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PostPosted: Mon Mar 02, 2009 5:10 pm 
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Mark II

Joined: Mon Nov 24, 2008 12:12 pm
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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


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PostPosted: Tue Mar 03, 2009 4:47 am 
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Mark II

Joined: Sun Jun 17, 2007 11:36 am
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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?


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PostPosted: Tue Mar 03, 2009 4:56 am 
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Mark II

Joined: Mon Nov 24, 2008 12:12 pm
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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?


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PostPosted: Tue Mar 03, 2009 7:30 am 
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Mark II

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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.


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PostPosted: Tue Mar 03, 2009 10:01 am 
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Mark II

Joined: Mon Nov 24, 2008 12:12 pm
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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.


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PostPosted: Wed Mar 04, 2009 4:10 am 
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Mark II

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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.


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PostPosted: Wed Mar 04, 2009 8:42 am 
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Mark II

Joined: Mon Nov 24, 2008 12:12 pm
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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.


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PostPosted: Thu Mar 05, 2009 4:48 pm 
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Mark II

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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?


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PostPosted: Thu Mar 05, 2009 7:12 pm 
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Mark II

Joined: Mon Nov 24, 2008 12:12 pm
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what do you mean safe? other than the fact that its a pain in the ASS 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.


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PostPosted: Sat Mar 07, 2009 3:59 am 
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Mark II

Joined: Sun Jun 17, 2007 11:36 am
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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??


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PostPosted: Sat Mar 07, 2009 9:17 am 
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Mark II

Joined: Mon Nov 24, 2008 12:12 pm
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ya know i wasnt thinking this whole time. the voltage rating on the caps are just that, the max voltage that they ca handle. so it doesnt matter if you go with a higher voltage rating, you just shouldnt go under. i dont know why i didnt think of that eirlier. i dont remember what i replaced mine with. i can let you know when i get my amp back from the tech


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PostPosted: Sun Mar 15, 2009 10:47 am 
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Mark II

Joined: Sun Jun 17, 2007 11:36 am
Posts: 65
well I finally pulled out that mystery resistor and took a photo of it, it only measures 2.2 ohms on my multimeter and judging by the colours it should be a 90kohm or a 900kohm resistor right? The colours are:
1) grey or silver
2) white
3) gold or yellow
4) red or orange
5) silver

What do you make of it? Anyways it was partially stressed but invisible to the eye untill the lead snapped off today.

Here's a photo of it:
http://www.flickr.com/photos/73835524@N00/3356870910/sizes/l/

thanks


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PostPosted: Fri Mar 20, 2009 12:24 pm 
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Mark II

Joined: Sun Jun 17, 2007 11:36 am
Posts: 65
Well I just did some continuity tests between the heater pins on V1 and the rectifier bridge for the DC voltage and here's what I found out:

- Pins 4 & 5 get continuity to 1 of the 6800uf caps(that looks correct) but not to the diodes (??)
- Pin 9 gets continuity to 2 of the diodes(that looks correct) but not to either of the caps (??)

I tested to all sides of all 4 of the diodes. Does anyone have any ideas as to whether these readings are correct?

thanks
Saverio


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PostPosted: Mon Mar 23, 2009 10:42 am 
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Mark II

Joined: Sun Jun 17, 2007 11:36 am
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Well now I'm really stuck, the circuit board is most probably a different version than that drawn up in the schematic infact the switching matrix is totaly different including the bridge rectifer, deosn anyone own the correct schematic for the mainboard version MESA DC 23C. The serial number of the amp is DC3 02363

thanks


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PostPosted: Tue Mar 24, 2009 11:45 am 
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Mark II

Joined: Sun Jun 17, 2007 11:36 am
Posts: 65
Hi well I contacted the mesa service centre here in Italy and as I thought he wasn't able to help me so I'm going through the board and tracing the paths manually.

Looking at the downloaded schematic, look at the switching matrix (last page) the negative voltage off the bridge rectifier goes to:

1) pins 4 & 5 on V1
2) pin 4 on the 4N33
3) a 22k or 22 ohm resistor


And the positive voltage goes to:
1) pin 9 on V1
2) the 1N4007 diode and relays

I've had to rebuild the bridge rectifier on separate PCB because the paths on the mainbaird are damaged and don't follow through properly. Now I've been able to trace down all the parts that are supplied by the bridge rectifier with separate voltages except for the 22k or 22 ohm resistor (and that's why I asked if anyone had the specific schematic for this mainboard version).
There are a few 22k resistors on the board, but how to tell which one is the right one?? Simple looking at the schematic and finding the 39k or 39 ohm resistors right? No that's wrong because there aren't any 39k or 39 ohm resistors on the board and there aren't any 40k or 40 ohm resistors either. So I thought of starting from the LDR's and going backwards and trying to find out what's what..... and in the link below is what I have found so far!!

http://www.flickr.com/photos/73835524@N00/3381972815/sizes/o/

As you can see it is quite different from the downloaded schematic, there are components which are not mentioned at all on the downloaded schematic.

let me know what you think

thanks


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