Re: TM5xx extenders


Bill Carns
 

Really good info - Thanks - I was about to make up one of those kits

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of ditter2 via Groups.Io
Sent: Monday, March 11, 2019 6:23 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] TM5xx extenders

A word of caution about these extender kits. I have seen these postings on e-bay from time to time and noticed that these kits use a simple circuit board whose edges are either sheared or more likely, routed with a tungsten carbide cutter. Mating the edge that plugs into the mainframe connector will likely damage it on the first insertion. The edge card connectors in the mainframe are plated with hard gold, to a thickness of 100 or 250 micro inches, depending on the vendor. This is a relatively thin plating for a connector that sees a lot of cycles. To be honest, the mainframe connectors used by Tek were not a good choice for this application. They were intended for use in semi-permanent applications, where the only cycling would be board replacement during servicing – not a plug-in where they may see hundreds of cycles in normal life. The vendor’s data sheet for the connectors stated that there design life is only 25 mating cycles. Note that exceeding this cycle count does not necessarily mean the connector would fail, but rather the specifications for contact resistance would not necessarily be met after 25 cycles.

Tek mitigated this short life by treating the male mating fingers on the edge of the plug in. If you look closely at any TM500/5000 plug-in, 5000 series scope plug-in and the Tek made extender cables, you will note several things. First, the fingers themselves are gold plated with 100 micro inches of hard gold over a nickel plate. Copper and nickel are much harder that hard gold and will deform it when mated. More important, the mating edge is treated in two manners. First, it is chamfered to a 45 degree angle with a carbide router bit by the board fabricator. This is essential to prevent the exposed glass fibers from the board edge from scraping the gold off the connector. Again, a single mating with an epoxy glass circuit board into one of these connectors can remove considerable amount of the gold plating from the connector, rendering if venerable to corrosion in high humidity environments.
While the chamfering reduces the likelihood of glass scraping the connector when mating, the step where the circuit board foil trace mates would still scrape when it mates. Tek prevented this by “rolling” the chamfered edge of the circuit board. The finished board after plating is ran through a pair of conical rollers that actually smashes the circuit board which reduces the thickness of the plated board fingers, allowing them to partially engage into the connector before beginning to deflect the spring function of the contacts. Thus, there is no edge scraping on the connector during the mating process. The board edge engages before it begins to force the contacts open.

Users who buy these extender kits can do some of these steps to improve them to prevent connector damage. Using a file, it is possible to put a chamfer on the mating edge. A short length of steel rod can be used as a roller to manually squeeze the connector edge. The only thing the user cannot do easily is add the gold plating, but the contribution of plating is minor compared to the problems of mating a rough exposed epoxy glass board into the mainframe connector.

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