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  • #15336
    AvatarGriff
    Participant

      Hi Ted!

      After a long wait and missing out on quite a few auctions, I’m finally the proud owner of a VC7 :D It’s going to look and sound great alongside my VC3, VC5 and P8. 8-)

      Sadly, it appears to have taken a bit of a tumble on the way to me, the right hand rack bracket is bent. A combination of ParcelForce and inadequate packaging. 😡

      I was hoping it would be purely cosmetic, but after a quick test is looks like there may be some more to this : Channel 1, all good! Can’t wait to have a play with those impedance settings.

      Channel 2 however, not so good: There’s a noticeable 50Hz hum, not only that, but it also appears to be microphonic. And by that I mean, if I tap the chassis or the even the table it’s sat on, I can hear it loud and clear. What do you think may be wrong? Input transformer perhaps? Seems to be the only thing unique to each channel that an impact could effect.

      Are you still able to source this transformers? Or would it be a case of replacing them both so the channels match?

      Your advise appreciated, as always Ted.

      Cheers

      Griff.

      #15987
      Ted FletcherTed Fletcher
      Keymaster

        These mic amps are usually microphonic to some extent! But if it’s really noticeable, then there is something wrong. More worrying is the 50Hz hum.

        I’m afraid I would have to have a look at it here (in West Sussex) before I could comment further. I do have a couple of spare transformers of the correct type, but most likely the problem is elsewhere. :(

        #15988
        AvatarGriff
        Participant

          Thanks for the swift response Ted!

          I took some brave pills and popped the lid off this evening. (The catches on those Switch Craft plugs had be vexed for a few minutes! But I worked it out eventually).

          I had a tap around the board with an eraser pencil, and for sure, there’s microphonics on both channels like you say. But CH2 is much louder.

          I’m fairly confident the microphonics are from the transformers, it’s loudest if I tap on them or the immediate vicinity.

          But like you say, I can live with microphonics to a certain extent, it’s the 50 Hz hum which is the real concern.

          I did notice that the AC for both lamps runs right long the top of CH2 input circuit and is tied to the various wires coming from that area, so I wonder if CH2 is simply amplifying AC from there. I guess a few minutes with a soldering iron could test that theory.

          Finally, the only thing visually amiss are the four output caps, they are all slightly domed, so it could be they need replacing. But conversely, the input caps look just fine.

          I did try the sum and difference outputs too. I can hear the hum on both, although it’s far quieter. Given all the output caps look suspect, it possibly proves nothing.

          At least channel 1 is good, so I can crack on with the vocal session booked for this weekend :D

          P.S. This reminds me of another similar issue on another bit of kit….I’ll start a new thread :)

          #15989
          Ted FletcherTed Fletcher
          Keymaster

            Further thoughts…..

            You have actually brought up a very interesting question….. I bet there are very few engineers who have had experience of ‘magged up’ microphone transformers!

            High quality transformers are made with laminations of different materials; partially soft iron and partially ‘Mu metal’. This material magnetises easily and retains its magnetism. Plugging in a microphone with the phantom power turned on can momentarily cause a current surge in the transformer which magnetises the core…. the transformer then operates like a microphone! Usually this is not a very serious problem as the microphone itself damps out the microphony, but unplug the mic and it becomes very obvious!

            I suggest, ignore it! de-magging the transformer is not an easy process and apart from ‘ringing’ when there is no mic plugged in, it should not be a problem.

            On the other problems, I would have to look at it to comment. :)

            #15990
            AvatarGriff
            Participant

              Thanks Ted, I’ll give it CH2 go in my next session and see how I get on!

              Another question: I don’t suppose you have any user manual for the VC7? I think I’ve figured most of it out, but it’s always nice to read what you had to say about the design and function.

              #15991
              Ted FletcherTed Fletcher
              Keymaster

                I have managed to unearth a copy of the original handbook…. with only one page missing!

                Drop me an email on ted@tfpro.com and I will email you a copy. :D

                #15992
                AvatarGriff
                Participant

                  Hi Ted,

                  Sorry for the late response, and thanks for the offer of the manual, I will drop you an email after this post!

                  I just wanted to feed back on my recent findings with hum on Ch2. I had observed before that the AC wire for the bulbs were twist-tied in place, so that they were laid right across Ch2 circuit (which looks tidy, for sure, but maybe not a great idea technically). As I had some crackling pots and was in there anyway, I decided to re-route the wires. There was enough slack to raise the black bulb wiring so that it was well clear of the circuit.

                  Well, guess what? The hum has almost entirely gone. There’s still a tiny amount more than Ch1, I suspect that is either due to the proximity of the mains transformer or the already discussed problem with input tranny 2 going microphonic.

                  I guess there was probably a change in wire routing between your final design, and how that was interpreted in production. 🙄

                  So I hope this helps anyone else with a VC7 and who notices a hum on Ch2!

                  And by the way – I used DeOxIt spray for the pots. Amazing stuff!!

                  EDIT : added a couple of pics to show the routing of the bulb lead.

                  VC7 1

                  VC7 2

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                  #15993
                  AvatarGriff
                  Participant

                    On another note : I haven’t been able to use the “FetHead” with the VC7. It worked great on the VC3, gave me a clean extra 15dB, much needed with the RE20 mic. On the VC7 however, it totally changes the tone depending on the impedence setting. None of them sound any good, so I’ve been using just the VC7’s own gain. I’m up around 55 which does raise the noise floor a bit.

                    I noticed use of the TL074 and MC33078. I’m sure much has changed in IC op-amp design since the VC7 was designed (as well as pricing and availability). I imagine there’s some value in upgrading these. Perhaps with less nose and/or more gain?

                    Is that feasible? Maybe wishfull thinking.

                    Or else , just any recommendations based on what you may have learned lately while working on the new voice channel? (Sorry, I do like to tinker :D )

                    #15994
                    Ted FletcherTed Fletcher
                    Keymaster

                      You are absolutely right about the wiring causing a problem….. production people don’t bother themselves with such unimportant things!!

                      You also brought up an interesting question about op-amps…. It’s interesting because contrary to what you might expect, there has not been any improvement in analogue op-amp design for years, and yes, I have been trying all sorts of types for the new mic amp and guess what…. I have come back to the MC33078. It may be about half a dB noisier than the LM833N or the LM4562, but it is readily available, reliable and stable so I like it! And with a good mic amp, the noise you are hearing is predominantly resistive noise from the elements of the mic itself. In the new mic amp, the electronic noise from the mic amp is actually at least 8dB below the noise of any mic I try. The big advances have happened in measuring techniques…. I can now measure noise and distortion far better than in the ‘old days’!

                      And ribbon mics can be a pain! I must look up the ‘FetHead’; that’s something I have not met. :)

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