Kitz Forum
Chat => Tech Chat => Topic started by: burakkucat on November 26, 2013, 04:44:25 PM
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I attach two images, below, for fellow Kitizens to examine.
It is a combined send/receive audio test set. I am told that the frequency is 800 Hz but as I do not own an "o'silly scope" with which to measure the output, the frequency remains unverified. I have listened to the audio output and it seems to be around that frequency and certainly the 10 dB difference in output signal strength is quite noticeable.
Connecting "Oscar"'s output to the input of the Level Measuring Set No 35A (http://forum.kitz.co.uk/index.php?topic=10878.msg213173#msg213173) showed that the 10 dB difference in the signal strength of the output was "spot on". :)
Looking at the rear label, I see that it must date from the time when Post Office Telecommunications had just been transformed into British Telecommunications but before she had been sold to the public. "British Telecom South East Test Equipment Design Centre" seems to imply that the concept of "the regions" was then still alive.
Does anyone have any memories? Or comments? Or donations suitable for the grotto, located in The Cattery? ;)
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Snap!
I have one exactly the same in every respect. The British Telecom logo dates it to the early eighties (the 'Buzby' era), when as you surmise the regions were still very much part of the fabric of BT.
Very useful it was too, for quick checks which didn't require anything more sophisticated (or heavy).
I can verify that the frequency is indeed the standard VF test tone of 800Hz, and that it's pretty accurate in terms of frequency, level, and impedance.
Nice find, b*cat - may I enquire how you came by it?
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b*cat smiles enigmatically, gives one of his best Japanese-style bows and utters the word: "eBay". ;D
Thank you for confirming the frequency, 4c. Perhaps I may interrogate your memory banks, please, for the input impedance, the output impedance and to what the output power is related? Possibly 1mW? :-\
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Of course you may. The memory banks have undergone a certain amount of depletion and distortion, but I'm OK with this one. ;D
Output and input (term) impedance is 600 ohms, and 0dB is indeed 1mW.
Strictly speaking it's 0dBm (1mW in 600 ohms), but the 'm' is usually assumed.
'Term' is used to measure an unterminated circuit (ie the meter is supplying the load), and 'Thru' for a terminated circuit.
A handy impedance check: If measuring on 'Term', switching to 'Thru' should see a 6dB rise in the reading. If measuring on 'Thru', switching to 'Term' should see a 3·5dB fall in the reading.
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Thank you for that further information.
A handy impedance check: If measuring on 'Term', switching to 'Thru' should see a 6dB rise in the reading. If measuring on 'Thru', switching to 'Term' should see a 3·5dB fall in the reading.
A very useful tip. :)
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Earlier this evening I performed a few tests with "Oscar".
Following the details shown at "Check Send", the send and receive wires (blue and red, respectively) were connected together and the unit was switched to "Meas Term/Send 0dB". Unfortunately my unit appears to be out of calibration. Instead of seeing 0dB +- 0.25dB, I observed +2.01dB. So either the send circuit or the receive circuit (or both) requires some adjustment.
Hmm . . .
Inserting a decade resistance box (Rheostat No 1A) between the send and receive wires showed that a series resistance of 380 ohms brought the display back to 0.00dB.
Next, the Oscar's send circuit was connected to the Level Measuring Set No 35A and with the latter configured to provide a 600 ohm termination, it showed +1.8dB.
Now I am uncertain of the calibration status of the LMS No 35A . . . but perhaps the Oscar's send circuit should be adjusted so that 0.0dB is displayed on the LMS and then the receive circuit should be adjusted so that it (the Oscar) displays 0.00dB when the send & receive wires are connected together.
Ideally I would need sight of the circuit diagram and the calibration instructions (or the maintenance manual) for the Oscar unit.
助けて :help:
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I was going to compare notes with my 'Oscar', but he seems to be bereft of life, and may indeed have already joined the choir invisible. :(
As a more sophisticated piece of kit, I would tend to trust the LMS 35A more than the Oscar, and assume that Oscar is sending somewhere around 2dB high, given the figures you quoted.
