Category Archives: Generators – High Frequency

Micro-Tel SG-811 Swept Signal Generator: another dead LH0021, and a design issue

After only a few hours of use since the last repair, the unit started to play up again. Simply, no output on all bands. A quick check revealed the issue, fortunately, no failure of the power supply, but again, the driver board for the oscillators, and again, the LH0021 power opamp isn’t delivering current.

By removing the wire that connect the LH0021 output to the YIG tuning coils, and feeding current from an external supply, all working great – fortunately. With no other stock at hand, I decided to move the power amp LH0021 from the (not normally used) filter driver board to the oscillator board, these boards are essentially the same design.

After that switch, it worked again – but only for another 30 minutes, then it failed again, another LH0021 burned out. How can it be??

No, I took all apart, including the mica washer, suspecting some short through the mica or similar issue (the heatsink is ground, but the case of the LH0021 is output). The mice is OK, but there is an issue with the screw hole and its plating. On the top side, is is plated as much as that it contacts the heatsink just slightly, probably, when it expands with heat, it causes the deadly short. Noteworthy – the driver board has the top side of the screw hole completely unplated!

Anyway, too many defective LH0021 yet, and this time I couldn’t find a cheap source. And not willing to pay USD 20 and take more chances for these parts to fail.

As luck would have it, there are some audio amps in my stock, quite common in lower-cost stereo amplifiers. About 20 W audio power, in an easy to use TO220 case, and despite being obsolete, these are ubiquitous, and low cost.

Normally, these are AC-coupled at input and output, and I didn’t find much reference to DC coupled uses. So I set up a little test circuit, and in fact, it provides a nice power opamp (unity gain stable).

Furthermore, the TDA2030 has both short-circuit and thermal overload protection. I wouldn’t call it indestructible, but chances are, that it would survive some adverse conditions.

Only trouble, there is no good space to mount the TO220 case to the heatsink. But a temporary setup will do for now.

With no other change of the circuits, all seems to work well, and also the frequency response seems OK. The LH0021 has about 15 kHz bandwidth, this can be easily met by the TDA2030A.

And in fact, it works well in the SG-811. All working and no need to align anything. Still, I would like to make sure the device has a proper heatsink. So, from a piece of scrap aluminum alloy plate, I machined a heat distribution plate, about 10 mm thick.

That’s the ready-machined parts, degreased with a bit of alcohol.

The distributor mounted well to the board, I cut threads into the metal block, so it is easy to affix to the board without any additional holes or modification.

To be not again trapped by some strange things, I also did some testing of the inrush current, power-on behavior and such (a current spike or reverse voltage may also damage the power amplifier). Also, mounted two more caps to the rails, and a dual-diode 48 V limiter.

However, the startup of the 18 V rail is good and clean.

Same fix applied to the filter board – there is enough space to fit the amplifier without any trouble.

Running at 18 GHz for a while, the temperature stabilized at about 60 degC, well in the range of good working conditions. A few hours later, the SG-811 is still working. So, with some luck, hopefully, a permanent repair.

HP Attenuator 33321SB AKA 33321-60026 ATTENUATOR: fixing a defective spare part

After some consideration, I decided on the fix of the the 33321SB attenuator. The defect – the plastic of the grey holders broke off, probably, a matter of age and a matter of stress in the special construction of the 33321SB with the side connector.

It is not the first attenuator that has such defect, so it seems to be a certain weakness of the HP design, albeit, a weakness showing up much after the design life.

Originally, I planned to fix it back home in Germany, with some precision machine tools – drill two holes, fit/glue two plasic rods, and the mount it in the original fashion. But currently, it is uncertain when I can go to Germany again, so I decided on an alternative fix. There are several unused HP attenuators around here, so I checked their internals for a contact of the same design (length). Take care, there are many different configurations of these attenuators that all differ in the length of the contacts, the distances between contact, and so on.

Found a good donor – now, cutting the piece with a razor blade.

The part is obviously fragile, better don’t touch the contact.

After assembly, it seems to work well and fits well. To avoid further risk of breakage, added some 2K epoxy glue underneath the metal spring – it is not visible on the picture. So I hope it will be a permanent repair. Otherwise, we will fix it again using better tools and methods.

