Silverface Bassman 135 Ext Spkr jack...?

[acorkos]

I have a Bassman 135 (LOUD) with a speaker jack problem. Plugging a speaker into the Main speaker jack yields a low distorted output. If i pull that plug and move it into the Extension speaker jack....normal LOUD output. the Extension speaker jack is one of those funny switched jobs (kinda looks like a gibson SG pickup selector switch)....and i think there is a problem with that jack. i replaced the Main speaker jack...no change.

can someone explain what the extension speaker jack is doing, and is there a suitable way to replace this jack with something a little more standard? there are a green and a green/yellow secondary taps....and it looks like the green/yellow is connected to the sleeve of the Extension jack as well as the tip of the main speaker jack.

thanks for your help

Bassman 135 Schematic

[52 Bill]

Check the extension speaker jack mounting, it should be isolated from the chassis.

The two jacks are set up so that when two speakers are used, they are wired together in series. If the isolation washers are bad or missing, it will ground the output of the amp causing the low power distortion signal that you describe.

The extension speaker jack has a switch that selects the low impedance secondary when there is only one speaker connected, and switches to the higher impedance secondary when there are two speakers connected to the amp (in series).

[pdf64]

I've not seen this exact model, but the Fender 75 and 140 use what I assume is same switched jack socket.
The ext speaker socket has a built-in changeover switch that changes the tap used on the secondary. Note that the main and ext speaker sockets are wired in series.
From your symptoms it sounds like the normally closed contact isn't closing properly for some reason.
The cause can be a bit of dirt/fluff, or a misplaced wire could be preventing the contacts closing.
The switch needs a close visual inspection, maybe with a magnifying viewer to check for mechanical obstruction.
If none obvious, the contacts can be cleaned by using a strip of thick paper (eg cut from a nice envelope) soaked in contact cleaner. Open the contacts by putting a jack into the socket, get it between the contacts, then remove the jack, thereby closing the contacts onto the paper. Slide it to and fro a few times.
If this doesn't work then you've a few options, but try that first.

[Enzo]

I'd wager the cutout contacts are not making. That extra couple contacts that are pushed apart when the EXT jack is used MUST come together electrically when it is NOT. Pull the wires off and measure resistance across those closed contacts - got continuity? In fact, open them up with a plug and stick a strip of cardboard between them. Pull out the plug to close them aroudn the cardboard, and wipe it back and forth to clean the contacts. A business card or a matchbook cover should work. If you have aburnishing tool even better.

[acorkos]

the isolating washer under the jack nut was missing. i found a nylon washer at lowe's....all is good now. i did clean the jack contacts as well, as you suggested, enzo. thanks everyone. i now understand how those funny jacks work.