Help with 1980's Mesa 4x12

[GoesTo11]

Hello,

I am a new user here and wasn't sure where this goes, as I may have been to blind to find the guitar cabinet section. Anyhow, for some years now I have had an older 4x12 and the top two speakers seem to not function. The rundown: Top two speakers are original Black Shadow 150W(each) 8 Ohm, bottom two are Celestion G12-80 80w(each) 8 OHM. I run this cab with a Crate GT1200h, 120 watt head. I am curious if maybe they arent getting enough power at the lower volumes I use (these days I only play in my small house and have a 1 year old so I dont ever crank it up). From what I can tell the wiring is in good shape, I do use a proper speaker cable. As an attempt to diagnose I used a 1.5V AA battery and got a sound when applied to the terminal. Any ideas? Thank you in advance!

 

[elvis]

I doubt that it is a power issue. Guitar speakers are VERY efficient. If you turn up a bit you should be able to tell if this is the issue. The top speakers should catch up to the bottoms as the volume goes up.

I recommend checking the wiring to see if the speakers are wired correctly.

Possibilities:
1. Top speakers are wired series and bottom in parallel, so bottoms get more power
2. Top speakers wired out of phase from each other so they cancel
3. Some other weird wiring

Use an impedance meter. 8-Ohm nominal speakers tend to measure as about 6 Ohms DC. Try the jack, then try each speaker (if you can get to them). Odd wiring will show up as quite different from 6 Ohms.

 

[GoesTo11]

Thanks for the tips, I am going to do more digging after work and will post any findings.

 

[GoesTo11]

As best as I can tell it is wired as series parallel. One positive runs from the jack to the top right speaker from that negative terminal it goes to the top left positive terminal when looking at the back of the cabinet. There is a negative from the jack to the top left negative. This is duplicated for the bottom speakers. With my multimeter all speakers show 7.6-7.9 Ohms except the top right which read 20.6 Ohms. At the jack I got 14 Ohms so something's not right.

 

[GoesTo11]

I disconnected the speaker that showed 20 and then connected the top left speaker. Now I have three working speakers so I am going to take the bad speaker for repair. I have to say it's sad to have to have the speaker repaired but working is better than original vintage I suppose.