Category Archives: Various

HP 8753C Network Analyzer: a new old YTO, and a new old firmware

After another trip to Germany, another HP 8753C to fix. This unit had option 020, 006, a 6 GHz unit, but there is no 6 GHz test set.

First, we need to get a suitable YTO, found a good ASF-8751M, from Israel. Cleaned it up and gave it a proper test.

It is a 4-8 GHz unit, but I easily got good power down to necessary 3.6 GHz. It is a well-behaved unit, with reasonable power consumption running of +15 and -5 Volts. The heater may be better run with 24 Volts, but there is only 15 Volts in the 8753C, and it is good enough it seems.

Some modification of the PLL board, as described before, to approximately double the tuning current, installed a 20 Ohms sense resistor, and installed a BD249C transistor on a good heatsink.

A quick drawing of the heatsink, should you need it. Use 1 mm aluminum sheet. Don’t cut yourself, when cutting the metal!

The YTO, installed in the veritable source assembly. Pretty confident that this will last for a while.

This time, all worked well and the pretune correction functioned immediately, no further adjustments needed. Phase lock seems very stable at all frequencies, scan rates, and band transitions.

Out of curiosity, did a phase noise test of the 8753C in CW mode (fixed frequency mode), getting well below 100 dBc. Pretty good. Maybe better than the original YTO.

For the current unit, I also wanted to update the firmware, and install the 010 option (time domain analysis). The option installation (and EEPROM backup), done like described in an earlier post, but desoldering the EEPROM, and changing three bytes…

The unit is still running pretty old firmware.

Should be easy enough to program some 27010 EPROMs, but the devil is in the detail. After a number of incorrectly programmed EPROM, finally figured out the once of the CD4015 CMOS of the EPROMMER had failed! Fortunately, I had some in stock to fix it.

After these efforts, the 8753C is starting up with the latest (albeit, dated) firmware, and all options.

A few tests with filters and such, a very useful and well working unit. The CRT also very good, no need to install a LCD.

HP 3325B Synthesizer/Function Generator: a quick fix, and a hot transistor

Recently, I got a defective HP 3325B, it is a very useful generator even for today’s standard. It features some highly linear ramps, has great frequency resolution and a powerful output (10 Volts p-p into 50 Ohm). This unit reportedly had major issues, no output, and failures with startup. So even before switching it on, I removed the panels to check. Nothing obvious at first glance.

After a quick power on, some smell from the output section, and clearly, there are some burned resistors, and one of the power stage transistor is terribly hot, so hot that the solder melts… don’t burn you fingers!

Removed the board altogether (take care not to damage the connection flat cables!), and even the solder had some spray by heat effect, so I cleaned the area well.

To get access to the resistors, and to also do a proper test, all the transistors in the area were removed, and the transistors desoldered. All cleaned up pretty well, the board seems to be of good quality.

The 3-440 transistor aka 1853-0440, cut open. It has a tiny chip, difficult to see the damage with my means, but it is shorted to base.

The resistors, the only issue is a slightly discolored 47 Ohms carbon composition resistor, part EB4701, a 0.5, 10% tolerance resistor. Quite expensive to get, and the part, despite some signs of heat, tested good and within tolerance. So I decided not to replace this transistor, because it has an effect on the high frequency performance of the circuit.

The power amp, it is a marvelous push-pull design. It relies on complimentary NPN-PNP transistors that have high frequency power.

Nowadays, the PNP RF transistors of this sort are rare, probably they even were rare and expensive during their time.

The damaged resistors, fortunately, after a good amount of searching, I found the bags here in by temporary Japanese workshop.

The transistors, these 3-440 are equivalent to the 2N5160, and I happened to have 3 of these back in Germany, new old stock. Purchased them some years back, because they are generally not easy to get.

After these replacements, I run the adjustments and performance checks as per service manual, with no trouble at all. Also the self test passes flawlessly. We can call the generator fixed.

Out of curiosity, I checked with ebay, and there are very reasonable offers of what appear to be Chinese copies of 2N5160 transistors. They have the Motorola label, but to my knowledge, the date code is much past the obsolescence of these parts at Motorola. So I am waiting to receive these parts, and will give them a good test and study, to see if these are good replacements, or just fake.

HP 6205C Dual DC Power Supply: a generous binding posts fix

The repair itself, it is not particularly noteworthy, because this supply has served me well in the last years, in fact, it had been switched “ON” all the time to power an experimental setup.
The initial repair of this supply has been documented before, and on the pictures there it is quite visible that this supply had damaged binding posts. Seems that the prior user dropped it on the front panel.

Now the noteworthy facts, a kind reader of this blog, an American fellow, had a few of these posts at hand, from a HP plotter. He kindly sent them to me, free of charge!

So, as a result of the kindness of the reader, and the standardization of the parts HP used in their equipment, the power supply is now in better shape than ever before.

Did a few tests, like, checking ripple current at full load, and electrical safety – ground resistance, but all looking good.

HP 8753C Network Analyzer: Serial numbers, options, EEPROMs

The HP 8753C comes with some software options 010, time domain (essentially, a built-in FFT function), and the even more useful harmonic analysis, option 002. These work without any further calibration, and used to be available as a code to enter to the instrument , with service function 56, to update the option status.

Thanks to a kind gentleman, such codes are available now, and normally you can add them to the 8753C without any expert knowledge and risk.

Unfortunately, for this instrument, the method to add options by code entry didn’t work. How come? As much as we know, the option code depends on the serial number, let’s check if the serial of the CPU board is the same as that of the instrument (ending in 00860). A first hurdle, how to read the serial – it is not showing upon startup for the 8753C, but you can get it by first executing service function 55, which will fail, and then go to Display-Title.

To my big surprise, the serial shown is incorrect, only 4 digits, missing the “8”.

Accordingly, we need to dig deeper, and the serial number and other information is stored on the U23 EEPROM, a 2kByte chip, Xicor.

It is a very long lasting device, no reason to believe that it will fail anytime soon, but there are always risks. First, I read all the coefficients via GPIB, and then carefully desoldered the chip.

Actually, desoldering went very well, even just with plain tools, a soldering iron and a manual solder sucker.

The programmer, put together from a few jumper cables, and an ATMEGA128A board. When reading, I hardwired the WE- write enable input to VCC, to make sure that no data are lost. There are also 6k8 pull ups directly on the ZIF socket, to make sure the input stays “High” even if the jumper wire is not connected well.

In the EEPROM, clearly there is the incorrect serial, it is not actually missing a digit, but has an incorrect character. Maybe it got modified when the CPU clock failed (remember that this board had a bad osciallator?

Now, we need to put in a single character, an “8”.

I don’t normally need to program 2816 EEPROMs, so rather than taking chances with some incompatible programmers, I made a small program, to just set a single byte, at a given address. In this case, writing an “8”.

With the serial number corrected, put the EEPROM back onto the CPU board – using a precision socket.

Using the secret code that only works with the matching serial – and with the write protection of the CPU board disabled – the option install worked perfectly fine.

Now, the 8753C shows the options upon startup, and the time domain and harmonic analysis functions show up in the menu as softkeys.

Afterwards, I checked the EEPROM contents again, there are only 3 bytes changed, in-line with what can be found in online forums. Also tried to activate the 006 6 Ghz option, not much use for me, but the option code is same as seen for the 8753D, etc. There are 3 bytes, right in front of the serial, with the upper half-byte bits all set (0xFx), and the lower half-byte encoding the options in a bit-wise fashion. With no options, the three option bytes are all zeros 0x00.

If you need any of these EEPROMs or related advise with the 8753x units, just drop me a line.

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 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!