atcomsystems.ca/forum Forums Other TDM Phone Systems 1A2 and Comkey Telephones & Systems 4A speakerphone volume fades

Popular Science [june 2010] has a list of elements in short supply.Estimates are that less than 100,000 tons of tantalum remain to be mined.This wiil force a return to aluminum and ceramic capacitors.

NOW THAT IS SCARY. Tantalum caps are a factor of 10 SMALLER than Aluminum caps. That means things are gonna get bigger, from IC's to TV's to Microwave ovens to car computers, etc, etc, etc. Someone will devise a tantalum recovery process and corner the market with a patent.

Okay, I will show my lack of knowledge here again and ask a stupid question: Is there anyway to test a capacitor with a standard VOM meter? Isn't the only way to test them is to run a known current through them? Sorry, I must have been asleep in the physics class when they discussed capacitors?

Just a REAL ROUGH test with a VOM. WITH NO POWER APPLIED!!!!!, first short capacitor leads to drain ALL charge. Then use VOM to charge the capacitor. Should rise to several megohms of 'resistance' within 1-5 seconds, depending on the Uf of the capacitor and the internal resistance of the VOM. Check it BOTH polarities. There may be circuitry that will prevent charging in one direction. And keep in mind that you are checking this with a 1.5 volt battery, and the actual operating voltage may be 20-30 volts. In other words, a capacitor that may break down in use and test ok with this jury rig! There are pieces of equipment that can thoroughly test the entire circuit, but they cost WWAAYY more than sending the unit to a repair center! And please tell us what you are using for a power source, Volts and ma/amp. If you are using a marginal P/S, that's got to be rectified first and probably will cure the problem.

Well, like Bill, I've isolated the problem to the 108AA unit. I picked up a cheap AT&T unit on eBay, plugged it in and it works perfectly. When I changed the case to match my WE system, I peeked inside and it too has the red molded cap-paks, so not much help there.

One day, if I'm bored, I may try to open the cap-paks on the malfunctioning unit and see if the individual elements are labeled.

I've sacrificed the dead 108AA unit and unfortunately don't know too much more.

One thing did jump out on a visual inspection - R41, which is right in the center of the center board was broken. 

Unfortunately, I did not see this until after I'd scavenged some parts to fix something else, so replacing R41 to see if that was the source of the fading problem was not an option. I'll leave this to phonenut260 to investigate.

The cap-paks were as I expected - potted with epoxy, so their secrets will go with them to the e-waste recycle bin.

The circuit boards are as fragile as I feared. Removing one of the paks did not leave a re-usable board - almost all of the pads lifted at 650 degrees, which is relatively cool, as soldering irons go. Granted, I don't have a professional-grade desoldering rig designed for multi-layer boards, but still, I was a bit surprised to see the pads and traces start curling away.

Steve -

Thanks for letting us know the final disposition. I think you discovered the dirty little secret of that era - the various component parts were built to be replaced - not repaired out in the field.

Sam