Date   

Re: Looking for sensitive audio RMS meter

 

On Tue, May 11, 2021 at 3:07 PM Matt <mhofmann@sbcglobal.net> wrote:

I have used an HP 3400A to measure the noise in circuits for several applications since the 1970s, both for business and pleasure. I found it useful for determining the equivalent input noise for various microphone preamps that I had built. Typically I used a low noise solid-state preamp on the front end of the HP 3400 with a low-pass filter on the input of the 3400A to reduce the bandwidth to the audible range. With this arrangement I could get 60 dB of gain on the preamp, and I could measure the equivalent input noise of the microphone preamp I was testing. I could get quite a bit of sensitivity with this arrangement. I would set the HP 3400A to 1 mVrms and add another 60 dB of gain with the low noise preamp, resulting in 1 uVrms full-scale sensitivity on the meter.
It seems like this is the kind of scenario I should be looking at.
What LNA were you using?

Liam mentioned the J-Can and he had parts for it available. I think
this should be the way to go.

Is it possible to modify the HP 3400A to have a dBV scale?

Thanks.

I used this arrangement for solid-state microphone preamps that I was designing and building as well as a tube based microphone preamp that later on I built for my boss.
I have also used an FFT based spectrum analyzer program on an old laptop PC that was useful in identifying the noise floor of these preamps.
I bought the HP 3400A on eBay a number of years ago for about $50 (I could have been a bit more).

Matt





Re: Looking for sensitive audio RMS meter

 

I'm looking to measure noise, but not THD.

I don't want to use a sound card as those generally cannot be calibrated.

The ones that can be calibrated cost much too much.

I see the points with the excess bandwidth making the measurement less
sensitive.

I'm looking to measure noise at least up to 30 kHz, but 22 kHz might
be fine too.

Perhaps the suggested 3456A or, as Liam suggested, 3457 are a good idea.

If I had money to blow I'd buy an Audio Precision box. My budget is in
the low $xxx for this.

On Tue, May 11, 2021 at 12:35 PM Marian Beermann <public@enkore.de> wrote:

I have two 3400A's (with nuvistor frontends), their noise-floor is around
60 µVrms, but they do have 20-25 MHz -3 dB bandwidth (so their noise is a
factor ~14 higher due to excess bandwidth). These have tons of gain (like
250-300x) after the input amplifier (which is a nuvistor plate follower
with unity gain on the lowest range, and 1:1000 attenuation at 1 V and
above), which is not conducive for ultra-low-noise performance.

Your best / cheapest bet is a "gain box" and a sound-card as others
mentioned.

Cheers, Marian

Am Di., 11. Mai 2021 um 05:06 Uhr schrieb Kerry Burns <
kburns@netspace.net.au>:

Hello Chris



I have an older 3400A. It works well for most audio measurements ( and
beyond, up to 10 MHz), but the lowest range is 1mV so probably not
sensitive enough for sub 1uV work.



Kerry



From: <TekScopes@groups.io> on behalf of bhome1959 <bhome@sympatico.ca>
Reply to: <TekScopes@groups.io>
Date: Tuesday, 11 May 2021 at 12:41 pm
To: <TekScopes@groups.io>
Subject: Re: [TekScopes] Looking for sensitive audio RMS meter



Hi Liam,

I would love a 3457A!



Anyone have a HP 3400A? That would completely suit Bob's requirements.
BTW, I am also looking for one of those that works. I bought one on
fleabay and it has serious problems.



-Chris






















Re: 465 Capacitor Refit Adapters

Bill
 

Thank you!!
Bill


Re: 465 Capacitor Refit Adapters

Michael W. Lynch
 

On Tue, May 11, 2021 at 08:37 AM, Bill wrote:


I have received my Capacitor Adapters 15.5mm triangle from CUOG on Ebay. How
are the pin headers used to make up the ground connections?