A thought occurs - what kind of DMM does the cattery have? I remember at work our Flukes were good to at least 1000 Hz on AC volts, and I often used one as a quick check. 0dBm would read 0·775V, and -10dBm 0·245V. The second calculator here (http://www.sengpielaudio.com/calculator-volt.htm) is useful.
I have a couple of Rheostat 2As, like yours only black, but no other test kit available at present, but I would have thought they'd be unsuitable for audio work as the resistance coils would surely have a fair bit of inductance.
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Thank you for those comments and the link, 4c.
As for the DMM, it is a Fluke 8020B. Due to its age, it is somewhat temperamental -- (1) regularly shows a phantom decimal point (2) does not indicate 0V AC when the probes are shorted together but with a known AC voltage (i.e. mains) it does indicate correctly.
I agree that the resistance coils of the Rheostat No 1A would insert an unknown series inductance into the circuit. I only tried it to see what value resistance would be required to bring the display back to zero dBm.
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The Oscar's "send pair" were connected to the Fluke 8020B, which was set to AC volts and switched on. The 8020B recorded a constant 0.020V with no input signal.
When the Oscar was switched on to send at 0dB, the 8020B recorded 1.524V (1.504V corrected) and when the Oscar was switched on to send at -10dB, the 8020B recorded 0.491V (0.471V corrected). :-\
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Ah, well that figures, as those voltages equate to around +6dBm and -4dBm ie 6dBm high (see Reply #3).
The DMM is presenting no load, so you need to terminate the circuit with the LMS 35A or connect the red Oscar wire to the blue.
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I wondered, briefly, about the high Z of the 8020B but decided to proceed with the experiment knowing that an expert would subsequently comment. ;D
Noting the above advice, the experiment was repeated with the LMS No 35A providing a 600 Ohm load. With the Oscar switched on at send 0dB the 8020B reported a (corrected) 0.762V and at send -10dB the 8020B reported a (corrected) 0.234V.
:hmm: Hmm . . . b*cat is a tad confused.
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...but decided to proceed with the experiment knowing that an expert would subsequently comment. ;D
Disclaimer: Any or all of the following might well be complete twaddle. ;)
:hmm: Hmm . . . b*cat is a tad confused.
Me too, on two counts.
I'm not at all sure that any 'correction' is needed. I've always understood that, especially on lower AC ranges, a meter doesn't begin to approach its normal accuracy unless at least 5%, say, of its range is applied, and what with noise as well, I've never expected to see zero on the 2V AC range.
Taking the actual reading of Oscar sending 0dB, then (0.782V), it equates to +0·08dB - ie pretty damn close.
But, Oscar and LMS are presumably still reading around 2dB high.
Overall, then, fairly inconclusive. :(
The there's the fact that b*cat's OP has sparked an interest, not to mention nostalgia, in such matters, and I've now got eBay searches bookmarked for 'Wandel & Goltermann', 'Wayne Kerr' and 'Hatfield Instruments'. This could be an expensive year. :o
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Thank you for helping the grumpy kuro neko. :)
Around 9 or 10 months ago, an eBay search turned up some Wandel & Goltermann test equipment. Both Asbokid and b*cat agreed that it was quality kit and deserved a home that would both appreciate & use it. After some joint deliberations, we both decided not to bid and both items went another 'appreciator'. ;D
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Earlier today I was studying an Operator Guide for Fluke 8021/22B DMMs. It had a section headed "dBm TO VOLTAGE CONVERSION (600Ω)" and that section had a "Volts-To-dBm Conversion Table" which contained the following data --
+5 1377.0
+4 1228.0
+3 1094.0
+2 975.0
+1 869.0
0 775.0
-1 690.0
-2 615.0
-3 548.0
-4 489.0
-5 436.0
-6 388.0
-7 346.0
-8 308.0
-9 275.0
-10 245.0
-11 218.0
-12 194.6
-13 173.4
-14 154.6
-15 137.7
-16 122.8
-17 109.4
-18 97.5
-19 86.9
-20 77.5
-21 69.0
-22 61.5
-23 54.8
-24 48.9
-25 43.6
-26 38.8
-27 34.6
-28 30.8
-29 27.5
-30 24.5
-31 21.8
-32 19.5
-33 17.3
-34 15.5
-35 13.8
-36 12.3
-37 10.9
-38 9.8
-39 8.7
-40 7.7
-41 6.7
-42 6.2
There was also the statement that "If the circuit impedance (Z) is not 600Ω, add the following correction to the dBm value from the Volts-To-dBm Conversion Table."
