Annoying noise in Dual rectifier 3 Channels (with solution).

[Pedro Vecino]

I had a Dual Rectifier three channels with a very unpleasant noise (stable and low frequency) dependent of gain and volume settings. The area of the rectifying diodes in the heaters line looked bad and after extracting them I have found that the board was blackened.
At some connection points I found low resistances (between 5M and 13M) and thought it would be the source of the noise. Diodes are well.
Contrary to exposed in the schematic are two 12AX7 that feed with DC (600mA + switching circuit) and the diodes generate too much heat dissipating in poor form.



The solution is to extract the PCB, cut the lines in that area and bypass connections aerially.



And install a new bridge. I used 3A FRED diodes to better dissipate heat and also serves to obtain 5.9V instead of the typical 5,7VDC (approximately) installed with rigid wire on sockets pins to support firmly the structure.



Finally itīs necessary retain the positive side for the channel switching circuit also aerially (it was taken direct from the positive side of the rectifier bridge). Itīs connected from the positive side of the 1N4148 diode associated with the marked relay until the end of the 22R resistor (+ 5,9VDC). It could also be taken directly from the positive on the socket but in this way does not interfere with welding.



That is all. I hope this can help someone. Sorry for my poor english.

[Leo_Gnardo]

Excellent work Pedro!

I'm not looking forward to seeing Dual Rectifiers from now on. Removing/replacing the PC board = long day in the no fun zone.

[Pedro Vecino]

Thanks, Leo. I said a prayer before connecting (always helps)

[drewl]

I've replaced those diodes in a few Mesa's.

I try not to wait until the board is burnt, sort of preventive maintenance and keep them up off the board to prevent heat damage.

[Jazz P Bass]

One of the few coherent things SGM ever passed along as far as Mesa amps: keep all resistors up off of any traces that pass under them.

[The Dude]

One of the few coherent things SGM ever passed along as far as Mesa amps: keep all resistors up off of any traces that pass under them.

Most likely plagiarized from someone else's post.

[Justin Thomas]

I still maintain that they sound great. Until they break. Don't bring it to me! That's a serious case of ten pound of $#!+ in a five-pound bag...

Seriously, though, I really respect guys who can work on such a tiny scale and in such cramped spaces. Nice, creative, and elegant solution, Pedro!

Justin

[drewl]

You want to see tiny?

Hahaha... I've got some microscopic surface mount stuff that's a real pain to work on!

[xjbear]

Had to rebuild a burnt one lately and used a 4 leg rec brick on top of the socket. Thank God for the photos you uploaded here because the traces ended up all nuked beyond recognition. Thanks again man!

[Pedro Vecino]

I return to this topic again to recover the lost pics in the first message and add two more in high resolution since I have found the same problem in another Triple Rectifier.
Yellow circles mark the cuts on the tracks. Eight on the bottom (although some is redundant for security) and two on the top.
The orange circles mark the capacitor pins that must be removed to access that area.

[Randall]

It's a damn shame we even have to do these mods and workarounds on these Mesas in the first place. I hate seeing one come in the door.

[nevetslab]

It's a damn shame we even have to do these mods and workarounds on these Mesas in the first place. I hate seeing one come in the door.

I'm with you on that one. I felt dreadful when Mesa Hollywood announced they aren't doing local service many months ago. I don't know if that changed, but they ARE such a PITA to service. Fortunately none of our Mesa inventory has come in for repairs since that time. I've never understood why they would sell amps that flat out oscillate & motorboat with typical control settings, or just engaging their voicing.

[drewl]

Nice update.
I've been seeing this problem more and feared that the board material was breaking down and causing hum.

Thanks.

[bsco]

Nice update.
I've been seeing this problem more and feared that the board material was breaking down and causing hum.

Thanks.

Well...I am not even going to touch one of these.......

[Pedro Vecino]

Another one. I almost know to do it by heart