You may be surprised at what you find at some of those places. I found an HP 5245L at one. They only wanted $25. I took it home, cleaned it up, calibrated its standard against a GPSDO, and it continues to serve a good purpose on my bench in the shop. If that q meter was not very expensive, it sounds like it would have been a fun project.?

OK, I did it!
?I put the jack in my vise loosely so that just the plastic caught the jaws of the vice. i put a drill bit upside down in the, hole for banana plug insertion.
I lightly tapped until the plastic cap came off. I did put another drill bit through the wire hole just to protect the bottom of banana jack hole, but it popped
off very easily, so no big deal. It would be nice if it was knurled, but it's quite usable in this form.
? I have the Q meter warming up to test for losses, but after the other all metal tests I have done, I don't expect any losses with these 'now' all metal banana jacks.
??????????????????????????????????????????????????????? Mikek

--
Thanks, Mikek

Though good ideas are being persued, I am still a bit sceptical a transformer circuit will be able to remain almost flat over 150Mhz range with almost no change in impedance, and if so, be able to be repeatedly built by others with varying skill or equipment levels.

Would it make sense to add a feedback winding on the transformer to monitor the output level right at the output to keep it constant? That might solve at least one problem. Another problem that will have to be solved is how to have a really short lead length while having a length that can be built to external connectors on a real bench test instrument.

The varying inductance might need a calibrated correction circuit, maybe a simple microcontroller that changes the meter response in relation to the measurement frequency. Once calibrated, it would correct for the varying impedance in the math before displaying it on a meter.? This might also solve the problem of the length of the secondary wire needed to reach the test terminals.? I guess I am assuming that getting the lead resistance/inductance to stay at near zero ohms across 150Mhz of range might be a loosing battle, especially when you try to build an actual test instrument for the bench.? Perhaps characterizing what happens with just a specific wire length to the test connector, with another one across the test terminals (or a known calibrated standard) would allow nulling out with a lookup table before the Q math is performed. Once this is figured out on an actual physical test instrument input circuit, the process could be duplicated by other people on the forum. Probably need a specific length +/- 1/4" at most with enough look up table compensation range to allow for wire impedance variance over the length/range. Then the physical transformer, secondary wire and test connector type/spacing would need to be duplicated in order to get similar readings from one Q meter to another once calibrated.

Another option might be that the QMeter will have to be separated into two or more ranges with each solved a bit differently. Possibly with a set of low and high frequency test ports.

I have looked at replacement designs for the HP 4815A probe and the 410C AC probe as good examples of difficult engineering problems. Lots of people of the decades have tried to duplicate the 4815A probe with no success. The 410C probe can be duplicated but only with the same design and one of the two possible tube types. Every replacement design has either a voltage level and/or frequency range problem. There may be a reason why HP and other vendors are not building wide frequency range versions of their older direct reading Q meters and leaving this to network analyzers packing lots of processing power.

I received some all metal Banana connectors today and tested them, no measurable loss caused when they are part of the test setup.


Then I retested the plastic covered? Banana jacks @ 1 MHz. I found about a 1.25% loss of Q, this is with a coil in the 1250Q range.
? Then I removed the lower half of the plastic from the Banana Jack.



i found the loss reduced to about 0.5%




I'm tempted to remove the plastic on the upper half, but, I don't want to make them unusable.
?These plastic coated Banana jacks are very good quality, The hole for the wire is a little over 0.2".
And, a clever design with a floating ring that does not rotate the wire when you tighten it.

--
Thanks, Mikek

I am surprised nobody suggested using PC/soundcard for this kind of testing. Quick googling returns a few solutions.


I used the latter to compare my piano room properties before and after some acoustic improvements.
You can save 2 or more sweeps for easy relative comparison. Saves a lot of time compared to manual sweeps using generator and voltmeter.

73, Mike AF7KR

I hope enough has been said about the FET 535A tube replacement circuit, that anyone interested has now been made aware of it.
?I printed it out probably 10 years ago, don't know from what site.
?If anyone wants put that good copy in the Files section, I'll remove mine.
--
Thanks, Mikek

John

I think my issue was not being a member of the HP Groups.io site. Initially when
hunting for the Files section Mike stated the file was located in, I could not find
the link for Files. It was not until scrolling to the bottom of the intro page for the
HP Groups.io site that I noticed the "Join" button and once I was declared a
member, suddenly the Files link for the group appeared. I think the link provided
by Mike did not work initially for me because I was not a member and not logged
in.
Sorry for the confusion.

Chuck

I don't remember having any trouble following Mike's link, but it doesn't work for me now.

The link opens to HP-Agilent-Keysight-equipment/files. Go down the page to the Boonton subfolder and it's there.

