Mesa Boogie Mark IV

[pnut5150]

I am searching for the 2 resistor values, this is a Mark IV B. The resistors are R123 & R124..I cannot find a schematic

[Chuck H]

It seems that no version of the Mark IV schematic indicates if it's A or B model and none include the power supply. I peeped a bit but didn't find any R123 or R124. Can I assume these are in the power supply? And if so it may be good to know why they need replacement before replacing them.

[pnut5150]

The circuit board around the two resistors has been repaired before, the trace was burned and damaged pretty badly. Somebody has also changed both of these resistors and 123 they have tied two resistors together, I was curious of the correct value. I am assuming they tie into V1, V1 is a 12AX7 that feeds from the input from what I can gather. V1 is not working therefor the amp has no sound.

[Chuck H]

Somebody has also changed both of these resistors and 123 they have tied two resistors together,..

I'm sorry. I don't know how to interpret this.

I am assuming they tie into V1

From the power supply?

V1 is not working therefor the amp has no sound.

I'm going out on a limb here with very little information, but... I'll guess those resistors ARE in the power supply because they don't appear in the available Mark IV schems that exclude the power supply. If V1 is not working because of a failure in the power supply it means that too much current was passing through that part of the circuit. The possibilities are a shorted preamp tube (not very likely), a bad cathode connection or shorted cathode circuit for V1 that caused the tube to "run away" and draw too much current (still not very likely) or a shorted filter cap in that part of the power supply (very likely). Simply replacing that trace and those resistors may not fix the amp because the burned up resistors are not the problem, but a symptom of the problem.

If V1 is not glowing at all then you may have a failure in the filament circuit for as yet unknown reasons. This may be an additional problem or related, it's impossible for me to say. But it would be unusual for a couple of burned resistor to cause only one tube filament to not heat. Unless the "B" model does something like operate the first preamp tube on a DC filament supply for noise reduction. So that might be something else to consider. And as with the other scenario it's probable that the burned resistors are not the problem, but a symptom. Discovering why the circuit is drawing too much current through those resistors is the only way to reveal the problem and repair the amp.

Not a schematic, but I hope it helps before you're done with this.

[pnut5150]

I have no clue of the resistors purpose due to no schematic. The values just seem way Off is why my curiosity of the correct value. From what I have gathered on the net, V1 has its own 12V supply direct from the power transformer though. None of the LEDS are working on the amp and no sound, I have gathered also that problem with the fried trace is a related symptom to no sound/LED,

[Enzo]

Way off, means? If they are way high, they could be damaged. If they are way low, it just means something is in parallel.

[pnut5150]

One was like 22ohm and the other was 330+k

[Enzo]

And what are they supposed to be? What are their color codes?


Also, when resistors measure funny, try reversing meter leads. If you get the same reading, fine. If it changes hugely, then there is some small residual voltage in the circuit.

[g1]

See attached pic. I don't trust them (any boogie schems) to be correct so no idea if the component numbers match up. I would trace out the related circuits and see if they actually go where the schematic says.
Pg.3, in Q10 circuit is R123, 5K6. Between E and F is R124, 6R7.

[pnut5150]

Thanks G1!
Enzo, I pulled the one that reads 22ohm, according to the resistor calculator I use it is correct, the other I’ll post a pic of how they replaced it..I hate when I run into a mess like this.

[Leo_Gnardo]

Thanks G1!
Enzo, I pulled the one that reads 22ohm, according to the resistor calculator I use it is correct, the other I’ll post a pic of how they replaced it..I hate when I run into a mess like this.

Me too. Couple years ago I had the annoying and time wasting experience of trying to fix a MkIV someone spilled their soup into. I think it was fish pho^. Smelled awful when the amp was warmed up. In any case there were several 39 ohm resistors flamed out, they were series current limiters to LED's within photocell modules. Plus several darlington transistors for which I could find no substitute, so I paid Mesa a reasonable price say 75 cents each plus $15 priority postage, the only way they would send them. Oh well, there goes the lunch money... Spent many a happy hour removing components from the circus board and swabbing it with 95% ethyl alcohol followed with a distilled water rinse to get conductive greasy pho^ film off the board. Eventually I got it working, mostly. A bit noisy but that's the best I could do. Customer of course none too happy I couldn't work a miracle. He won't be back. Can't win 'em all. Good luck with yours!

[Chuck H]

Great coming through with the schem g. We need to keep it on the DL I think since Mesa (apparently) doesn't want it in the public domain or it would be available with a *oogle search. Maybe pnut could report that it's uploaded to PC and it could be removed? I wouldn't want the forum or any repair techs here to have to fuss with their relationship with Mesa

Since neither R123 or R124 is 22 ohms the schematic or the circuit is already in question. If R124 is the one reading over 330k that would explain V1 and anything on those low voltage sources not working (provided the schematic is accurate?). Which could include some of the peripheral things you mentioned.

[pnut5150]

Haha sounds like you had a major mess!! Bottom of this board is damaged pretty bad from previous repair, they scratched board pretty severe and looks like a small hole in board as well. Board is black around the two resistors in question as well. I’m sure I will pull my hair out, Mesa is a mess to begin with and customer service won’t help except for trying to save your hair and get you to send it in lol

[Chuck H]

Mesa is a mess to begin with and customer service won’t help except for trying to save your hair and get you to send it in lol

That's a disheartening read Mesa was always good to me WRT customer service. Freely providing schematics (accurate?) and they even repaired a known hum fault in an amp of mine for the cost of shipping!!! And I had bought the amp used! There were times when I had to excuse myself from the conversation because their techs were so talkative and engaged. Sorry to hear that things have changed to this. That is, if you did call them? If the Mark IV-B has an inherent failure in these circuits I would have expected the old Mesa (Boogie) to help generously as a matter of honor.

[pnut5150]

I have it saved to my phone if it needs taken down, thanks! I haven’t called Mesa on this one, I called about a Roadster a few years ago, schematics are top secret. After I called multiple times they finally released the power section part of the schematic I needed. Very friendly people, just not helpful with schematics I guess I should say.