HP 8566B (85662A) Spectrum Analyzer: fixing the 10 Hz issue, A4A7 3 MHz filter assy

The 8566B/8568B analyzer both use the 85662A spectrum analyzer display section, which is not just a display but also takes care of the IF processing. For the 10 Hz to 1 kHz bandwidths, a 5 pole xtal filter is used. A rather delicate assembly that dates back the the earlier analyzers, 8565A, in somewhat modified form. As a side note, HP had a strong tendency to utilized time proven circuits, some of them, over periods of 20 to 30 years… it helps with the repairs, once you get used to a certain assembly, the same pattern is repeating in multiple instruments. One of the examples it the A4A7 assembly of the 85662A (p/n 85662-60004). It is a rather ingenious design, and is critical for the 10 Hz resolution which makes the 8566B/8566B units so useful to resolve close-in spurs, like mains spurs.

The unit currently on my bench showed issues in the 10 Hz bandwidth – not enough gain. First, I assumed it to be an alignment issue, and spent quite a while re-adjusting the circuit. To no avail (well, it helped to improve the passband shape, which is now perfectly symmetrical again).
Almost wanted to give up. But not quite.

Checked the gain of the A4A7 with one of the stages, at a time, bypassed by a substitution circuit, a 47 n capacitor, in series with a 2.8 ohms resistor (see earlier entry). And, quite surprisingly, the gain of the 10 Hz bandwidth increased dramatically when shorting the 5th of the xtal poles.

8566b peaking cap

After careful inspection, notice the peaking cap. It is at its lowest value – this might be the issue – each of the poles has at least 3 adjustments: center, symmetry, peaking, and 2 of the poles, also a gain adjustment…. Maybe, the 5th stage (which is working at all the other bandwidths), is just not set to peak!!

85662-60004 a4a7 assy 5th pole

The 68 p capacitor, it is a factory selected component, and 68-82 p is the allowable range. This assembly had a 82 p fitted, but only at the 5th stage… well, just a few pF too much.

Where to get a 68 p cap (a silver mica…) now, one big ocean away from the well-assorted stock back home in the main workshop? Well, always good to have some old, spare HP boards at hand:
8566b scrap board
…one of them now is missing a 68 p cap….

After some re-tuning, running the calibration routine, look at the result, before and after:
8566b corr coeff before and after repair
…there are the missing 2 dB. Problem solved!

HP 8566B Spectrum Analyzer: YTO/YTX tuning, flatness adjustment, and an OCXO

The 8566B I am dealing with here as parts from at least three units, so no wonder that the YTO/YTX drivers are all completely out of adjustment. So much that the LO sometimes locks on an incorrect multiple of the reference, or that it doesn’t lock at all.

Well, the adjustments are all well described in the manuals, rather straightforward.

8566b corr coeff
The amplitude offset at 10 Hz, a bit more than I want it to be, but this is related to the A4A7 assy of the 85662A, not to the 8566B itself.

8568b flatness adjusted
Might still tweak it a little bit, when all repairs are complete, but for now, a quite satisfactory result.

The OCXO, it is mounted in a set of 3 rubber isolators, here are the rough dimensions, if you want to fit a custom OCXO….
0960-0477-1 osc 49-61c dimensions

8566b 0960-0477-1 osc 49-61C 10 mhz ocxo

From the service manual – there are at least two versions of the 8566B, one using the HP 10811, and the other, using an Ovenaire OSC 49-61C.
8566b a22 assy ocxo

As far as I know, the oscillators have more or less the same performance level, but the connectors on the motherboard are different (still carrying the same voltages – the 10811 has small add-on regular board, 5062-1909), and there may be also differences in the holding brackets.
5062-1909 10811-60111 board
Notice the different plug! I have a spare 8568B motherboard around that supports this connector style.

One of the many test results, the 22 GHz noise floor.
8566b baseline noise 22 ghz

Not bad at all, about -118 dBm. Also checked the power line spurs and the noise characteristics, all considerably better than specified.
The only downside: total weight, of the 8568B: 112 lbs, and two strong fans.

