> Date: Mon, 26 Oct 2009 08:36:49 +1300
> From: Bruce Griffiths <bruce.griffi...@xtra.co.nz>
> Subject: Re: [time-nuts] LPRO101 Lamp Exciter Frequency
> To: Discussion of precise time and frequency measurement
> <time-nuts@febo.com>
> Message-ID: <4ae4a8d1.5070...@xtra.co.nz>
> Content-Type: text/plain; charset=ISO-8859-1
>
> Roberto Barrios wrote:
> > Date: Sun, 25 Oct 2009 14:42:08 +1300
> > From: Bruce Griffiths <bruce.griffi...@xtra.co.nz>
> > Subject: Re: [time-nuts] LPRO101 Lamp Exciter Frequency
> > To: Discussion of precise time and frequency measurement
> > <time-nuts@febo.com>
> > Message-ID: <4ae3acf0.3080...@xtra.co.nz>
> > Content-Type: text/plain; charset=ISO-8859-1
> >
> > Roberto Barrios wrote:
> >
> >>> Date: Sun, 25 Oct 2009 10:24:38 +1300
> >>> From: Bruce Griffiths <bruce.griffi...@xtra.co.nz>
> >>> Subject: Re: [time-nuts] LPRO101 Lamp Exciter Frequency
> >>> To: Discussion of precise time and frequency measurement
> >>> <>
> >>> Message-ID: <4ae37096.5030...@xtra.co.nz>
> >>> Content-Type: text/plain; charset="iso-8859-1"
> >>>
> >>> Roberto Barrios wrote:
> >>>
> >>>
> >>>> Date: Sat, 24 Oct 2009 09:35:03 +1300
> >>>> From: Bruce Griffiths <bruce.griffi...@xtra.co.nz>
> >>>> Subject: Re: [time-nuts] LPRO101 Lamp Exciter Frequency
> >>>> To: Discussion of precise time and frequency measurement
> >>>> <time-nuts@febo.com>
> >>>> Message-ID: <4ae21377.7070...@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
> >>>
> >>> -------------- next part --------------
> >>> A non-text attachment was scrubbed...
> >>> Name: RblampExciter3.png
> >>> Type: image/png
> >>> Size: 22923 bytes
> >>> Desc: not available
> >>> URL:
> >>> <http://www.febo.com/pipermail/time-nuts/attachments/20091025/982d5b41/attachment.png>
> >>>
> >>>
> >>>
> >>
> > Roberto
> >
> >> Hi Bruce, Antonio,
> >>
> >>
> >>
> >> Bruce, you are right, there should be no need to modify the original
> >> design. The attached schematic is the one I have printed and it faithfully
> >> represents the actual circuit in the LPRO I have. C14 is actually 82pF (I
> >> took it out to measure it), but capacitance from E3 to ground is 157pF.
> >> The added capacitance must come from the line going to R13. I can keep
> >> unsoldering components until I uncover where that added capacitance comes
> >> from but looking at the Clapp oscillator formula, it should'n make a big
> >> difference in the oscillating frequency.
> >>
> >>
> >>
> >>
> >>
> > How did you measure the capacitance from E3 to ground?
> > What was the measurement frequency?
> > NB there is a series resonant circuit between E3 and ground.
> > At low frequencies you should measure the capacitance of the tuning cap
> > plus the strays.
> > Near tank resonance the equivalent capacitance will be much larger.
> > Also you will have the input capacitance of the MOSFET connected across C14.
> >
> >> Antonio, I'm 100% sure I'm not looking at the second harmonic. In the
> >> spectrum analyzer, coupling with a small loop, I can clearly see the 60Mhz
> >> signal coming from the physics unit, but absolutely nothing at 70Mhz,
> >> nowhere. And a big peak at 157Mhz clearly located at the RF MOSFET / lamp
> >> location. If I load lightly the gate of the MOSFET, that peaks falls into
> >> the noise, so it is the fundamental frequency. It's harmonics are indeed
> >> there also. (Am I missing something ?)
