[time-nuts] LPRO101 Lamp Exciter Frequency

iovane at inwind.it iovane at inwind.it
Sat Oct 24 20:41:10 UTC 2009


I think you are reading the second harmonic.
I would recall that:
- the LPRO loses lock if the lamp gets too hot. This is 
indicated by a color more close to red than to pink (which is
normal). This could happen if the temperature control circuit 
has some fault. I would suggest cheching the voltage at the 
base of Q703: it should drop to about 5V from the initial 11V 
after warm-up. If the voltage remains around 11, then R721 
61.9K (bottom of PCB) could be interrupted. Onboard it should 
read 14K if good, 18K if broken.
- the lamp could glow even if the lamp assy is cold, and the 
LPRO doesn't lock. This is easily checked touching the lamp 
assembly after one minute of power-on, you should burn your 
finger. If you don't burn your finger, then check the heating 
circuit, particularily the small 100K resistors on the bottom 
pcb near the lamp assy.
Be aware: once again I would remind that the LPRO doesn't lock
if the cover is off and there is some light coming from an 
incandescent light or any light that has ripples or pulses.

Antonio I8IOV

> 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
>  		 	   		  
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> 
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