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AKG WMS40 guitar UHF wireless systen (not mini, the older one).

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  • AKG WMS40 guitar UHF wireless systen (not mini, the older one).

    Have my old wireless unit. Transmitter and Rvcr kit. Transmitter is PT40, and Rcvr SR40.

    Had not used it in a few years since I was primarily just cable. I grabbed the wireless unit and fired it up.
    It works but has an annoying and audible hiss.

    I tried resoldering, reflowing the Xm and RCv but it is still has an audible hiss with any guitar being played.

    Mostly when you ring a note and it dampens. As it dampens you hear this hiss. I don't recall the unit doing that.
    New batteries of course and PSU is also fine and has been substituted. Tried various locations. Same thing.

    Ideas?

  • #2
    Here is the SM.
    Attached Files

    Comment


    • #3
      Working on RF systems requires different test gear to set up or diagnose properly than used for audio systems. The first thing to suspect after sitting for a long time is the crystal oscillators in each have drifted off frequency, which noise would be the first symptom. It uses a convention local oscillator frequency modulated by the processed audio coming from the mic preamp section that has two important differences between a normal mic preamp and the transmitter preamp. The mic signal has a hp filter then is compressed then high frequency pre-emphasis is added before the signal is used to vary the carrier frequency of the crystal oscillator a small amount with variations in audio amplitude and at a rate equal to the frequency of the audio. The transmitter then send the signal through a series of multiplier stages that increase the frequency of the RF signal up to some frequency in the 710-860Mhz range. Any noise in the audio chain will be multiplied in level also. If the transmitter or receiver's crystal oscillators drifted off their original channel frequency, distortion and noise will increase until it drifts off to the point where intelligble audio is no longer recovered.
      Is it distorts on sound peaks? If yes, it probably the system needs to be put back on the same exact frequency. If the noise level is high but audio is not too distorted but the noise varies with voice level, it is probably on frequency but the compressor in the transmitter and the expander in the receiver are probably not synchronized. To get good signal to noise ratios the transmit audio is compressed in dynamic range and decompressed in the receiver. If they are compressing and decompressing by different amounts from each other, hiss would be expected at higher than normal levels. For the compressors to be working properly, the transmitter mic gain, transmitter modulation deviation, and receiver demodulator need to be seeing the signal level. They won't be if one of them is off frequency. You can also get more hiss if the receiver sensitivity or the transmitter power are too low.
      With the right test gear like a communications Service Monitor like my Motorola R-2008, it would take 10 seconds to test all that and put it exactly on the right frequency and deviation and audio level. But you are working someone in the dark without RF test gear.. One crude test would be in an open room, how far can you get away with the transmitter before the signal deteriorates or cuts out?
      Transmitter/receiver carrier frequency drift is the most common cause for the problem you are describing.

      Comment


      • #4
        Range is still pretty good 110' or so.. It's issue is more with as notes are played (regardless of distance from Xm Rcv) they seem clean until they start to dampen or played softly, then there is an audible hiss as the note decays or if softer notes are played there is hf noise or fuzzy hiss the coincides with the notes.

        I read on another forum these units are not serviceable, and are supposedly in a range of no mans land. I think the last part was confusion of UHF vs VHF units. IIRC VHF units were the ones with other electronic device issues and phased out of music retail. This one is UHF and they still sell UHF units.

        I don't have the type of equip needed to test deviation, etc. So, there's no way to really test the crystal osc? I wonder if I could just shotgun a pr to try if they can be found. I'd rather not have to trash this system, but that may be the end result. Thanks

        Comment


        • #5
          Originally posted by guitardad View Post
          I read on another forum these units are not serviceable, and are supposedly in a range of no mans land. I think the last part was confusion of UHF vs VHF units. IIRC VHF units were the ones with other electronic device issues and phased out of music retail. This one is UHF and they still sell UHF units.
          There is a range of UHF frequencies that were banned for wireless use. The 700mHz band (698 to 806mHz). If your unit is in that frequency range, no one is allowed to repair it. They are illegal to use or sell.

          https://www.audiolinks.com/articles/fccwhitespaces/
          Originally posted by Enzo
          I have a sign in my shop that says, "Never think up reasons not to check something."


