Mesa Quad switching problems

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Uruz

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Hi guys,

I have been reading posts on this forum found via google searches, but never actually registered until now :)

The below story is a work in (slow) progress, but I thought I'd let you know how it develops and you may point me in the right direction as well. :oops:

I have had a Mesa Quad preamp for almost 10 years now. I am very happy with the sounds, but have faced intermittent switching issues. Sometimes it is ok for a long time, but then all of a sudden it will go into 'no control mode' (usually during gigs of course). Usually, all 4 channels will activate simultaneously, but it has also happened that e.g. Lead 2 is always on (ultimately, all 4 channels will go on at some point). I am not sure, but either vibrations and/or temperature differences seem to trigger the amp to go into 'no control mode', or coming back to normal again.
By the way, I use either the FU2A footswitch or the switching jacks on the back with a Nobels MS4 midi switcher. This issue can occur in either configuration. I have brought this amp to a guitar tech at a local shop who is familiar with boogies, but so far no luck. The intermittent part in this story makes failure analysis difficult. By simply moving the amp from my basement to the shop, it can recover.

When the 4 channels are on, and don't seem to be going back to normal again, I have a relatively 'simple' solution (not advised, unless you know what NOT to touch). After opening the amp, I can push down and slightly bend the PCB approximately in between where the wires from the switching matrix are soldered and where the 4706N logic IC is located. This will make the LEDs on the front go on and off erratically, and at some point they will stay off regardless of applied pressure and my amp will switch channels normally again for some months...
In the image you can see the double row wires from the switching matrix at the top. I push down slightly to the right of them at the edge of the PCB. You can see the left most IC is the 7406N hex inverter buffer (see below).
https://drive.google.com/file/d/0B8XZ0P6Cs74pTDFvbktBRlBLRjdwY3hTMzJybnpvaGQ3R1NR/view?usp=sharing
Manipulating the board at other locations (e.g. around the tube sockets or opposing sides) does not seem to be as effective as the specific position mentioned above.

Using a multimeter I have tried to check voltages across most of the optocouplers (this used to be my primary concern), but could not find (or understand from the schematic) what might be going wrong. I also tried cleaning all the tracks where dust had accumulated over the years and specifically flux residues (and other gunk) close to where the wires from the switching jacks connect to the PCB. Mostly using cue tips and ethanol.
Recently, I found the below thread and the comment from "jvk" that the 7406N hex inverter buffers IC may be causing problems.
http://forum.grailtone.com/viewtopic.php?f=3&t=22584

It looks like all my experiences with the amp may be explained by an issue with this chip. Perhaps the connection in the socket (best case), or that the socket itself has a problem (hopefully not requiring re-soldering from the backside). My next step is to try and pry the IC out of its socket and clean it a bit with ethanol. Fingers crossed.

Let me know if you have any other suggestions. What can I do to e.g. check the zener diodes mentioned by jvk in the other thread?

Thanks!
Rik
 
In retrospect, I guess I should have posted in the rack section :roll:

Small update: I removed all three ICs one after another. The 7406N came loose very easily when compared to the others... The Quad functioned fine after pulling it out and re-inserting, but started switching again after about 1.5 weeks (when I touched the PCB near the chip with the wooden part of a cotton tip, it jumped back to working order).
I removed the 7406N again and used a straight edge to slightly bend the leads so it would fit more snug into the socket (and scrape a bit against the sides of the socket connects I hope). For the last ~2 weeks it seems to be fine again without any unwanted switching. Let's wait and see.

Maybe a stupid question, but does anyone know why sockets were used instead of directly soldering onto the PCB? Are these chips meant to be replaced every so often or easily swapped for a new generation or other variant or something?

Thanks
 

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