> >>
> >>
> >>
> >> I've measured the temperatures of both the heated lamp and the cell
> >> assemblies and they are correct, 100? and 70? respectively.
> >>
> >>
> >>
> >> I've carefully removed the lamp and it looks OK. The coil has perfect
> >> geometry and the wire is mint. Something I've noticed under the microscope
> >> is an almost invisible crack in the transparent plastic disc located
> >> between the lamp and the cell assemblies, going vertical from top to
> >> bottom. I wonder if that is normal, but since it is completely invisible
> >> when looking perpendicular to the disc, and the housing is not sealed, I
> >> don't see why it should have any effect. (Am I right ?)
> >>
> >>
> >>
> >> I tried looking at the photocell output directly on the scope but I see
> >> only what it looks like weak noise, which I think it's more or less normal
> >> (Or should I see something clearly?) Bruce, I've got no schematic for
> >> anything beyond the photocell, it will be difficult to follow the path of
> >> that signal...).
> >>
> >>
> >>
> >>
> > Even when it locks the transmission dip will be much less than 1% so
> > most of the photocell signal will be noise.
> > When locked modulation frequency component seen by the synchronous
> > detector will be very small.
> > Without a circuit for the synchronous detector and preceding amplifier
> > troubleshooting this part of the circuit will be difficult.
> >
> > What does the waveform at test point 5 look like?
> >
> >
> >> Yes, after fiddling, I always close the enclosure and pray for the 10Mhz
> >> output to stop dancing...
> >>
> >>
> >>
> >> I'll see tomorrow what I can find regarding the modulation of the
> >> microwave signal. Anything else to look for?
> >>
> >>
> >>
> >> Thank you guys for your comments and guidance.
> >>
> >> Roberto, EB4EQA
> >>
> >>
> >>
> > Bruce
> >
> > Hi Bruce,
> >
> >
> >
> > Having no other thing to try, I managed to lower the exciter frequency to
> > 70Mhz, just to find that the lamp won't ignite at that frequency. Anything
> > lower than about 100Mhz causes no light output at all. I found that
> > trimming the capacitor to maximum capacitance causes the frequency to lower
> > to about 120Mhz, and the LAMP V reading at that point is maximum. If the
> > capacitor was bigger, and the frequency could be lowered even more, LAMP V
> > would increase further. I though that this capacitor was to be tuned for
> > maximum lamp output, but it isn't. I left it more or less where it was at
> > the begining, about 140Mhz. Everything seems to be fine, it would be great
> > to confirm if other LPROs work at 70Mhz or actually at 140, like mine.
> >
> >
> >
> > At TP5 there is only DC component, 7.26V. Not very complex for a
> > waveform... Are we on track? What should appear there?
> >
> >
> >
> > Thank you,
> >
> > Roberto, EB4EQA
> >
> >
> Ciao Roberto
>
> Since the repair manual shows a waveform at this point which appears to
> be a sinusoidal modulation multiplied by a square wave whose transitions
> are in phase quadrature with the sine wave, perhaps this is the output
> of the synchronous detector before filtering.
>
> Naively one might expect some modulation evident at this point even when
> the microwave frequency is detuned??
> However the rubidium cell transmission may not exhibit any detectable
> modulation if the probe frequency is too far from the resonance.
>
> Can you detect any modulation of the microwave signal?
> Can you measure the frequency of this signal?
>
> Bruce
>
>
>
>
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> End of time-nuts Digest, Vol 63, Issue 94
> *****************************************
Hi bruce,
There has been a misunderstanding on my side. I read "Test Point 5" and went
directly to put the probe on "TP5" which indeed only has DC component and is a
completely different part of the circuit. But yes, I had already looked at
"Test Point J5" and I have those fancy waveforms too (which is good but does
not help).
I am unable to work on signals that high in frequency, but since (what appears
to be) the output of que synchronous detector shows activity, we already found
all we could look for...
Thank you for your patience :-)
Roberto, EB4EQA
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