If a service manual says inject a 1k sine -20dbm at point X, what is that?

[gooseEL34]

Ok.

I am somewhat knowledgeable about electronics.  I can read and trace a schematic, troubleshoot to an extent, and have a HP 206 sine generator, a Fluke 77 and a old Tek scope.

I have a digital delay meant for the guitar/audio world.
It was having an odd symptom in that the delay time (which is variable in 1ms increments with a up/down rocker switch), was behaving badly.  Specifically, it would not hold the specific delay time (for example, if I set the delay to 300 ms, it would float up to 310 and fall to 270 etc...

I suspected something with the power supply.  Sure enough, I opened it and found a few caps that had puked their guts.

I was reading the service notes, and after I replace the caps, I plan to set it up per factory specs.
Two of the tests require a 1k, -20dbm sine to be introduced to the input jack.  Another asks for a -10dbm

Most of the time, I have read amplifier schematics and they say "insert a 1k sine at .250v into the input"

What is -20dbm??  Thanks guys.
Tim

[T3sl4co1l]

The source impedance is going to matter based on what the circuit is doing, so, whether they mean, apply a wave of so-and-so and adjust amplitude until the voltage as measured at the injection point is -20dBm, or they just mean, set it for -20dBm and patch it in... I don't know.  But other than that...

dBm = dB relative to 1mW.  So, again, the source impedance matters as far as what voltage or current it presents... but for 50 ohm generators, 1mW == 0.223V (RMS).  So -20dBm is 1/10th the voltage or 22.3mV (RMS).

Tim

[kff]

This table may be helpful:

http://ifmaxp1.ifm.uni-hamburg.de/DBM.shtml

For 50 ohm impedance and -10dbm, you need 70.7mV RMS or 100mV peak.

[Po6ept]

Guitar equipment is usually high impedance, but different equipment can use different values.  A few "typical" values are 44K, 60K, and 220K ohms for active devices.  If you know the input impedance, you can use this calculator to determine the voltage.  http://www.coretechgroup.com/dBm_Calculator.php

[albert22]

Most of the time, I have read amplifier schematics and they say "insert a 1k sine at .250v into the input"

If I am not wrong aprox .250V rms corresponds to -10dBm into 600ohm.
AFAIK some audio measurements use 600ohms reference impedance.
It seems that you need .250vrms for -10dBm  and around .08Vrm for -20dBm

 

[EEVblog]

What is -20dbm??  Thanks guys.

[gooseEL34]

Ok..  Thanks for all the info.

I guess the problem is that I do not know how to determine the input impedance of this unit.
Would that be written on the schematic somewhere (which I do not have)....
Thanks
Tim

[CZ101]

Ok..  Thanks for all the info.

I guess the problem is that I do not know how to determine the input impedance of this unit.
Would that be written on the schematic somewhere (which I do not have)....
Thanks
Tim

-20 dbm in audio generally means - as terminated into a 600 ohm load.

I've found this calculator handy for this http://www.random-science-tools.com/electronics/dBm-Watts-volts.htm

I recently went through a similar quandary trying to calibrate an older mixing console.

[gooseEL34]

Well darn.... that cap did not in fact puke its guts, it was merely the goop used to keep the thing in place.  Plus it was a 47uF non-polarized electrolytic and not in the power supply.

The schematic is here: 

http://www.synfo.nl/servicemanuals/Roland/SDE-1000_SDE-3000_SERVICE_NOTES.pdf

The symptom is this:  When the delay time is set (using a rocker switch on the front panel), it will hold at that setting but then start fading (delay time decreasing). 

I cannot see any burned or broken or otherwise damaged parts from a visual inspection.   

Anybody have any thoughts?   My gut would say that there is a bad DC supply somewhere, but I don't really know where to start (I mainly work on hand wired tube amps that have 1/100th of the components of this thing (and no ICs nor transistors).

Thanks