          Comment


          • #6
            Originally posted by g-one View Post
            There is a range of UHF frequencies that were banned for wireless use. The 700mHz band (698 to 806mHz). If your unit is in that frequency range, no one is allowed to repair it. They are illegal to use or sell.

            https://www.audiolinks.com/articles/fccwhitespaces/
            734.6 Mhz

            looks like I will be buying new.

            Thanks, so that might might explain the hiss, fizz, and popping noises on occasion. Like I said it had been years since I used it last.

            otoh, what about changing Xtals in Rvc Xm?
            Last edited by guitardad; 01-26-2015, 05:54 PM.

            Comment


            • #7
              Depends were you are since the band covered is only under the jurisdiction of the US FCC and those are still sold in other parts of the world. Sell them on eBay to other countries.

              You can change the crystals but you would also have to change all the tuned circuits in the front end of the receiver and the whole transmitter multiplier chain and the output stage tank circuit. Not worth the time and effort.

              You have a good test already that they do not need RF alignment, they have range and do not have distortion on the signal which combined tells me they are on frequency. If they sound good with signal but on the signal decay the hiss comes in it means the compressor in the transmitter and expander in the receiver are not tracking. That can be as simple as a level adjustment in the receiver or transmitter preamp. They should be pretty easy to repair since they use standard parts. Newer models have a number of parts like masked programmed processors that are not generally available. So few shops in the audio field have both the gear and RF knowledge that the manufacturers do not want any one fix them but the factory. In reality they are simple devices and a shop set up for it could do well repairing them if they had access to the cosmetic and case parts that break most often.

              Comment


              • #8
                Originally posted by guitardad View Post
                734.6 Mhz

                looks like I will be buying new.

                Thanks, so that might might explain the hiss, fizz, and popping noises on occasion. Like I said it had been years since I used it last.
                As Stan said, your transmitter & receiver may have drifted over time in storage. And/or your receiver may be picking up noises from other commo gear that's now in use for fire, ambulance, police, military. So far nobody's come knocking on your door looking for the sources of guitar-playing that have been showing up on their currently-assigned commo frequency, thank goodness for that. Dodgy sounding guitar signal plus unexpected hiss is possibly the result of the old "two-transmitters-on" problem. Yours and - who knows who has the other one. Don't find out the hard way, sell that old rig as Stan said, overseas where it's still legal to use those frequencies.
                This isn't the future I signed up for.

                Comment


                • #9
                  The problem is that that band, 698-746 Mhz has been sold. What used to be public resources(the radio spectrum) has been converted to a commercial asset and was auctioned off to some corporation which "owns" that band. It is really strange how something that occurs as part of nature, like a musicial note, can be removed from public ownership and sold for exclusive use by a company. Anyway, the 698-746 Mhz band was auctioned off with few bidders, probably an inside job. I would not doubt that some corporation will be allowed to buy the rights to 440hz A and remove it from public domain for exclusive use by some company.

                  Anyway, you know it is on frequency since there is not distortion and the range is good. Expander time constant is determined by signal level so it really sounds like compander mistracking. The gain should reduce in the receiver as the envelope decreases, just as the compressor in the transmitter does the opposite, reducing gain as the signal envelope increases. The hiss is coming from excess gain in absence of signal.

                  Comment


                  • #10
                    Originally posted by km6xz View Post
                    The problem is that that band, 698-746 Mhz has been sold. What used to be public resources(the radio spectrum) has been converted to a commercial asset and was auctioned off to some corporation which "owns" that band. It is really strange how something that occurs as part of nature, like a musicial note, can be removed from public ownership and sold for exclusive use by a company. Anyway, the 698-746 Mhz band was auctioned off with few bidders, probably an inside job. I would not doubt that some corporation will be allowed to buy the rights to 440hz A and remove it from public domain for exclusive use by some company.

                    Anyway, you know it is on frequency since there is not distortion and the range is good. Expander time constant is determined by signal level so it really sounds like compander mistracking. The gain should reduce in the receiver as the envelope decreases, just as the compressor in the transmitter does the opposite, reducing gain as the signal envelope increases. The hiss is coming from excess gain in absence of signal.
                    Yeah that was something I noticed with many wireless units. Dynamic vol playing tended to create hiss. Some worse than others. I even get some of that with the L6 relay sys which supposedly does not use compander circuits. The unit hisses though regardless of vol setting on the guitar, and sort of accompanies the sound produced. Doesn't seem like interference. Not near any public service xmitters either.

                    So what can I do with the other unit to align the compander.

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