With the spare part repaired, and installed in the 8662A, the generator is working well again, at all power levels.

HP 33321SB aka 33321-60026 Attenuator: HP 8662A Synthesized Signal Generator fix

Finally, I received the attenuator set for the HP 8662A repair. The very special 5+40+40 db attenuator with a side connector (rather than the usual design of these attenuators with two top SMA connectors).
However, a quick test showed that the 5 dB step is not working right. The “through” path is fine, but when I engage the 5 dB attenuator, is is something like 45 dB, unstable. Maybe a blown segment? That would be an easy fix.

So, I opened it up carefully, and found a broken contact. It seems the side connection is pushing a bit on the contact, and over the year, this design caused the plastic to fail. Tough to fix without precision drills and machine tools. So it will need to wait for the virus to clear, and for me to go to the German workshop again, to attempt an repair.

There are two small plastic studs, and they broke off, maybe the plastic got brittle over time with the pressure of the side contact pushing.
Let’s think about how to fix it, if at all possible.

Note – the seller was kind enough to refund half of the price, without having me to ship back the part, fair enough. At least some spare coils and segments should I need to fix other attenuators in the future.

HP 8662A Synthesized Signal Generator: almost fixed!

Still working on the 8662A, a unit that is in very good shape but someone tampered with the power supply in a non-expert attempt to fix it.

After a little bit of waiting, CMOS ICs worth a few dollars arrived, and a single 4049 CMOS fixed the power supply. Probably, the defective caps on the power converter board destroyed one of the transistors, and then lead to a voltage spike that destroyed the CMOS (which has a connection to the sense line) – only that gate got destroyed, the other gates still work.

There was no need for any adjustments, the voltages are still accurate to the resolution of the meters I have around here.

Back up and running, the machine is working fine and giving good output. At least after installing a coax line to bridge the missing attenuator.

There are some small issues that were easily fixed. A few non reactive keys that were traced to a bad contact of the connector related to one of the lines of the key matrix.

… and some issues of the modulation display. It is stuck at “20”, but the 1st digit (+-1) is working. Turned out to be a defective trace of the multilayer motherboard. Maybe someone damaged it by pulling on the ribbon cable, or similar bad handling – it is one of the outermost pins.

Easily fixed with a wire and some silicon glue.

Some study on the attenuator – to my great surprise, the 8662A uses a retrofit set, to replace the obsolete attenuator at the time of construction of this very late production unit. And, it does not(!) use the attenuators of the 8663A. These don’t fit, neither in values nor in size, because there is an interference of one of the SMA connectors with the bandpass filter that is also mounted to the inner front plate. The missing attenuator, a 33321-60026 is a 85 dB unit, one segment 5 dB, two segments 40 dB each – these are switched simultaneously. A really special part, and difficult to assemble even if you have several other step attenuators because of the length of the 40 dB chip…

I don’t actually need to attenuator that much, but luckily found this set on ebay from a seller in Israel. The kind guy offered it for a very reasonable price, great! So we can restore the unit to full function.

Meanwhile, the attenuator set has arrived in Germany, where I will pick it up during the next business trip – coming up soon. In any case, I will also need to pick up numerous screws, because the unit arrived with several screws missing, even some quite essential ones.

Anyway, there are plenty of HP screws around.

HP 8662A Synthesized Signal Generator: An almost new generator, with a pretty old power supply, and a serial number mystery

This is another one of the marvelous HP 8662A generators, I got it eventually for free because it had parts missing but otherwise, a good unit.

The missing parts were the mains filter, and the transformer for the OCXO. The filter, I just replaced it with a fairly standard commercial unit, should be good enough for now. The transformer it is a bit more tricky, because of the limited space and rate type. By luck and coincidence I found a spare in Japan. 25 Euros, fair enough.

Be aware that there are many versions of the input assemblies – and my unit has a 1982 vintage power supply, with a 1996 back plane. This is not going to work well, so I needed to study all the wires and connections. No wonder someone had given up on the repair job before.

