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.790 MHz check (TP10) continues to widely oscillate

igmo

igmo

Member
Feb 2, 2013
20
7
13
Leesburg, Va
Going through alignment procedures beginning at synthesizer on a 148GTL (5-pin side mic) from Sams Photofact. All good so far until I check for .790mhz at TP10. Reading is all over the place. Going to voltage checks next but perhaps someone has a tip.
 

Never have learned to fix PLL circuits without a 'scope. If the signal at TP10 is too low, the PLL chip has a hard time locking onto the selected channel frequency. Might be what's going on here, might not.

I find that L21 is frequently adjusted way off from its proper peak setting. Since it won't affect either the S-meter or the wattmeter in a direct way when you turn it, a tweaker may leave that one set in some random position after "trying" it to see if it boosted a meter reading.

A 'scope at TP10 is the best way to get the slug in L21 set at its proper peak. Lacking that, a RF Voltmeter would also tell you this.

A DC/AC multi meter won't.

We check this first whenever we see this kind of PLL misbehaving.

73
 
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nomadradio said:
Never have learned to fix PLL circuits without a 'scope. If the signal at TP10 is too low, the PLL chip has a hard time locking onto the selected channel frequency. Might be what's going on here, might not.

I find that L21 is frequently adjusted way off from its proper peak setting. Since it won't affect either the S-meter or the wattmeter in a direct way when you turn it, a tweaker may leave that one set in some random position after "trying" it to see if it boosted a meter reading.

A 'scope at TP10 is the best way to get the slug in L21 set at its proper peak. Lacking that, a RF Voltmeter would also tell you this.

A DC/AC multi meter won't.

We check this first whenever we see this kind of PLL misbehaving.

73
Click to expand...
Agreed and thank you! Incidentally, I found the issue…a faulty connector on my freq counter. Driving me nuts until it finally broke away.
 

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