Category Archives: HP 8569B Spectrum Analyzer

HP 8569A Spectrum Analyzer: two mysterious resistors

Again, some repairs of a 8569A spectrum analyzer. This time, no problem at all with the front selectors and contacts that break all too often, but with the CRT showing only a defocused vertical line. Some probing around – the analyzer appears to be find but something wrong with the deflection circuit? This is on the A5 assembly, and with no extender board around, quite difficult to probe, and a bit dangerous because this assembly is using +120 VDC. Any short may damage the supply, and any touch may give you a shock. So better exercise some care. By soldering wires, I was able to trace the signal and it seemed to be OK up until to some of the last amplification stages of the X amplifier. Maybe something wrong with the transistors – no these are good. Probing around on the board, checking all values of components close-by: the 75k resistors (fairly large sized 0.5 Watt resistors) are high-resistance, both seemed to have failed at the same time – strange. Eventually, this is becoming a quite frequent failure, already the 5th case or so over the last few years of open high-ohmic, say, 100k and about, resistors that are subject to 100 V or similar “high voltage” usage.

With no suitable 75k resistors available, I went with a combination of 82k parallel to 880k, resulting in 75.01k, close enough. And the 82k resistors that need to carry most of the current are really good old low inductance transistors suitable for such 100 volts service.

The repair, it is not particularly beautiful, but it works and at no cost, with available parts. Finally, some hours of test to make sure there is nothing else at fault in this unit. All good, test passed.

HP 8659B Spectrum Analyzer: mostly, the known issues

Mostly, the well-known issues for this 8569B: a bad fan, a bad 5.2 V supply capacitor (see the 5.2 V rail ripple below!), some issues with the display adjustment, and a bad control assembly with contact broken off. The control assembly, interestingly enough, somebody else had fixed some part of it before, from the handwriting, an American.
Still some minor issues with the Z axis control (brightness control), but this will be fixed soon, and then the analyzer will be thoroughly tested and will find good use again.

HP 8569B Spectrum Analyer: sweep issues on larger spans, A18 full multiband assy issue

A quick repair story from a kind contributor (Martin, you can find him at

(1) I have a unit that generally works OK but has stopped displaying spans above 2MHz/div.

(2) I still get a display on the higher frequency settings e.g. span/div frequency etc etc are displayed at top. However, no trace is displayed.

(3) If I manually sweep using the 100 MHz test signal / comb generator or external frequency source I can still identify the peaks of the signals in the higher span/div settings.

Note that there is a “NARROW” signal used in these machines, which will switch state when span is set above 2 MHz/div. So, best start with looking at all the circuits that are affected by the NARROW signal.


The problem was with the A18 (Full multiband assembly board).
When I scoped out the “over sweep blanking signal” (TP1) the signal was high as soon as the unit was set to >= 5MHz/div.
The signal into the board was OK though.

Anyway, I checked U3B opamp and the signal on the output was stuck high on all the higher ranges.

After inspection of the schematics and downstream circuitry ……
* Removing the multiband board allowed viewing of the higher span/div settings, without the full / multi band and over sweep blanking functionality.
* Isolating the over sweep blanking connector pin from the A18 board to the backplane allowed all ranges to be viewed, but without the over sweep blanking function. Note: The display still looked OK without the over sweep blanking.

Anyway, I changed U3 and U7, but still no luck with the operation. Then I re-checked all diodes and feedback resistors around the opamps.
On inspection, I found that the feedback resistor R29 for U3B (see schematic below) had failed open circuit and was effectively putting U3 into an open loop configuration and thus saturating the output.

The resistor has been replaced and it all seems OK at the moment.

8569b a18 full multi-band assy

This is a quite uncommon fault, on a low-power resistor – maybe a singular fault, or a lot of defective resistors, who can tell for sure.

HP 8569B Front Panel Assy Repair: rotary switches

Another 8569B repair, dealing with the aging plastic of the front panel assy rotary switches. Having the variable ref level encoder fixed earlier (ref level encoder repair), the level rotary switches were fixed, by using some small (metric) M1.2×0.25 brass screws.

Most important – the holes to be tapped for the screws need to be drilled quite accurately, using a 0.8 mm drill. This is best done on a milling machine or precise drill press, or with a very steady hand. If you don’t have a drill press, make sure all is as straight and perpendicular as possible.

A M1.2×0.25 tap is then used to thread the parts, make sure to start the tap perpendicular to the surface (a single tap, or machine tap will be fine – no need to use tap sets).

M1.2 screws will fit pretty snug through the contacts, so you might need to use a small screwdriver or similar tool to slightly enlarge the holes of the contacts.

