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
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