[time-nuts] Need - PRS10 magic smoke

Glenn Little WB4UIV glennmaillist at bellsouth.net
Tue Jun 14 00:04:30 EDT 2016

My guess is the burned choke is in series with the Tantalum capacitor.
This is a very common failure mode for these Tantalum capacitors.
There are a number of things that will cause this failure.
1). power supply ripple too high (Tantalum capacitors do not like ripple).
2). Moisture absorbed by the Tantalum capacitor.
3). Voltage rating of the Tantalum capacitor too close to the supply 
voltage. ie a 6 Volt capacitor used on a 5 Volt supply.

The solution is to replace the choke and the Tantalum capacitor.
Replace the capacitor with a higher voltage capacitor.

A lot of high end test equipment is prone to Tantalum failure because 
the design engineer used the same Voltage safety margin for the Tantalum 
capacitor as would have been used for an aluminum electrolytic.

The Tantalum capacitor requires a higher safety margin because the 
Tantalum capacitor will short (as you experienced) by a spike above the 
allowed for safety margin. An aluminum electrolytic will absorb the 
spike, the Tantalum might short.

You should be able to recover the PRS10.


On 6/13/2016 6:32 PM, Skip Withrow wrote:
> Hello time-nuts,
> Well, broke out a PRS10 that I had put away for a project some time ago and
> upon power up found that it had issues.  Status indicated low lamp output
> and high lamp temperature.  Duh!  Upon removal of the lamp assembly it
> became abundantly clear that the magic smoke had gotten out.
> If I recall correctly, seems like there was another PRS10 with corrosion
> issues in the same area.  My guess is that the heat does not help this
> situation.  I suspect that I may have a full-on heater, which may have
> caused the tantalum cap (and possibly other) failures.
> Looks like this may be the weak spot in these oscillators.  I'm very
> hopeful that a rebuild will bring it back to life.
> Thought you would enjoy the pictures though.  Two are attached.
> Regards,
> Skip Withrow
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Glenn Little                ARRL Technical Specialist   QCWA  LM 28417
Amateur Callsign:  WB4UIV            wb4uiv at arrl.net    AMSAT LM 2178
"It is not the class of license that the Amateur holds but the class
of the Amateur that holds the license"

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