Sick Mark IV

[rtcook]

Hello all,

I just purchased a non working 1993 Mark IV. I was looking for a little guidance for a speedy repair. Maybe someone has run across this issue before and can help me out. Here's what I know. When I bought the amp, I brought it home and plugged it in. It was silent. No noise but the usual hum. The lights light, it switches channels, EQ light switches, loop light lights, all heaters come on in the tubes. My first thought was to check the power tubes. I checked them on my Hickok and they all tested well. So, I swapped out all preamp tubes for brand new Tung Sol's. Now, I have some volume, but it is very weak. It's like at bedroom level, but on "10" and poor bass and sounds tinny. I verified all knobs work, EQ works, presence, etc. My first worry was the output tranny was shot, but now that I have sound I am thinking it must be ok. Is that a fair assumption? Also, I am now questioning the power tubes even though they checked out. I have a brand new set coming from Mesa soon. So, based on what I have described anyone have any pointers for me before I attempt to check out this maze of circuits? I'm hoping this will be an easy fix. The guts of this thing look more complex than anything I've worked on before. Plus, it doesn't look real easy to work in. It's really stuffed full.

Thanks in advance for any help,

Roger

 

[Restless Rocks]

Verify that the Pull Silent Record switch ( on the Output control) is pushed in and working. Also try a jumper cord in the FX loop.

 

[rtcook]

Did both already. Pull Silent Recording works. Like I said, I have some sound. So, when you pull the switch it quiets the amp and when you push it in you get sound, just not very loud with the amp cranked to 10. Sounds like it has no power. Did the FX loop trick as well... nothing.

Thanks,

Roger

 

[rtcook]

Anybody?

 

[dodger916]

Have you tried posting this question in the Vintage Amp sub-forum?

 

[rtcook]

No, I did not. My amp is a 1993 and the vintage is 1991 and prior. I didn't want to make anyone mad.

Roger

 

[Monsta-Tone]

I would definitely try a new set of power tubes.

You might also try swapping 1 known good preamp tube in at a time. I've had a bunch of preamp and power tubes that were bad right out of the box. The old saying that "They don't make them like they used to," is very true when it comes to tubes.

You might also be able to rule out a few things by using the rotary knob on the back to select each channel and see if maybe one channel has more output than the other.

It's pretty hard to diagnose a Mark IV without seeing it. You might see if there is a Mesa tech in your area that someone here recommends.

 

[rtcook]

The amp has all new tubes from Mesa Boogie. The only differerence it made is that it go louder. It's only at bedroom level with all volumes dimed. All controls work, channels switch, etc. The only problem I can see it that it is not loud. I have worked on amps, but have not explored this one. If anyone can give me an idea of what they think the problem is I can start testing. I'm thinking the output tranny is not shot since I have some volume, but I am not sure. Any other suggestions before I tear into it?

Thanks,

Roger

 

[BoogieDown]

Perhaps it's an output transformer issue? Also, is this a a combo or a head? Gave you tried different speakers?

 

[Monsta-Tone]

I had a DC-2 a long time ago that sounded great at lower volumes, but would crackle and sputter and cut out at higher volumes. It had a burned coil in the OT.

I would definitely rule out any cables, guitar, speakers, etc. if you haven't already.

 

[rtcook]

Its a combo and I have tried different speakers with the same result. Cables are fine and so is the guitar. I would like to eliminate the possibility of the output tranny being bad. Anybody know how to test it?

Thanks,

Roger