Category Archives: HP 8753C Network Analyzer

HP 8753C Network Analyzer

HP 8753C Network Analyzer: an Avantek fix

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Still continuing on the repairs of the HP 8753C. No luck with the Watkins-Johnson YTO, maybe one could get it to work after some severe modification of the phase lock assembly, but this is not my intention. So I rather try another YTO, a Avantek part, purchased for 25 USD.

Some changes are necessary – at least the dampening of the YIG needs some work. Set the 8753 to the source tune mode, and determined the capacitor value to get stable current regulation (in the source tune service mode, the PLL is open).
From about 100-120 nF onwards, in parallel to a 100 nF-2.2 kOhm network, perfectly stable. Added 200 nF, ceramic capacitors, directly at the YTO coil, and modified the R-C network on the A11 assembly a bit (replaced the 1 k resistor with a 2.2 k).

Here a quick schematic, the voltage regulators, and a few Zener diodes to protect the YTO driver. The main modifications are:

*1 cut a trace on the A11 assemly, re-route to the current sense resistor, to allow for some adjustment (sure you can use fixed resistor, once the necessary current has been found.

*2 replace the current sense resistor – most YTOs have 50 mA/GHz current requirement, about double of the original part.

*replace the YTO driver transistor, with a part that can be more suitably mounted on a heatsink, or provide some heatsink to the existing TO-3, if you can manage.

*adjust the R-C/C network across the YTO main coil, good values are 100-200 nF in series with 1-5 kOhm, in parallel with 10-250 nF. Check for clean and fast current regulation, with the PLL open (source tune mode in the service menu).

After the preliminary checks (adjust the current of the YTO for about 4 MHz output, with the source tune mode set to 300 kHz), immediate success – there are absolutely no PLL issues, phase lock is established smoothly.

There is a 3 dB pad, the YTO has plenty of power, and it will help to reduce any frequency pull.

Below, to adjust the pretune DAC, you need to remove the EEPROM write protection, this picture from the web is quite useful.

At first try – the pre-tune test passed!!

Even better – all internal test passed!

Some check at 1 GHz, the spectrum looks clean, there are no parasitic oscillations or sidebands.

The analog bus of the 8753 series, input 16, is quite useful to check the tuning voltage and YTO status. It needs to be a line (the voltage is directly proportional to the YTO coil current).

To test the unit, measured a 1.9 GHz bandpass. All good.

Also the GPIB interface and plot function, working well.

After all the electronics, a little bit of work on the lathe – the cable that connects the test set to the VNA had broken screws, two new long screws fabricated – I used a bit thinner round stock, to make the task easier. Note that the cable is still available from Keysight, EUR 238 a piece.

Remaining task – to add the options 010 and 002, time domain analysis, and harmonic analysis, both are software options and can be added with a secret key code….

HP 8753C Network Analyzer

HP 8753C Network Analyzer: spare YTOs

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Not giving up on the HP 8753C Network Analyzer repairs. Especially, because these units are really top class in terms of specifications and serviceability, easy to use, and well handled by GPIB bus software. Not sure why you would by and later model, if you have a working 8753C or similar model.

Some study of the YIG oscillators used in the 8753 series analyzers, while these are nothing special neither in output or tuning range (3.8-6.8 GHz, and a quick test on the source assembly shows that everything above 8 dBm seems to drive the source mixer to saturation), they have a ~23 mA/GHz tuning current, much less than the 50 mA/GHz (20 MHz/mA) of many industry standard and common YTOs. No idea why that is, a bit less power consumption, a bit less copper?

Looking around, found this marvelous Watkins-Johnson YTO, for just about 15 EURs. It was a bit dirty with wires badly soldered to it, but easily cleaned up.

Power is good, measured through a 6 dB attenuator, and pretty stable all over the range (won’t even need it up to 8 GHz).

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There were no data on the power supply, so I needed to test it out. Heater voltage, seems to work well with 15 Volts, supply voltages -5 and +12 V give stable operation with sufficient margin (there seem to be internal voltage regulators).

The tuning current 50 mA/GHz, about double of the 23 mAh/Ghz of the HP YTO. So, there will be more heat dissipated, let’s do some calculations around the YTO driver. Located on the A11 board.

