That's a good point, Rainer. I don't have the phase-matched cables necessary. I did try making some up a while ago, but unfortunately they were not 'flat' all the way up to 1300Mhz. It's a lot harder than one might think!

Hi??Attilio,
A reverse RIAA network is what I use.? You can check the accuracy of the EQ curve as you are doing, or if you characterise the reverse RIAA network it becomes more simple.? One thing the network will do for you is reduce the noise floor of the 8903B so you can get better readings.

One thing I had to do is move to a modern audio analyser such as a U8903B or Audio Precision in order to get noise floors low enough.? I was lucky enough to grab an RTX 6001 when they were briefly available, coupled with Multi-instrument (M.I.) software, it is an effective tool.? ARTA also works for quick jobs.? I understand that these are very expensive options, so I hope the 8903B with the reverse RIAA? can give you measurements adequate for your needs or expectations.? I dearly wish I could afford a Keysight U8903B.

-Chris

I think I have the original Hp APP NOTE describing measuring S parameters using the 8754A. If I can find it, I shall scan it for here. The little T/R test set that they sold worked nicely, but it didn't have automatic switching as controlled by the 8754A itself. Nor could it change signal directions to make the complimentary measurements (S22, S12), which was done by hand of course. The 8748A appears to do exactly that, although I never saw one in the flesh in my career. I wonder how rare those are?

One could do that, but it isn't preserving the 8754A as it was originally intended to be used, it is making a hybrid VNA with the automatic switching of a 8753 etc. When we bought 8754A in the 1980s, it was the cat's meow for doing measurements up to 1 GHz. We stocked up on good directional couplers, like the Narda 3020A, and did whatever test we needed with a single path setup. I was designing high power cavity amplifiers for FM broadcast transmitters at the time, so it was great for sweeping the big circuit for resonances and gain. People designing antennas had them strapped on their range to sweep the antenna with a solid state power amplifier to boost the level. Using external directional couplers, that was very easy to do using the 8754A. I even saw one up on a tower, as heavy as they were.

Then came the Hp3577A. It had calibration built in, that could be run to normalize the setup. We bought that for the transmitter work, only ran to 200 MHz though. I moved on to another company, where we had a Hp8409A VNA, which was a Hp8410C network analyzer, test set, switches, Hp8620 sweeper, and a 9836C computer. It was two racks of equipment. Clunk clunk clunk. The Hp8510A came out and revolutionized measurements, all in a single rack on wheels. I donated our 8409 to a university. Quickly after 8510A, the Hp8753A revolutionized the world again, and they were hot items! I moved to another employer and by now I have operated every network analyzer until the latest models from Keysight, and I just got one of them at work, getting used to it. The only one i never laid hands on was the 8753D and some of the PNA models after the 8753 was out of production.

If it were me, and it isn't, I would not put a lot of work into trying to make a 8754A do things that weren't intended, when you can find a lot of amazing network analyzer capability either used or new from so many sources. People are making measurements with a hundred dollar devices now, although they are no where as good as a real bench VNA. I do recommend that you pick up a couple of Narda 3020A broadband directional couplers, or the HP equivalents, to make good use of the 8754A for insertion loss, gain, reflection, all done by hand. It will make you understand the measurements very well using that box!

Sadly, an old engineer at work bought a 8754A back in end of the 1980s, when it was about to be discontinued. They didn't have the foresight to spend on the 8753 at the time. That 8754A sat in a cabinet, hardly ever used. I ran across it about 5 years ago, with the original manual. As much as I hated to do it, I sent it to our surplus/salvage group and hopefully someone out there got a unit in perfect condition after it was auctioned off at Bentley's in Albuquerque. I strapped the manual to it with tape as our salvage/surplus operations tends to throw out all the paper and cables. Cool old box for sure. Probably fun to repair too, so much RF and analog in it.
John Lyles

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I'm pretty sure I didn't have the scope set up optimally to be able to measure any ripple on the regulated outputs taken from the Test Points, but there didn't seem to be any sign of instability on any of them ("from a distance" as it were). The voltages were slightly out, though:

A8TP1 (+22V rail) read +23V
A17TP4 (+20V rail) read? +20V
A17TP4 (+12V rail) read? +12.275V
A17TP2 (+5.2V rail) read +5.175V
A18TP5 (-5.2V rail) read -5.16V
A18TP1 (-40V rail) read -39.6V
A18TP4 (-10V rail) read -9.88V

I'm no expert, but I can't see anything here which is far enough out of whack to cause the YTO unlock error. :-/

S parameters, as you know, can be characterized by S11, S21, S12 and S22. The two port test sets have enough directional couplers and power splitters inside to measure all four, by switching the direction of the RF feed, if directed to do all four measurements by the VNA controller. However, the 8754A was built long before that became a necessity for fast bench measurements. There are still just two ports on the test set, I am sorry if I confused the topic. I meant four S parameters being measured.

It is true that the freq response curve of RIAA standard deviates from flat, so that THD is not exactly what you think. However, RIAA preamps are typically measured that way. You could build a passive deemphasis network after the preamp to flatten it again and feed into the input of the test set. But you shouldn't be getting horrible THD numbers from a preamp in any case, and usually this indicates noise has crept in. A lot of work goes into microphone and phone cartridge preamps to reduce the noise from the IC's since they have high gain. Did you try filtering using 20KHz LPF for example? Does it have a powerline component to the noise, looking on the scope of the nulled output of the test set?
John Lyles

Uploaded in files section all N28F010 dump of 54720A? FW ver. 4.07

Oops, my mistake: the part-number for the MRC is 1820-2312, not 1820-2131.

The 1820-2131 is the microcontroller in the same counters, it is a mask-programmed Mostek MK3870.
Sorry for any confusion this may have caused !

Joel Setton

On 2/20/22 01:28, Lincheng Xiu via groups.io wrote:

After 6 hours troubleshooting, it is fixed. The reason is C35 short. 100uF/50V electrolytic capacitor.
Here is what I traced the partial schematic of HP83751A power supply.
?After C35 short, the PWM controller does not drive the MOSFETs.
Also checked the ESR of C1, which is too much, 2 ohm, it will soon fail, just replaced with a new one.

Wow, I see electrolytic capacitors like that dry out and lose capacitance all the time, but I don't think I've ever seen one shorted. Nice troubleshooting!

-Dave

--
Dave McGuire, AK4HZ
New Kensington, PA

Thanks! I was having a devil of a job trying to find those test points! I never had such a problem with the old loose-leaf binder type manuals.
I'd expected to see *some* ripple present across the filter caps, but 10% was a surprise I must say.

Typo correction: the 12V rail is at A17TP6 (not A17TP4).

As Tom said, this is exactly as it should be!

You should not care much about ripple that you see before the outputs of the different regulators on the A8, A17 and A18 boards. However, you should see negligible ripple (less than 3mVp-p is typical) on the following test points:
A8TP1 (+22V rail)
A17TP4 (+20V rail)
A17TP4 (+12V rail)
A17TP2 (+5.2V rail)
A18TP5 (-5.2V rail)
A18TP1 (-40V rail)
A18TP4 (-10V rail)

When you test for ripple, stop the instrument from sweeping (e.g., by selecting "single sweep") as sweeping? introduces slight noise into some rails that is sometimes confused with ripple.

See the "Performance Tests and Adjustments Manual" section titled "Low-Voltage Power Supply Adjustments" subsection "RF Section" for the voltage tolerances for each supply rail and the locations of the different test points mentioned above (BTW, I could never find in the manuals a specification of the allowed ripples) .