Tag Archives: msa-0505

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).

HP 8561E Spectrum Analyzer: 100/300/600 MHz system fixed!

Progress with the HP 8561E – the 100 MHz to 600 MHz system, driving the 2nd converter, and other sub-system including the 300 MHz output.

Initially, it looked like a failure of the 600 MHz doubler, so I decided to open up the RF case of the A15 assembly, and to go to component level troubleshooting.

According to the block diagram, the 8561E first uses a trippler to convert the 100 MHz VCO output to 300 MHz, and then a doubler to convert the 300 MHz to 600 MHz. Note that there are various revisions of this board, not all use the same frequency multiplication scheme.

Very soon it became clear the the double is not getting enough 300 MHz power to work. So, to check it, I injected a 300 MHz signal after opening the signal chain after the tripler (there is an attenuator, just desoldered the middle resistor of the PI configuration attenuator, and checked all the components around this area)

Only about 2 dBm are needed to drive the tripler, there is an amp stage in front of the doubler. A quick test – the doubler is working just fine!

So, probably a fault in the trippler? At least, there is 300 MHz present. What is going on? Let’s go one step further back – removed the tripler transistor, marked “Hb” which is a NE85635 transistor.

Let’s drive the 300 MHz circuit from an external generator. This is running at quite high power – about 18 dBm!

With the injected signal, all is working fine! So, the 300 MHz transistor probably failed? By luck and coincidence, I found a spare 2sc3603 transistor, marked “Oq”, and soldered it in.
Surprisingly, the old transistor, once desoldered, tested just fine. And, to confirm this, the 8561E still not working!

The 300 MHz system can be conveniently monitor by checking the 300 MHz cal output with another (working) spectrum analyzer.

Well, we need to go back one more step – to the 100 MHz amp.
The tricky fact – the 100 MHz system is working, but after some careful measurements and calculations (I don’t have a precise active probe here), the power at the output of the 100 MHz amp is clearly low. This needs to deliver well over 10 dBm of power, otherwise, all the following systems won’t work properly.

Fortunately, the 100 MHz amp is a fairly common part, a MSA-0505 gain block.

These MSA-0505 are used in many HP circuits, just took one from an old board:

The A15 board, still with the 100 MHz gain block, and the tripler transistor replaced…

A last step – replaced the Oq transistor, with the old/original HP part (300 MHz circuit).

Finally, some tests – the 8561E (at least the low band up to 3 GHz is working again, and the CAL output is in spec (0.02 dB difference to another calibrated 300 MHz source!).