Clearly, the transistor will have to dissipate more heat. Do avoid any big changes, let’s adjust the current sense resistor to about half the value – resulting in about the same voltage drop per GHz.

The driver transistor, nothing special, a NPN TO-3 part, used without a heatsink.

There is no good space to fit a heatsink for TO-3 transistor, so I replaced the transistor. Used a BD245C, with amplification hfe of 40, well good enough. The SOT-93 case, it can easily be mounted on a piece of sheet metal (aluminum) to provide enough cooling.

Everything well insulated and mounted. Using the TO-3 screws to hold the transistor/heatsink assembly.

The current sense resistor, the key part for any YTO driver, needs to be low drift, low thermal coefficient. Otherwise, there will be all kinds of drift. HP use in some of their equipment sense resistors specified to 2 ppm/K, the best I could get is about 15 ppm/K with Dale RH-10. One day I will check their actual thermal coefficient. Now I just burned it in a bit, and selected one that looked perfectly stable with any load changes up to 1 Ampere.

The A11 board can be modified fairly easily, even reversibly – replace the sense transistor, from 40 Ohms to 20 Ohms, cut a trace (the current sense voltage to the opamps, close to the board connector – the via is handy to attach a wire). And a 18 Ohm/390 Ohm/200 Ohm -10 turn pot arrangement to set the proper currents.

Another small modification, the 1 k/100 network across the main coil – replace the 1 k resistor by 2.2 k.

The 8753C provides +15 V and -15 V at the YTO connector. Assembled a small power regulator board, to provide the necessary +12 V and -5 V.

Testing…

Several adjustments of the 10T trimmer, but there are issues – no stable lock below 3.3 MHz, and the Service Function 58 won’t do the pre-tune corrections. Note that you can use the source tune mode to monitor the YTO with the PLL disengaged. Ideal for adjustments of the YTO slope. Still no stable lock. Working reasonably stable at times, but all a bit temperamental.

Various lock issues, and the self tests won’t work well.

Some study of the manual, and quite some time spent to add more components, changing PLL filters, and so on. But no luck. Below 3.3 MHz, there is a also a gain change of the PLL, by Q10 FET, also modified a bit there, some improvement, but not as stable as it needs to be.

Maybe, some specific issue of this YTO, at least after detail study of the tuning currents, some magnetic hysteresis, or similar. Not an uncommon problem. Not all YTOs are suitable for fast, precise sweeping and phase-locked operation.

After all, let’s give it another try, with another YTO. Found this beauty for USD 25, a great AVANTEK YTO, ASF-8347M, with solid output power. Hermetically sealed, in a magnetically shielded casing.

HP 8753C Network Analyzer

HP 8753C Network Analyzer: all the dead YIG Oscillators

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Having had the non-working HP 8753C on the bench for while, almost getting a bit desperate. The machine is in pristine condition, including the test set, and the CPU board issues were an easy fix. But workings source assemblies, or YTOs, are to no avail. If at all, they are way too expensive. Finally, even got a 2nd 8753C (this unit with option 006 and 002, and with a good display, good to have, in any case) – but same issues there – no signal, the YTO dead.

Here are the two YTOs, both not working at all.

Doing some search on the internet, seems that these oscillators have general lifetime issues, if you want to call it an issue after 30 years… I expect these to work 50 years and longer.

The 8753C series instruments used a variety of YIGs, all pretty similar, but different in their parts number. From hearsay, HP optimized the cost a bit and used these so call economy YIG oscillators. They are a bit suspicious, because they are not hermetically sealed.

The inner circuit, nothing to fix here, all put together with molybdenum substrate, gold wire, and a special transistor that doesn’t seem to provide enough gain any more. Sure I tried to rearrange the sphere (red arrow), and checked all the wires, but no apparent issue. It really seems to be a HF transistor issue inside the microcircuit, at the green arrow.

Be mindful when bidding for any of these 8753 units – they may have all dead or soon-to-die YTOs. And even during their prime time, when spares were still available from HP, the source assemblies 08753-60003 were only available as a whole unit, and certainly HP used to charge several 1000 USD per piece.

HP 8753C Network Analyzer

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

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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 300 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.