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 <[email protected] > wrote: > Roberto Barrios wrote: > > 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]> > > 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 -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
