DC - 3 Hum problems on Rhythm Channel

[Jimmy74]

Well I took all the readings on the power tubes and here's what I found:

V7
Pin 2 = -10.74 voltsDC
Pin 7 = 438vDC
Pin 9 = 378vDC

V8
Pin 2 = -10.57vDC
Pin 7 = 433
Pin 9 = 378

V9
Pin 2 = no voltage???
Pin 7 = 436
Pin 9 = 379

V10
Pin 2 = -13.1vDC
Pin 7 = 437
Pin 9 = 378

Obviously V9 is the where the redplating occurs..... but what could be the cause? The 220k or 330k resistors?

 

[Jimmy74]

Well today I found out what the bias problem actually was, the power tube board had been screwed in too hard and caused the circuit paths to break and scratch off, so with a bit of wire I re-wired the paths and now the bias problem has disappeared but the hiss problem is still there!!

 

[Jimmy74]

Well I also took the readings of all pre-amp tubes and mainly pins 1 & 6 of all pre amp tubes were slightly under the readings stated on the schematic:

Tube Pin 1 Pin 2 Pin 3 Pin 4 Pin 5 Pin 6 Pin 7 Pin 8 Pin 9

V1 125 ---- 1.0 0.1 0.1 125 --- 0.7 0.1

V2 122 ---- 1.0 3.2 3.2 124 --- 1.0 3.2

V3 120 ---- 1.0 Heaters 81.6 --- 0.7 Heater

V4 131 ---- 0.9 Heaters 131 --- 0.9 Heater

V5 280 ---- 2.4 Heaters 71.1 --- 0.5 Heater

V6 247 ---- 2.3 Heaters 241 --- 2.3 Heater

What do you think?

 

[Jimmy74]

Ok folks here's a really good question for you all.... the schematic states that heater voltage is to reach from V2-V6, I suppose that's a writing error, but in that case I'm not getting heater voltage on V1...... any ideas?

 

[adam972]

ya i see what you are saying that the schematic omits V1. The DC-5 schematic includes V1....and i am really confused why the DC-3 does not, unless it is some amazing really stupid typo...which makes just as little sense to me as not have V1 heaters....were you getting 3 vAC from ground to all of your heater pins?

 

[Jimmy74]

Yes all the other tubes inluding the EL84's correctly have heater voltages! This is really strange, does anyone know for sure that V1 should have heater voltages?

 

[adam972]

well just the basics of tubes, V1 would need the fillament wires, so it must just be a glaring omission by mesa boogie...

 

[Jimmy74]

Ok everyone, I've taken the board out of the amp and the first thing I've noticed is that the heater connections on V1 are very different and not connected to heater paths of the other 5 tubes. Here are some photos:

http://www.flickr.com/photos/73835524@N00/3322375700/

http://www.flickr.com/photos/73835524@N00/3322375698/

if anyone could give any suggestions, maybe a tech at Mesa would be able to put some light over this mystery??

 

[adam972]

hey i found it! on page 7 of the schematic, you can see that V1 is getting its heater power from the switching matrix...why it is done this way i dont have a clue. The switching matrix is hooked up to the 3vAC filaments from the transformers, so V1 should be getting heater juice just like the others. Were you saying you were getting no readings for you V1 heaters? or were they reading and you just couldnt see where they were connected in the schematic?

 

[Jimmy74]

I'm getting close to no readings on all 3 heater pins, 0.1vAC, where do you think the problem could be, in the bridge rectifier or the 6800uf caps?

 

[adam972]

if you are getting bad readings there, i would start going backwards until you get to the 3vAC input points on page 7 of the schematic. If you are getting a bad reading there, i would be suspicious, as this looks like it comes right off the main heater line. I guess this means that your V1 heaters are running on DC....from what i can gather from page 7

 

[Jimmy74]

Well the wires all give continuity untill they get to the rectifiers and caps so my guess is that one or both of the caps has failed or the diodes aren't doing what they're supposed to do. BTW the diodes are soldered to the main heater line... so that means V1 is probably getting 3vDC and not AC..... would the tube still work?

 

[adam972]

from what i see, Mesa intentionally gave V1 DC heaters....not sure why, but it looks like they did. If you can read the 3vAC before the diodes then i suspect that one or more of the diodes is messed up.

What worries me, is that if this whole time you V1 tube has not been functioning, you really should have been hearing a very quiet amp. Let me ask you, if you set both channels to the same settings, do you hear a noticable volume jump from one to the other?

 

[Jimmy74]

Well when I received the amp there was no volume jump, the rhythm channel acted properly just with more hiss than the supposed to be more noisier lead channel. I changed out the 100k resistors as advised by monsta tone and that's when I started having other problems. So it could be a short along the line of the heater voltages to V1 but it could be a messed up rectifier diode in that section.

 

[adam972]

you might have done this, but since V1 appears to have DC heaters, did you set your multimeter to DC...just an offhand shot. If V1 has been dead this whole time, i would have thought you would have seen very poor output. you should be reading 3V DC on the V1 heater....at least it looks that way since it gets recified to a -3 +3 v output.

 

[Jimmy74]

I suppose MEsa used this method inorder to keep V1 as quiet as possible using DC heater voltage.... I'm having a hard time finding those 6800uf caps and another strange thing I noticed is that the schematic rates them at 16 volts, whereas the caps on the board are 6.3volts.

 

[adam972]

well thats seems odd....do they look newer and may have been replaced? if the schematic is calling for 16v and they are only rated for 6.3v i would def swap them out for the correct caps.

 

[Jimmy74]

Yeah well I did get some 6800uf 25 volt caps but they're way too tall, I think that's why mesa chose to put 6.3 volt caps instead. I think the ones in there are original..... does anyone know a safe way of pulling the board out of this amp?

 

[adam972]

what do you mean safe? other than the fact that its a pain in the *** to pull the board on a DC. Usuually you wil have to unbolt all of the pots, desolder all of the power, standby and channel switches, indicator lap, channel LEDs, then squeeze off all of the board supports, then you can usually lift the board from the front, leaving all of the rear connections. Go slow, as most of those wires are prob pretty old and might have bad solder joints to begin with.

if you wanna get really crazy, and lucky, you might be able to pull a preamp tube and have just enough room to snake a solder pen up there without having to remove the board...i wasnt so luck for any of my soldering jobs, but it does look like you could reach a few of the components. Thats the one thing that sucks about the radial capacitors, with the leads on the bottom, you have to pull the board and cannot solder on top.

 

[Jimmy74]

Well I did pull all the pots, switches and unsoldered the lamp wires and succeeded in pulling the board out.... great experience!!

Yeah the radial cap underneath is a real PITA to get at without taking the board out.... does your amp have 6800uf 16 v caps or are they 6.3volts aswell??