RF deck.


Getting this exact device and programming tools ready was out of question, some of these PROMS might still around, with datecodes of the 80s, but really not worth the effort and cost.


Step (1) – A little test rig was set up, with the recently repaired HPAK 8904A as a DC source (can source -10 to +10 Volts, in very fine steps), and the DC voltage connected to the leveler correction control voltage line. The Wavetek is designed for servicability, and there is a nice jumper to disengage the actual level correction DAC, and the feed an external voltage instead. An EIP 545A was used as a frequency counter and power meter. Cable and EIP 545A power meter accuracy was tested with my best calibrated source at hand, and found to be within +-0.5 dB over the 7 to 12.4 GHz band.

Step (2) – Data were collected by setting the Wavetek to various frequencies, mostly in 0.5 GHz steps, and the control voltage was adjusted for the power meter to read about 0 dBm. The data were then used to calculate the coefficients of a forth degree polynomial, and converted to the 256×8 bit format. The Wavetek uses a DAC0800LCN DAC, and the output voltage (after the on-board opamp, a LM307N) was found to be very close to 10.00 V with 0xff input, and nearly -10.00 V, for 0x00.

wavetek 907 cal

Step (3) – The tricky part. How to get a replacement for the 28L22 PROM? There are mainly two choices, one option would be to use a little microcontroller, that can easily function as a pseudo-memory, or use something more permanent, in this case, an EPROM. I had some 2532 around, therefore, not much effort. Only a small fraction of the 2532 will be used, 2 kbit of a total of 32 kbit. What a waste!
Unfortunately, the 2532 isn’t close in size to the 28L22, nor are the pins arranged in a similar fashion – but this can be solved with a little adapter board, and a few wires. Not the most beautiful solution, but who cares, it works!


Leveler – Missing part!

As it turns out, somebody must have opened up the Wavetek 907A before, and it is missing a crucial part – the detector, for the leveling circuit! Checked out with the manual – there used to be a 6 dB pad (to get better SWR for the detector), and a positive-type tunnel diode detector (SMA input, SMA output). Nothing I have around here surplus.


Wait – there is a spare HP Schottky negative-type detector around in a drawer back home in Germany, and coincidence allowed to have it carried over to the US. This litte device as SMA input, SMC output, and with a little SMC to SMA adapter, at least a mechanical fit.

A little mod will is required to get this going, changing the polarity of the input amplifier. No big deal:


Next step – there are correction coefficients stored in the Wavetek (in ROM), to compensate for signal losses along the RF chain, and to keep the output calibrated. This will require some more thought, to be continued.

HPAK 8904A – Power supply repair

With the failed parts identified, we need a replacement for the FES8DT diodes: 200 V, 8 A, 0.95 V drop, 35 ns recovery time, 125 A surge current.

Something more rugged – because already on the datasheet, it says “Ultrafast, rugged” – what do you want more – are the BYV79E-200 diodes.
These are 200 V, 14 A, below 0.9 V drop, 30 ns, 150 A. These should last for another 20+ years, and come in the same packaging, SOD59, aka TO220AC.


Fortunately, I had some spares around from other activities, but you know – the best source is not xbay, but just some junk switchmode supplies that should be sitting around in any good electronics workshop – these are always keepers, and have come in handy as a source for parts, for more urgent (commercially relevant) repair jobs, rather than for the HPAK 8904A…

For the 8904A – one important thing! Never short out the RAM backup power on the main board – you will need to go through an activation routine, otherwise, your instrument will be rendered non-working. Don’t now why HPAK didn’t use EEPROM at the time, maybe, these were not yet invented… Following the instructions in the user manual (Thank You, HPAK, for providing all these manuals free of charge on the web!!!), the backup battery was replaced with a new Lithium cell. In the end, it might be a good idea anyway, to open-up the case every 10 years, get the the dust out, and a new battery in.

With new Schottkys, new RAM backup, new serial (and new code), the instrument is now fully functional, option 001 and 002, and will provide good service, hopefully, for decades to come!


Anyone with a broken 8904A – feel free to contact me!

HPAK (HP Agilent Keysight) 8904A Multifunction Synthesizer

The HPAK 8904A – you will find the detailled specs elsewhere on the web, it is a one-of-its-kind instrument, an early type of arbitrary waveform generator, capable of frequencies up to 600 kHz, but its mainstay is the audio region.
Why is it so unique, well, it has a complex (some say, difficult to use, but are there any easier ways?) user interface that let’s you program all kinds of test signals without any time lost for generating complex waveform bit by bit, uploading it to a modern arbitrary generator, and so on. The HPAK 8904A support all the common modulation features, AM, FM, Phase, and additionally, DSBSC (double sideband supressed carrier) and Pulse modulation. It can gernerate FM stereo test signals, various other types of composite signals, DTMF tones, just to name a few. In fact, HPAK has published a catalog, with some of the common waveform (will try some of them later).

Most importantly, your plain 8904A might not be able to do all these things, because all the nice and special functionality is only available with option 001, which is, believe it or not, a software option (to my best knowledge, one of the earliest occurrences of a really powerful software option feature for HPAK equipment).

The unit I’m using came with option 002, two channels, but not with option 001 – well, it’s software only, all you need is a serial and key, and some intructions – which I will be happy to provide to you, when needed.

But before we start: the unit discussed here came from an undisclosed source, and failed power on test (no noise or other signs of functionality). Once opened up, big surprise, HPAK was using a Computer Products switchmode supply, 90 Watts, Model XL51-5601. Fair enough, this is a quality supply, but somethings must have gone wrong – no power.


Troubleshooting a switchmode supply, with my somewhat limited workshop here in the US – well, it’s worth a try. No schematic, so hoping that it failed because of one of the more obvious defect modes. And, fair enough, here are the cuprits: FES8DT Schottky diodes, the secondary rectifiers – 2 of 3 were dead.


The FES8DT are 200 V, 8 A, 85 pF, 0.95 V drop devices. 35 ns recovery time. 125 A surge current. Seems they don’t fully hold up to the requirements.