8569b rotary switch repair

Final stage of assembly, still with some of the wires unsoldered…

8569b control assy

8569b control assy front

HP 8569B Spectrum Analyzer: reference level adjust repair (broken contact)

A very common fault for the 8569B (and 8565A, 8569A) are defective front panel assemblies because of missing or broken contacts. One particular case is a broken input attenuator switch assembly contact, for the manual/variable attenuation level encoder. This variable attenuation adjustment actually uses a potentiometer (green part), and this mechanically coupled to a BCD encoder, formed by a few contacts and a gold-plated circuit board.

This repair is part of a re-assembly tasks – a gentleman out there had disassembled the switches, and I promised to help out with the re-assembly. But as often, things are a bit more difficult than they first appear, and it turned out not to be only an assembly job, but a repair job.

8569b assembly job

8569b contact repair level adj knob

One of the contact fingers is clearly not making any contact to the circuit board.

8569b contact repair level adj before

To fix, just support the contacts with a piece of circuit board, or other plastic or metal sheet of adequate thickness.

8569b contact repair level adj supported

Then, using 2-component epoxy glue, apply a good amount along the section holding down the contacts. with some contacts still properly mounted, this will hold down the others, for a lasting repair. Sure, you can also use a small amount of JB Weld or similar compound, but I would always advise to apply it evenly over all contacts rather than just trying to fix an individual contact – this will provide added strength.

8569b contact repair level adj

Make sure to properly cure the epoxy before putting things together again. I usually let is sit for close to the heater for a few days.

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 8569B Spectrum Analyzer: almost the same, not quite identical

It took the best part of two hours to find the reason for the mixed mode malfunction-the lacking sweep time denominator issue. By comparison of all 84 lines going from the main front panel assy no. 08565-60002 to the analyzer, found an issue with the J1-43 line, MNL SWP (manual sweep).

8565a j1-19 man swp8569b j1-43 mnl swp

Above, old version (08565-60002 A-1645-45), below new version (08565-60002 B-2430-53).

8565a old board
8565a new board

8565a front switch assy modified

8565a wire

No big deal, added a yellow wire, to connect the pushbutton contact to the J1-43 line.

8569b working

A quick test, and it is working just fine!

Now, the only thing remaining are the knobs, but this will have to wait a few more weeks.

H 8569B Spectrum Analyzer: some progress, and a few more items to fix

A brief summary of the somewhat tedious job of fixing the 8569B front controls, which are a great feat of engineering but the plastics are prone to aging:

First, replacing the gear, it is a 48-pitch, 48-tooth spur gear, glad that I had one spare, but they are still available:
8565a gear
8569b gear

Next step, some contact cleaning, using a soft eraser, and some isopropanol.
8569b cleaning contacts

The frequency control – for some reason, there are different versions around, one that has wires attached, and one the has pins – interestingly enough, the 8565a control fitted has wires, but the 8569b requires pins – also here, good to have a spare assembly around, with the pins…
8569b frequency control

Defective bias pot – also here, a spare fitted.
8569b bias pot

Some of the contact fingers broken off-also there were fixed, and everything put back together.
8569b back together

Finally, noticed that the 10 dB segment of the 70 dB input attenuator (5086-7365) is stuck. Nothing dramatic, just one of the little O-rings holding the contact actuators broken off, and parts of it stuck in the contact.
8569b 5086-7365

Finally, a quick test – everything seems to be working-
8569b display

-unfortunatly, still missing something – the sweep time indicator is not showing a time denominator (┬Ás, ms, sec), and the analyzer remains in the digital mode for all sweep time settings – it should switch to mixed analog-digital mode (at the time the 8569b was designed, there was no easy way to run analog-to-digital converters any faster than a few kHz without causing exorbitant cost, so the 8569b uses the digital storage mode only at the slower sweeps).
8569b display missing

Remaining items:

(1) Identify the issue with the sweep time indicator and missing transition to mixed analog-digital mode. Maybe related to the 8565a control fitted to a 8569b? – Checked by substituting a control assy from a good 8565a – working just fine with the 8569b – accordingly, this is not the reason for the fault. There seems to be a defect on the main front panel board. Maybe a bad switch contact, or a broken trace. Will be quite a pain to test will all the cables, switches and screws.

(2) Manufacture the control knobs for the frequency-bandwidth-span-atten-ref level setting. These were missing and don’t have spares at hand/these wheels are getting brittle anyway. Will make a new set from aluminum alloy, during next stay, in Germany, the only place with access to adequate machine tools.

HP 8569B Spectrum Analyzer: working essentials

After two very busy weeks, finally, a chance to have a look at the 8569B analyzer (with the 8565A control). No display, no way to find out if at least the essential items are working – these include the input mixers, the YIG filter and oscillator, and other GHz frequency components.
First tests showed that the power supplys are all working and well adjusted; still no display.

Connected a scope to the rear panel output – sweep is working!

Some little repairs of the CRT section (nothing really interesting to write about) – look at this sight:

8569b working crt

Really great, a bright CRT, and well-focused. And, it is actually showing a signal – basically, a working unit. At least, a start. Now we just have to see how to get the controls fixed.

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!