Mark IV problems

[Mag]

I purchased a used Mark IV and it has some issues that I hope this forum can help with.

Here are my issues:

- footswitch will not go to lead, no red led on footswitch or front of amp, footswitch feels good upon tap on switch, can be manualy switched with knob on back then Lead led will light and play, R1 and R2 footswitch work ok
- EQ front panel led lights up but real dim, stays on most of the time,
- R1 master level has no control over loudness
- Lead voicing switch on back feels "loose" on switch, meaning no firm click, and switch moves between up and down ( sure its broke)

Prev owner states tubes were just replaced.

Last question is were is the serial number located at, its a short body.

I've been playing a wide body for sometime, but it was a shared amp we payed 950 for a couple years ago, this one cost me 600 from a friend.

Thx in advance


Mag

 

[GD_NC]

I'll start with the easy question. The Serial number is next to the output transformer if you look underneath the chassis.

For the other problems. Unless you really know amp repair, I'd take it to a Mesa certified shop or send it back to Mesa for repair. Those don't sound like minor issues.

A few things I learned from owning a couple of Mark IV's over the past 20 years...

The switching system in a Mark IV is fairly complex and dependent on many optical relays. When/if these relays start to fail you start to see dim/blinking LEDs, channel switching problems, loop level issues, etc.

The rocker switches do wear out eventually but they are easily replaced.

LED's stop working and occasionally need to be replaced or re-soldered

 

[Fronzil]

for the EQ LED on the amp..when its dim, just push it gently in towards the amp haha..it works..may go dim again in a week or so, but just repeat the process

 

[Mag]

Thanks for the quick replies......

GD_NC,
Thanks for serial # info, will check..... @ taking to the Mesa shop, well op amps and flip flops are easy for me.
I took the foot pedal apart last night and checked the two circuit boards out. Saw the three IC's, heck they even have removable sockets, so its as easy as replacing a tube, I will stop by Fry's to pick up a couple, cant be over $20 for all three..... regarding the footswitches for the R!, R2 and Lead, they are all momentary units, but are they all N.O ( normally open, or closed ) , dont wish to unsolder if I dont need to....

Couldn't find a schematic for the foot pedal after a number of searches, even sent mail to mesa asking, will advise as to what I find out for you all.....

Also asked mesa for a detailed breakdown of the unit assembly like one Fender sent me for the Stage 100. Really dont need it, but it would be nice....

Fronzil,

Will try on the EQ led, thx too

Mag

 

[Mag]

Just got my parts from Mesa today, we had alittle prob with getting the right switches and R1 Master Pot. Will be installing this weekend.


Word of wisdom: give Mesa your serial # so they can match it with the right parts, even if they dont ask for it ....



Mag

 

[Mag]

Plan A
Solder the 100K ohm pot, and the mid gain - lead voicing switch in, wasn't bad at all if you have done this kind of work before. Fired it up but still had the same issue; cant switch to Lead with footpedal FU-3. Don't run these amps without a speaker or load attached from what I've read.

Plan B:
Borrowed a FU-2 from a friends Mark IV Wide, the 5-6 pin Din connector was the wrong sex, FU-3 requires is a male plug, and FU-2 has a female end.

Plan C:
Replaced 3 IC's on FU-3 Footswitch unit, no help.

After careful examining of the schematics downloaded from Mesa, noticed two of the same IC's that were on the FU-3, were also on the main logic board, in the same general area as the switch on the amp. Swapped those out with the ones I pulled from the FU-3 and that did it. The FU-3 pedal now works on all three channels.......

Now onto my 300W power amp, its status lite is red for channel A .....


Mag

 

[fretout]

Here are the things I did to fix the Mark IVs in the past (for you or anyone else that might have these same problems with their nMark IVs)

- footswitch will not go to lead, no red led on footswitch or front of amp, footswitch feels good upon tap on switch, can be manualy switched with knob on back then Lead led will light and play, R1 and R2 footswitch work ok - May be a bad LDR. Either replace it yourself ort take it to a shop to repair.

- EQ front panel led lights up but real dim, stays on most of the time - Press on the bulb itself and it should light back up to normal operating brightness. Vibrations can rattle it loose again, so if it starts to dim/shut off, press it back in place.

- R1 master level has no control over loudness - Replace master level pot, or check leads to make sure they did not become "unsoldered".

- Lead voicing switch on back feels "loose" on switch, meaning no firm click, and switch moves between up and down ( sure its broke) - replace switch.

As for the serial number, there should be a sticker near the Transformer that has the typical "Mesa/Boogie Spirit Of Art In Technology" phrase on it with a chrome stripe. In the chrome stripe will be the serial number (it should read as IV-XXXXX). If the sticker is missing...it may be a really HOT amp.

I was about to buy a Mark IVa medium head a few months back for some guy from Russia (I think). The amp sounded great, looked great, ect. I looked at the back of the chassis and noticed that the serial number sticker was gone, so I asked him what happened to it (expecting he would say that it fell off or something), and he immediately got defensive and said that he never stole the amp. I never implied that he stole the amp, but apparently someone had a guilty conscience!

As far as I am concerned, no serial number = no purchase.