[R-390] R-390 Problems - Relay

[Mark Huss]

You are right about putting current through the contacts, but the 
antenna relays in the R-390's are designed for no current through the 
contacts. So it is not that much of a problem.
When i monitored the coil current, I was thinking along the lines of the 
contact pressure changing, thus changing the pressure on the oxidation 
layer on the contact. Less pressure would equal a change in resistance 
through the oxidation, and would account for the change in attenuation 
through the contacts. This can be caused by a decrease in the strength 
of the magnetic field, especially if a bias magnet is used in the coil 
core, or magnetization of the contact actuator arm. This would be 
evidenced by an increase in the pull-in/drop-out current of the relay. 
If this was the case, cleaning the contacts is ony a temporary fix. 
Replacement is the only option. I had run into this problem on the 
control system of a sixty-foot dish I had worked on once. Drove 
everybody nuts as you would clean the relay contacts (salt-water shore 
location), and everything would work for a few weeks, then the 
intermittent operation would reoccur. You would think that all the 
relays in the chain should have just been replaced in the signal chain 
at the first sign of trouble, but the origional relays were designed to 
be operated off plate current (65VDC, if i remember right), and were no 
longer available. Ended up degausing the bad relay with a coil of wire 
and a wall plug. Saved the Taxpayer several hundred thousand dollars. It 
lasted another twenty years before they replaced the entire dish. By 
then, the dish would intermittently freeze, a friend who still worked 
there told me. The fix was to walk out to the Dish, take a broomstick 
off the hook on the wall, stick it through the hole in the 'Coffin' ( a 
five by three by three metal box on the floor with dozens of relays in 
it), and rotating the end to bang everything inside until it worked!
Reminds me of the 'Good Old Days', where you would walk up to an 
inoperable R-390A with your toolkit. Ask the operator to step aside. Run 
through the front-panel checklist to confirm the R-390A would not turn 
off. Open up your toolbox. Pull out the six-inch steel ruler and a 
pencil. Carefully measure the front panel and mark an X on the front 
panel. By now the operator is standing at your sholder, wondering just 
what the h** you are up to (which is exactly what you have been waiting 
for!). Then you pull out the little rubber mallet from the toolbox, and 
with a show of great concentration, tap the front panel between the Mode 
switch and the Limiter pot. At which point, the Dial Light goes out. And 
with a look of great satisfaction, put away everything, turn to the 
operator, and confidently tell him it is fixed! Use left hand to close 
operator's jaw. And walk away, as off in the distance you hear 'Hey, It 
Really Is Fixed!!!'

Kenneth G. Gordon wrote:

>On 16 Aug 2006 at 15:13, Mark Huss wrote:
>
>  
>
>>Had a similar problem with an R-392, very similar to an R-390.
>>Receiver works fine, then suddenly looses sensitivity. Later comes
>>back up. Traced it to the Break-In relay contacts being dirty. It
>>suprised me a bit as I would think that the contacts would either make
>>or not make as long as the contacts did not move. Considered that the
>>voltage on the relay coil might be 'iffy', but the relay held to
>>16VDC. Burnished the contacts and everything works now.
>>    
>>
>
>One thing we often forget concerning relays is that in order for the 
>contacts to STAY in any sort of low-resistance condition, there MUST 
>be some current flowing through the circuit that the contacts are 
>switching. If that current is below a certain level, the contacts quickly 
>oxidize and their closed resistance goes up...sometimes way up.
>
>And this has nothing to do with coil's current or applied voltage.
>
>Ken W7EKB
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Some people are like a Slinky .. not really good for anything,
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