1990 Mark IVa -- Jack on the bottom of the chassis...

[dunkonu23]

Hi,

I have a 1990 Mark IVa which I am pretty much restoring. On the bottom of the chassis above the power transformer and the foot switch XLR jack is a 1/4" unbalanced jack. I found the same jack here:

http://forum.grailtone.com/viewtopic.php?t=3842&postdays=0&postorder=asc&highlight=mark+iva+chassis&start=30

I can't find out what it's for, though. Here's a couple of pictures I took today...

Tube side of chassis:

Underside of chassis:

Does anyone know the purpose of the jack?

Thanks!

Scott

 

[ketiov]

Hi, mine is a bit later than yours, but not by much. I think it may have something to do with the graphic EQ, seeing as it is below the switch on the back that has to do with the GEQ and there is a little triangle/arrow pointing at it.

 

[dunkonu23]

Thanks for the reply. I just looked and the little caret has "Remote" by it. If it's for EQ that means it's in addition to the other EQ switch port.

Scott

 

[MrMarkIII]

Uh, why not plug a foot switch into it and find out?

 

[dunkonu23]

I did think about that before posting. 8) The answer is easy... I don't know if it a momentary contact switch or an actual closed/open switch. I don't want to damage the amp.

Scott

 

[MrMarkIII]

If you plug a latching f/sw into a jack meant for a momentary, all that's going to happen is you will have to click the switch twice instead of once to switch the, uh, switch. (Or maybe it's the other way around.)
The caveat I would offer is to put the amp on standby when plugging in or out. Otherwise, you could get a speaker-ending POP.
Hey, could that jack be for reverb on/off?

 

[dunkonu23]

Good question... it doesn't seem to be a reverb on/off because it's wired straight to the main board--you can see it in both my picture and the pictures in the link to the other thread in my first post. In looking at the schematic, there appears to be no reverb-on-off circuit, so I would assume if it were a reverb switch, it would have to be in-line and open, so the reverb wouldn't work. Oddly enough, I just replaced the reverb tank today and it works, so there you go.

I dunno... I may just have to take it apart tomorrow and trace the circuit. I was hoping to avoid that, though. I'll try the switch tomorrow, too. I'm getting braver as the night wears on.

Thanks!

Scott

PS To clarify, there was a question about the jack I'm concerned about in the thread I quoted, however there was no definitive answer. One person thought it was for satellite EQ and one thought it was another eq switch.

 

[MrMarkIII]

If this is what it looks like, its definitely for switching the Graphic EQ on a Satellite.
Case closed.

 

[dunkonu23]

That's what it looks like... I guess the case is closed! Thanks!

Scott

 

[Shep]

It's for a link cable to go from your mk Iv to your satellite combo..so you can turn both amp's eq with one click!.. pretty boring.aha

 

[dunkonu23]

Thanks, Shep. I suppose what got me about it was other than the fact it's connected to the main board, it looks like installation was an afterthought.

Scott