Category Archives: Various

HP 8561B Spectrum Analyzer: a smoking power supply

Recently, busy days with all kinds of business trips and vacation in between, but finally time to go back to the workshop and enjoy some repairs in free time.
I got this analyzer for cheap, but it is not working unfortunately. When I plugged it in, smoke came out. Not a good sign, but let’s first find the source of the smoke. Easier said than done, because this “compact” unit has many fragile boards, and many screws, but I managed to get down to the innermost part, the power supply. Still wondering how HP designed this unit, it must have been a mere engineering nightmare, but these units are surprisingly reliable, 30 years old, or older.

Well, it didn’t take long to find the culprits, some old RIFA caps!

One blown, the others not looking much better. So I decided to replace them all, including the 2n2 Y-rated caps. The 100 nF X2 caps have a 20 mm raster, not a common size nowadays. We may as well replace them with original RIFA parts. Not cheap, at about 3 EUR per piece, but the unit is definitely worth it.

The power supply compartment is specially shielded, and I used the opportunity to clean out the dust.

A few days later, the new caps arrived (Y caps were still in my stock, WIMA brand).

The old board is looking marvelous with the new caps mounted.

A moment of truth – furtunately, the caps were the only issue, all working fine!

HP 8753C Network Analyzer: a dead FOX and a dead YTO

This will probably be a lengthy and complicated repair, because we are looking at a non-working 8753C. It is a great unit, in best possible shape, and came with all the original cables and a APC-7 test set. Even high quality APC-7 to N and -BNC adapters were included. Only downside – this unit is not showing anything on the screen.

Some quick checks later, found that the power supply is perfectly fine. Only, the A9 CPU assembly shows no activity. So I decided to take it out of the box, and power it with a lab power supply to see what’s going on. Absolutely nothing, no bus, no data. No clock??? Wait a minute. The clock is generated on the A9 assy itself, what can cause such silence? Probing around, absolutely no clock signal at all, not even at the osciallator (which is a standard DIL14 oscillator module, with the odd frequency of 19.6608 MHz).

Remove the oscillator, and it is completely dead.

Immediately, I ordered a couple of these oscillators at negligible cost, because I don’t have this cracy frequency in stock. To see what else is wrong with the unit, some temporary test with a 3314A signal generator (using the sync output). And, great news, the 8753C is starting up, with a very good and clean and focused display. The red arrows show the activity LEDs working, and the black cable supplying the clock.

Some basic tests later it is clear that the source has no output. It should sweep from about 30 kHz to 3 GHz, but no signal. The pretune DAC is working, also the driving signals are working fine (supply voltages and current). The source is all located in the A3 source assy. Made in USA, while the rest of the machine had been made in Japan.

There can be 4 issues with the A3 assy. (1) something with the control board, (2) something with the microcircuit, really bad, (3) the fixed oscillator, ok, (4) the YTO yig tuned oscillator, intermediately bad – can be replaced with a spare YTO but these don’t come cheap.

Test the fixed oscillator – always good to have all kinds of cables and adapters around!

For such tests, best use the pretune mode – disable the PLL. You should see good output with variable, slightly noisy (no PLL) frequency.

Next test, the YIG itself. Fortunately, we have the pinout from some old HP schematics.

No good news – no signal. I even opened it up, but no visible damage (except a kind of low cost construction YTO, and very thin gold bonding wires). I suspect the main transitor is bad, not enough gain anymore to make it oscillator – a well known issue of these HP economy-type YIGs.

Replacement parts are difficult to get for the 5086-7473, and no wire bonder and special tooling here to put in a new transistor. So my best attempt will be to use a good high end Avantek YTO to replace the original part. Probably, this will need some tuning of the coil drive circuit, but the 8753C is fairly robust in this regard. Let’s see if we can accomplish this – it will need to wait until August, because the various spare YTOs are all in Germany, in the main workshop. Stay posted.

HP 4192A LF Impedance Analyzer: a leaking backup

Finally, to complete my collection of HP Impedance Analyers, I found a 4192A really cheap. As always with cheap things, there is a catch – this unit has some scratches, and doesn’t power up.

Well, usually no big deal, so I placed a bid and some time later the big box arrived. Similar to other HPY (Japanese-made) impedance analyzers, this unit has a lot of empty space inside, and is big and bulky, but at least, this simplifies repair.

Opening up the covers, the main issue is quickly found – the NiCd memory backup batteries have leaked some alkaline substance to the board and case, reading to some damaged components.
Fortunately, the corrosion is not looking too bad, at least the PCB traces are present, and the solder joints seem to conduct electricity.

The front view, you can see the scratches and dirt, but an overall complete unit. No boards missing. Despite their age, these units are normally still traded at 1-2 kUSD, and list price used to be close to 15 kUSD in the late 80s. New units of similar accuracy and range will easily cost you the same, in 2019 dollars.

