Fixing an RF generator.

Discussion in 'Technical Repair' started by mbohuntr, Aug 11, 2018.

  1. mbohuntr

    Thread Starter Senior Member

    Apr 6, 2009
    430
    32
    Afternoon all,
    I am refurbishing an EICO 330 RF generator for my bench, I ordered it from EBAY. I had the expected dirty rotary switches, and stuck pots. What I found was I think this was a kit, or somebody has "Repaired" it.

    The board is fine, caps all appear good, no leaks or bulges, and after I cleaned the switches, and freed up the pot, I got all 5 bands producing a signal. the generator has 5 ranges from kHz to 54 Mhz with a 400 Hz modulated wave built in the circuitry

    My issue is the 16MHz -54MHz band. The varicap and vatiable inductor have been gutted, and it has an Air coil with no cap at all??? At 16MHz, there is virtually no output, it peaks at around 30MHz, and puts out way under the 300millivolts it is supposed to on high. At 54 MHz, it is around half of the peak of 30MHz. somewhere around 20mV P-P.

    If I wish to replace the inductor, and Varicap, I do not know the value of the inductor. The cap is 1-8pF, but no value for the inductor. I'm guessing around .2uH, but don't know for sure and what Q should I go for, low or high? Should I assume the resonance to 30MHz, and with a low Q get a wide band that attenuated at either and?

    The schematic is enclosed, somebody changed it..The L5 inductor is now a hand wound coil, tapped at 2 turns, and the varicaps do not follow the schematic, the one that goes to L5 is gutted, no innards, and disconnected. The other outputs are strong and accurate to my Hantec scope, I put a 10 Meg pulldown resistor in the C band across the inductor to get it oscillating, it works much better now. I found similar slugged inductors, on line, I just do not know the proper value. Any ideas?

    http://www.nostalgickitscentral.com/eico/schematics/eico_schematic_330.pdf
     
    Last edited: Aug 14, 2018
  2. sghioto

    Well-Known Member

    Dec 31, 2017
    1,039
    157
    Are there any values marked on the other inductors? Can you see the windings and tell how many turns there are? If so you may be able to calculate the value based on that.
    SG
     
  3. mbohuntr

    Thread Starter Senior Member

    Apr 6, 2009
    430
    32
    No values, only a part number that does not come up on a search...
    Eico cat . Part number 36085...
     
  4. sghioto

    Well-Known Member

    Dec 31, 2017
    1,039
    157
    Maybe you can find another one on Ebay and use it for parts or vice versa.
    SG
     
  5. bertus

    Administrator

    Apr 5, 2008
    19,521
    3,977
    Hello,

    Here is the complete manual, wich includes some calibration tips.

    Bertus
     
  6. mbohuntr

    Thread Starter Senior Member

    Apr 6, 2009
    430
    32
    Thanks! It says to tune the inductor and cap to zero beat, but someone gutted both. There is no cap connected other than the main tuner, and the inductor has no core, and looks like a 16 ga. air core..
    I was thinking that if I took the mid point of the range... say 35 MHz, and found the resonant frequency capacitance at the mid point of the tuner. (295pF) so 150 pF, I should be able to reverse engineer the inductor value.. Or am I completely off the mark?
    SG... Heathkit SG6 has a similar coil set up, and frequency range value, but no values.... *surprised*
     
  7. mbohuntr

    Thread Starter Senior Member

    Apr 6, 2009
    430
    32
    Thread bump... I tried various combinations in a colpitts oscillator on my bench, the best I was able to come up with is 2) 68pf caps and an 1uF inductor giving me around 6-7 Mhz... When I put a 300pf air tuning cap in place of the lower cap, you only get around 20Khz of adjustment before it drops out. I have an acute lack of knowledge here..... I cannot get anywhere near the 16-54 Mhz adjustment. needed..

    Update, after doing some more circuit tracing, It appears the band uses C9, and C3 in parallel to adjust the frequency, and the inductance range is .18uH - .29uH I reverse calculated it using a resonant frequency calculator.. Does anyone know if this is correct? Right now it uses a hand made air coil, and no 1-8pF cap.
     
    Last edited: Aug 14, 2018
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