The later version:

Mounted these parts – yet, I didn’t turn the unit on, because of a blown fuse. After some inspection of the power supply, the common issues. A few bad caps on the driver assembly, replaced with a pair of new 22 uF axial caps.

Other issues – one of the main switching transistors is dead – ordered some spare transistors, new old stock, from Greece, and took them to Japan during the last business trip to Germany… these are MJ16012 power transistors. 800 Volt, 15 Amp 175 Watts. I don’t trust any copies of these, but rather bought good old Motorola parts. 1986 vintage.

Good practice with the 8662A power supply repairs – checked the control assembly.

The oscillator is working, 40 kHz present, but no drive output – how come? Some quick checks clearly show issues around the U1 inverter, a defective logic gate. Someone must have tried to fix it before, but seems he didn’t have the proper tools and solder – all sticky and bad, so I cleaned it with alcohol.

Desoldered the U1 4049 CMOS, with quite some difficulty because the drill used was quite small, and difficult to desolder.

Some strange observation – all the versions and date codes. The power supply control assembly, maybe 1997 vintage. The power supply base assembly and low voltage circuit, 1982 vintage. The A7A3 power converter assembly, 1993 with some parts dating back to 1986…

The serial – 34xx would suggest 1960+34=1994, but this can’t be, because the frame was only made past 12/1995.

Checked two random assemblies – also these were made in 1997.

Probably fair to say, a 1997 unit, and someone tried to fix it some time after that with a 1982 supply. Why he changed the power supply baseboard, and the removed the oven transformer, no clue.

The defective gate of the control assembly – order a few new CMOS circuits. Plenty of them back home in Germany, but here, it is faster and cheaper to order them new from Thailand… in addition, ordered a few 4013 and so on just in case.

Lastly, this unit also has an attenuator missing. Not uncommon for the 8662A. It is a 33321-60028 65 dB 5 V attenuator. 5/40/20 dB elements. Maybe I will find one on eBay, but actually, I have not much use for it because this 8662A will be running at +10 dBm all the time as a reference LO for my VCO test rig and phase noise analyzer.

HP 8642A Signal Generator: Further repairs

After quite a bit of traveling and other time consuming endeavors, back to the 8642A. After the repairs described earlier, the unit was working fine, until some some escaped from the A2 assembly, which has the low frequency source – one of the tantalums blew. No problem, this can happen for a 30 year old unit that hasn’t been used for while. With close inspection, I discovered not only the defective tantalum, but also two resistors to be defective-burnt.

These resistors are in the supply circuits of some of the amplifiers, to remove ripple from the power supplies.

These are the parts, R17 and the blown C17, and R23. Maybe C27 has some issues as well?

The burnt parts marked in red:

… same case for R23. Note that the resistors had about 12 ohms and 15 ohms, so they did not fail open, and this is why the A2 assembly had no functional defect.

Further tests showed that C27 is perfectly fine. Maybe some overload occurred out the output, or the resistor R23 has some weak design (these are only 50 mW resistors!).

The repair is easy, just put in 22 Ohm resistors (0.4 Watt metal film – don’t have any 19.6 Ohm resistors here, but for the given purpose, 22 Ohms is good enough). The 10 uF tantalums C17 and C27, I replaced by 10 uF multilayer ceramic capacitors.

Carrying out the full self test (Shift-Preset-Shift-SPCL-330-Hz).

It takes a while to test the unit, several minutes, but all tests passed with no issue. Modulation output and amplitudes are fine as well.

As you can tell, I have also fixed the backlight (same as before – some high efficiency white LEDs and two 120 Ohm resistors; keep in mind, the common pin for the backlight is ground, and the bulbs are driven by -5 Volts, so, make sure to get the polarity right).

Last but not least – I also replaced the front frame of the instrument, I had a spare back home in Germany, and carried it over to Japan at the occasion of the last visit in Germany, end of March.