The board affected, the A7 power supply assy. A switchmode supply. According to the manual, HP used a switchmode supply to reduce the weight and make the unit more portable (???? – what is portable about this box).

The bay holding the power supply, you can clearly see some traces of corrosion, but it is only superficial. The NiCd electrolyte has a tendency to leak out and then slowly creep with moisture all over the place.

These are the General Electric troublemakers!

Best cure for such leakage – wash with plenty of hot water.

Then scrub with a toothbrush, and scrub with vinegar (don’t use any concentrated acid). Vinegar will neutralize any traces of alkali electrolyte.

This is some of the worst placed, but fortunately, the traces were not affected much, and even the leads have a lot of good metal left.

Many good and well known parts in this unit – the CPU

… many Eproms holding very few kbytes each…

Pricy DACs.

And, the first fix – replaced the NiCd batteries with a commercial NiMH pack. There is a 1 kOhm resistor on the board, charging from less than 5 Volts – so this will be fine even for NiMH (less than 0.03 C trickly charge won’t cause any significant deterioration of NiMH cells).

Also – replaced 3 cracked RIFA 15 nF Y-rated caps.

Further repairs will have to wait until I come back from Germany in a few weeks, because some parts on the power supply board show damages, a ceramic capacitor (10 n, 100 V) that didn’t like the electrolyte and a diode (similar to 1N4148).
The electrolytic caps still look OK, but we will see in a while.

HP 6634A System Power Supply: A few almost-bad RIFA caps, and a 100 Volt, 1 Amp, source-sink supply, and a generous load of transistors

A quick look at a really nice piece of kit, a 100 V, precision regulated power supply, can be floated to +-240 V, and can provide 100 Watts of power, or sink power, about the same range.

The front panel and handling is like any other HP system power supply from this era, and there are models 6632A (20 Volts) and 6633A (50 Volts) that share virtually the same control circuit. All is GPIB controlled, of course, and this unit has front and read outputs. I am going to use it for a capacitor tester (to study the voltage bias effect and hysteresis of ceramic capacitor materials), so I need a fairly reliable unit because it will run unattended for a while.

The top view, there is a massive heatsink, for 100 Watts of dissipation…

The transformer, it is the highest standard and insulation I have ever seen.

There are 8 power transistor, in a really massive output stage (4 complementary pairs, 2 each on each heatsink-the heatsink is sub-divided in two sections), each of them capable of handling 250 Watts of dissipation.

The output stage, it is a really generous design, considering that this is a 1 Amp supply (most designers would handle it with two transistors).

The only thing I don’t like about the unit, the RIFA X and Y rated caps. These are all cracked (still not shorted, but I don’t want to take a chance). So these will need to be replaced.

Otherwise, all is good with this unit, almost no dust inside – I believe this instrument had very low hours, or has been used in a very clean environment (not even a trace of dust on the fan).

HP 3326A Two-Channel Synthesizer: replaced, and replaced again!

Recently, two assemblies of a non-working HP 3326A were fixed by replacing their 15 uF tantalum caps – a good number of them had failed, presumably, because of a bad production lot of these capacitors (see earlier post)

Unfortunately, during the test run, some sporadic failures of the power supply, with overcurrent indication flashing. Then, permanent failure of the -15 V rail – as it turns out, by a short in the assemblies we had just fixed! An again, a discolored tantalum capacitor. Replaced it, and a few hours later, the same issue, with another capacitor of the same kind.
My mistake, I had use a bag of cheap China-sourced 15 uF, 25 Volts dipped tantalums, but these seem to be no good (unlike other Chinese electronics good that have attained good quality in recent years, provided you don’t by the cheapest kind). Maybe it was my mistake to buy the cheapest tantalum capacitors, but not much choice if you need 34 pieces to fix some old equipment – I don’t want to pay EUR 1.45 each from top brand parts from Mouser or similar sources.

With some luck, I found reasonable prices KEMET T350 series Ultradip II capacitors, these are known to be reliable.

You can see the size differences – the KEMET part is much bigger than the Chinese 25 Volts part – it is more similar in size to a 15 Volts KEMET part. Probably, the design was put a bit to the limit.

With the capacitors replaced and another 48 hours of run in test – no issues at all and the 3326A can be considered fixed and working for now.

HP 8754A 4 MHz to 1300 MHz Network Analyzer: final repairs, and a function test

Finally, the spare parts arrived, and the repairs of the HP 8754A could be finalized. The LM339 comparators, fitted to the boards…

The cap of the mains filter had many small cracks – replaced. For some reasons, the original filter had a Y-rate cap across the mains supply – Y rating is usually for connection from mains to earth. So I replaced it with a X2 rated cap for service parallel to mains.

Some tests – the 8754a is a very nice unit, because of its instantaneous response to the dial settings, rather than the delay of any digital network analyzer. Even the most modern of all units still don’t such a direct feel compared to the fully-analog 8754a.