Getting close to installing them.
The pin headers solder into the vias of the adapter boards, Once the caps are soldered onto the boards, The ground connections are made by traces in the adapter boards themselves. I made a small soldering jig that would hold all 4 pin headers and allow them to be soldered easily and perfectly perpendicular to the adapter boards.

--
Michael Lynch
Dardanelle, AR


Re: 465 Capacitor Refit Adapters

n4buq
 

I did something a bit different:

https://groups.io/g/TekScopes/album?id=64929

It's not the prettiest, but if those caps ever need to be replaced, it won't involve soldering on the main board (provided snap-in caps are still available at that point).

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Dave Peterson via groups.io" <davidpinsf=yahoo.com@groups.io>
To: TekScopes@groups.io
Sent: Tuesday, May 11, 2021 9:41:48 AM
Subject: Re: [TekScopes] 465 Capacitor Refit Adapters

Bill,

I put up some photos of my recap job in the folder:
 https://groups.io/g/TekScopes/album?id=258720

There are examples of the new caps with the adapters on (these are pre-snap
in sized adapters), and a view of the board with the caps soldered in. I had
to trim the leads of the snap-in caps to fit the adapters. Plenty of
material left to make good solid electrical and mechanical attachment.

I backed up the large holes with a piece of foil tape. The thick aluminum
foil used to seal duct work (not duct tape!). I used a hole punch to make
nice round patches, then stuck the lead through the middle. The adhesive
melts right away, but the foil did a good job of not allowing the solder to
overflow onto the backside of the board. I didn't fight too much to fill the
large holes entirely. The electrical connection is beyond established, and
the mechanical connection is also very solid. The caps are significantly
smaller and lighter than the originals.

Hope the pics help.
Dave


On Tuesday, May 11, 2021, 06:37:52 AM PDT, Bill via groups.io
<ko4nrbs=yahoo.com@groups.io> wrote:

I have received my Capacitor Adapters 15.5mm triangle from CUOG on Ebay.
  How are the pin headers used to make up the ground connections?

Getting close to installing them.

Bill











Re: 465 Capacitor Refit Adapters

Dave Peterson
 

Bill,

I put up some photos of my recap job in the folder: https://groups.io/g/TekScopes/album?id=258720

There are examples of the new caps with the adapters on (these are pre-snap in sized adapters), and a view of the board with the caps soldered in. I had to trim the leads of the snap-in caps to fit the adapters. Plenty of material left to make good solid electrical and mechanical attachment.

I backed up the large holes with a piece of foil tape. The thick aluminum foil used to seal duct work (not duct tape!). I used a hole punch to make nice round patches, then stuck the lead through the middle. The adhesive melts right away, but the foil did a good job of not allowing the solder to overflow onto the backside of the board. I didn't fight too much to fill the large holes entirely. The electrical connection is beyond established, and the mechanical connection is also very solid. The caps are significantly smaller and lighter than the originals.

Hope the pics help.
Dave

On Tuesday, May 11, 2021, 06:37:52 AM PDT, Bill via groups.io <ko4nrbs=yahoo.com@groups.io> wrote:

I have received my Capacitor Adapters 15.5mm triangle from CUOG on Ebay.  How are the pin headers used to make up the ground connections?

Getting close to installing them.

Bill


Re: Unusual use for a Tektronix scope?

Siggi
 

On Tue, May 11, 2021 at 9:48 AM Matt <mhofmann@sbcglobal.net> wrote:

An unusual application for a Tek scope: simulating the depth of water in
the lake?
That's ingenious, using the scope as a delay pulse generator.


Unusual use for a Tektronix scope?

