[time-nuts] LPRO101 Lamp Exciter Frequency

Ron Hensley rchensley at gmail.com
Mon Oct 26 21:42:02 UTC 2009


Hi



I have reading the mail on the rubidium oscillator. I just finished
repairing my FRS. The manual for the FRS only gives basic operation. However
an older manual for the FRK tells that the lamp should have a current
between 125mA and 145mA. This is adjusted with C11, the cap on the lamp
board. The wire going to the lamp support board to feed through cap C14 is
removed from C14 a 500mA meter is inserted. At power on the voltage should
be +22V. After the lamp has ignited and the resonator photo cell has
detected the lit lamp the power supply should switch to +17V. The current of
the lamp (really it’s the current of the oscillator) is adjusted to the low
side of the 125-145mA range with C11.  I found out that a 10:1 scope probe
connected to a SM and touching the lamp housing I was able to see and
measure the 70MHz signal. The FRK manual says to turn C11 to one of the
listed frequencies listed, keeping the lamp current as above. These
frequencies are 69.5, 71.0, 76.5, 77.5, 78.5, 79.0, 84.5, 87.0, and 91.5MHz.




My FRS unit would not lock at 10MHz. I found the problem to be the 70MHz
oscillator would have a clean signal, but as the lamp housing warmed up the
70MHz oscillator would go into a parasitic oscillation mode. Once in this
mode the resonator photo cell, for some reason can not detect the lamp
light, even though it is lit. C11 would tune total range from 69.5 to
91.5MHz, but I could not stop this parasitic oscillation. The only way to
fix it was to place a Fe bead on the choke lead L5 (.15uH) connected to the
base of Q3 (2n3375) oscillator. I was able to tune C11 to 78.5MHz to obtain
132mA lamp current. At this time the resonator photo cell detected the lamp
light and the power supply then switched to +17V. And the 10MHz locked



I don’t know if this is the problem but it’s a good place to start.



1)      Check the 22/17V power supply switching.

2)      Check to see if the lamp is ignited.

3)      Check for parasitic oscillation mode of the 70MHz oscillator.


If you have any questions, send me an E-MAIL.


RCHENSLEY at GMAIL.COM




On 10/26/09, paul swed <paulswedb at gmail.com> wrote:
>
> 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
> >
> >
> > _______________________________________________
> > time-nuts mailing list -- time-nuts at febo.com
> > To unsubscribe, go to
> > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> > and follow the instructions there.
> >
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
>



-- 
Ron Hensley
408-857-2261
408-248-1382
RCHENSLEY at GMAIL.COM


More information about the time-nuts mailing list