Mark IV lead Channel help!

[Brewjangle]

Recently I bought a MIV widebody combo and had to immediately send it off to be repaired due to the EQ going out after about 15 minutes because of a bad LDR. Well, after a month of waiting I fianlly get it back only to have traded a working EQ for a lead channel that doesnt work. Im would like to know if anyone here can give me some troubleshooting tips to try before admitting defeat and having to send it off for another month.

When on the lead channel I get some sound but its very low volume and has almost no saturation. Even cranking the volume and gain do nothing more than make what little is there slightly more present. Ive switched tubes hoping it was just a bad one, but that didnt help.

If you have any tricks or tips to try please!!!! let me know.
Thanks in advance for any and all advice!

Brew

 

[Boogiebabies]

Sounds like they ruined another LDR, or damged it with the heat from a Soldering gun. That amp as a ton of LDR's and you really need to know what your doing to replace them. I'll take a look at the schematic and see what LDR's are responsible for the Lead and EQ. They also have to be the right LDR as Vactec makes single element and dual element LDR's so the wrong LDR may have been put in.

The MK IV is not the easiest amp to work on.

 

[Brewjangle]

Thanks so much for the reply, Boog. I had a feeling that was the problem. Guess it goes back to them this week.
Are there schematics of the amps online?

 

[Blueracer]

Hi Brew,

Sorry to hear that you are having troubles with your Mark 4. They are truly a great amp although I am sure you are somewhat frustrated with yours at the moment. There are several sources for schematics on this amp. Here is a link to one of these.

http://www.schematicheaven.com/boogieamps/boogie_mkiv.pdf

As has been pointed out there are a lot of LDR's in this amp. The ones that makes sense to me as being suspect are either LDR 11 or 12. These are on sheet 1 of the schematic and kind of above and in between V2A and V1B. The purpose of these LDR's is to ground out the lead gain stages (V3A and V3B which are on the second page) when in the Rhythm 1 & 2 modes. When in the lead they both should be off with LDR 10 being on to complete the circuit to the output of the lead stages. Now if LDR 11 or 12 is on when in the lead mode this will not only ground the output of the lead stages but also the output of V1B which is being summed with the lead stages to the input of V2B. This effectively stops all signals coming from the amp's input at this point.

To check if this is a reasonable possibility as your problem do the following. Get a preamp and connect its output to the fx loop's receive input on the amp. If you don't have access to a preamp try a pedal that has some gain. Choose the loop on the amps peddle and hook a guitar to the preamp's input. Make sure the amp is in the lead mode since this is what we want to test. The volume should be substanntially louder and controllable through the output level control. Now if you do get some volume doing this at the very least it will point to the preamp/lead section as the problem. This is tubes V1, V3 and V2A. Since the amp seems to work in the Rhythm modes V1 and V2A are not good canidates all of which points to a switching issue in V3.

Note one might argue that LDR 8, which controls the input to the lead circuit in V3, as the culpret. However you would still get some signal since the path through V1B is still active and being routed to the input on V2A. I don't think this switch is a likely canidate.

While testing of this nature will not cure your problem at least you can start to isolate it. If all goes according to the plan above it would eliminate the power output stage, the eq stage and V2A as being the culpret. Hope this is somewhat helpful.

Gary

 

[Blueracer]

Just spotted another potential LDR that could be the problem. This is LDR 4 which hooks up the lead tone stack to the input of V1B. As has been pointed out there are a LOT of LDR's in this thing. If you are good with electronics you might try to jumper across this. If not leave it alone.

Also some new tests. Turn your lead treble, mid and base all the way up. Try varying the lead gain and see if it makes a difference in volume. Now at maximum lead volume, treble, mid and base try varying the lead drive. This is going to be somewhat subjective but if the lead drive has more of an effect this would support LDR 4. If both have relatively little affect it would point to the LDR 11 or 12 as the culpret. Again I hope this all is helpful.

Gary

 

[Brewjangle]

Blueracer, Thanks. I dont feel that I could do anything with this type of electronics. Its way above my level at the moment, but hopefully that will change so I can do this type thing myself. On to the amp. I have a triaxis. Will this work as the preamp? I imagine it should. Just making sure. Also, I have plugged in and turned the tone controls to 10 and then played with the gain. It does get more full and louder. Still, at the level i have to run the volume to really hear anything is deafening when I switch to one of the other channels.

Ill hook up the preamp later today and see if I can narrow doen the problem. Thanks for the site to the schematics!

Brew

 

[Brewjangle]

Blueracer, Thanks. I dont feel that I could do anything with this type of electronics. Its way above my level at the moment, but hopefully that will change so I can do this type thing myself. On to the amp. I have a triaxis. Will this work as the preamp? I imagine it should. Just making sure. Also, I have plugged in and turned the tone controls to 10 and then played with the gain. It does get more full and louder. Still, at the level i have to run the volume to really hear anything is deafening when I switch to one of the other channels.

Ill hook up the preamp later today and see if I can narrow doen the problem. Thanks for the site to the schematics!

Brew

 

[FPFL]

Well, after a month of waiting I fianlly get it back only to have traded a working EQ for a lead channel that doesnt work. Im would like to know if anyone here can give me some troubleshooting tips to try before admitting defeat and having to send it off for another month.
Brew

Brew,

I'm assuming the "They" in your quote is Mesa Boogie? If so, WTF? I'm a little stunned that they fixed it by breaking it differently. Again, if its them, have you called them? I'd be pretty pissed.

Either way you are a more peaceful man than I and I tip my hat to you.

-P

 

[Blueracer]

Hi Brew,

The Triax as a preamp will be fine. In addition to trying the gain do try and adjust the lead drive as this is the control for the additional lead gain stages in V3. See how much difference it makes. It is clear that the signal is getting choked off somewhere. These test are really to try to determine where the likely place is. It is also meant to show how to systematically approach these problems and isolate them and how to eliminate things that are working. Hope this is helpful. Unfortunately I think you will be stuck with sending the unit back. The good part is it will be worth it in the end. The Mark 4 is an outstanding amp (when it works).

Gary

 

[jamme61]

don't forget to check your preamp tubes. I had the same thing happen when I knocked one of the 12ax7's out. I had the amps chasiss out and when I put it back in I knocked the tube out. It's worth checking the 12ax7's.