Mark III EQ problems

[bchulse]

Just got a Mark III Red stripe 1x12 Imbuya/Wicker combo on
Ebay. Didn't come with any footswitches. Everything appears to
work fine except the EQ. Doesn't work with the front toggle
in any of the 3 positions. I'm assuming it should work in the
"EQ In" position regardless of whether there is a footswitch
connected or not. Is this correct?
Or is it possible the EQ was turned off via footswitch the last time
the amp was used and can only be turned back on with the F/S?
That doesn't seem like it would be the case.
Thanks for any comments....

 

[acorkos]

with no footswitch connected, the front panel switch is functional. if you don't notice a change in tone when switching the EQ in, there's something wrong with the EQ circuitry.

 

[steve_k]

With the switch down on a red stripe, the EQ should be always on. You will notice gobs of difference. You need to let the seller know and carry it to a repair shop.

Steve

 

[bchulse]

Thanks guys. That's what I suspected. I've got 3 other Mark
amps and of course it's easy to tell when the EQ is active.
I'll take it in for a good going over.

Happy Holidays!

 

[steve_k]

Thanks guys. That's what I suspected. I've got 3 other Mark
amps and of course it's easy to tell when the EQ is active.
I'll take it in for a good going over.

Happy Holidays!

Who was the seller on ebay? Waiting on my IIC+ to show up and hope all is well with it.

Steve

 

[Boogiebabies]

It's most likely the LDR supply or a bad LDR.

To start, the LDR voltage needs to be checked at the large 680 Ohm resistor near D203 and in front of a large 220uf
63V capacitor(Center far left on PCB). One side of the resistor should read to ground around +30V and the other side around +12V. If not, you need to change to the 680 Ohm
resistor with a 2-3 Watt power type as this resistor gets a lot of heat.

If your other switching functions work, it's more likely a bad LDR for the EQ ground.
On the top right corner there is a LDR near D30 where a Yellow, Green and Gray wire connect to the board from the
EQ fader board. Jump the two leads on the backside near the purple wire from the lead master and see if the EQ kicks in.
If not, jump each leg over the LDR to the opposite leg as sometimes the cell still has a very large off resistance and a jumper in parallel
won't work.If the EQ kicks in, change the LDR with a Vactec VTL5C4. You may need to change it anyway to cure the issue.

The best place to get them is Allied Electronics.

http://www.alliedelec.com/search/productdetail.aspx?SKU=9800716

 

[bchulse]

Steve_K, Ebay seller received amp through an inheritance and was not
a player, so I don't blame him for not knowing. But he did say that the
amp had gotten a clean bill of health by a tech before it was posted.
I also got a IIc on Ebay a few weeks ago and everything works perfectly.
Good luck with yours!
Boogiebabies, thanks for the detailed information! I'm dropping off the
amp this AM at my local tech. I'll pass this info on to him....

 

[238839]

hmmm...just to be the Devil's Advocate...is there a 1/4" jack plugged into the EQ jack on the back of the amp? If there is, unplug it and the EQ on the front should be OK.

 

[Boogiebabies]

hmmm...just to be the Devil's Advocate...is there a 1/4" jack plugged into the EQ jack on the back of the amp? If there is, unplug it and the EQ on the front should be OK.

Good point. Maybe the footswitch contact is bent as well.

 

[bchulse]

Nothing in the EQ jack. I didn't slide out the chassis.
to check to contacts. Dropped it off for a checkup at
my local tech. The tubes are pretty old and the stem
on the treble pot was bent also, so it has some maintenance
issues. Should have it back in a week or two.
Thanks for all the suggestions!