HP 8642A Signal Generator: To be, or not to be a parts unit

The HP 8642A is the cheaper brother (or sister) of the 8642B (see here, I have two of the 8642B around and still use them quite a lot, one in Japan, one in Germany), it is essentially the same unit, but the “B” has a built-in doubler to effectively double the frequency range. The 8642A works up to 1057 MHz, good enough for most HAM purposes. It has all the desirable HP high frequency goodies inside, including, a set of precision 140 dB attenuators, and a huge number of parts that would come in handy for repair of other RF gear, everything of highest quality, low noise transistor, high reliability tantalums, a box of cables and connectors, and at least 20 kg of case aluminum. So, I did not hesitate to buy this unit for the scrap price of the aluminum contained. It also has a low distortion modulation source, which is also very useful, and has a may high quality relais and opamps.

The unit is somewhat dirty, seems it had been sitting in some storage room for a while, and looking at the fan inside, it also has seen some hours of operation (which is not necessarily a bad thing).
The front frame has a mechanical damage, some part is missing – fortunately, no damage to the front panel. But I have some spare HP System II frames, let’s see.

Strangely enough, one module is quite shiny, the case aluminum had some other surface treatment – also, it has a later date code (1989), compared to the other units (1985-87). Upon close inspection of the connectors there are slight scratches – seems this module has been replaced. The 8642A had a field repair program based on module exchange (even the specs were guaranteed after such exchange), quite likely that this module had failed after a couple years of service.

After a quick power up test – nothing to report, the unit is not powering up at all. Took all the panels off, and checked the voltages – nothing present. Checking around the rectifiers and capacitors – all is good here, but the voltage regulators (+-5.2 V, +-15 V, and +-50 V) won’t start up, even when I try to force them. Checked the rails – disconnected the cable (ribbon cable) from the supply assy (A17) to the power distribution board. The 15 V line has a hard 0 Ohms short!

15 Minutes later – checked each module. And the shiny one has the short! A bad 10 uF Tantalum (25 V rated, running at 15 V – should usually be good enough). Replaced it with a 15 uF, 25 V Kemet – no 10 uF Tantalums here in my Japan workshop.

Still, before we proceed, let’s be careful with the power supply. Not that it starts up, and has some issues, and all the modules are gone. Easier said than done – there are sense wires going to the power distribution board, and, a ground sense wire going to the rectifier board (A18). I didn’t bother to study the schematic and notes too precisely, there it says: sense ground, connected to a screw and to the chassis. Of course, I had removed this screw, and now wonding why the supply won’t work…

That’s how this screw and trace looks on the schematic.

Fixed it with a jumper wire, still no success.

Fortunately, only minor trouble, a dead Zener in the 15 V crowbar (using a Zener-Thyristor-SCR circuit, marked red below). And, by design of the supply, if the 15 V is dead, all supplies stop.

After this fix, the supply is starting up, and all voltages are accurate to 5 mV, with no adjustments… this is real quality. And with the supply, the unit is starting up, and passing the start up self test, and even the extended self test (preset-shift-330-Hz), no issues.

Not so high quality are the elastomeric materials used – two kind of foam, one of low density, which completly desintegrated to a black glue like substance (same applies to the 8642Bs I have, so it is a material age issue, not related to the storage or use condition). First, scratching off all the old stuff with a credit card. The bottom cover was a mess, so I don’t show pictures (couldn’t touch the camera with the gloves).

Everything cleaned off. Below, these are the craps (including a chocolate bar cover, which you will need after this messy work).

The new foam pieces (not shown), were all cut to the precise shapes, and mounted with double-side tape (carpet tape).

There will be some further repairs needed (the backlight is not working, and I need to get a good front frame from my German junk pile), but some initial tests were done. Phase noise is good, at least as much as I can check, vs. a 8662A, tested at some random frequencies (10 and 56-odd below).

Note, at above 10 kHz, the 8662A has higher phase noise than the 8642A, so the test can only show the overall function and absence of phase noise issues (for the 8642A) above these frequencies.

There are issues with the attenuators. And flatness, see below. Even with a rather crude spectrum analyzer as flatness indicator, all within 1 dB easily, over the full span.

All in all, still a good unit, and I won’t yet use it for parts and spares.

