[time-nuts] Z3805

[Charles P. Steinmetz]

Bob wrote:

>I've heard much talk in this group about the power supplies on the Z3805.
>*   *   *   my Z3805   *   *   *   started producing a real bad smell after a
>few hours, the classic burnt transformer type.  *   *   *   I opened the
>unit and found the 25W 5V, +/-15V (made by AT&T) was real, real hot.
>
>  Is this the supply driving the heater inside the 10811?

There appear to be quite a number of different boxes available 
labeled "Z3805" these days, so information you get may or may not 
apply to the particular unit you have.  My Korean-made Z3805A takes 
nominally 24-27 V DC power and has a dual-oven 10811 OCXO.  I presume 
-- but do not know for sure -- that the ovens run from the raw DC 
supply, not from a DC-DC converter.

If the smell did come from the DC-DC converter block, it suggests a 
failure either in the converter itself or in the circuitry that is 
powered by the 15 V supplies.  I think the former would be more 
likely -- the converter should have internal protection that would 
step in before it got hot enough to smell, in case of a load fault.

A failed oven controller in the 10811 could also produce a burnt smell.

Whatever it was, it will very likely give a repeat performance at 
some inopportune time, unless you chase down the trouble first.  Not 
easy with an intermittent fault, but particularly if it was one of 
the 10811 ovens acting up, it would be nice to catch it before it 
happens again.

Hints:  (1) use your nose, up close.  Does one part have a residual 
smoky smell?  (2) Look very carefully, using a magnifier.  Does one 
part have ID markings that look faded or scorched, or small 
cracks?  Is anything leaking out of the DC-DC converter?  Also look 
carefully at resistors and polar capacitors.

Best regards,

Charles