HP 8566B Spectrum Analyzer: 2 partial units and some spare parts

This story starts with a set of rather valuable 8566B parts that I received for free a litte while ago:
8566b spare parts

A partial unit, stripped of of most of its RF parts, and missing some boards, and missing the OCXO.
8566b partial unit1

For a long time, I have been looking for another partial unit that can provide the missing boards, the OCXO, and some bits of hardware to complete the instrument. Not worried about the 85662A display units, because I have a perfectly working spare unit around, or could use the unit of the 8568B.
Finally, a unit showed up, also missing some boards and parts, but luckily, not the boards that I needed -except, also no attenuator, and no OCXO here.
8566b partial unit 2
8566b partial unit2

That’s the start, the empty space that is going to hold the RF treasures:
8566b empty space

The YTO assy, missing the YTO, and other bits.
8566b yto loop incomplete

This part, the 5086-7226, to do it fully justice, one would have to talk about it for a few hours. It is a not only gold plated inside and out, but HP used two kinds of solds, of different melting point, to assemble the inner workings in subsequent steps, without melting the already assembled parts….
8566b 5086-7226 YTF

Some more pictures – the YTF driver.
8566b ytf driver front
8566b yig driver back

The 1st and 2nd converter assy, ready for the semi rigid lines to be attached.
8566b 1st and 2nd converter

A high quality input relais and a band pass filter.
8566b input relais and bp filter

Well, unfortunatly, don’t have a spare 8566B/8568B OCXO around, and they go on xbay for no less then USD 50, often, no less then USD 100, that’s ridiculous.
May this unit, which is very low phase noise, very stable, from a HP 3585A analyzer, can be made fit? Ovenaire OSC 73-52.
8566b spare ocxo 0960-0465 ovenaire osc 73-52 10 mhz

After a LOT of fiddly work:
8568b assembly progress

8568b testing

A first signal!! Amazing! Frequency is off by 80/300 MHz – the unit will need a proper alignment, but the PLLs are all locked, which is a great start.
8566b a signal

And, the noise – the effect of harmonic mixing can be clearly seen, so the input stage and mixers seem to be all working!
8566b noise

More to come!

HP 8568B Spectrum Analyzer: a fair number of adjustments…. done.

Having the 8568B basically working again is great. Not so great were the correction coefficients, all seems to be a bit out of alignment.

Before alignment…. quite some deviations at 100 kHz, at the lower frequencies, and so on.
8568b corr coeff

This is the order of the coefficients:

First, checked the log amp circuits, and all seems fine. No adjustment made.

The, some small tweeking of the A4A5 assy that controls the step gains.

Then, the major part, the xtal filters of board A4A7:
8568b a4a7 assy 85662-60004

Each of its crystals can be switched to 10-30-100-300-1kHz bandwidth, at virtually constant gain.
To do the alignment properly, each xtal has to be tuned separately, and some bypass networks are needed.

8568b bypass

8568b atx cable connector

The bypass network, a 2.8 Ohms resistor in series with a 47n ceramic cap – just use an old ATX power supply connector to get some suitable contacts. These come free of charge, whereas HP used to charge a dollar each, or more. Note that I did not have any 2.8 Ohms resistors at hand, so I used two 5R6 in parallel.

This is how it looks during the adjustment:
8568b a4a7 adjustment bypass

When all is done, the bandpass responses should be symmetrical, which they are, and the amplitude flat for all bandwidth. After running the calibration, these coefficients were found:
8568b final corr coeff

As you can see, it all looks great, except for the 10 Hz bandwidth. Carefully re-checked the aligment – 30-100-300-1000 is perfectly flat, but whatever I do, it seems A4A7 has extra loss when set to 10 Hz bandwidth. In priciple such situation can happen with misaligned crystals, or with some low-frequency issue of one of the amplifiers, which are rather unlikely. Maybe just some aged xtals? Will give it another try later, with flatness checks with some of the xtals bypassed, to see if the issue is caused by any particular of the stages. Found one of the 2.2 µF tantalum caps on the A4A7 to have high leakage current, replaced it, to no effect.

