Category Archives: Generators – High Frequency

HP 8663A Signal Generator: another power supply repair

2016/03/08 Simon Leave a comment

Always good to do a proper test of equipment after repair – especially, after a power supply repair – see recent post 8663A pwr supply repair. In most cases, all will be fine, but this time, another failure followed the initial repair: the -10 V rail showed an error, only providing about -8 V to the instrument, not enough, to keep it going. An issue with the A7A1 assembly, linear regulator board, hp part 08662-60157 (the HP 8662A uses an identical assembly).

After some checks it was clear that the final pass transistor Q2 was OK, and that there was no current limit issue (by checking the voltage drop over the sense resistor, R36).

So, I guessed, something must be wrong with U2, the actual regulator. This is a 1826-0016, alias LM104H. Not quite a common part, at least, I didn’t have a spare on hand. Found some new old stock online, about USD 3 per piece, well, not too bad. It arrived a few days later, but, the exchange of U2 was to no avail – still no regulation.

Well, I should have done a proper check earlier – turns out, the transistor Q7 (2N2904A) didn’t provide enough current for the final stage, despite being fully driven by U2. This time, I had more luck and found a 2N2904A in my parts collection (datecode: 7050 – 46 years old – but still working, hfe=170).

Some final test with a 25 Ohms power resistor to test under load, before risking any damage to the venerable 8663A. And, long story short, all is good now.

HP 8663A Synthesized Signal Generator

HP 8663A Signal Generator: switchmode power supply repair

2016/02/22 Simon 6 Comments

A heavy guest on my bench, a 8663A signal generator. These generators are exceptionally clean, perfect for close-in noise measurement and receiver checks. Still today, hard to find a cleaner source, especially not, if you are on a less than USD 30k budget.

Symptom – easy to describe. Unit turns on, but only briefly, then switches off; over voltage protection light activated at times; a lot of noise on the DC rails when shorting out the safety circuits.

After some probing, the culprit could be located: one of the input capacitors. While this is a common failure mode of other equipment, these caps don’t fail too often for such HP equipment, because only best-in-class components were used, and these are typically run cool, for long life. Still, one of these 32DR6593 SPRAGUE Compulytic caps failed (resistance about 100 kOhm, virtually no capacitance, rapid self-discharge when charged to 50 V for test, framed red in the schematic below).

These were replaced by 81D series Nippon Chemi-Con (Vishay) caps. To call this a successful repair, you might wish to check the ESR specifications – the SPRAGUE had about 0.25 Ohms, the Nippon 81D (680 µF, 250 V) has about 0.198 Ohm, good enough. Note that the 600 µF screw-type terminal caps might still be available, but they are pretty expensive, so I opted to for Nippon Chemi-Con, USD 2.50 per piece, surplus, rather than USD 50 for a pair of screw-type caps. I still think it is a good compromise, because this is not about restoring old equipment, but to make this unit working again, quickly, and at lowest cost.

Some repair is also needed on the A7A3 board – there are 22 µF caps that provide a low impedance DC input to the switching transistor, these are essential for operation (framed green in the schematic). They still work, but were hot, and stressed, possibly overstressed, by the dead main cap. Their can be replaced by any good cap, I use Shiangchen GSA T axial caps, 105 deg C rated. Note that the schematic calls for 15 µF, but 22 µF (measuring about 28 µF) were present in the circuit.

With the power supply disassembled, always a good idea to take out the boards for cleaning, and for re-tightening of the screws holding the various TO-3 regulators in place. Some of these were pretty loose (no wonder, with 30+ years of thermal stress on the boards).

After the repair, add thermal compound to the heatsink/cover – this power supply has a rather critical thermal design. Then, make sure to check the insulation resistance and electrical soundness/earth leakage, which is always a good idea after repair of switchmode power supplies.

HP 8662A Synthesized Signal Generator

8662A Synthesized Signal Generator: another rather straightforward repair

2015/02/08 Simon 1 Comment

These 8662A generators keep coming in, seems that after so many years (like, 30 years) of 24/7 service, some little things are coming up. But not really sure if the frequency of failure/repair really increases, because with all these 1000s of parts, most of the units have seen some repair already, and it is just in the ordinary course of things that there can be defects every once in a while.

