Tag Archives: spectrum analyzer

HP 8569B (8565A) Spectrum Analyzer: curing the knob disease

Having seen quite a few of the 8569A, 8569B, and 8565A analyzers in the shop recently, once common issue are the controls. The 3-knob operation is one of the features that makes these long-established machines still desirable for today’s work, in particular, for general test and troubleshooting in the microwave region, up to 22 GHz, and above. At the same time, these knobs were made of plastic, and they age – most of they show cracks, which will sooner or later require difficult repair. In the current case, I am dealing with the unit described earlier, it is a 8569B, and someone had fit a 8565A control pad – all of the knobs more or less cracked and useless – I might use their remains to fix upcoming instruments.

So, what do to? Well, decided to go for a rigorous approach, and provide completely new knobs. These use a coaxial design, with 1/4″ outer shaft, and 1/8″ inner shaft. For the small controls, just ordered a few Augat/Alco knobs, which are quite sturdy and easy to mount.

8569b augat alco knob

The large knobs, I custom machined back home in the main workshop, using my little CNC lathe (kind of an overkill) – they are made from POM/Delrin plastic, with some brass inserts, and 4-40 set screws. The knobs are knurled at the outside – one turned out a bit black, because I didn’t clean the tool properly… fair enough.

8569b large knobs

The only downside – there are no markings on the knobs, like for the originals, but such markings aren’t really needed for the 8569B which has a full on-screen display of all settings. Also the ‘lock’ positions for coupled BW-Span still works!

8569b new knobs detail

8569b new knobs mounted

All in all, I’m pretty happy with this repair, estimated useful life of the new knobs – 30 years, and replacements can be made, as we go. But maybe, by then, the unit might be really obsolete, although that’s the thing I am least sure about!

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: 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

HP 8565A?? HP 8569B?? Spectrum Analyzer: a mixed box

For next to nothing, I got hold of a HP (Agilent, now: Keysight) 8565A 8569B mixed analyzer:

8569b front panel 2

8569b panel

As you can see, the main unit, including the CRT and electronics, is a 8569B, but someone fitted a (rather incomplete) 8565A control assembly. Not a big deal, normally, but the control assembly fitted here has virtually all the common defects: missing contact wipers, and missing/defective knobs.

Another common defect (can all be fixed):
8569b wheel

A quick test – the CRT seems to be working, albeit, it is now dark after a few minutes of operation – most likely, just a dead capacitor, but the CRT itself is definitely pretty good. Also the CPU seems to work just fine.

8569b yig

The unit has a lot of RF goodies, like a 22 GHz 3-stage YIG filter, a YIG oscillator, a 22 GHz mixer, various 22 Ghz coaxial relais – but with another parts unit already at hand, this unit seems to be to precious to scrap. Well, need to think about it, always wanted to provide an electronic replacement for the aging 8565A 8569A/B control assemblies. Maybe, a good project for next winter!