Mesa Bass amp repairs - Bass 400 and Fathom (M6)

[dr teeth]

Long overdue bass amp repairs. And something to look for if your Fathom/M6 cuts out.

I picked up a Fathom that was explained as it was working fine and then just cut out. The previous owner took it to an amp tech who was stumped, called Mesa and decided the amp would be too costly to repair, an obvious punt. It's not the first Mesa that stumped a tech for which careful physical examination found glaring cause of electric mayhem. I just love me some good Electric Mayhem- can you picture that?

The Fathom has this red open coil component and automatically you think inductor when you see a coil. Did this make it to the M6 version after they pulled the Fathom? It looks like an obvious fault point and you would think there is a less fragile way to implement an inductor into the circuit if that's its purpose

I jumpered it which you can see and it sprang to life gloriously into a RR 1516. Time to solder it. Alternatively does anyone know what this is exactly and if replacements can be had?

 

[dr teeth]

Next up is the Bass 400 which I bought locally in Denver non-working a few years ago. It's one of the "2 ohm" versions, dated 7/89 and B114x, so one of the last ones. It has the 561180 OT so I'm wondering what OT the non- 2ohm versions used and if the 400+'s or early 400+'s use the same 561180.

 

[dr teeth]

The obvious issue with this 400 are the shot 100ohm (brown black brown) carbon comp R's for each side of the heater filament power socket lugs that span the first 2 power sockets. There are only these 2 heater CC R's in the power section.

I've seen 2W carbon comps on a green stripe crumble so popped half watt CC's on a Bass 400 power socket is no shocker and I decided to up the handling to 1W as a preventive engineering upgrade. You can see they are a bit bigger.

Anyone know the purpose of these filament R's?

Moreover, are there any known issues with the Bass 400, anything normally replaced that I should do while in here? I know the electrolytics are 30 yo but I'm talking aside from those. When I push on one of the non-push/pull pots it presses in a little more than I would expect. Is this indicative of a pot needing a rebuild/re[place? I notice MB is rebuilding pots on the 80's marks more than infrequently and wondering if it's a physical feel, listening, or ohm testing that leads him to which ones need rebuild.

 

[dr teeth]

I've heard there are MIM green disc caps in the Marks that are junk but don't know the details. Are there specific ones to look for and are these here the green caps of death and doom?

 

[dr teeth]

wrapping up loose ends-
the fathom was an easy solder. If I could order a new part I would just replace it but I've never seen an open coil inductor like this and wouldn't know what to call it to look for one let alone rating.

 

[dr teeth]

For the Bass 400 the new heater 1W carbon comps are in.

This amp came with 5 7581A's and I've tried to get 6 str 418's but only have 4. Being at the end of the product run I wonder if this was "voiced" for the 6L6 as I assume the 418's were getting scarce. I know you are supposed to be able to swap 6l6GC or 6550A but that doesn't make sense to me unless the bias point was a compromise and/or specific testing valves are needed.

Of all my mesa's this is the one that stands out begging for a bias pots install and/or get some TAD 6550A's.