The channel element of our ham repeater club's 110 watt VHF Micor repeater oscillates constantly and can be heard within 25-50 feet of the station when the unit is in standby mode.
Some Motorola trained techs tell me this is a common occurrence and not worth troubleshooting, others say there's likely a bad transistor someplace in the exciter.
Comments?
Oscillating Channel Element On Micor 110 watt VHF Repeater
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railtrailbiker
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MICOR
Its normal for the MICOR repeater. You might check the shields to see if they are RF tight.
If not clean up the finger stock/chassis to decrease the siginal leaking out.
If not clean up the finger stock/chassis to decrease the siginal leaking out.
Dennis
It may depend on how the station is configured or reconfigured. Personally I prefer the xtal to be oscillating all the time since it will be stable and on frequency when the transmitter keys up instead of shifting (drifting) a couple of 100 Hz on frequency.
This doesn’t mean that it couldn’t be caused by some problem instead of being intentionally configured that way. It definitely isn’t a bad transistor in the exciter since the channel element ground is applied to the exciter and not generated there. If you don’t like it, you need to sit down with the “Control and Application” manual (partnumber 68P81025E60) and see why the F1 oscillator ground is active. Typically it comes from the Squelch Gate but other optional boards can also generate this signal.
Now if you can measure about 400 mW coming out of the exciter going to the PA input on the left side while idle, then you do have a problem since the final transistor in the exciter is not keyed when the station is idle.
Nand.
This doesn’t mean that it couldn’t be caused by some problem instead of being intentionally configured that way. It definitely isn’t a bad transistor in the exciter since the channel element ground is applied to the exciter and not generated there. If you don’t like it, you need to sit down with the “Control and Application” manual (partnumber 68P81025E60) and see why the F1 oscillator ground is active. Typically it comes from the Squelch Gate but other optional boards can also generate this signal.
Now if you can measure about 400 mW coming out of the exciter going to the PA input on the left side while idle, then you do have a problem since the final transistor in the exciter is not keyed when the station is idle.
Nand.
My two bits worth:
The channel element is supposed to be "keyed".
The channel element ground switch is Q8 on the Squelch Gate card.
(One of the following must be present to key the station: Squelch Gate, Guard Tone Decoder, DC transfer.)
Keyed A+ forward biases Q8 grounding the channel element.
Keyed A+ originates on the station control board Q18.
It seems that your station is being keyed by "keyed A-" which originates on Station Control Q11. Keyed A- is connected to the emmitter of the exciter output transistor.
Perhaps Q8 is shorted, or the F1 select (channel Element ground) is shorted, accidentally, or on purpose.
The channel element is supposed to be "keyed".
The channel element ground switch is Q8 on the Squelch Gate card.
(One of the following must be present to key the station: Squelch Gate, Guard Tone Decoder, DC transfer.)
Keyed A+ forward biases Q8 grounding the channel element.
Keyed A+ originates on the station control board Q18.
It seems that your station is being keyed by "keyed A-" which originates on Station Control Q11. Keyed A- is connected to the emmitter of the exciter output transistor.
Perhaps Q8 is shorted, or the F1 select (channel Element ground) is shorted, accidentally, or on purpose.
Aloha, Bernie
Bernie is right on, and for whatever reasons, I've found several Micor stations with the Control backplane bridge soldered or jumpered to keep the F1 channel element grounded all the time. Some look hacked, some look factory. Check the backside of the backplane for jumpers or solder bridges. You'll probably need the manual for the particular backplane in order to find what may not be obvious, and I know how rare those manuals can be.
curmudgeon.....and I like it.