sundance-communications.com/forum Forums Other TDM Phone Systems 1A2 and Comkey Telephones & Systems 4A speakerphone volume fades

I was given a 4A speakerphone and am having trouble making it work right.

The parts in question are:
108AA speaker set
680AE mic
2500SM phone

If I press the ON button, the unit picks up the line and I hear dial tone. At first, it's loud and clear, but after a few seconds, the volume drops out and can't be adjusted with the volume control. If I hold the ON button, the volume ramps back up (It's not an instantaneous change) and is controllable, but that's not too useful since holding the button acts as a mic mute, not to mention defeating the whole hands-free point of a speakerphone. <g>

Where should I be looking to find the problem? (And is it likely to be a wiring or connection problem, or is it some sort of electronic component failure?)

Steve, I have a similar problem - well, exactly the same problem. Luckily I have several units and after switching them out, the problem appears to be in the 108AA speaker. However, that's as far as I could solve the problem. I was going to post a similar question here but you beat me to it; hopefully, other people might give us some ideas. I have about three 108s with the same problem. One thing to look at however: are all your components WE or are some PC (Precision Components)?

My gut reaction would be failing electrolytic capacitors. The polystyrene ones should outlast most of us, but the same cannot be said for most aluminum electrolytics.

I would be interested to hear what happens if you go through the 108 and replace said caps, 'shotgun-troubleshooting' style. Be careful about desoldering/resoldering on those flex circuits.

Happy tweaking.

What Bruce said, there's a reason that Tantalum has become the preferred material for electrolytic capacitors!

Try cleaning the common switch contacts on the TT dial.

OK, Arthur, I'll bite the hook. Where are the common switch contacts on the TT dial? And why would this affect the aforesaid problem? It did this no matter what 2565 I hooked it up to.

Sorry, I must have missed that part of your explanation. I thought you said you were using (one) "2500SM set" not "multiple 2565 sets."

Please disregard my suggestion about the TT dial common switch.

Arthur, maybe I confused you. I am not the original poster but said that I had a 4A with the identical problem. Unlike the OP, I have a number of 2565s, 108s, and 680s, and I was able to try different units of the problem 4A with known working components. I isolated the problem to the 108 speaker by so doing.

Aha. Understood.

Bruce -

Failed caps was one of my initial thoughts as well, but there's only one discrete electrolytic in the unit, and it appears to be part of the power supply. I'm guessing if it failed, the extent of its failure would be audio hum.

There are three molded reddish plastic blocked labeled as CAP-PAK. Before I try to desolder all those legs (16 per module) any idea what the individual values might be? With 16 legs, I'm assuming 8 caps in a module, but if there's no way to tell what any given one should be inside of a molded lump of plastic, it's a pointless task to pull one off the board, not to mention risking damage to the multi-layer board.

There's also about a dozen of what look like TTL decoupling caps scattered about the boards. Naturally, they all wear KS numbers, but as an educated guess, they're .01uF ceramic, or maybe tantalum. Like the power supply cap, I'd guess they're not the culprits.

Does anyone have an actual schematic with real-world component values for these things? I don't think I'd have much luck calling Jameco and asking for Kearny System part numbers. <G>