Re: 7A13 relay substitution report

Fabio Trevisan

Thanks Dennis,

For the heads up and for the kind offer...

I think I`m covered, with the 10 x NEC EB2 relays I got, and the 2 OMRONs G5V-1.

What puzzled me more (until I gave up on the Tek ones) was that, when I removed the Tek relays and exercised them in my test jig, they seemed to operate quite well, even at 10 or so cycles / sec, the contact bouncing would still be less than 10% of the period, meaning that they would settle in less than 5ms.

When inserted into the 7A13 however, as a general rule they would all perform well upon power-up, but after three or four minutes (sometimes up to 10) they would start acting...
But on the first 5 to 10 minutes, we cannot even use the plugin because it's still warming up and the 7A13 takes sometime until we can set the Var Balance, the Step Atten Balance and X10 balance.

The way how the relays are used on the 7A13, being kept all the time on or all the time off, depending on the range, makes their coil heat up considerably... and it seems the material the armature is made loose magnetic properties as they heat up (I'm not even sure if that can happen... I know magnets lose power with heat, but not quite sure about electromagnets) and the magnetic pull gets weaker.
There's also the PTC effect on the coil wire contributing to the diminishing of the magnetic pulling force.
Since the armature actuators are split into two opposite ends, and each one may "see" a slight different opposing force (from each of the contact springs), this causes sometimes one contact to remain actuated, while the other can give up altogether, not only breaking contact sometimes, but even enough to make a poor contact back to the N.C. contact (so half of the relay is make, the other half is break)..
It doesn't help either that the pivoting point is not very precise and plays somewhat, which allows one of the ends to "break" while the other side is still on "make".

I wasn't being able to go through the input attenuator's compensation adjustments until it started acting up, and that's what made me give up.
I would love to keep it original and even thought about getting some of them from Sphere or Qservice... But how to be sure they would also not fail in no time?

It's so unfortunate that no current relay share this same symmetrical / mirrored layout of the Tek ones, because wiring the new ones to the old pinout is never straightforward.
For each relay I`m having to draw 4 different combinations of positions and contact choices, to chose the one that has the shortest connections and less cross-overs.
So far I have been lucky to make most of the connections bridge no more than 2/10ths of an inch and no more than one signal cross-over but still, I'd guess there's at least 1/2 inch of added signal path and some added capacitance too.
Given the difficulty to get the wires routed the shortest path even counting on the 3D space between them, I doubt that making adapting boards won't make it even worse so, I won't even try it.

I`ll keep posting the results as I move on.



On Wed, Oct 17, 2018 at 05:32 PM, Dennis Tillman W7PF wrote:
Hi Fabio,
I think we will all benefit from your adventure into replacing the Tek relays.
If you get stuck and cannot find an appropriate substitute at any point
me off list at dennis at ridesoft dot com and I will try to help however I
Dennis Tillman W7PF

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