Anritsu MG3681A Digital Modulation Signal Generator: a digital box

Along my search for used&broken&cheap test equipment in Japan I came across a Anritsu MG3681A 3 GHz generator, it has quite impressive specs, and the unit offered had the full CDMA-W digital package, allowing a whole lot of experiments with digital modulation. The price was right (just about EUR 100 plus shipping!), so this was soon to become the the first Anritsu gear in my workshop.

The information from the seller – at least it powers on, and frequency is locked. The unit also has the Option 02 OCXO, a really high quality 10 MHz source, so even if all the machine is defective and broken, there is still good use for it, just using the parts for other projects.

Further tests confirmed that the output is low, albeit, it is locked and even the ALC (level control) appears to be working.

Without having a good insight into the workings of this unit, also because of lacking schematics and details, I decided to do some tests. After removing about 100 screws or more (the unit has double and tripple shielding to avoid RF leakage), I got access to the attenuator and signals going to the attenuator (see block diagram). Obviously, something is wrong with either the attenuator (a burned segment or reverse power protection element introducing loss?) or the detector, or the ALC circuit itself.

Checking the diagnostic messages – the MG3681A thinks all is good! The only strange thing is that I don’t get any ALC error message (unleveled message, even when dialing in +17 dBm output, which corresponds to about 12 dBm output at present).

A through check of the attenuator shows that it is working in 1 dB steps – i.e., the MG3561A is only using a 1 dB for linear adjustment of power, and all the other attenuation is by the switched/mechanical attenuator. This true up to +5 dB, above 5 dB, the attenuator is set to 0 dB and the gain of the output amp is determining the power. Same below -135 dBm. Attached detail from the service manual shows the detail.

After all, I was able to confirm good working condition of the attenuator.

Next, some tests of the output level of the RF amp assy, which feeds the attenuator. And, not surprisingly, the output power is already low before entering the attenuator. All is leveled and working as it should, and also flat regulation with frequency (!), but too low power.

Following the service manual, there is a complicated procedure for output power adjustment. It requires some special software, and two more Advantek instruments that are hard to come buy, and actually, we don’t have any issue with flatness or response, but just an offset of the level. So we should be able to correct this somewhere in the analog circuit, say, in the opamp doing the ALC control, or in the DAC setting the output level.

Studying the circuit a bit, with the block diagram, and some general knowledge about such analyzer. The function blocks are clear. And a quick test showed that the level detector itself is working. So we need to troubleshoot the level control loop and opamp. Another interesting observation – the digital control of the final amp is actually done by light beams (IR diode sending, IR receiver), to transmit digital information noise-free to the final amplifier. Note the gaps in the cover, marked in the yellow circle. That’s where the light passes from the digital control to the amplifier section.

This level control circuitry is part of the modulation assembly, which is a fairly complex assembly. But, what is this? There are several micro-size adjustment pots. Could it be that one of the relates to the ALC loop? How can we find out? Easy enough, we monitor the output with a power meter, and turn each of the pots a bit, of course, not without clearly noting its original position. The 3rd adjustment pot – it does affect the output power! And, surprisingly, I can easily increase the power by about 6 dB.

With this adjustment identified (I could not find any reference in the service manual to these adjustments), I rechecked and readjusted it output power at a range of frequencies from 1 MHz, 10 MHz up to 3 GHz, and flatness is in fact very good! Also, I get the expected performance now when increasing the output power past about +14 dBm – an uncal-unleveled message is coming up, indicating that the maximum output power has been reached (the MG3681A can provide about 14-15 dBm leveled power over the full range).

A good amount of logic in programmable devices (firmware of MG3681A can be updated, if you have any such firmware, please share with we!).

Another view of the inside of the instrument – it is what I call the Japanese test equipment design, a lot of empty space, all well arranged, and many different types of assemblies and hardware with great attention to detail and sophistication, and some manual corrections with superfine wire. Because of such design, it also needs two fans – one for the power supply, and one for the main unit.

The CDMA-W functionality. Fully working at least as much as I can tell from spectral analysis of the digitally modulated signal.

Some study of the phase noise performance. The MG3681 is not too bad, it has fairly low SSB noise, even close to the carrier.

