Category Archives: Spectrum Analyzers

HP 8569A Spectrum Analyzer: Any organ (ehh, CRT) donors out there

2017/06/03 Simon 3 Comments

This 8569A came from the US, purchased by a local HAM operator, and in need of repair. Some switches don’t move nicely, so it seems liked a regular repair job of a 8569A.

First issue, 5 minutes after turn-on, where not the switches, but a blown mains capacitor (aging X2 grade capacitor). Well, this is easy to fix.

With the instrument powered on, it quickly became evident that the there is some distortion of the display, in particular, in the upper section of the CRT. This is not a common fault, and hard to explain other than by a broken deflection system of the CRT.

To be absolutely sure that no other assemblies are causing this defect, the X-Y deflection signals were coupled to a working 8569A, and indeed, no distortion visible.

The CRT display of a working 8569A – with the X-Y signals of routed from the unit that shows the distortions:

Sure, the display is not quite synced (also because of the extension leads with alligator clips). How to proceed? Well, this will need a 8569A CRT for repair, from a donor unit. Let’s hope, at least for the HAM friend that one will come around soon.

HP 8566B Spectrum Analyzer

HP 8566B Spectrum Analyzer: A19 board, YTO unlock, bad precision trimmer(s)

2016/10/11 Simon Leave a comment

Not the first HP 8566B on the bench, and not the first at all showing the famous “YTO unlock” message. Most of these YTO unlock message issues can be traced to defective capacitors, but not this time.

With the 8566B, take my advise, don’t touch any of the assemblies if you aren’t really sure which one is at fault, it is a fairly complex machine. To troubleshoot, a microwave counter is handy, to check the LO frequency.

Next, the PLL was disengaged by disconnecting the cable from the sampler/LO pll. Still, no good LO frequency output. This leaves two main assemblies to be checked, the LO pretune DAC, and the YTO driver assembly, A19 and A20, respectively.

Quickly traced the issue to the A19 DAC assembly, and luckily enough, had a spare one around, albeit, an older version. After swapping the boards, it was confirmed that the A19 assembly is really the faulty part.

Next – desoldered all the capacitors at one end, but, to my surprise, all useless work, all caps in best working order, even after 25+ years!
Checked various components, and finally, found some issue with the precision trimmers – seems a cold or aged solder joint. To be sure the the fix is as permanent as possible, all the trimmers were removed, the solder connections cleaned, and all installed back in. Easy fix, all working again.

With this unit, there was no intention to do a full calibration, but as an extra service, I checked the power at the reference signal outlet – see below. Quite amazing how accurate, and pretty sure that this unit hadn’t been at a cal lab for at least 15 years….. this is really a superb level of lasting precision and quality, and ingenious engineering.

HP 8569B Spectrum Analyzer

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

2016/02/22 Simon 1 Comment

A quick repair story from a kind contributor (Martin, you can find him at www.mjbrf.com).

Symptoms:
(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.

Solution:

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.

Note:
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.

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 8568B Spectrum Analyzer

HP8568B Repair Story: 249, 275 unlock

2016/02/10 Simon Leave a comment

Quick story from a contributor, no pictures:

8568B Spectrum Analyzer. When switched on – following error codes were displays:
249 unlock
275 unlock

Findings (in German; in short, there was an issue with thw 275 MHz oscillator, C10 had been replaced before, along with Q5; replacing C10 with a high-quality adjustable cap and increasing capacity a bit, this increasing feedback, the issue could be resolved; root cause: oscillator did not start oscillation reliably when powdered up, because of a lossy C10 cap).
##########################################
Der Übeltäter war auf der Platine A18
275 MHz Phase lock Oscillator..
Nach diversen Abgleichen fehlte plötzlich
das Ausgangssignal. Als ich das Modul
herausnahm, sah ich,
dass jemand vorher schon herumgelötet hat.
Das C10 war durch einen billigen Keramik-C
ersetzt, ausserdem war ein neuer Transistor
Q5 eingesetzt.
Am Werktisch liess sich zeigen, dass der
Oszillator nicht immer anschwang. Mit
Betriebsspannung ab und wieder anschalten
scwang der Oszillator dann wieder an.
Ich habe das C10 durch einen hochwertigen
Trimmer ersetzt, die Kapazität etwas höher
eingestellt (mehr Rückkopplung)
und der Fehler war beseitigt..

Es ist auch bemerkenswert, wie relativ stabil
der Osz. freischwingend läuft, trotz der
doch einfachen Schaltung.
##########################################
Thank You Bodo for sharing this.

HP 8569B Spectrum Analyzer

HP 8569B Front Panel Assy Repair: rotary switches

2016/01/09 Simon 5 Comments

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.

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

HP 8569B Spectrum Analyzer

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

2015/11/27 Simon 3 Comments

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.

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

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

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.

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 Spectrum Analyzer

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

2015/05/11 Simon 2 Comments

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.

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.

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!

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

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

2015/04/07 Simon Leave a comment

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.

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!!

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:

…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:

…there are the missing 2 dB. Problem solved!

HP 8566B Spectrum Analyzer

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

2015/04/05 Simon 1 Comment

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.

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.

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….

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.

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.

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.

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

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

2015/04/04 Simon Leave a comment

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

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

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.

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

The YTO assy, missing the YTO, and other bits.

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….

Some more pictures – the YTF driver.

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

A high quality input relais and a band pass 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.

After a LOT of fiddly work:

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.

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

More to come!

SimonsDialogs

Category Archives: Spectrum Analyzers

HP 8569A Spectrum Analyzer: Any organ (ehh, CRT) donors out there

HP 8566B Spectrum Analyzer: A19 board, YTO unlock, bad precision trimmer(s)

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

HP8568B Repair Story: 249, 275 unlock

HP 8569B Front Panel Assy Repair: rotary switches

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

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

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

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

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

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