DC - 3 Hum problems on Rhythm Channel

[adam972]

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

 

[Jimmy74]

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

 

[Jimmy74]

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

 

[Jimmy74]

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

 

[Jimmy74]

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