Category Archives: HP 4191A Impedance Analyzer

HP 4191A Impedance Analyzer: power supply fixed

In a recent post,HP 4191A, I have introduced a 4191A with defective power supply. In the meantime, a spare power supply regulator assembly arrived, and the repairs of the originally fitted, defective supply have been completed.

First, some research into the HP p/n 1826-0043 Opamp, which is considered to be a LM307 comparable part. But what is acutally inside the can? Let’s crack it open.

The die has a small marking, reading “LM107C”. The LM107 is very similar to the LM307, except for a wider temperature range, and somewhat extended power supply limits. In the current application, we can safely drop in LM307N opamps. It is also clearly visible that there are no bonds for external frequency compensation (like for the LM301).

The replacement power transistors have been discussed in the earlier post, and now all has been cleaned and new heat conducting grease added.

All the replaced parts marked, after all, I should have also replaced the fuses and fuse holds which show signs of corrosion and bad contact.

A quick test, on the 4191A, left hand side is the working spare, the right hand side, the repaired original supply. I didn’t connect it to the 4191A board, but just to the transformer to confirm the basic working condition without putting the 4191A to any dangers – the repaired supply with but just a spare, and probably it will never be needed… at least not for this unit.

There were no other defects with the 4191A, and now some tests with the 4191A working instrument. It is essentially a very precise one-port vector network analyzer, with a thermostated reflection bridge and some other features that make it suitable for component measurements. There are some (expensive) HP test fixtures, but essentially, you don’t need these because most of the components will need to be tested on some custom boards, or directly soldered to a test connector, to avoid parasitic effects at the frequencies of interest. I just used a set of SMA flange-type connectors.

First some standards were fabricated – a short, by applying a generous amount of solder the connector to completely short it, an open, by cutting off the center pin and machining it flat (maybe need to add a cap or other structure later, to avoid any parasitic capacities, for now, during calibration, you just need to keep the hand away (and any other ground planes). The load 50 Ohms – from two 100 Ohms SMS resistors in parallel.

Some test objects, a 680 pF SMD 0805 capacitor (NP0), a 100 Ohm SMD 0805 resistor, and a wired 100 Ohm metal film transistor.

The test itself, run with some excellent Excel software (by a certain Harry Percival) and Zplots (by Dan Maguire, AC6LA) via GPIB bus.

The 100 Ohm SMD resistor, it has pretty good performance out to 1 GHz.

The wired transistor, inductance is adding to the parasitic behavior.

… same in linear frequency scale.

The 680 pF capacitor.

Also did some drift checks, over 10 hours (after about 2 hours of warm up), I could not find any detectable drift of the Z values, so the instrument seems very stable at least with reference to the stability of the 50 ohms load measurement.

HP 4191A Impedance Analyzer: 1-1000 MHz

Recently, I found a defective HP 4191A for a low price, and thought I should try to fix it – these devices where manufactured by HP’s Japan division, and for years, I have been looking for a working unit, but to no avail (prices in the range of 2-4 kEUR, and new instruments of this class demanding well over 10 kEUR). So, let’s see if we can fix this box.

This is how it should look like, from the cover of the HP Journal.

This is the unit currently, already opened it up to have a look inside.

Top view – a good amount of empty space, which is there to fit the high frequency resolution option.

The CPU boards, it seems to be the newer version, compared to the boards described in the service manual.

Rear view – this also carries the power supply (voltage regular A23 assy). Already removed it, it was only held on by two screws. As it turns out, it is not working, the 12 V rails are missing. And some other voltages are not good (5 V is fine, so the CPU is working and seems undamaged).

The unit is reasonably clean, but the power supply, it is dirty, and rusted. Not sure why.

Especially the opamps have signs of corrosion – two had even non-conductive, fully rusted legs. These are 1826-0043, which is a pretty generic HP part and can be replace by LM307H (or the DIP version, much lower cost, LM307N).

I have never seen such rusted opamps, maybe the instrument (or at least this assembly – the rest of circuits has no sign of rust) was kept close to the ocean, in salt spray?

After more careful analysis, some trouble with the 12 V (positive and negative) pass transistors.

Seems someone tried to repair it before, but for some reason, judging by the date codes, didn’t replace them.

The NPN, no problem, it can be replaced by a 2N3055. But the PNP – it is HP 1853-0252, alias SJ1798. Not sure how to get one of these.

With some further analysis of the maximum current, and other parts of the circuit, I believe we can safely replace it by a very common and low cost MJ2955. This is the complementary TO-3 transistor to the famous 2N3055, and by all I can tell, it should work just find in this power supply, as a simple pass transistor.

In the meantime, the board has been soaked in some isopropanol, and brushed with a soft brush, and all the rusted opamps removed. Ready for the new parts to be soldered in, once they arrive.

Another thing to look at – the NiCd batteries aren’t good any more. These will be removed or replaced, once the other parts are working, mainly to avoid any future risk of leakage of these cells, messing up the instrument.