Using a meter for calibration .
 

Ok as some of you will be aware in the last epic fail of mine . We discussed using a meter as apposed to a scope for setting these decks up . So i got one . I got a Seimens and its supposedly deadly accurate and in spec. Now I have a couple of questions if you wouldn`t mind clarifying them for me .

So . It says in the manual that the reference for setting up the Revox B77 is -20dB . So that means that I set my meter to the -20dB scale yes ?

Which of the resistance buttons am I to use , 600 , 10 or 100k ? My sig gen can output either 600 ohms or 50k I believe

I`m using the 0.3 - 3.4khz range (for 1khz test signal)

Thanks for any other advice you can help me with with it . Its a massive piece of kit but seems to function as it should . £50 I paid for it .

Re: Using a meter for calibration .
 

those are not pics of the settings i`m using by the way , just the choices available

Re: Using a meter for calibration .
 

Ah, you have a psophometer!

Affectionately known up the production line of another firm which made such instruments as a p*ss-off meter!

The connectors, a trio of 4mm holes for banana plug type pins in a common housing are known as 'Siemens 3-pole'

Psophometric weighting is a shaped frequency response designed to model the human's perceived dislike of noise. It roughly follows the characteristic of the human ear. It will have been made for use by a telephone system operator.

For your use, it's a rather nice audio millivoltmeter. You won't want the noise weighting filters, so stick to the three unweighted ranges. 15Hz to 30kHz looks good

You also have the choice of reading peak or RMS voltage. I suspect that's a true RMS converter doing the RMS option for you, not a crude re-scaling of peak.... Noise measuring instruments really do have to have proper true RMS to make sense.

600 Ohm input is to make telephone systems happy. It's their native impedance.

10k and 100k Ohms are intended for bridging onto circuits which are already loaded. Use whichever of these you feel like. To decide which is appropriate you have to know what the source impedance is of whatever you're testing.

The 30Hz-15kHz range would have been used for testing land line circuits for broadcast links from say studios to transmitters. The 300Hz to 3.4kHz range is for plain old telephone circuits.

Your only problem now is the balanced input....

David

Re: Using a meter for calibration .
 

Thanks David. Thats brill . I tried in vein to find a manual for it , but your help will be brill .

Am i right in terms of using the -20dB range for this B77 ?

David

Re: Using a meter for calibration .
 

Ahh nevermind I figured it out .

I`m using the following settings and its giving me the correct results . Magic !

Thanks David !

Re: Using a meter for calibration .
 

Yes, that's showing a measurement of 0dB with the range selector set to 1v (0db), for -20dB you can set the gen output to register at the -20dB mark on the scale using the same voltage range setting of 1volt or you can switch the voltage range down to 0.1v (-20dB) and adjust the gen output for 0db on the scale.

Lawrence.

Re: Using a meter for calibration .
 

Thanks Lawrence . I feel like I just got born ,. this is so amazing , I should have got one of these a long time ago . sailing though the set up now .

sorry just read that properly ...its asking for 77.5nV @ 10khz .....??

Re: Using a meter for calibration .
 

That sounds about right, 77.5mV = -20dB, divide the voltage scale by 10 if setting -20dB on the 1v range, or divide the voltage scale by 100 if on the 0.1v range.

Lawrence.

Re: Using a meter for calibration .
 

thats got to be a typo hasnt it ? Thats like -130dbm or something isnt it ?


yeah its asking for nV ....not my typo ...lol ....

has to be theirs ..

Re: Using a meter for calibration .
 

here ...lol

Re: Using a meter for calibration .
 

That's definitely a typo! It says 77.5mV in the French translation to the right...

Re: Using a meter for calibration .
 

good spot ...haha ...i didnt look there I cant read french numbers ...hahaha :-)

Re: Using a meter for calibration .
 

Having an issue , this has all gone swimmingly so far and i got to sat I do far prefer using the meter . Now I have the hang of the buttons etc , its a breeze , however ....

I`m now onto the bias ....I have the output from the sig gen putting -20db onto the tape . When I turn the trim pots for fast / slow bias adjustment both left and right , fast and slow . the needle doesn`t go up to a peak it just goes DOWN , The meter is adjusted as per Lawrence suggested . -20db range , needle on the `0` for the -20db level . I also looked at it with my scope and it does the same thing so its a fault somewhere .

I turn all the pots anti clockwise , then start to turn and the needs just drops slowly not increases . The post have been changed to piher ones , im wondering if someone has put the wrong ones in maybe ?

Anyone know whats going on here and what might be causing it ?
Thanks

Re: Using a meter for calibration .
 

All done ....another manual fail ...it doesnt tell you to switch to 1khz .....sigh

Re: Using a meter for calibration .
 

The most sensitive range has 30mV full scale on the meter, so 77nanovolts isn't going to be noticeable.

THe 0dB reference is 775mV RMS which is 1mW into 600 Ohms, and is the usual reference level for telephone circuits. 1mW into a phone handset earpiece is a good comfortably audible signal.

The bottom left set of buttons handle balanced /unbalanced inputs, you have it set OK for unbalances, with signal on connector pin B


David

Re: Using a meter for calibration .
 

If you like that unit, have a look at the HP8903A or B version.
Very low distortion audio generator from a few hertz to 100kHz.
DC voltmeter
AC voltmeter (microvolts to volts) up to 100kHz True RMS for noise
Total harmonic distortion (automatic, self-tuning)
SINAD (signal to noise and distortion) measurement for setting up comm radios
Signal to noise ratio (in dB if you want)

All programmable and can be controlled remotely. Oh, and the right option has psophometric filters if you switch them in.

Bit dearer than psophometers on the second hand market, but not out of reach... cheaper than a good Revox, I suppose.

David

Re: Using a meter for calibration .
 

Thanks , this one really is great now I`ve got the hang of it . Took me a minute to understand whats what of course but its a darn site easier than using the scope . I still used the scope for the bias traps , it seemed the sensible thing to do as it would give me a chance to also look for noise etc , but the meter is just brilliant , for £50 I really can`t complain I dont think .

I sorted the above problem by the way , the manual doesnt tell you to switch to 1khz . so I founds the peak in the end , I`ve just got to sort the record levels now they are really low for some reason . especially on one side so I`ll check the transistors and I haven`t recapped that board either .

So far the meter is just great though . Thanks David


David

Re: Using a meter for calibration .
 

Yes a meter is sometimes easier to use than a scope, but the scope shows you a lot more and gives you better clues so you can smell a rat when something is wrong. It's a matter of learning to interpret what a scope gives you. You'll have noticed that the repair shop type people reach for a multimeter first, the designers and heavy technical types reach for a scope and the RF people go for a spectrum or network analyser. Don't give up on the scope, it's still worth learning how to really make it work for you.

David

Re: Using a meter for calibration .
 

Ok just a little more help if you could please .


When I have the meter set for -20dB / 100mV range .

how does this effect the divisions ? are they still 1dB each or ???

Sorry if its a dumb question , just need to get my head around it .

Re: Using a meter for calibration .
 

Yes 1dB each so -21db, -22dB, -23dB etc, the -20dB mark on the scale is -20dB below the -20dB ref point which on the 0.1v range is 0 on the dB scale which means that on that voltage range the -20dB mark on the scale is -40dB below the 0dB 0.775v reference on the 1v range, remember that dB's are just added or subtracted.

Lawrence.