/g/HP-Agilent-Keysight-equipment/files/Boonton/HP%20%20Boonton%20%20535%20for%20BRC%20260%20%20Q%20meter/Boonton_535_fet_repl.pdf

John KK6IL

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And learn to manage your wife. Most important stressant for many, at least for me.

Tam

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Tam HANNA 

Enjoy electronics? Join 21k4 other followers by visiting the Crazy Electronics Lab at 

Arrgh, I should have given you this one!
/g/HP-Agilent-Keysight-equipment/files/HP%20%20Boonton/HP%20%20Boonton%20%20535%20for%20BRC%20260%20%20Q%20meter/Boonton_535_fet_repl.pdf
--
Thanks, Mikek

On Tue, Nov 1, 2022 at 10:13 AM, Chuck Moore wrote:

Sorry to have pestered you.

?No problem, I should have checked it before posting, this one worked for me.
/g/Test-Equipment-Design-Construction/files
?????????????????????? Mikek

?
--
Thanks, Mikek

Well Mike, I need to pay more attention. I could not find the Files link on the
HP groups site and wondered what was going on that you could see it and
I could not. Upon scrolling further down the page I found a button to "Join" the
group and voila. there was the file's section. Sorry to have pestered you.

Chuck

On Mon, Oct 31, 2022 at 01:50 PM, Mikek wrote:

Boonton_535_fet_repl.pdf
--

Mike

The link provided takes me to the HP/Agilent/Keysight Groups.io home page, not the file of interest.
Would you be kind enough to check the URL again and repost if there is a correction available?

I did a bit of searching on the HP/Agilent/Keysight Groups.io site and found some interesting links
to the module and also commentaries from a few folks that had encountered them. Unfortunately
a search of the site for:? Boonton_535_fet_repl.pdf produced? no more than e-mails discussing it.

Regards

Chuck

?

Thanks folks, for those interesting factoids.
Perhaps now is a time to remind that *for my purposes*, I don't care about absolute measurements, but rather I want to make relative comparisons against particular mic models (such as Shure MC-30). And my "use case" is just voice intelligibility.

Pete

Long time back I worked for a Company that supplied microphones so there was a need for a microphone comparator to measure the performance of returned? or repaired microphones. We used a 18mm birch ply box about 2m long and 0.8m x o.8m. It had a small sealed box speaker at one end. the box was lined with? sheep's wool held in place with fabric, it was around 200mm deep.? Test microphones were placed side by side about? 600mm from the speaker through a hatch.
The speaker was driven by a B&K BFO, the reference mic was used to level the field at the microphones, the test mic could be plotted on a recorder and displayed on a meter.

For quick tests a switched frequency oscillator was used as due to the kinds of mic that were being tested a test at 125 Hz, 1KHz and 8KHz was sufficient to verify functionality, a full sweep took appreciably longer.

Open air tests can be good, a good approach is to bury the speaker so that the emitting face is level with soil level, a sand pit is very useful. ( this method is invalid for speaker testing as it totally alters box stiffness issues!)? However in practice it's pretty useless as there is so much noise from cars, vans and people alongside wind noise? and rain that dynamic range is very limited. Even more limited when people start moaning about the noise you are making!?

Note that the lined box does not need a perfect lining, it is there to partially supress standing waves and to assist in keeping external noise out and to avoid the need for a stiffer box.? You don't need a half wave of damping at all, just enough to stop organ pipe resonances. The mics are equidistant from the source and very close together. The most difficult part is keeping the wool density uniform, that is a Trade Secret !?

That was the formal test however there was a very useful "informal test for the experienced" , that was two identical mics, two mic amps and a very good pair of headphones. You just listened with the mics together, conversation or noise quickly revealed any significant differences. In the Broadcast Industry that was how some mic faults were diagnosed.

So how do you know the frequency respo initially?? There is the three transducer method, use two reversible transducers ( small speakers typically and the test mic. You do three loss measurements, A-B, A-T, B-T where T is the test mic.? A-B gives the transducer loss, 1/2 of that is the transducer loss at the test frequency. If A and B are near identical you can skip the B-T measurement. The transducers are mounted on Couplers, not in open air- makes a LOT of difference.

That was all? around 50 years back, way before PC's and way before impulse or multi-tone methods were developed.?

Mics. Many electrets are very flat, there can be a slow loss of sensitivity, -1dB per year was often mentioned. The electret stores a forming charge which will decay so the absolute sensitivity will decline with time. A dynamic ( moving coil) mic is normally very stable providing it is not dropped. But the best device is a well made capacitor mic that is polarised with a constant voltage. A version on the DC polarised capacitor mic is the so called RF bridge where the capsule is part of a tuned circuit, this can have the highest sensitivity of all microphones and can go down to 0.1Hz . MEMS can be very good, they can go way higher in frequency than the nominal 10-20KHz electrets, they are the normal mic used in Bat Detectors, some have? built in ADC's which may not be what is needed in your application.

Regards, Alan G8LCO.