DC 3 help, lead channel missing gain

[nisatis]

Hi there

I just got a Mesa DC-3 combo that is a great little amp but this one is weird on several points that I noticed when comparing it to another DC-3. The other one is a revision B and the weird one a A:

1/ I wonder if the cab is the original one or if it's coming from a Mark combo as the tolex and grill cloth do not look like ones that are usually found on a DC-3 and also there seem to be a gap between the chassis and the baffle filled with some foam, which is not the case on the other DC-3 (see pictures) and doesn't look like a nice finition as I would expect on a Mesa. Also there are some holes on top of the cab where the screws to fix the chassis are fitted. Do you think this is the original cab?

so here is the link where the pictures are: http://s1077.photobucket.com/albums/w471/nisatis/

with

my DC-3 (with holes on top):

close up for the gap with foam in it:

same close up with the revision B one (no gap):

2/ the gain available on the lead channel is really weak, when I put the gain on 10 it more or less corresponds to the gain on 5 on the other one. All the preamp tubes are 12AX7 while on the revision B one, there is a 12AT7 in the PI, so it should be the opposite, mine should have more gain than the revision B? The rythm channel sound about the same on both amps. I doubt that the difference of gain in the lead channel is due to the difference of versions, I am not a high gain player but there is definately not much gain on tap for this one and it doesn't seem to be a tube issue (I swapped all the 12aX7 in turn with another one to make sure the culprit was not one defective preamp tube), so what else could it be, could the amp have been modded somewhere?

Also, I don't know if this could be linked to the lack of gain but the blue lamp indicating that the amp is on is much weaker than on the revision B one

Thanks for any help

Nils

 

[nisatis]

no opinion anyone?

 

[Monsta-Tone]

Nice amps! It does look like a Mark cabinet. Might have been a custom order. Either way, it looks sweet!

Sounds like you might have a preamp tube issue.

V1, V2, & V3 affect the lead channel.
Try swapping a known good tube in to each spot, one at a time and turning the amp on. If the gain comes back, then you have found the issue.
You could easily use one of the preamp tubes (not the AT7) from your other one. This way, you can do it right away and you don't have to wait for a new tube to arrive.

Do a search here and you will find tons of information.

 

[nisatis]

Hi again,
well I finally got the time to open the beast and as I suspected there was some changes made to the circuit, see picture below where I highlighted the resistors I think were changed:

1: the 470 K was disconnected on 1 side and a 360 k added in serial
2: the 7.5k resistence seems to be a replacement of another one, but which one (it says 680 on the board but that's really a big difference then)?
3: seems like 2* 1.5k have been removed, or only 1?
4: seems to be a 13 k but replacing which value?
5: 2* 330k in //, again what was the original value there (82k?)?

Does anyone have a picture of this area of the circuit or could tell me the original values for these to allow me to put back the original values in? or I guess I should be able to figure it out from the schematics but I find it difficult to make the link with the circuit...

note: with these mods the lead channel sounds good but with only a very light overdrive, so this could be interesting to s.o. who wants to really tame down his DC and just plays blues...

 

[bendo]

This is a Stock DC3 manufactured 9-99. Any other photos and I have to pull the chassis. Yours appears to be pretty heavily modified. Good Luck

On another note, whomever modified that board was not very skilled in their endeavor. I hope you're better with an iron than they were :mrgreen:

 

[nisatis]

Thanks for the picture, this is very helpful!
I hope to do a better soldering job, but I am no expert myself....!

 

[D2dark]

Spend a ill more on that beast and send it to Mesa. They can bring it back to life for sure !

 

[nisatis]

Well I put back the original values for 1, 2 and 3 and this restored enough gain tap for me. I left the 4 and 5 as is since these are obviously just affecting the tone stack but I am happy as it sounds now so I prefer to avoid extra de-soldering/soldering... although I still wonder what was the purpose of theses changes on the tone stack (doubling the resistors values between the treble, bass and middle filter caps , what's the effect?? maybe less influence from one tone control change on the other 2's..?).

For Mesa support, I live in Belgium and unfortunately we don't have as good support here as in the U.S., and I don't see myself posting an amp to the U.S.! So I guess I just have to learn repair it myself, it takes some time but it's fun and cheaper! (I also have an old DC-5 that pops loudly when switching channels...so that's some fun in perspective to find out and repair this one!).

Anyway thanks for the helpfull answers from you lucky DC owners!