Date: Sat, 24 Oct 2009 09:35:03 +1300
From: Bruce Griffiths <[email protected]>
Subject: Re: [time-nuts] LPRO101 Lamp Exciter Frequency
To: Discussion of precise time and frequency measurement
<[email protected]>
Message-ID: <[email protected]>
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Roberto Barrios wrote:
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> Hi all,
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>
>
> I've got an LPRO101 that refuses to lock and you sure will be of great help.
> These devices are quite cheap but I'm trying to learn in the repair process.
>
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> I've followed PE1FBO's repair guide and everything noted there seems ok. I
> could not find a single suspect component. These are some notes I've taken on
> the unit after a 20 minutes warmup:
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>
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> - Power input current during warmup is 1.2A and 0.4A after it.
>
> - 10Mhz out swings between 10.000191 and 9.999875, taking 40s to go up and
> 60s to go down in freq.
>
> - Lamp voltage is a steady 6.7V.
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> - The lamp glows a few seconds after powering the unit.
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>
>
> Placing a pickup look over the PCB, the analyzer shows peaks all over the
> place up to 2.5Ghz (it's limit), so the thing is alive.
>
>
>
> There is one unexpected thing I found... The frequency of the RF power going
> into the lamp is 157.3Mhz, very stable. From the repair guide, it should be
> 70Mhz. I checked it with everything on hand (scope, counter, spec. analyzer)
> and there is no doubt about it. A clean sine of about 16V peak to peak, at
> 157.3Mhz can be found at the output (source) of the BF160 MOSFET.
>
>
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> Could this unexpectedly high exciter frequency cause the inability to lock or
> should I look somewhere else?
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> The deviation from the expected 70Mhz seems too big to me, but should I tweak
> the oscillator tuning capacitor (C901) to try to lower the frequency?
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The oscillator is a Clapp oscillator and the (0.6-4.5pF) series tuning
cap has a large influence on the frequency.
Unless the coil has shorted turns or another component has gone open
circuit its seems likely that the oscillator has been mistuned.
> Thank you all,
>
> Roberto EB4EQA
>
>
Bruce
Hi Bruce,
Thank you for taking the time to look at this and answer my message. Thank you
for pointing to the oscillator type, thanks to that, I've made some
calculations. I've measured the inductace of the coil and it turns out to be
460nH. Given the capacitor values, doing the math, the oscillator is tunable
from about 129Mhz to 310Mhz by adjusting capacitor C901. I've found that there
is about 157pF where the 82pF capacitor is, but that has very little effect on
tuning range. I've tried adjusting C901 and the lower I can get is 125Mhz, as
expected.
Could the correct frequency be in that range, and not 70Mhz ???? If you confirm
it should be 70Mhz, I'll add some capacitance to 901 to get the oscillator down
again to 70Mhz. About 90pF should do.
Could this actually be the problem in the unit (the lamp glows...)
Thank you & best regards,
Roberto, EB4EQA
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