This unit add issues with the reference sum loop (A6A5), showing up in intervals of about 10-20 MHz. Knowing that a 10/20 MHz reference is used for the ref sum loop assy, this was the first and most likely cause of the defect. So, I substituted the 10/20 MHz signal, but, to no effect. Therefore, the defect is most likely located on the A6A5 assy.

One option would be to switch the assembly with one of my 8662A/8663A units, but these are currently in use and I don’t want to pull out assemblies that are difficult to fix and need adjustments, etc.

So, next, checked the pre-tune circuit, and, issue found. TP14 is at a constant -2 V, rather than variable voltage as per requirement given in the service manual. Digital input is OK – maybe the fault is in the semiconductors – one transistor, and 4 FETs switching a current source.
With spares at hand, these suspicious parts were quickly replaced. Effect: no effect. Big mystery.

Now, connected an external resistor, to measure the pretune DAC (4 FETs), and, it works! Even more mysterious.

Finally, also checked the traces and the resistors, and found the ‘2K’ Trimpot to have about 40k!! With the wiper contact having some effect, but not much.

Desoldering the Trimpot – all seems to be OK with it. Maybe some kind of aged solder joint? Whatever it was, the Trimpot is now fixed, the A6A5 assy re-adjusted, and the 8662A working again!!

HP 8645A Agile Signal Generator

8645A Agile Signal Generator: finally, all fixed

2015/01/31 Simon Leave a comment

After quite a bit of work with the front panel (see earlier post, massive transport damage), and repair of its most beautiful and complicated inner workings, the remaining item to be fixed: the main output connector (N-type connector). The original one, damaged beyond repair, and the bracket holding it, severely bent, and a broken-off screw stuck in it.

Screening through my parts stock, found a rather old but still good panelmount N connector. It has a slightly different inner construction (the original 8645A has a quite long gold-plated coaxial air-line in the connector; replacement connector has no gold plating), but within reason, this should not matter.

This how it looked before:

The bracket and assembly was fixed back home, in good old Germany, in the mechanical workshop, during xmas vaccation.

Now, looking at the result, quite pleased with the outcome of quite a few hours of work (and some expenses, for a replacement HP System 2 front panel frame, see earlier post):

Note that there are two handles now mounted to the unit, not so much for carrying it around (about 70 pounds!), but to protect the front panel.

HP 8663A Synthesized Signal Generator

8663A Synthesized Signal Generator: a 1 year long repair story

2015/01/28 Simon 2 Comments

Over a year ago, I got hold of a defective 8663A. It seemed beyond repair, but hardly any equipment is, provided, you put in sufficient effort. The 8663A is certainly worth any resonable effort – it is a marvelous piece of test gear, providing full evidence of human ingenuity. It is said that a team of no less then 20 of the best HP engineers took about 5 years, with full support of the mighty HP organization at the time, to develop the 8662A and later the 8663A generators. Ever since then these were the gold standard for any low close-in noise source, for phase noise measurements, and so on. Keysight is offering a replacement now, the E8663D, about USD 50k, nothing compared to the 8662A or 8663A, for its historic value, and comparing the sheer mass of metal, the amount of gold on the assemblies, and the variety of strange little parts.

After some analysis, these were the main defects of the unit discussed here:

(1) A missing attenuator. The 8663A uses a pair of mechanical attenuators; these come in a set, together with a calibration ROM; with one of the attenuators missing, we might need to check flatness and level accuracy once replaced.

(2) Some intermittent failures of the A6A4 output sum loop. Seems to depend on frequency but not limited to any particular range. Supplied some test signals to the A6A4 assy and the fault really seems to reside with this assembly.

(3) A defective output amp. P/N 08663-67002. This is a real disaster. The output amp is a microwave microcircuit, with HP GaAs FET transistors. Needless to say, such assemblies were very expensive already at their time, and spare assemblies, despite long search, are fully unobtainium.

