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EV-638 low output?

Discussion in 'Microphones, Speakers & Audio Processing' started by K7BGV, Feb 13, 2020 at 10:29 PM.

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  1. K7BGV

    K7BGV Ham Member QRZ Page

    EV-638 (2).JPG I recently acquired an EV-638 HI Z mic to use it with my HW-101.Does anyone know what would be the approximate output,( mV level )out of this mic? I crank the mic volume all the way up and it’s kinnda weak.I’ve checked its impedance and it measures about 21.7K at 1 khz. 20.0 k at 100Hz and 28.9k at 10khz which seems to be correct...The mic looks lightly used and is wired correctly per HDP-21A manual.
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Its output rating (Hi-Z) is -57 dBv, which is a bit lower than many dynamic mikes but not terrible.
    EV 638.PNG
  3. K7BGV

    K7BGV Ham Member QRZ Page

    Thank you for posting the 638 data sheet...Indeed, -57db is low, I wonder if it has been degraded after so many years. I have to speak loudly and very close to the mic...I replaced the 6EA8 with the 6GH8A .. Interesting that the Heath selected this mic for the HW-101...
    Last edited: Feb 14, 2020 at 2:44 AM
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Check all the resistors around V1A to see if they're still the right value or have drifted.

    Heath used ordinary carbon composition resistors, especially R1, R2, R3 and R931 may have drifted far enough out of spec that the tube can't have proper gain. Also if C2 has drifted or leaked (it's an electrolytic, and they do that), stage gain will suffer.
  5. K7BGV

    K7BGV Ham Member QRZ Page

    I did not mention that I basically re-built this XCVR .I stripped it to the "bare bone" and replaced all the original resistors with quality ones (metal oxide), installed new capacitors ,except the ceramic caps, and completly wired it with teflon insulation wires;did the same to its HP23A PS and the SB-200...I also checked all the tubes and replaced the deffective or weak tubes. Checked all tube sockets ande cleaned the contact pins on switches. It works very well on all bands, per the equipment manual except this mic low output problem...I'll check the mic speech ampl,voltages though... I plugged in a Shure 450 mic and it works better but its output is 20dB higher (-37dB)...

    Attached Files:

  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The E-V mike was considered a really good microphone, as is the case with almost all E-V products.

    Why Heath selected that as an offering in their catalog...I really don't know. But it may be because the president of E-V was a ham (same guy who started Ten Tec -- Al Kahn, K4FW) and they cut a deal.
  7. W8AAZ

    W8AAZ Ham Member QRZ Page

    Have you tried any other mic with the rig to see as a comparison? In case it is something with the radio? I do not know of too many ways a dynamic mic gets weak, either for some reason the magnet got weak or there is out and out damage to the mic from abuse or misuse or something. I would expect a mic like that to last 100 years with proper care. They are usually well sealed and the plastic diaphram they used in that time period does not deteriorate. Maybe dirty connections or bad solder joints. I have seen out and out dead dynamic mics from the hair size wires coming out of the element to the solder pads breaking. Or probably if someone hooked to a rig with DC bias on the audio line for electret mics could pass a current thru the coil and overheat. But alot of those type of mics use a stepup transformer between the element and the output, too. Thus any DC might damage the transformer but not the element. I would rig another microphone up for it and see what goes.
  8. K7BGV

    K7BGV Ham Member QRZ Page

    I did. I hooked up a Shure 450 and I had the mic/cw level control set at 12:00 o'clock to keep a meter indication around S3 to S6 at peaks. When I use the EV mic the mic/cw level control is set at 3:00 o'clock and to barely get a S3 indication on the meter.
    I looked at the signal level at C13 (input into the balanced Mod with a scope and the signal looks a little lower than the Shure mic produces but it is still a good signal. Shure mic is 20db higher which explains the difference in amplitude but as I said the EV signal also looks good at that node.
    The EV mic is very clean, looking almost new...
  9. K7BGV

    K7BGV Ham Member QRZ Page

    Thank you for all the answers. It seems like there is some problem with V1 and the VOX circuit since the VOX relay "chatters" when EV-638 mic/cw control is set at 3:00 o'clock and it works normally when mic/cw control is set at 10:00 o'clock. There is about 300mv pk-pk /60hz noise present at R213/214 junction and 1nF ceramic capacitor was installed between that junction and ground. I could not find that capacitor on the schematic so it seems like it was just added there. I'll follow the HW-101 service bulletins and see if the problem is being corrected...


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