For comparison, the phase noise of a 8642B, which is a good HP generator, not the best for close in phase noise, but it is a low noise and heavy and sophisticated machine, even without any digital modulation.

No sophisticated phase noise test gear set up here, but let’s study it on a 8561B analyzer, at 2 GHz frequency. At 100 kHz span, the noise of the analyzer dominates, and both the 8642B and MG3681A show very much the same levels.



Close in, at 10 kHz, the MG3681A appears superior, maybe, by about 10 dB. Not bad!



With all the functions established again – some clean up: removed all the dust, checked all the connectors, and put the thing back together.

Now, all is hidden again under several sheets of metals, and all the many screws (not only many pieces, but also many kinds) back in. Let’s hope we don’t need to open it soon again!

Some useful documents:

Anritsu MG3681A Technical Description

Anritsu MG3681A Service Manual

Anritsu MG3681A Datasheet

The 08662-60001 low noise VCO: the heart of the HP 8662A

This is certainly one of the electronic antiques and marvels, which had the most hidden and non-spectacular impact on mankind and development of mobile communications in the 80s and 90s – the VCO of the 8662A/8663A, the defacto standard for low close-in phase noise signal generators.

The concept, it is a switch reactance oscillator, i.e., the inductances will be switched, to cover the 320 to 640 MHz range, rather than doing this all by varactor diode tuning.

For low phase noise, you need an oscillator with high Q – this is achieved by strongly biasing the varactor diode over the full tuning range, and by low resistance PIN diodes (two in parallel, see below!), that switch the thin-film inductances embedded between lexane disks.

Note the small indent on the golden patch on the PCB? This is where the copper bolt reaches in to adjust the frequency offset.

All this is housed in extruded aluminum, end a layer of special steel which absorbs magnetic fields. So far, I never had the chance to look inside of these – but a kind reader this block, Martin, shared this picture and I put it up here for those interested.

The designer of this marvelous and magic device, his name, Dieter Scherer, a German fellow of HP, unfortunately, I have never met him and don’t know if he is still alive. Sure he left behind great achievement and a legacy of high frequency engineering.

HP 8662A 8663A Power Supply A7A3 Assy: base transformer defect

A little note, Thanks to a kind fellow sharing this repair info with me, related to the A7A3 assy of the 8662/8663A generators: the 9100-4018 base drive transformer. There are two of these on the 08862-60289 board, protected by a fuse and diodes, nothing should acutally happen to them, but things can go wrong.

Below, the two versions of the A7A3 boards, left, the older 08662-60289, and the more recent 08662-60604. This post only refers to the -60289, the base drive transformers of the newer units look the same, but have part number 9100-5291 – don’t know if the can be exchanged.

8662 8663 pwr supply

A snipet of the power supply circuit, there are two identical base drive transformers, center tapped on one side.

08662-60289 base drive schematic

Cross reference – this is a NSN part, the most prominent manufacturer seems to have been Fil-Mag, which use to be a Sprague company long ago.

9100-4018 master cross ref

5950-01-267-1279 aka 9100-4018 transformer

Note that these little beasts come at a quite hefty price! That’s well over USD 100, just for the two base drive transformers – I hope, HP did not pay the list price, or anywhere close to it.

Here is a rare view of the interals, after heavy work with sandpaper and other means (these transformers are potted, but as with all potting compounds it can be removed, if you have plenty of time, a good supply of tools and don’t mind the dust and dirt).

8662 8663 pwr fil-mag 42z994 internals

It uses an OJ41408 bobin/pot core, PC14/8 size. Wire is about 0.1 mm size. The pot cores are still available from Magnetics Corporation,, and the material is just a regular mid-frequency ferrite. So you might be lucky with just using any average good pot core with about 5000 permittivity (e.g., N30 ferrite).

8662 8663 pwr oj41408

mag-inc 41408ug pot core

mag-inc j material

Fingers crossed that you will never need this information, because it is quite a laborious effort to reverse-engineer the internals, and to fabricate a new transformer manually. Tempted to say, I could manufactur them well below list price, if someone would need a 1000 pieces….