For now, it is good enough – specification at 10 Hz more relaxed anyway, and I never do 10 Hz bandwidth measurements without correction enabled, just considering that any modern analyzer basically relies on a good number of calibration and correction data stored somewhere in the instrument, and applied to all measurements.

HP 8568B Spectrum Analyzer: an amazing find, a few repairs, and a restored marvel of RF engineering

It appears that the US is a land of plenty when it comes to somewhat dated test equipment, otherwise, it would be hard to explain why someone would sell a 8568B analyzer, including a display unit, for just a few dollars. A great find!
In the as-received state, after removing the 8568B and the 85662A display unit from two huge boxes, it was starting up, but did not show any signal, and no annotations on the screen. The latter turned out to be a rather easy fix, a little defect in the intensity control circuit.

First step – adjusted all the CRT circuits, focus/intensity control circuits, and the analog/digital display scaling and stroke generator.
The CRT is of a quite amazing quality, not sure if it is the original CRT – it has a hand-written label sticking to it, which could indicate that it has been replaced at one point in time.

The major item, no signal (but a typical background noise trace) – this can be anything, but unless in cases of several neglect, it is hard to destroy the mixer or other hard to fix ciruits of the 8568B (typically, the attenuator, and the build-in limiter are absorbing any overload power).

Switching the input attenuator, some signal found at -70 dB attenuation! Strange, so there is something wrong with the attenuator.

Similar to the CRT, also the attenuator seems to have been replaced before:
8568b atten 5086-7815 70 db 24 v 4 ghz

8568b atten assy

Opening it up, with the necessary care, what a strange thing – the contacts are not making any contract… the screws indicate that someone has tried to fix it before, or maybe damaged these contact fingers, while trying to fix it.
8568b atten defective 2

This explains why only at the highest attenuation setting, there is a signal: the contacts work when pushed agains the 10-20-40 dB attenuator pads, but they don’t make contact with the pad bypass (“0 dB”).
8568b atten defective

Using some fine-tip tools, re-adjusted the contacts so that they close the by-pass of the attenuators.
8568b atten adjusted

Before re-assembly, make sure that there are no dust particles, and that the mating surfaces are perfectly clean. Best use a small, soft brush.
There is no need to over-tighten the screws. This attenator is the 4 GHz version, and not particularly critical. For the 22 GHz version, of same design, best check for SWR and insertion loss, and carefully tighten all screw with just enough torque to hold the assembly together.

Some checks, some adjustments – and the instrument passed the self-calibration with no issues. The coeffcients are not zero, but close enough, and cross check with a well-calibrated 8642B shows that the amplitude accuracy is perfectly fine, no issues with flatness, any of the attenuator settings, or when switching through the various bandwidths.
8568b corr coeff

Some of the other parts, the 1st LO – a YIG oscillator.
8568b yig

The reference, and OSC 49-61C, unfortunately, I can’t find any spec data for it, but appears to be a rather low phase noise oscillator, with more than adequate stability.
8568b ovenaire osc 49-61C

As a further note, should you be in the market for a 8568B or 8566B analyzer, make sure that it comes with the 85662-60093, 85662-60094 bus and interface cables.
8568b 85662-60094 85662-60093 cables
These cables don’t look like anything special, but are commonly sold for over USD 100 a piece, even in used condition. Often, the cables are lost when the instruments are put in storage, and auctioned later. Fortunatly, the current unit came with all the cables, even with a set of power cables!

A short glance on the main board, it is a marvel of engineering and a pleasure for the eye, all traces layed out by hand, fully gold plated, amazing quality and attention to detail. Might last another 100 years of 24/7 use.
8568b main board