Category Archives: Test Equipment Repairs

For some, it is like solving crossword puzzles: fixing defective test equipment. Preferably, mid-70 to early 90s vintage.

HP 8662A Synthesized Signal Generator

HP 8662A 8663A Power Supply A7A3 Assy: base transformer defect

2016/05/21 Simon 4 Comments

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.

A snipet of the power supply circuit, there are two identical base drive transformers, center tapped on one side.

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.

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).

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, mag-inc.com, 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).

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….

HP 3562A Dynamic Signal Analyzer

HP 3562A Dynamic Signal Analyzer: a blackened transistor, and a dead S-RAM

2016/05/09 Simon Leave a comment

Here, Thanks to Michael from Zurich, Switzerland, and for the benefit of everyone with a 3562A showing similar signs of disrepair:

The 3562 A shows a fault code, is on the A2 (SYSTEM CPU/HPIB) error code 19, which means Monitor RAM Test Multiple Monitor RAM failures.
All voltages checked, and they are OK, the ripple is OK and the clocks are OK too. Everything was OK, no smoke, but I still had to solve two issues.

Issue (1)
Non-working Display Unit HP 1345A

First of all I guessed that this issue was coming from non-working A2 (CPU) but installing the test jumper to get the test screen did not work. Measured the voltages on top of the HP 1345A. These did not show any issues (no shortened tantalum capacitor) on +5V/+15V/-15V. Checked a few more voltages but not the +105V.
So I removed the HP 1345A unit to do a visual inspection and noticed the defect on the A3 board (Low Voltage Power Supply). Q1/Q2 did not look so good.

For sure without +105V we do not get any picture from the HP 1345A. I removed the faulty Q1/Q2 and solder some test cables for +5V/+15V/-15V/+105V to A3 to power the assembly from an external power supply. Lucky me, no other defect and the test picture came up.

I decide to order SG3524 (pulse-width-modulator), MJE180, and all capacitors of the A3 to replace them all. After the rework on A3 I carefully powered up the +15V supply which is used for the DC/DC converter to generate the +105V and measured the current.

See the re-worked A3 assy:

No issue seen anymore, the +105V is working. I added a 2.7k resistor to create a nominal load (approx. 37mA) on the +105V path to adjust the +105V. Just to know what was causing the burned Q1/Q2 I swapped the new SG3524 with the original one and I see that the current was increasing like hell when slowly increase the +15V voltage. So I guess the major problem was a defect SG3524 here.

Issue (2)
Faulty A2 board with hex error code 19

After installing the now working HP 1345A back into the HP 3562A I got a bit more detailed information about the A2 problem.

It says that there is a problem on the low byte SRAM on A2. To ensure that nothing else causing this problem (bus issues) I removed the A2 from the cabinet and it can be operated completely standalone. From the LED on A2 I still got the same error code as before (when A2 was installed) so at least there is no other
board causing this issue and I really can focus on the A2 board. First I checked all signals on the two 32kx8 SRAMS (U212/U211) with a scope but I did not see anything defect, everything looked so far good (no shorts, activity on all signals, etc).
So I attached my nice Philips logic analyzer.

Playing a bit with the logic analyzer, but did not get any more results so I believed what the monitor test logs said and replaced the low byte SRAM (U211) with a new one (ordered 70ns ones from Mouser).

After replace the SRAM the self test is passing on the A2 and it’s now time to install the A2 board back again in the
HP 3562A cabinet (and crossing fingers!!!!!).

With the changed SRAM, my HP3562A boots up without any other errors and issues and is ready to be used again!

Various

HP 436A Power Meter: smoke and stench – X-rated cap failure

2016/04/04 Simon Leave a comment

By coincidence, another HP 436A power meter – this one, emitting smoke and terrible stench! The culprit was easily found, a defective X-rated cap. One of the known-bad epoxy covered capacitors that tend to blow after about 30 or 40 years of service.

The residue, oily stuff, terrible smell. Use plenty isopropanol or methylated spirits to clean – otherwise, the stench will stay with the instrument for years, and I can’t say that it is a healthy smell.

The cap is of the well-known PME271M series. Still available, but hopefully, with improved construction.

A replacement is easily found – taken from an old switchmode power supply. Make sure to take a “X2” cap, not an ordinary cap. Only X2 caps are specified for mains voltage service, and self-exinguishing, anything else will present a major fire hazard, don’t compromise on the choice of capacitor!

Fix complete – new cap soldered in, and insulated with some electrical tape. In general, I tend to avoid electrical tape where possible, but in this case, it appears to be the only viable solution.

Various

HP 436A Power Meter: a strange analog ground issue

2016/03/19 Simon Leave a comment

This power meter had been received with strange defect, a permanent overrange error, irrespective of any settings or input to the sensor. Sure enough, in most cases, this would be because of a dead sensor – but not here.
The 436A is a really simple instrument, at first glance, but with its design dating back over 40 years (mid-1970s), it has a remarkable complex design to achieve the A/D conversion, and to use something close to a CPU, at the time, called a state controller.

What was wrong with this unit? Something with the analog ground driver.

Checking the A2 and A3 assemblies, it turned out that the analog ground was floating, at about -6 V. Strange! And, simple enough, grounding the analog ground on either A2 or A3 solved the issue! For a temporary fix, a wire was added, from the board edge connector, to chassis ground. Need to look at the analog ground driver…

Using chassis ground for general grounding – an indication of the dated design, and some of these board use 3 or 4 separate grounding path to keep noise down…

After this fix, working again (still need to check out what it wrong with the analog ground driver).

