Author Topic: Should TT Pads ever be cleaned or maintained? - 2500MM circa 1981  (Read 421 times)

Offline JimG2100

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Telephone newbie is getting a (n apparently working) Western Electric 2500MM circa 1981 (81254 stamp) ready for home service. Have the phone open while waiting for a replacement jack to arrive, and thought to, at a minimum, check for any loose screws or contacts, and clean.

My search terms may not have been the best, but couldn't locate a general document on touch-tone key pads.

Should a TT pad be cleaned or maintained in any way BEFORE any trouble is noticed?

The posts I've found at various places around the web did not seem consistent - especially regarding contacts. Some cautioned no TT contacts were designed for any maintenance and will easily be damaged. Some advised a gentle paper or card friction clean. This (referencing low-volume/static problems) suggests a spray contact-cleaner on the leaf-springs. Another said never use contact cleaner on TT parts as it will damage the delicate metals.

Perhaps each apparently contradictory source actually was referring to different types of contacts in the assembly, or different models (analog, IC, etc.), in a way that didn't assume a reader unfamiliar with the mechanisms (like me).

(Apologies if this should have been posted in "Restoration".)

Offline Charles

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Re: Should TT Pads ever be cleaned or maintained? - 2500MM circa 1981
« Reply #1 on: August 08, 2017, 09:42:27 AM »
I have an early 80s Touch Tone Princess phone. I have never done anything (as far as cleaning goes) to the pad and it still works great. These phones were made to last with little to no maintenance.  I would just try using it and see if there are problems before attempting to clean.

Alex G. Bell

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Re: Should TT Pads ever be cleaned or maintained? - 2500MM circa 1981
« Reply #2 on: August 08, 2017, 10:32:58 AM »
The cliché "If it ain't broke don't fix it" applies here. 

Also, broad sweeping generalizations about TT dials are bound to be wrong.  For starters, there were two basic types of metallic contact keypads.  The earlier ones had frequency selection contact springs bent in the form of a "V" with a thin gold plating.  The plating may eventually wear off, causing base metal which tarnishes to be exposed.  When this happens the contacts will need periodic cleaning to maintain an oxide free surface which makes good electrical contact. 

However if the gold is not worn off, cleaning will only serve to wear it off since the gold is pretty soft and thin.

Later keypads however had precious metal contact tips welded to the ends of the contact springs.  These may need cleaning and will not be worn away using non-abrasive cleaning methods.

So one must inspect the keypad contacts to determine which type they are before considering cleaning.

Later digital (IC) keypads (earlier Western Electric 72-type) use snap-dome technology, which is AFAIK impossible to clean.  Later 72 types, ITT 42 & 46 types and some S-C types use conductive elastomer (rubber) which can be restored using special chemicals or cleaned sometimes with rubbing alcohol but disassembly for access is difficult.

Alex G. Bell

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Re: Should TT Pads ever be cleaned or maintained? - 2500MM circa 1981
« Reply #3 on: August 08, 2017, 10:35:28 AM »
I have an early 80s Touch Tone Princess phone. I have never done anything (as far as cleaning goes) to the pad and it still works great. These phones were made to last with little to no maintenance.  I would just try using it and see if there are problems before attempting to clean.
Conditions of use affect life.  I.E., "YMMV".

Offline JimG2100

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Re: Should TT Pads ever be cleaned or maintained? - 2500MM circa 1981
« Reply #4 on: August 09, 2017, 03:39:26 AM »
Wonderful. Thank you for taking the time to educate on the varied types. The posts I had browsed make much more sense now.

The one in this 2500MM seems to be a common Type 35 dial (35Y3D, dated 9-81)

I will not disassemble - it ain't broke! - but as the dust cover is clear, did note that the leaf springs on the bottom side have developed some dark spotting.

"Conductive elastomer" sounds awfully like the inexpensive graphite-rubber button pads in TV remote controls, but as it's a telephone, it's probably quite different or some much more robust implementation.
« Last Edit: August 09, 2017, 04:38:35 AM by JimG2100 »

Alex G. Bell

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Re: Should TT Pads ever be cleaned or maintained? - 2500MM circa 1981
« Reply #5 on: August 09, 2017, 03:40:38 PM »
Wonderful. Thank you for taking the time to educate on the varied types. The posts I had browsed make much more sense now.

The one in this 2500MM seems to be a common Type 35 dial (35Y3D, dated 9-81)

I will not disassemble - it ain't broke! - but as the dust cover is clear, did note that the leaf springs on the bottom side have developed some dark spotting.

"Conductive elastomer" sounds awfully like the inexpensive graphite-rubber button pads in TV remote controls, but as it's a telephone, it's probably quite different or some much more robust implementation.
You're welcome.

Not sure what you mean by "bottom side".  The "common switch" is on the back or bottom.  The frequency selection "matrix" contacts are around the edges. 

IIRC, the earlier dials such as the 35Y3 had hand wired common and matrix contacts and "V "shaped gold plated contact point while the later dials (35AF3 maybe?) which had welded contact points had machine welded instead of hand wired connections between the contacts and the circuit board.

Conductive elastomer is not essentially different from what is used in various electronic devices including AT&T and other major brand office phone system telephones.  Snap dome was first used in calculators, especially TI, who promoted this technology and had a division which made OEM snap-dome keyboards for various applications.
Don't know what would cause spotting but I think the gold is selectively plated only at the contact point areas so tarnish along the spring member is plausible and would not matter.

A 2500MM or any 2500 for that matter could have any generation dial.