[time-nuts] LPRO101 Lamp Exciter Frequency

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Oct 23 20:35:03 UTC 2009


Roberto Barrios wrote:
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> Hi all,
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> 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.
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> 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:
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> - Power input current during warmup is 1.2A and 0.4A after it.
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> - 10Mhz out swings between 10.000191 and 9.999875, taking 40s to go up and 60s to go down in freq.
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> - Lamp voltage is a steady 6.7V.
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> - The lamp glows a few seconds after powering the unit.
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> 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.
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> 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.
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> Could this unexpectedly high exciter frequency cause the inability to lock or should I look somewhere else?
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> 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?
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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,
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> Roberto EB4EQA
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>   
Bruce





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