[time-nuts] OXCO Issues -- Latest

Bob Camp lists at rtty.us
Sun May 12 09:20:48 EDT 2013


There should be a glass probe or bead thermistor on the oven. It's likely buried in epoxy. The epoxy may be silver loaded. Make sure the thermistor hasn't come loose from the epoxy. Give the leads of the thermistor a gentle tug. They can be sitting in solder, but not be fully wetted with solder. My guess is that the thermistor isn't the issue, but it's worth checking.

Next up is that multi turn pot. If it's got a problem, the wiper is likely where it's at. Again, a long shot. Quick check is to mark the trimmer, rotate it, and return it to it's original position. 

Depending on when this one was made, it's either got a discrete transistor based error amp, or an op-amp in the loop. Either way there are resistors that set the gain. They may have changed value with age. My bet would be an op amp controller with a carbon comp feedback resistor across the amp. The poor thing may be 2X what it was when new. Gain is 2X what it should be, loop isn't stable….

With any troubleshooting, guessing like this isn't really the way to do it. My real point is that if you are careful, there's nothing crazy in there to work on. 


On May 11, 2013, at 11:42 PM, Frederick Bray <fwbray at mminternet.com> wrote:

> Okay,  I thought that perhaps the heating element wasn't working, so I pulled the oven apart.
> In fact, the internal unit was warm and it got hot when I ran it for a couple minutes w/o the foam around it.  All of the solder connections which I can see look good.  I am not sure I want to try to remove the internal PC board.
> There are two adjustments visible once it is apart.  One is a trimpot and the one that you can normally adjust with the case closed appears to be a piston type capacitor.  It actually appears to be better constructed than I thought.
> I'll take a few pictures tomorrow for those who asked.
> Fred
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