Matt
 

Last summer I purchased a 38 year-old sailboat with a depth sounder that was only partially working. It looked like the unit basically worked, but the LCD display was completely dead, so you didn’t know how deep the water was. I had a user’s manual for this 40 year-old unit, but there was no schematic. I figured that I might be able to replace the LCD, but held little hope of finding a direct replacement for it; I didn’t even have a part number for the display. I purchased a generic 3 ½ digit direct-drive LCD for about $6 and proceeded to figure out how to map the pin-out from the dead display to the new one.
The obvious approach was to get the unit under repair attached to a transducer and put it in various depths of water between 4 feet and 400 feet. Well the only transducer I had is permanently mounted in the boat which is out of the water for the winter, so I had to figure out how to simulate the return echoes that the depth sounder would see. I traced as much of the circuitry on the circuit board as I could before I went nuts trying to identify the pin-outs of the various transistors. I found a point that looked like it generated a pulse to the processor from the return echo. I build a little circuit to invert the pulse and then soldered it to my candidate circuit location.
I took my 2465B and attached one channel to the output of the depth sounder that went to the transducer (which I was on the boat and not available) and I could see the several hundred volt pp waveform generated as the transmit pulse and I triggered the scope on that. I then took the B-sweep trigger pulse off of the back of the scope and used that to drive the inverter attached to the return echo detector output to simulate the pulse that normally would be generated from the echo. Putting the sweep in “B start after delay” I could simulate any depth of water using the delayed sweep, from 1 ft, to 399 ft, the limit of the depth sounder.
Then I attached the input pins of one digit of the new LCD, which I obviously had the pin-out for, to the dead LCD, looking for activity on the new LCD to isolate which pins on the old LCD corresponded to the segments for the least significant digit. After many observations simulating various depths using the B-delay, and going from digit to digit, I was able to ascertain which pins on the old LCD corresponded to the appropriate pins on the new LCD.
If I had found a drop-in replacement for the malfunctioning depth sounder, I probably wouldn’t have done this. It took any hours to do. Now for the hard part, figuring out how to mount the new LCD in the unit and wire it up.
An unusual application for a Tek scope: simulating the depth of water in the lake?
Matt


Re: Looking for sensitive audio RMS meter

Jean-Paul
 

To OP cheater cheater: What is the exact application, most audio work does not need microvolt metering.

Since 1970s, used HP3400A, Balantine 323, more Audio precision SYS 322,

still have Tek AA501 TM 500 plugin, many useful filter options.

The Fluke TRM meters like 8920A include DB, Dbm Dbrel as well as volts. The low range has a 2 MHz BW limit.

Much more useful in design and troubleshooting is a high gain diff scope plugin like the TEK 7000 series 7A22 with a 10 uV range.

https://w140.com/tekwiki/wiki/7A22

I may have some working Balantine 323 in our archive. Please contact me PM OL with your contact info and zip code for a quote.

Bon Chance,

Jon


465 Capacitor Refit Adapters

Bill
 

I have received my Capacitor Adapters 15.5mm triangle from CUOG on Ebay. How are the pin headers used to make up the ground connections?

Getting close to installing them.

Bill


Re: Wanted - Tektronix 305 Oscilloscope Knobs and Caps

Reddy
 

I have a parts mule . I will see what's left on it this evening.


Re: Looking for sensitive audio RMS meter

Matt
 

I have used an HP 3400A to measure the noise in circuits for several applications since the 1970s, both for business and pleasure. I found it useful for determining the equivalent input noise for various microphone preamps that I had built. Typically I used a low noise solid-state preamp on the front end of the HP 3400 with a low-pass filter on the input of the 3400A to reduce the bandwidth to the audible range. With this arrangement I could get 60 dB of gain on the preamp, and I could measure the equivalent input noise of the microphone preamp I was testing. I could get quite a bit of sensitivity with this arrangement. I would set the HP 3400A to 1 mVrms and add another 60 dB of gain with the low noise preamp, resulting in 1 uVrms full-scale sensitivity on the meter.
I used this arrangement for solid-state microphone preamps that I was designing and building as well as a tube based microphone preamp that later on I built for my boss.
I have also used an FFT based spectrum analyzer program on an old laptop PC that was useful in identifying the noise floor of these preamps.
I bought the HP 3400A on eBay a number of years ago for about $50 (I could have been a bit more).