Correction Factor (dBm) = 10 log10 (600Ω/Z)
The data in that conversion table was ripe for plotting and thus was so done, with the use of the following script --
#!/bin/bash
dBm_gnp() {
echo "set terminal png"
echo "set nokey"
echo "set title 'Terminated Into 600 Ohm Impedance'"
echo "set ylabel 'Power (dBm)'"
echo "set xlabel 'Voltage (mV)'"
echo "set yrange [-45:+5]"
echo "set xrange [0:+1400]"
echo "set ytics 5"
echo "set xtics 100"
echo "set mytics 5"
echo "set mxtics 4"
echo "set grid ytics back"
echo "set grid xtics back"
echo "set grid mytics back"
echo "set grid mxtics back"
echo "plot 'dBm_mV.txt' using 2:1 linecolor 1 with lines"
}
# Create the graph.
[ -s dBm_mV.txt ] && dBm_gnp | gnuplot > dBm_mV.png 2>/dev/null
exit 0
The resultant graph is attached, below.
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I checked that table (formula: Power = V^2 / R) and it's correct to a very close approximation.
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Just purrfect. :)
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...b*cat's OP has sparked an interest, not to mention nostalgia, in such matters, and I've now got eBay searches bookmarked for 'Wandel & Goltermann', 'Wayne Kerr' and 'Hatfield Instruments'. This could be an expensive year. :o
Not too bad so far. :)
Had to bid up to £30 for this Hatfield Measuring Set 1017A (http://s132.photobucket.com/user/4candles/media/Kitz/MS1017A_zps32c18dbb.jpg.html). Works fine with two PP9 batteries (under the cover top left), but I'd like to source a Power Unit 91A if possible. It goes into the battery compartment, and is in fact about the size of two PP9s - just the kind of thing our resident dark ovine might happen upon in his travels. ;)
The real bargain, though, are the Wandel & Goltermann PS-20 and PMP-20 (http://s132.photobucket.com/user/4candles/media/Kitz/WampGbricks_zps6ce1690a.jpg.html) oscillator and level meter, known colloquially as 'bricks'. Very portable, and amazing build quality.
It seems the seller had found an aluminium case containing the 'bricks' (with manuals), a handheld RF oscillator, a capacitance meter, two sets of cased RF adaptors, and a couple of test leads. He'd no idea what they were, and the case had been in the loft for several years - indeed the PP3s in the bricks had an expiry date of 2000. No sign of corrosion though - presumably the case provided a good seal. Started at £9·99 and I was the only bidder. :)
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Nice looking equipment. :)
It may be a silly question but will you actually use it? :-\
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I acquired them mainly because of the large part they played in my working life, and their intrinsic historical and technical interest.
They will be utilised though, as such things were at home, in my days as one of Beattie's drones. Nothing serious, just footling around experimenting and testing various audio and telephonic devices.
One such project in the pipeline is investigating the differing ring tones encountered in certain conditions, and when calling some exchanges. It's been intriguing me and another ex-drone, especially as I believe, from my limited knowledge of digital switching, that the ring tone is always generated at one's own exchange.
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One such project in the pipeline is investigating the differing ring tones encountered in certain conditions, and when calling some exchanges. It's been intriguing me and another ex-drone, especially as I believe, from my limited knowledge of digital switching, that the ring tone is always generated at one's own exchange.
Yes, I believe I have read somewhere that the "ring" tone is generated locally. Now you have mentioned that project I can understand the need to measure the different tones!
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I've just noticed that there are two copies of the Hatfield Instruments Measuring Set No 1017A & 44C Operator & Service Manual (http://www.ebay.co.uk/itm/Hatfield-Instruments-Measuring-Set-No-1017A-44C-Operator-Service-Manual-/291143521691) available on eBay as "buy it now" items.
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Thanks b*cat. :)