HP/Agilent 6060A System DC Electronic Load: a quick repair

This is a 300 Watts, 60 Volts, 60 Amp electronic load, a quite handy device to have, especially, a HP/Agilent brand item. There are many cheap electronic loads, but I would rather recommend to get a good instrument, if you want to put some power supplies to real tests. Otherwise, you load may fail earlier than the supply.

The instrument we are dealing with here, a low cost auction fid – it had a bad front connector. These instrument use HP 60 Amp binding posts, these are quite rare and expensive (about EUR 40 per piece from Keysight), and the plastic gets brittle over time, and with overtightening it can break. The instrument had front and rear connections, I only need one set – so it will be an easy repair by just moving the good binding posts to the front.

Also, we find that all the X and Y rated capacitors have hair cracks, and are of RIFA brand, so these may fail soon – let’s replace them all.

The power is dissipated in several MOSFETs, all mounted to a large heatsink. Essentially, a small 300 Watts room heater, which is great to have these days in cold Japan.

The front connectors, after repair (just moved the rear connectors to the front, rear connectors, I don’t need them).

New caps soldered in – quite a difficult task because some vias are part of large copper fills, without thermal relieve, and I don’t want to preheat the whole board.

Finally managed to solder-in the X and Y capacitors.

A test at 40 Volts, 6 Amps, running for several hours with no issues at all!

Agilent 4352B VCO/PLL Signal Analyzer: working!

After a short xmas vacation, several spare parts arrived, including, 10 amp solder-in fuses, and thermal glue (704 silicon glue).

The glue is needed to mount the defective/blown thermal fuse to the power resistor. This resistor usually stays cool but will heat up in case of a power supply failure.

The fuse protects the primary of the switchmode transformer, it is a 10 Amp fuse, and it took a while to find it – it is located in a hidden place underneath the transformer.

Now, with the fuse installed, the thermal fuse glued to the resistor, and the two drive mostfets replaced, the Artsyn 24 Volts supply is starting up just fine. All self-tests passed!

Next step, let’s update the firmware, and do some tests.

The firmware version 2.11 is the latest one available, but it needs to be loaded from a 3.5 inch floppy – I have a USB floppy drive here, and one single disc which I purchased from Sri Lanka. Took a few attempts to convince the 4352B to read the disc and load the firmware. But finally, success!

Many tests could be done, here just a simple test with a 15 MHz signal from a 3585A vs. a 8642B generator. Seems to work well, and easy to use.

Now we can close the case, and use the device for VCO characterization, phase noise measurement, etc.

HP 6038A System Power Supply: all fixed!

After some weeks, the spare parts arrived – RIFA X2/Y2 rated capacitors (now made by Kemet), a full set (see earlier post, 6038a repair).

The new X2 capacitor, let’s hope RIFA has improved the resin and durability. Albeit, the old capacitors lasted for a long time…

And a fan, from China. The fan, upon close inspection, it has a broken frame, but fair enough, I will use this one while a replacement is on the way.

A lot of dust removed from the case and boards, all completely disassembled. The X and Y capacitors all replaced – the old capacitors are still working, but cracked and it is good practice to replace them, unless, you want to risk a lot of smoke and stench (usually, at least no fire risk).

Always good to use high quality tools – I only have low quality tools here, and bits that crack!

All cleaned and put together…

…finally, some testing. It is working, the fan is providing a substantial amount of cooling, it is definitely big enough for the unit.

HP 3326 Two-Channel Synthesizer: a bad lot of tantalum capacitors

This HP 3326A was found for a ridiculously low price, non-working, so I decided to pick it up, in case I need some spares for my good 3326A, or as a source for some HP parts. But when it arrived, it was in such good shape that a repair appeared worthwhile.

The symptom, it just doesn’t start up, the +15 Volt and -15 V rails shorted. Brief check showed that the power supply is working. There must be a short somewhere in one of the modules. How to find such short?

The 3326A has a cast aluminum cage construction, which houses all the modules in separate cavities, all heavy cast metal! To find the defective module, we first have to undo 100s of screws… and usually the last module will be the one at fault.

Half an hour later – found that both (!) phase detector boards – these are identical for channel A and B – have shorts.

Some more probing later – the reason a couple of shorted tantalum caps, 15 microfarads.

These are in general high quality capacitors, but it must have been a bad batch. So, let’s desolder all the 15 uF caps, and solder in new ones.

Even with the dead caps cut-out, the 3326A is working again! No issues with any of the self tests, including the service self test (push button self test-%-6 to activate).

Some maintenance is also needed on the power supply. Checked all the transistors which are known to develop issues with the sockets. And added some thermal compound to the 5 Volt rail transistor which was running a bit hot.

Also, the connector to the transformer has the typical bad soldering and cracked solder joints, all now re-soldered with a generous amount of good old lead containing solder.

This unit even has option 001, the precision ovenized reference.

There is no manufacturer datasheet available for this Japanese OCXO, but the HP manual has all the data. It is not an ultra-stable timing standard, but by far good enough for a two tone synthesizer.