[time-nuts] LPRO101 Lamp Exciter Frequency

[paul swed]

I guess the thing I have run into on these is there is a small variable cap
in that circuit.
With age they get noisey and jump around.
That needs to be tweaked.
I have one with this issue and it would be quite troublesome to replace that
cap.
But maybe one day.

On Sat, Oct 24, 2009 at 5:24 PM, Bruce Griffiths <bruce.griffiths at xtra.co.nz
> wrote:

> Roberto Barrios wrote:
> > Date: Sat, 24 Oct 2009 09:35:03 +1300
> > From: Bruce Griffiths <bruce.griffiths at xtra.co.nz>
> > Subject: Re: [time-nuts] LPRO101 Lamp Exciter Frequency
> > To: Discussion of precise time and frequency measurement
> > <time-nuts at febo.com>
> > Message-ID: <4AE21377.7070800 at xtra.co.nz>
> > Content-Type: text/plain; charset=ISO-8859-1
> >
> > Roberto Barrios wrote:
> >
> >>
> >>
> >> Hi all,
> >>
> >>
> >>
> >> 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.
> >>
> >>
> >>
> >> 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:
> >>
> >>
> >>
> >> - 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.
> >>
> >> - The lamp glows a few seconds after powering the unit.
> >>
> >>
> >>
> >> 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.
> >>
> >>
> >>
> >> Could this unexpectedly high exciter frequency cause the inability to
> lock or should I look somewhere else?
> >>
> >>
> >>
> >> 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?
> >>
> >>
> >>
> >>
> > 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
> >
> >
>
> Roberto
>
> Your lamp exciter differs from the one attached.
> Unless a fixed capacitor is faulty you shouldn't need to change it.
>
> In principle it doesn't matter too much what the lamp excitation
> frequency is as long as the coupling coil is suitably proportioned.
> If the oscillator operates at a frequency other than the design value
> the coupling to the lamp may be reduced.
>
> It would appear that the design frequency differs from that in the
> repair manual (unless the coil is faulty).
>
> The fact that the 10MHz oscillator frequency ramps up and down suggests
> that there is something wrong with the frequency lock circuit.
> Try looking at the photocell signal processing chain.
>
> Is the microwave signal actually being modulated?
>
>
> Bruce
>
>
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