[Amps] Fuse holder surprise

John Simmons jasimmons at pinewooddata.com
Tue Jun 2 23:21:11 EDT 2020

I built 25 350W VHF paging stations. They all used solid state amps and 
we finally settled on Duracomm DPS75 power supplies. The switching power 
supplies were much more reliable than analog supplies and much easier to 
schlep into walk-in remote sites. I never, ever used a fuse holder, 
depending instead on the current limiting of the supply. In almost 20 
years I never, ever had any problem due to over-current. However when 
using batteries, you absolutely have to have a fuse.

With high-current systems you have to eliminate every voltage drop you 
can. All the transmitters I built had 350 amp quick-disconnects on the 
PAs for fast, east changeouts of failed units. Paging has a lot of 
thermal stress and the PAs created maintenance issues. I learned a lot 
about fixing MICOR 100W PAs maintaining  IMTS systems. In these 
situations I tend to think that the old conduction cooled tube PAs were 
better. Of course, high voltage power supplies are another animal but 
are (were) much more reliable.

Ah, the old days. Thanks for reading.

-de John NI0K

Jim wrote on 6/2/2020 8:17 PM:
> Oh... And I don't use fuse holders between my Icom or Kenwood HF rigs 
> and the power supply either. I use Astron switching power supplies 
> (SS30 or SS50) and or an RS50A
> Thanks
> 73
> Jim W7RY
> On 6/2/2020 8:05 PM, Artek Manuals wrote:
>> I dont recall at the moment ...what is your point?
>> On 6/2/2020 9:04 PM, Jim wrote:
>>> How about during normal operation? What power supply?
>>> Thanks
>>> 73
>>> Jim W7RY
>>> On 6/2/2020 7:48 PM, Artek Manuals wrote:
>>>> During the Bench testing I have an Astron R-35 more than up to the 
>>>> brick's challenge of 10-15 amps and fairly  short power cable less 
>>>> than 18"
>>>> Dave
>>>> NR1DX
>>>> On 6/2/2020 8:38 PM, Jim wrote:
>>>>> What are you using for a power supply to run this amplifier?
>>>>> Thanks
>>>>> 73
>>>>> Jim W7RY
>>>>> On 6/2/2020 7:33 PM, Artek Manuals wrote:
>>>>>> This is going to seen trivial to many of you but some times the 
>>>>>> most puzzling problem has the most simple solution, worth sharing.
>>>>>> The " 60W-brick" ( an old Mirage C106) on the 220 "lets chase 
>>>>>> DX"-repeater had given up the ghost. No power on light ...hmmm 
>>>>>> checked and the fuse on the back panel fuse holder� looks 
>>>>>> absolutely perfect. So out of the rack and back to the home shop 
>>>>>> for a more detailed autopsy. After 5 minutes of poking around 
>>>>>> with the VOM it seems the fuse is actually a zombie fuse, meaning 
>>>>>> it looks OK but in reality it is open with no sign life or the 
>>>>>> cause of death, usually 15 Amp fuses die pretty spectactularly. 
>>>>>> Oh well pop in a new fuse and it is off to the races. But the amp 
>>>>>> is only putting out 40-45 W and it is supposed to be 60W? Playing 
>>>>>> for 10 more minutes and nothing seems out of the ordinary..oh 
>>>>>> well it is 30 years old anyway button it up and move on to the 
>>>>>> next project right? As I put the cover back on I happen to absent 
>>>>>> mindedly tighten the fuse holder cap and it is VERY WARM ...now 
>>>>>> wait a minute fuse holders aren't supposed to be that warm? 
>>>>>> Further measurement shows� a little better than .5 volt drop 
>>>>>> across the fuse holder under load. A little loosen-tighten cycles 
>>>>>> and it is down to .25 volts . Upon closer inspection the spring 
>>>>>> inside the fuse holder looks dark and crusty ( like the one in 
>>>>>> your flash light after the batteries leak). So I have some really 
>>>>>> nice fuse holders in the junk box with bright shiny copper 
>>>>>> insides , replace the fuse holder , no more voltage drop and the 
>>>>>> brick is at 70W out.
>>>>>> The moral is the old fuse holder was corroded internally to the 
>>>>>> point that it's contact resistance had gone up enough under load 
>>>>>> to heat the fuse contact end and melt the solder inside the fuse. 
>>>>>> The bigger take away is that if something seems a bit odd keep 
>>>>>> looking. This one was easy but in 50 years of tinkering I had 
>>>>>> never seen a bad fuse holder and now I wonder how many I over 
>>>>>> looked over the years 8^)
>>>>>> Dave
>>>>>> NR1DX
>>>>>> manuals at artekmanuals.com
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