Update: found the issue – lower right and corner of above diagram, this is the analog ground driver (also supplying analog ground to the A3 assembly (via mother board) – transistor Q1 found dead, a 1854-0003 (which is equivalent to 1854-0637, JEDEC 2n2219A, or any other ordinary 0.8 W NPN transistor).

Soldered in a 2n2219A, and removed the temporary ground wire. Fix done.

After a full calibration and extended test, the instrument is rock stable, both for zero point, and 1 mW input. Also checked linearity, and it appears to be better than any means available here to check… most likely, better than 0.1 dB.

Output of the 50 MHz 1 mW cal source – cross checked with a calibrated HP power meter, 437B , and in agreement within 0.01 dB – good enough!

Some other issue with this unit – a stuck analog indicator. After disassembling the front panel, used a razor blade to open up the plastic case of the indicator, and some mechanical adjustment of the inner workings fixed the issue.

The 7 segment decoders, these use heat transfer compound, for some pretty unusual way for HP design – being pushed vs. the front panel for cooling. To make sure these stay cool, I added some fresh white stuff.

The decoders are quite remarkable anyway, for their time – these are latching decoders with constant current output, high level integration for the early 70s….

HP 8663A Synthesized Signal Generator

HP 8663A Signal Generator: another power supply repair

2016/03/08 Simon Leave a comment

Always good to do a proper test of equipment after repair – especially, after a power supply repair – see recent post 8663A pwr supply repair. In most cases, all will be fine, but this time, another failure followed the initial repair: the -10 V rail showed an error, only providing about -8 V to the instrument, not enough, to keep it going. An issue with the A7A1 assembly, linear regulator board, hp part 08662-60157 (the HP 8662A uses an identical assembly).

After some checks it was clear that the final pass transistor Q2 was OK, and that there was no current limit issue (by checking the voltage drop over the sense resistor, R36).

So, I guessed, something must be wrong with U2, the actual regulator. This is a 1826-0016, alias LM104H. Not quite a common part, at least, I didn’t have a spare on hand. Found some new old stock online, about USD 3 per piece, well, not too bad. It arrived a few days later, but, the exchange of U2 was to no avail – still no regulation.

Well, I should have done a proper check earlier – turns out, the transistor Q7 (2N2904A) didn’t provide enough current for the final stage, despite being fully driven by U2. This time, I had more luck and found a 2N2904A in my parts collection (datecode: 7050 – 46 years old – but still working, hfe=170).

Some final test with a 25 Ohms power resistor to test under load, before risking any damage to the venerable 8663A. And, long story short, all is good now.

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 8663A Synthesized Signal Generator

HP 8663A Signal Generator: switchmode power supply repair

2016/02/22 Simon 6 Comments

A heavy guest on my bench, a 8663A signal generator. These generators are exceptionally clean, perfect for close-in noise measurement and receiver checks. Still today, hard to find a cleaner source, especially not, if you are on a less than USD 30k budget.

Symptom – easy to describe. Unit turns on, but only briefly, then switches off; over voltage protection light activated at times; a lot of noise on the DC rails when shorting out the safety circuits.

After some probing, the culprit could be located: one of the input capacitors. While this is a common failure mode of other equipment, these caps don’t fail too often for such HP equipment, because only best-in-class components were used, and these are typically run cool, for long life. Still, one of these 32DR6593 SPRAGUE Compulytic caps failed (resistance about 100 kOhm, virtually no capacitance, rapid self-discharge when charged to 50 V for test, framed red in the schematic below).

These were replaced by 81D series Nippon Chemi-Con (Vishay) caps. To call this a successful repair, you might wish to check the ESR specifications – the SPRAGUE had about 0.25 Ohms, the Nippon 81D (680 µF, 250 V) has about 0.198 Ohm, good enough. Note that the 600 µF screw-type terminal caps might still be available, but they are pretty expensive, so I opted to for Nippon Chemi-Con, USD 2.50 per piece, surplus, rather than USD 50 for a pair of screw-type caps. I still think it is a good compromise, because this is not about restoring old equipment, but to make this unit working again, quickly, and at lowest cost.

Some repair is also needed on the A7A3 board – there are 22 µF caps that provide a low impedance DC input to the switching transistor, these are essential for operation (framed green in the schematic). They still work, but were hot, and stressed, possibly overstressed, by the dead main cap. Their can be replaced by any good cap, I use Shiangchen GSA T axial caps, 105 deg C rated. Note that the schematic calls for 15 µF, but 22 µF (measuring about 28 µF) were present in the circuit.

With the power supply disassembled, always a good idea to take out the boards for cleaning, and for re-tightening of the screws holding the various TO-3 regulators in place. Some of these were pretty loose (no wonder, with 30+ years of thermal stress on the boards).

After the repair, add thermal compound to the heatsink/cover – this power supply has a rather critical thermal design. Then, make sure to check the insulation resistance and electrical soundness/earth leakage, which is always a good idea after repair of switchmode power supplies.

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.

SimonsDialogs

Category Archives: Test Equipment Repairs

HP 8662A 8663A Power Supply A7A3 Assy: base transformer defect

HP 3562A Dynamic Signal Analyzer: a blackened transistor, and a dead S-RAM

HP 436A Power Meter: smoke and stench – X-rated cap failure

HP 436A Power Meter: a strange analog ground issue

HP 8663A Signal Generator: another power supply repair

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

HP 8663A Signal Generator: switchmode power supply repair

HP8568B Repair Story: 249, 275 unlock

HP 8569B Front Panel Assy Repair: rotary switches

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

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