Matt


Re: 547 Recap

Jim Adney
 

On Sun, May 9, 2021 at 06:42 PM, J wrote:

* If I recap the timing caps while I'm doing this - what voltage rating should I use? It's not specified in the parts list.
I'll second the recommendation to NOT replace any of the timing capacitors. In the first place, I doubt if any of those are electrolytics, so they don't have that tendency to dry out and fail over time. Secondly, I believe Tek generally supplied these in matched sets, to assure the correct 1-2-5 ratio steps.

I have a couple such Tek sets of matched timing caps, bought decades ago via ebay. I bought them because they were cheap, they were Tek, and they were local (no shipping.) I've never needed them and I probably never will. If anyone needs a set, the part number I have is 295-0134-00, dated Dec 7, 1984. There's a Tek note attached to the bag that says, "DO NOT MIX SUBPARTS BETWEEN SETS." Each set contains 4 of the special silver metal cased Tek caps plus one brown silver mica. I'd be willing to sell them if anyone needs a set.

Anyone know what scope these were used on?

I also discovered that I have a LARGE random bag of those very nice Tek silver metal caps. I don't remember where these came from, but each appears to be marked with a Tek part number. So if anyone needs some such thing, please send me the approx length and diameter, to help pick it out of the haystack, and the part number, and I'll see if I have one.


Re: PG506 Repair - I had a Great Day today/

tek_547
 

Thanks Albert for your research. Together with the information from Michael I have plenty of material to investigate my PG506. After a few days I report again.
René


Re: Looking for sensitive audio RMS meter

 

Hi Ed,
You're exactly right. I do audio work for a living, and measuring signals down there is no joke. Audio analyzers to measure and characterize signals down there are running $30K +, and older models will not go down that far. Normal bandwidth is 100 KHz with the Keysight U8903B up to 1.5 MHz (options).

You might consider using a good DVM hung on the monitor output of an HP 3400A. So you use the analogue meter as a front end. You should be able to calibrate that lot. Noise will limit your resolution. Of course, you can filter that using whatever curve you want to lower the system noise.

-Chris


Re: Looking for sensitive audio RMS meter

Marian Beermann
 

I have two 3400A's (with nuvistor frontends), their noise-floor is around
60 µVrms, but they do have 20-25 MHz -3 dB bandwidth (so their noise is a
factor ~14 higher due to excess bandwidth). These have tons of gain (like
250-300x) after the input amplifier (which is a nuvistor plate follower
with unity gain on the lowest range, and 1:1000 attenuation at 1 V and
above), which is not conducive for ultra-low-noise performance.

Your best / cheapest bet is a "gain box" and a sound-card as others
mentioned.

Cheers, Marian

Am Di., 11. Mai 2021 um 05:06 Uhr schrieb Kerry Burns <
kburns@netspace.net.au>:

Hello Chris



I have an older 3400A. It works well for most audio measurements ( and
beyond, up to 10 MHz), but the lowest range is 1mV so probably not
sensitive enough for sub 1uV work.



Kerry



From: <TekScopes@groups.io> on behalf of bhome1959 <bhome@sympatico.ca>
Reply to: <TekScopes@groups.io>
Date: Tuesday, 11 May 2021 at 12:41 pm
To: <TekScopes@groups.io>
Subject: Re: [TekScopes] Looking for sensitive audio RMS meter



Hi Liam,

I would love a 3457A!



Anyone have a HP 3400A? That would completely suit Bob's requirements.
BTW, I am also looking for one of those that works. I bought one on
fleabay and it has serious problems.



-Chris



















Re: Looking for sensitive audio RMS meter

Ed Breya
 

cheater cheater wrote:
"I want to be able to measure total noise down to -110 dBV, or possibly
-130 dBV if possible. This means resolving single microvolts and
tenths of microvolts."

