Category Archives: General

HP 8754A Network Analyzer: gold, sapphire and still low output

Some performance validation of the recently fixed 8754A revealed that the output is leveled at 0 dBm, but it doesn’t provide any more than 2 dBm, when you turn the knob to higher levels… it should provide at least +10 dBm leveled, and +13 dBm typical.

So, what is wrong? The signal source is mostly located on the A7 assembly, two VCOs, a mixer, and several amplifiers and levelers.

The osciallator is working, as we can see, but the amplifier circuit, a golden box, number 5086-7235, is not amplifying sufficiently. HP did not consider this field-repairable, so the manual only has some rough information about its contents.

To find out more, we need to crack it open – it is not welded, but glued with a generous amount of silver epoxy.

From right to left, the preamplifier, a filter (LC low pass to remove the VCOs and higher mixer products), and the power amplifier with detector and a -20 dB tap for the PLL-marker circuit.

I checked all the bias voltages and currents, these seem OK. The main amp substrate (sapphire?) has a crack, but is it not fully going through the material, and the gold layer is thick, and the crack is not cutting through the critical sections.

The filter inductances, gold traces on alumina, with some bonding wire. I assume, hand made… It is a pitty I don’t have a microfabrication facility at my disposal, and a wire bonding machine…

Various testing has been performed, to find out the power levels, using a 50 MHz precision source, and a fine tipped probe to check the levels on the substrate (using a microscope, and a steady hand to avoid damaging the bond wires).

From the data it is obvious that the preamp is not amplifying, but absorbing power. This is good and bad, because the final amp needs to provide clean amplification to avoid spurious, so I don’t want to mess with it, and there is also the detector diode, which is essential for the flatness of the unit, also something that is not easily fixed, if you introduce some parasitic resonances or the like.

The fix – scraped off the transistors and most of the gold from the preamp, and soldered a short wire across the substrate, I think it is about 50 ohms impedance. Then, I inserted a set of 2 integrated microwave amplifiers (a MSA-0505 and MSA-0386) to provide about 19 dB gain.

The maximum output +13 dBm at virtually all frequencies (a small dip around 1 GHz).

A test at various power levels, with a good spectrum analyzer (don’t have a calibrated power meter here, but this analyzer is pretty well calibrated). Amplitude is 1 dB per division. The 8754A is calibrated at 0 dBm and 10 dBm, at 50 MHz.

0 dBm leveled output, 1 to 1400 MHz… pretty good.

10 dBm output, also, great flatness.

Finally, at test at 5 dBm – it’s accurate and flat!

Now, we will let this run for several hours at maximum output, to see if the repair is permanent, then the amps will be sealed with epoxy (just plain epoxy, no silver epoxy).

Agilent 4352B VCO/PLL Signal Analyzer: a great find (hopefully!)

I found a great deal on a “totally faulty” 4352B, well, it is a bit a cat in a sack, as we say in Germany, you never know what you get. It shipped from Manila, even better! List price, I think it was close to USD 50k!

It’s a clean unit, except for about 100 stickers and seals!

All heavily shielded, many kinds of screws and metal plates to keep to good waves in, and the bad waves out.

The brain of the machine – quite sophisticated – Japanese engineering (this unit has been manufactured by HP Japan!).

After some study and test, it is clear, it is dead because there just isn’t any power. The power supply is a two-stage supply, first stage, a switchmode universal voltage to 24 Volt (190 Watt) supply, bz maker Artesyn, then a really top quality 2nd stage, made by Agilent. And guess what, the 24 Volt supply is faultly!

This supply uses power factor correction, and two IRFBE30 MOSFETs to drive the transformer – one of them is shorted shot. Otherwise, no damage to be found on the board so far, except the thermal fuse, which must have cut out immediately (even the primary fuse is OK – so there is hope that this will be an easy repair of the 24 V supply).

Ordered some spare thermal fuses and MOSFETs, quite common IRFBE30 type – low cost. Let’s see if this will let us get the power back, if not, worst case, we can always install a new 24 V 200 W supply – there is enough space in the case.

In any case, these VCO analyzers are a great deal – there is a 26.5 GHz (!! APC 3.5 !!) power splitter inside, value, still available today, EUR 1350, and a 13 GHz detector, value, 450 EUR. But let’s keep fingers crossed that this won’t become a parts unit anytime soon.

Moving to Japan: Busy times…

Recently, not so much activity in the workshop, for a simple reason – I was moving to Japan. Still keeping the German main workshop, it is only a temporary work assignment, but temporary can mean two or three years in this case. Anyway, Japan is a great place and this move is to the real Japan, Ube, Yamaguchi, not some expat community in a big international city.

With the help for kind colleagues and the big enterprise, all has been set up in the meantime and life is carrying on along the usual path…

The Japanese house, it is of traditional style, which means, it is hot in summer and cold in winter, but at least you get a better connection to nature, and it is a very healthy life to have fresh air and wind moving trough the big open windows rather than to sit in a hermetically close skyscraper.

Not to miss, the Japanese garden!

… Vegetable garden …

The temporary workshop – I can’t go to such far away places without at least a phase noise measurement test set.

My latest acquisition, an electro-mechanical device called “Toyota Aqua”, aka Prius C – very good fuel efficiency, thanks to its hybrid drive train. Sorry, no service manual for this one but it’s great to explore the beauty of Yamaguchi Prefecture.

Historic view of the workshop – Joh. Eisele Porzellan Grosshandel, Ludwigshafen

Found this great view of the current workshop (ground floor, left windows), dating back to before 1918! The building itself dates back to 1910, established by Joh. Eisele, who used to operate a major trading house for porcelain, glass, and stone ware, including a decoration workshop for such items.

While the back factory (with the railway tracks) is missing today (maybe bombed out it WW II?), the main building still exists today, in quite recognizable shape and form, even still with the name of Joh. Eisele in golden letters!