The number one tool of anyone fixing modern
electronics should be an ESR meter of the
Dick Smith variety.
Arguments about about ESR vs capacitive reactance
are numerous, but immaterial. When a capacitor
goes bad, its ESR will usually be 2 to 100 times
higher than normal.
Shorted capacitors will also be obvious, as the
ESR meter will show their ESR to be 1/4 (or less)
the normal value for a capacitor of that value.
Capacitors are the least reliable part made today.
Also, use your eyes, and scan the field. If almost
all of the capacitors used are, say, Nichicon, and
you find one or two that are YooSun, or Samson, or
Jokcon, or something else weird, it means the supply
line for Nichicons of that value was blocked up for
some reason, and purchasing bought what they could
find.
-Chuck Harris
Michael Williams wrote:
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Found the problem!
C448 had failed short.
Many, many thanks to all the guys who contributed to this thread ¨C especially Mark and AmirB. You guys pulled my head out of the rabbit hole with that MUX nonsense. I¡¯m so glad it was something as simple and easy to fix as a bad cap. I had fears of bad ASICs, and other such unobtanium floating through my head.
If anyone is interested in my fault-finding approach this evening, read on. Otherwise, please accept my thanks ¨C this group has been very helpful to me.
My probing the other day had shown no signal at R525, and when I was unable to see anything resembling an 800MHz source signal (due to bad probing) I started checking every voltage I could find, which led me down the rabbit hole with 2ns_VCO_Vcc and the MUX nonsense.
After listening to Mark B and Amirb (and the many others who contributed to this thread) I had a more organized plan of attack for tonight. After double-checking the supply voltages, I started working from the other end of the 50MHz signal path (the 1/16 divider). I confirmed the presence of the 800 MHz 2nd VCO by making an ad-hoc RF probe with a loop of wire connected to my SDR Play, I then walked all the way through the 50Mhz path. After seeing signal on one side of R523, and no signal on the other side, I began to suspect C448. A quick power-down and check with a multimer confirmed my suspicion. I may not know one end of a MUX from another, but I do know that capacitors should not act like low-value resistors. I pulled the cap, and the analyzer started to behave itself. Little did I realize I was so close to the problem when I probed R525 the other day!
I have a replacement cap on order. I assume it¡¯s purpose is part of a filter along with L55 and R523 to clean up the 50MHz signal. The analyzer would probably function reasonably well without it, but I may as well replace it while I have the RF module open.
Cheers,
Mike Williams
AG5QX