First things first – the attenuator. Found a spare attenuator that should be resonably close to the orgininal one. Did a quick check – it has very flat frequency characteristics anyway, so the ROM calibration coefficients might not be too significant (the 8663A has +-1 dB level spec, but typical accuracy appears to be more like +-0.2 dB; relative levels about +-0.1 dB). Someone tool the attenuator from the ‘dead’ unit, including the bracket – well, I didn’t spent too much time (as you can see) to fabricate a replacement. Good enough.

The intermittent fault of the A6A4 assembly – these assembly seem to be the weak point of the 8662/8663 series – I have already fixed a few of these. With schematics around, no big issue to fix – still it took a few hours this time. Turns out, the pre-tune DAC, which is a discrete circuit using 4 FET switches (1 per bit), showed erratic behavior. This was traced to one of the FETs, of the common 1855-0020 type – I took one from an early 80s HP parts units (8569A).

This is the bad guy:

The most critical item, the 08662-67002 amp. This has a low frequency (<120 MHz), and a high frequency input (>120 MHz), which are routed to a common output, providing about 20 dBm of power, at low distortion (about 35 dBc), over the full band from DC to 2.5 GHz.

After opening up the microcircuit, it is pretty clear that the last of the FET transistors is blown, and shorted to ground. This is all sapphire substrate, wire bonding, high frequency art. Beyond my capabilities (do you have a wire bonder at home, and a steady hand, and the skill and knowledge? Please teach me!). However, this world is not all bad, and rescue came along, back in good old Europe – in its South-West corner.

Turns out that a HMF-1200 is a suitable replacement for the proprietary-unknown original HP part.

Here the work of the kind friend, who certainly has tremendous skill and is a master in his field:

This is the associated board, with a PIN switch, and some bias regulators.

After all these repairs, and some adjustments (which took another few hours; including amp bias, lock detectors, ALC, FM VCO, …), the machine is working again – uptime so far, 48 hours at full power – consuming 500 Watts, and 100 mW at the output. Like powering a 100 W light bulb, from a 500 kW supply…. not quite efficient but a good heat source for the house, during these cold winter days.

To come: some flatness and level checks of the attenuator, but don’t expect any bad surprises.

An interesting document, found during the search for spare assemblies – the US Air Force also seems to be looking for repair, for 4 pcs of the amplifier assemblies, for their F15 Tactical Electronic Warfare System Test Set, P/N 001-006730-003.

May be a good business opportunity, but not for me!

HP 8642B Synthesized Signal Generator

HPAK 8642B Synthesized Signal Generator: backlight replacement

2015/01/06 Simon 2 Comments

The 8642B is an excellent generator, very clean, at least at offsets >1 kHz, hard to beat. It is also very heavy, thanks to a special modular concept that HP was pushing at the time. Their intention was to make the unit more serviceable, with the result that the generator is super heavy, and so expensive that it never was a real commercial success for HP. Frequency range is from below 100 kHz, to 2115 MHz. Pretty useful, with amplitudes from -140 dBm to 20 dBm.

The generator has a rather large (for the time) LCD display – fully story can be found in the HP Journal, December 1985.

The backlight has a very thoroughly designed light diffusor, which directs light from two 5V axial bulbs evenly to the LCD. Sure enough, these bulbs can burn out.

Some webpages claim that such bulbs would only last a few 100, maybe 1000s of hours, but such statements are incorrect. The bulbs used, 5 V, 115 mA, will typically last about 40000 hours, much longer than common household light bulbs.

These little bulbs have 1.9 Lumen each, not bad. To replace with a LED, 14000 mcd, at 25 deg angle, are about 2.2 Lumen. Close enough.

Found some 3 mm superbright white LEDs:

0.04 USD each!! Amazing!

Also these won’t least forever, white LEDs do lose intensity over time, like, 50% remaining intensity after 20000 hours.
I decided to run them below the rated current, at about 16 mA (120 Ohm series resistor with 5.2 V supply). Maybe this will make them last a bit longer.

