2500 keypad with bad tones

[MaximRecoil]

I don't know the manufacturer, but I've attached a picture of the backside of the keypad.

The bottom three keys (star, zero, and pound) all generate tones that are noticeably higher pitched than they should be, and none of those three keys will break the dial tone. The rest of the keys seem to work fine.

My guess is that it has a bad tone generator chip (TCM5087N) and/or a bad crystal oscillator. I looked at the solder side of the PCB and a few of the legs on that tone generator chip have been reflowed by someone in the past (the phone has a sticker on the bottom that says it was re-manufactured by Bayless Industries, so I assume they did the re-soldering), so that chip seems to be the likeliest suspect (I hope that's all it is because replacing that chip would be an easy fix). There a few transistors, but I don't know what they do in the circuit. I don't consider the rest of the components (resistors, diodes, and a Mylar capacitor) to be likely suspects, as those rarely go bad.

What do you guys think?

[rdelius]

Looks like a SC/Comdial pad .Most likely the chip. It might be easier to find another pad

[MaximRecoil]

Looks like a SC/Comdial pad.

Thanks for the information.

Most likely the chip. It might be easier to find another pad

If replacing the chip fixes it then that's as easy as falling off a log (I worked in a PCB factory for 2 years, and did tons of rework of this type). I'll socket the new chip too like I always do when replacing directly soldered through-hole chips. The only annoying part is waiting for the new chip to be delivered. I ordered 10 of them for good measure.

Finding another keypad isn't easy as far as I know, because it's metal like a payphone keypad, which is why I want to fix it:



The button presses are light and have a good tactile feel, and they make a metallic sound when you press and release them. I've wanted a metal 2500 keypad for a long time but I've never been able to find one. If you know (or if anyone else knows) of a place where they are readily available, I'd like to know where, because I'd definitely buy another one (assuming they aren't outrageously priced).

[MaximRecoil]

The new tone generator chips I ordered came in the mail today. I desoldered the old chip, soldered in a socket, and plugged in a new chip, and now the keypad works perfectly:



It's currently wired to an ITT 427 type network. Does anyone know if it can be wired to a WE 4228 network?

[HarrySmith]

Which one did you replace? The larger one in the center marked Phillipines? I can't even tell, good work, glad you got it fixed. Where did you find the chips?

EDIT: I answered my own question by looking at your original picture.

[FABphones]

Adding a socket was a good idea. If it ever needs replacing again it will be an easy swap over.

I love those old style PCB boards, easy to work on. Surface mount looks good, and have their benefits, but they are a throw away job if anything goes pop.

[MaximRecoil]

Which one did you replace? The larger one in the center marked Phillipines? I can't even tell, good work, glad you got it fixed. Where did you find the chips?

EDIT: I answered my own question by looking at your original picture.

I found them on eBay; 10 of them, new old stock, for $8.96 shipped, from Hong Kong. I didn't need 10 of them, but considering U.S. sellers were selling just one of those chips for close to that amount, I figured it wouldn't hurt to have 9 spares for a dollar or two more. The chip has been considered obsolete since at least 1990:



Newer tone generator chips have pulse dialing capability as well as touch-tone.

Adding a socket was a good idea. If it ever needs replacing again it will be an easy swap over.

Exactly. Plus it avoids subjecting the new chip to the heat of soldering. They can take the heat, to a certain point, but that doesn't mean it does them any good.

I love those old style PCB boards, easy to work on. Surface mount looks good, and have their benefits, but they are a throw away job if anything goes pop.

When I worked in the PCB factory I did rework on both through-hole and surface-mount components. Surface mount is fine if there are only 2 "legs", but with more legs than that you need specialty tools (or very low melting point solder, like "Chip Quik") for desoldering. And sometimes SMDs are downright tiny, and my eyes don't deal with that as well as they did in the late 1990s when I worked in that factory.

[HarrySmith]

Absolutely! 10 of them for $9.00 is a deal in my eyes! I don't understand how these guys ship from Hong Kong and other far east places for free when the product is so cheap. I would not be able to send a package to there for under $9.00. Congrats on a good deal and getting it fixed.