This sounds like you want a "THD+noise" type measurement as with an audio analyzer, but such measurement may be out of reach, depending on the situation. You may need to consider the fundamental limitations involved, which have a lot to do with the measurement bandwidth and impedance of the source. If you're working in a high impedance system, say 1 megohm, and 100 kHz BW, you can't directly reach these kind of numbers. For instance, typical high-Z (100 k - 100 M impedance) amplifiers may have input noise around a few nV/root-Hz RMS. In a 100 kHz BW, this results in 316 nV/nV/root-Hz. Say you have a good amplifier with 6 nV/root-Hz, so the noise is 316 x 6 nV, and there's nearly 2 uV RMS at the front, just from the amplifier. Then add the noise due to the source resistance. If you're looking for p-p noise values, then another 5 to 6 times factor is involved.

If you reduce the effective BW and/or impedance, you can get much higher resolution (less noise), which is what you'd see with a lock-in amplifier or spectrum analyzer, when dealing with CW signals. For broadband noise, you're stuck with the situation - there's no signal processing involved, just the noise according to nature.

The simplest things to help are to reduce the BW to the smallest possible - say 20 kHz only, for audio, and work in the lowest impedance possible. The very low noise opamps and JFETs available want to be in low impedance environments for best performance - don't bother trying to make megohm input amplifiers with them. If you can work all this in 50 or 600 ohms, that would be good, but it won't be as convenient as poking a high-Z scope probe around. A good practical limit data point is that 50 ohms has about 1 nV/root-Hz - you're not likely to get lower without special methods and very low-Z signals. I built a 1000X gain lock-in preamp long ago, using a 1:100 transformer for voltage gain, followed by a low noise JFET (2SK269) and OP27 amplifier. It can achieve 300 pV/root-Hz, but only from a very low (1 or 2 ohms) resistance source - the noise is dominated by the resistance of the transformer winding and its protective fuse, and interconnect wiring.

So anyway, I don't think you'll find a regular meter that will measure far enough down with low enough noise, but if the actual measurement is somewhat different, there may be options. Audio analyzers, lock-ins, and SAs may do the trick, depending on the actual deal. If it really is just a broadband RMS conversion, and the working impedance is low enough, then the best bet may be to build a low noise preamp to get it in-range for a DMM.

Ed


Re: Wanted - Tektronix 305 Oscilloscope Knobs and Caps

 

I see this on eBay, which appears to have the full set of colored SEC/DIV and V/DIV knobs with red concentric knobs. The price is pretty good to get undamaged knobs with a "free" parts mule thrown in.

https://www.ebay.com/itm/233141250240?hash=item36484d28c0:g:ufwAAOSwnwpcbqnC


Re: Looking for sensitive audio RMS meter

Kerry Burns
 

Hello Chris



I have an older 3400A.  It works well for most audio measurements ( and beyond, up to 10 MHz), but the lowest range is 1mV so probably not sensitive enough for sub 1uV work.   



Kerry



From: <TekScopes@groups.io> on behalf of bhome1959 <bhome@sympatico.ca>
Reply to: <TekScopes@groups.io>
Date: Tuesday, 11 May 2021 at 12:41 pm
To: <TekScopes@groups.io>
Subject: Re: [TekScopes] Looking for sensitive audio RMS meter



Hi Liam,

I would love a 3457A!



Anyone have a HP 3400A? That would completely suit Bob's requirements. BTW, I am also looking for one of those that works. I bought one on fleabay and it has serious problems.



-Chris


Re: Looking for sensitive audio RMS meter

Tom Lee
 

Because "Bob's your uncle" (only makes sense if you're from the UK).

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 5/10/2021 19:42, cheater cheater wrote:
On Tue, May 11, 2021 at 4:41 AM bhome1959 <bhome@sympatico.ca> wrote:
Hi Liam,
I would love a 3457A!

Anyone have a HP 3400A? That would completely suit Bob's requirements. BTW, I am also looking for one of those that works. I bought one on fleabay and it has serious problems.
Why does everyone keep on calling me Bob? What the heck...



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