Note that the backlight is software-controllable (special functions 134 and 234). This is how it is implemented (let me know if you need to full schematics of the 8642A or 8642B):

The resistors (1.8 Ohm) make sure that the lamps operate at 5.0 Volts, not 5.2 Volts, and don’t interfere with the operation of the LEDs. Quite amazingly, running at 5.2 V vs. 5.0 V would reduce the life span from 40000 hours, to 25000 hours!

Agilent sold these bulbs for about USD 18 each!!

No mechanical modification of the 8642B at all, the LED and the resistor fit well into the cavity (don’t worry about the lens of the LED – the light will find its way.

The result:

– not to my full satisfaction (non-uniform brightness, looks a bit dark on the left hand side, albeit, very easy to read).

After a bit of head scratching – turns out I soldered in the left LED with incorrect polarity – so it can’t work.

With this little mistake corrected, all is good:

Now, let’s hope that these LEDs will last. Never mind, I have a bag of spares!

Note: the display assembly is said to be rather ESD sensitive. Make sure not to damage it!

Wiltron 6659A Programmable Sweep Generator

Wiltron 6659A Programmable Sweep Generator: EPROM images captured, some rubber items

2014/11/17 Simon Leave a comment

The 6659A is running on no more than 10 kilobytes of code, stored in 5 pcs 2716 EPROMs. These were programmed in 1984 – just about 30 years ago. Just in time to capture a copy. Well, not just of their exteriors.

Their contents –
wiltron6659a
-also contained are the data of the 4 frequency correction EPROMs.

Looking through the board, a very intriguing part: a DAC72C-CSB-I

It is a 16 bit DAC, 30 ppm INL, 7 ppm/K gain drift. Merely, a resistance ladder, with a build-in 6.3 V reference. Still going for about 40-50 USD each! Very impressive case, for 1984 vintage, and still working just fine, to the current day.

While silicon chips last, rubber is subject to aging – in particular, the 4 rubber stand-offs that were holding the fan. Removal was no easy task – they were glued in with some Loctite, but eventually, I could remove them, and they were replaced by regular screws – not too much vibration anyway.

Wiltron 6659A Programmable Sweep Generator

Wiltron 6659A Programmable Sweep Generator: 10 MHz to 26.5 GHz, in a single sweep…

2014/11/09 Simon 4 Comments

This instrument just needed a few adjustments, still, very interesting to look at.
It is of a very classical design – 4 YIG oscillators (2-8, 8-12, 12-18, 18-26.5 GHz), a coupler, a detector, and all the driver and ALC circuitry to make this work.
There is also a Wiltron-brand downconverter that provides the 10 MHz to 2 GHz output, by conversion of a 4610-6600 MHz input, with a 4600 MHz LO.

The YIGs are all of best quality, Avantek parts.

The coupler, for the ALC loop, a Krytar ultra-broadband part, with a biased detector.

The generator provides ample power, 10 dBm or more, over most of the bands; about +6 dBm, at above 18 GHz – and, around 23 GHz, there is a dip in the power curve (see scope screen, showing the ALC/power signal vs. horizontal sweep). Checked the bias (was set at a constant +11 V) – changing it, and going up to +15 V, no change. Also checked the power directly at the YIG output – still, the dip. So it seems, nothing we can do about it, but for most practical purposes, about 0 dBm will be plenty, at any frequency.

After some more alingments, the frequencies and bands are spot-on (no need to re-programm the linearizer EPROMs – all YIGs are still tracking perfectly fine), some some cleaning, using 50% isopropyl alcohol – done.

The only thing left to be done – a back-up of the 2716 EPROMs that still hold the firmware, after about 30 years.

HP 8645A Agile Signal Generator

8645A Agile Signal Generator: power-up, lots of mechanical repair – finally, calibration, and some first tests

2014/11/02 Simon Leave a comment

The 8645A has been a real challenge. Not electronically, but with all the mechanical damage.

Most of the defective parts are now fixed or replaced, the last one missing:

The front connector and bracket, badly damaged – this will need to go back with me to Germany, don’t have the right tools here (anyone out there with a metal workshop, in the greated NYC area, let me know!). Will see if I can bend this back, or machine a replacement from aluminum alloy. Also, one of the screws is broken off and stuck in the thread – all pretty laborious to repair.

