Background: I help maintain five Z-10 transmitters (all running at 10kW TPO) which all have a nasty habit of "popping" fuses. At first I thought the problem was faulty SCR's - replacing them didn't help. Some suggested power spikes on the incoming mains, however logging the incoming power feed over a few weeks didn't reveal anything.
As the fuses are ceramic it was hard to say whether the fuse had melted slowly or exploded violently (which sometimes helps reveal the nature of the fault). Out of curiosity I cracked open a few of the failed ceramic fuses and was surprised with what I saw. It appears the fuse element was intact and had separated from the end-cap.
Anyone familiar with the Z-10 PSU knows there are 4 transformer taps the PSU controller can choose to use to change the output voltage. As the taps change so do the fuses the current flows though. It was concluded the fuses were failing due to thermal cycling creating stress on the end-cap joint. I checked other failed fuses and they had suffered from the same fate (incidentally the other fuses were from a different manufacturer, which reduced the likelihood of the problem being confined to a particular brand or batch.)
Despite having become rather proficient at removing Z-10 PSU modules for repair, it was happening far too frequently and I decided enough was enough. After discussing the matter with some colleagues I opted to redesign the PSU board to use a different type of fuse which would better withstand high operating temperature and thermal cycling. The fuse chosen is known as a JCASE automotive fuse (details here http://www.littelfuse.com/products/fuse ... s/895.aspx) and the internal construction of the fuse better caters for thermal cycling without creating stress on the fuse element.
Today I finished building and testing the first prototype board. It functions on the bench just fine, the next step is to put it into a transmitter to assess its long-term performance. If this manages to solve the problem in the long term then I might look at making boards available for sale in case there is anyone else in a similar position.
Here is a photo of the partially completed board:
Happy New Year!
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