If a service manual says inject a 1k sine -20dbm at point X, what is that? - Page 1
Forum section: electronics / repair

author: gooseEL34
author_rank: Regular Contributor
timestamp: January 22, 2015, 12:37:07 am
content: 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 -10dbmMost 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

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 22, 2015, 12:43:29 am
content: 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

author: kff
author_rank: Contributor
timestamp: January 22, 2015, 12:49:03 am
content: This table may be helpful:http://ifmaxp1.ifm.uni-hamburg.de/DBM.shtmlFor 50 ohm impedance and -10dbm, you need 70.7mV RMS or 100mV peak.

author: Po6ept
author_rank: Regular Contributor
timestamp: January 22, 2015, 12:53:00 am
content: 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

author: albert22
author_rank: Regular Contributor
timestamp: January 22, 2015, 11:22:25 pm
content: QuoteIf 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

author: EEVblog
author_rank: Unknown
timestamp: January 22, 2015, 11:27:43 pm
content: Quote from: gooseEL34 on January 22, 2015, 12:37:07 am

author: gooseEL34
author_rank: Regular Contributor
timestamp: January 24, 2015, 09:17:47 pm
content: 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)....ThanksTim

author: CZ101
author_rank: Contributor
timestamp: January 25, 2015, 06:15:52 am
content: Quote from: gooseEL34 on January 24, 2015, 09:17:47 pm-20 dbm in audio generally means - as terminated into a 600 ohm load.I've found this calculator handy for thishttp://www.random-science-tools.com/electronics/dBm-Watts-volts.htmI recently went through a similar quandary trying to calibrate an older mixing console.

author: gooseEL34
author_rank: Regular Contributor
timestamp: January 27, 2015, 09:57:31 pm
content: 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.pdfThe 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