The temporary set-up – connected a SMA f/f adapter, to get the signal out.

Electronically, major success! Not going into any details for now, explaining the inner workings of the 8645A would take hours. But, it is now back to a stage where power can be supplied to the circuits, and watch out, this is the first result:

… after several minutes, still no change – checked the manual – and the self calibration can acutally take 5 minutes or more, so I am patiently waiting. Finally, this result:

Sounds good.

Went quickly through all of the basic functions, and the machine seems to put out almost +20 dBm over the full band, stays perfectly phase locked, and, as far as I can tell with the 8565A, it is pretty clean.

Machine under test –

– showing a ~1.7 GHz signal, FM modulated, 30 kHz depth, 1.5 kHz audio.

For now, repair-final check on hold, with more tests once the connector is back in place.

The many hours spent so far, certainly worthwhile,for a machine that cost about USD 39.5k in 1990, close to 90k nowadays… all its major capabilities, well described in the HP 1990 catalog.
8645a hp catalog 1990

HP 8645A Agile Signal Generator

HPAK 8645A Agile Signal Generator: pushing it back in shape, Chinese spares

2014/11/01 Simon Leave a comment

The 8645A – see earlier post, reached my workshop in a really bad shape, electronically dead, and severely damaged. For any reasonable effort, damaged beyond repair.

However, I do this for a hobby and there is no rush, and I just can’t take this thing apart for scrap metal – the case suffered damage, but the inner parts, no major mechanical issue.

For most of their 1980-1995 equipment, HP used a so-called “System 2” modular case, mainly, die-cast aluminum alloy. The 8645A is not much different – the bottom and top lid are different (to provide better shielding than the regular System 2), but the front frame, it is the same.

Looking around at xbay – found a complete front system 2 front assembly, System 2, from a 8782B Vector Signal Generator. The Chinese vendor, he seems to make a cut by taking apart equipment, and selling it for parts – it wasn’t quite cheap, but still, for a 8645A, to get it back into service, it seems pretty much worthwhile.

It arrived already after about one week.

Not in best shape, but usable.

This assembly also has quite a few BNC connectors, and a precision N connector, all very handy to fix the replace the snapped-off parts.
As it turns out, the N connector is of a different length, but never mind, I have some more spares around.

The front panel assembly, it is apparently one of the weak points of the 864x series.
Two layers of rather soft aluminum, a ~0.8 m plastic sheet that also acts as filters for the fluorescent displays, and a thin outer layer with the labels.

This is quite easily pushed in, if force is put on the BNC connectors – it is always best to safely secure the instrument at the corners during transport, and to attach handles.

The layers are held together by glue, and it seems, HP has used a very sticky, but not permanent type – with great care, it can be separated. Key thing is to keep the sticky surfaces clean, and they are best protected by a polyethylene foil.

To flatten things out, a carefully adjust iron is a handy tool – with the plastic sheets put in between two layers of non-stick paper.

The more laborious part – the flattening of the aluminum panels, with a hammer, and a piece of wood, and about 2 hours of work.

The result – all things glued back together, pretty happy with the result.

Sure, there are some little blemishes, well, fair enough!

Now we just need to get the inner workings going…..

SimonsDialogs

Category Archives: Generators – High Frequency

HP 8663A Signal Generator: another power supply repair

HP 8663A Signal Generator: switchmode power supply repair

8662A Synthesized Signal Generator: another rather straightforward repair

8645A Agile Signal Generator: finally, all fixed

8663A Synthesized Signal Generator: a 1 year long repair story

HPAK 8642B Synthesized Signal Generator: backlight replacement

Wiltron 6659A Programmable Sweep Generator: EPROM images captured, some rubber items

Wiltron 6659A Programmable Sweep Generator: 10 MHz to 26.5 GHz, in a single sweep…

8645A Agile Signal Generator: power-up, lots of mechanical repair – finally, calibration, and some first tests

HPAK 8645A Agile Signal Generator: pushing it back in shape, Chinese spares

SimonsDialogs – A wild collection of random thoughts, observations and learnings. Presented by Simon.