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Thread: Dragon 485 low recovered FM audio

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    Default Dragon 485 low recovered FM audio

    Hi there, I've had a dragon 485H for a while now and whats is really annoying me is that the recovered FM audio is very low.
    While using the FM mode for most situations I have to have the volume control turned almost full to get enough audio while mobile, ok I havn't been using an external speaker which would probably help, but I find the rectified audio from AM and SSB to be quite adequate so I assume that an external speaker shouldn't be nessasary.

    Tonight, I have just peaked the coil T3 (I assume this is the FM discriminator adjustment) which is accross pins 5 & 6 of the fm detector/IF chip which is a BA403, it has inceased the recovered audio slightly.

    Does anyone know a method of increasing the recovered audio anymore, without damaging the radio???

    After looking at a techincal document regarding the BA403, pin 7 is the output, which has quite a large resistor, 1.5 meg in series linking it to the next stage of the circuit, I'm not quite sure reducing this resistor is a good idea at this point.

    I have found a service bulletin which discusses mod'ing the frequency response in FM mode from being too high and making it deeper or vice versa, it's a bit hard to interperate because the bulletin was in german only.
    Thanks in advance



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    Junior Member bit_pit's Avatar
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    Iv'e got a 485 , but gotta admit I have never heard anyone on FM to see how it sounded.
    Do you have a cct diagram? I have one here somewhere so I'll see if I can find it.

    I guess a quick way to test would be to tack a variable resistor across the 1.5M one and see what happens as you adjust...
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    Quote Originally Posted by bit_pit View Post
    .
    Do you have a cct diagram? I have one here somewhere so I'll see if I can find it.
    ...


    Hi there, I don't actualy have a cct diagram for the "H" model, I have only found one for the "S" model which is a little different, there is also a "PLUS" model which looks like it has a different display, infact it looks like a very nice display...
    My display is the one that only has the 5 segments even though it looks like it has more....

    For some one in the know, If I wanted to add an external analogue meter to this radio to either replace or run in parallel with the origonal meter, would I need to add a buffer amp, or could I just tap directly from the input to the CPU/display driver to driver the meter?
    Thanks
    Last edited by toyboy2131; 08-04-08 at 08:03 AM.

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    It is possible that the discriminator is too broad in frequency response causing some of the low recovered audio issues. Signals with low deviation will sound low level, so narrowing the discriminator, (if that is what the mod you a referring to actually does) may have the effect of increasing the recovered audio. The only problem is if you then receive signals that have wider deviation than your modded discriminator they will sound clipped and distorted.
    Common sense isn't !!!

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    Quote Originally Posted by eastlake View Post
    It is possible that the discriminator is too broad in frequency response causing some of the low recovered audio issues. Signals with low deviation will sound low level, so narrowing the discriminator, (if that is what the mod you a referring to actually does) may have the effect of increasing the recovered audio. The only problem is if you then receive signals that have wider deviation than your modded discriminator they will sound clipped and distorted.
    Hi there, I don't think that the mod actualy narrows the discriminator, it only talks about moding the frequency response, I suppose what it is doing is flatening the curve, so that the high end isn't quite so high and brings up the bass some what, atleast that is what i think it's on about...

    this text is copyed from the actual service bulletin from Alan Electronics.

    "Problem: Too low FM modulation frequency response at low (bass)
    frequencies: If the FM modulation should sound too high , you can equalize the
    audio frequency response curve by increasing two capacitor values. C 702 (680 pF)
    to 820 pF and C 709 to 0.47-1F on the additional FM board"

    The only problem is that this information is for the "S" version of the 485, the "S" version has a small daughter board for FM, mine is all located on one board, and as mentioned before I don't have a diagram for the "H" version
    hope that helps to clear things up a little.

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    As an after thought, How would you "narrow" the discriminator?
    I assume this involves replacing certain Capacitors & or resistors? with different values.

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    take a photo and post it !.
    most radios in australia are now 12.5khz you will need to change the 455khz filter.
    TRUTH ALWAYS wins over LIES and religion is one big LIE!

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    Junior Member bit_pit's Avatar
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    I finally found the schematic, but it is likely the same as yours, marked Albrecht (which is why I couldn't find it) 485 S High Power version.

    I used it to fix a squelch problem on mine a year or two ago, seemed to be the right radio, but I was only looking at that section of the radio so it could be a variation...
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