Modulation Transformer Talk Back.
What sounds like modulation talk back may be the
overload relay RY-5 vibrating at your audio
rate.
To cure this problem wire in a 100 to 200 MFD 20V or more in parallel with the relay coil. This will bypass the audio fluctuations in the B+ line to ground instead of going through the relay coil and causing the relay to “talk back”.
BC-610 modulation Transformer Talk Back ... more
The BC-610 does seem to love the high freq audio component and this will cause the mod transformer to “sing” along with your speech.
Here is something to try…these values are calculated for the Bc-610E transformer.
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Bias regulation of the 100TH's .......
An easy way to regulate the 100TH bias is to use three 12V 5-Watt Zener diodes connected from center tap of T8 to ground. This will give –36V of regulated bias and a resting current of about 45 ma. Increase the Modulator Bias control on the front panel to 40V to 45V to make sure the Zeners are drawing some current and regulating the voltage. Check this voltage before connecting the Zeners also check your bias voltage required before wiring in the Zeners (my bias voltage was –36V for 45ma. Resting current)
If you operate at 2500V in AM you will need four 12V 5-watt Zener diodes (for –48V bias and resting current of 35 ma.). Connect a switch (spst) across one diode. This will permit you to short out one diode when in the 2000V position for –36V.Use 4 diodes in the 2500V position (-48V). When in the 2500V position and operating AM, increase the modulator bias front panel control to about -55V(check voltage with Zeners out of circuit) this will allow the Zeners to maintin the -48V bias.
If you DO operate CW...
If you do operate CW you must add another switch to remove Zeners from bias circuit because R34 needs to bias the 100 TH's to complete cutoff. Another option for CW is to wire the Zener diodes in before the relay RY3 or at the R12 slider.
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