Jcm 900 16 ohms output does not work!

[ilarranaga]

Hi guys,

I have a JCM900 (4100) amp and a JCM900-1960A. Since the post "Marshall Amp Forum > The Amps > Marshall Amps > Amp-cab impedances mismatch (or not?)", I changed the way I use them, and now I use the amp and the cab in 16ohms configuration.
Today, as almost every day, I switched on the amp and I started playing my Les Paul. All was right until the amp started to sound intermitently and after few seconds, it stopped completly.
I checked the cables with other amp and there are Ok. So, I tried to change the impedance configuration, configuring the cab as stereo use (8 ohms) and selecting the 8 ohm output in the amp. With this configuration all is ok, it sounds well and I used it for 2 hours with no problems.
After this I tried to set again the 16 ohms configuration, but it did not work.
So, it works with 8 ohms configuration but it does not work with 16 ohms configuration (only sometimes intermitently).
With these symptoms, I think that the OT is OK, since I think that if it was damaged, nor 16 ohms neither 8 ohms would work, but I am not sure. Perhaps it is damaged...
I think that perhaps the 8-16 switch is damaged or maybe the green cable is not well connected to the OT pad.
Tomorrow I will open the amp and check this points.
What do you think about it?
Should I check something more?
The problem is that I have two shows this weekend and I need my amp working properly as soon as possible, no time to go to a store or service...

regards

 

[impetus maximus]

the jack could be bad or it's solder connection could be loose.
you might wanna double check your wiring on the cabinet. a multi tester can
check the ohm's on your cabinet without opening it up.

if you don't have a backup amp, get yourself one.

good luck

 

[ilarranaga]

Hi,

I checked the output with a multimeter and it seems to be OK. I measured de impedance between GND and the 8 ohms pad and between GND and the 16 ohms pad (from the jack) and it seems to be ok, the impedance is low enough to think that the OT is ok.
So I measured the input impedance of the cab... and here is the problem!!
In stereo configuration, each input is around 7-8 ohms. Remember that the system work with this setting.
In mono configuration, the input impedance of the 4 ohms input is 7-8 ohms!! and the input impedance of the 16 ohms input is... is open circuit!
My diagnosis is that or the solder wrong and I have a bad contact, or the switch to select mono-stereo is broken.
I will solder the contacts and check again the input impedance.
Do you see something wrong in my reasoning??
Thanks!!

 

[Purgasound]

Good thing you checked the cab. Open her up and check each speaker individually to make sure they are all ok. More than likely you have a blown speaker coil.


I would suggest hard-wiring the cab for 16 ohms eventually.