Category Archives: HPAK (HP Agilent Keysight) 8662A

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….

8662A Synthesized Signal Generator: another rather straightforward repair

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.

8662a a6a5

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.

8662a a6a5 assy schematic

8662 defective trimpot

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!!

HPAK (HP Agilent Keysight) 8662A Synthesized Signal Generator: ref sum loop adjustment, output amp/peak detector repair

Introduced in about 1980, the 8662A is a marvelous machine, it is an ultra-clean signal source, with very low close-in noise. This is the kind of oscillator used as a reference for phase noise tests, narrow channel receiver testing, and so on. Just a quick glance at it inner working, and it is clear that only the most brilliant engineers must have been working on this apparatus in those days. Sure enough, this did not come cheap, about 30 k$ in 1981, something like 75+ k$ nowadays…

Even more interesting, these machines are still in use today, and are still valued for the same reason – hardly any synthesized generator exists that has similar close-in noise.

The block diagram – essentially, there are two ultra low noise switched inductor osciallators that form the reference and output sum loops, together with with some fractional and n divider circuitry, to allow for the fine resolution. The output sections has a heterodyne band, and other bands derived by doubling or dividing the 320-640 MHz fundamental output of the output sum loop oscillator.
8662a block diagram

As with all wonders and complex machine, they sometimes stop working. This one had two issues reported, inaccurate output power (and error #10B – ALC error for some frequencies/settings), and a ref sum loop unlock condition at certain frequencies. The latter issue had been persisting for some time, but the ownder didn’t use it at the critical frequencies, the ALC loop error just recently seems to have come up, for now external reasons, apparently, during a measurement.

(1) The output sum loop A6A5, error #06. This turned out to be a drift issue, of the low noise REF oscillator. This had drifted out of pretune span, and needed just very slight adjustment of the tuning screw (under a heavy metal cover, rear panel of the instrument). Such adjustment is not mentioned in the manual, but easy enough, set the generator to 640 MHz, check that the pretune is properly adjusted (adjustment is done for -3.75 V at 320 MHz), introduce a -34 V tuning voltage signal, and adjust for a frequency a bit above 640 MHz, say, 642.5 MHz, to give the phase loop a bit of room to operate.

08662-60417 oscillator

Great care must be exercised, because the adjustment uses a copper (!) screw – red frame in the above picture, most likely, acting as a capacitor vs. a metal surface inside of the unit (don’t open up these oscillators – they are wraped in Mu metal or similar exotic material). Don’t overtighten the retaining nut, copper is pretty soft.

After that, the ref loop is stable again, even when sweeping through the full band, for some hours.

(2) The more tricky part, as it turns out, the ALC loop. This is implemented on assemblies A4A1 (which has the peak detector), and A4A7, the ALC control, this has a DAC, to set the target voltage, and a regulator, to keep the output signal at the level set by the DAC output.
8662a a4a7 probing
After some time of probing around, and substituting the input of the A4A1 assembly by a calibrated 100 MHz signal, it is clear that the A4A1 peak detector delivers insufficient voltage, and that the voltage is frequency dependent.

A quick glance at the schematic:

8662a output amp

Blue frame – the bandwidth limiting circuit, enganged at below 10 MHz – first guess: something is at fault, in this circuit. Well, unfortunately,after even more probing, and even swapping some transistor (they tested good but you never know) – the focus shifted to the part in the red box. The peak detector diode! This is really bad news, because it is part of the output amp microcircuit, part number 08662-67008. Carefully desoldered, and checked – the detector diode has 0.3 V forward, which is fine, and 0.7 V backwards – this is a far too low reverse breakdown voltage. Something must have happened with this diode, maybe, it is just age. So, I openend up the circuit (the lid can be lifted off with a knife, glued on with silver epoxy), and inspected with a microscope – no obvious defect, all kind of nicely wire-bonded parts on sapphire(?) substrate – nothing I can fix with the tools at hand.


But, as luck comes along, found two of these circuits on ebay. Very mysterious. They look like old and used and ripped out of the boards (with a screwdriver, not by desoldering), and I suspected that the two might just be damaged parts – but 14 days right of return were offered, so not an issue. Even then, it doesn’t really make sense to rip the out of the board, every reasonably skilled and knowledgeable person would rather desolder them carefully, and fit a spare. However, these spare still had the through hold plated vias on the connectors, from the board. Glad the feed-throughs at the bottom of these microcircuits are so sturdy! Well, the only explanation I have – someone saw the golden parts, on some odd circuit boards, and only wanted to keep the gold, but not the boards. Fair enough.

And, this is almost the end of the story, the spares arrived within a few days, and I cleaned them up, and fitted the most “used” looking 08662-67008, and, quite to my surprise – working just fine.

The only thing that remained was the amp bias adjustment, the offset adjustement (both on A4A1) and the CW power adjustment (A4A7) – also checked the other alignments of A4A7, but they were all still fine.

In the end, still one output amp in the box, for the next 8662A, and the current one, back alive.