Mesa Walkabout Power Amp Section is Distorting

[bass_69]

I have an old silver faced Walkabout which I like so much I bought the WD800 to replace it. The other day I went to try the Walkabout again after months of non use. The speaker lead may have been dodgy and shorted the output. Not sure though. With a much better lead I tried it again and now the power amp just distorts horribly. It seems to have plenty of volume. I have run a seperate preamp into the return and it still distorts. I have also run the preamp out of the Walkabout into a Crown XLS1002 and it sounds great. So it must be the power amp section that has the problem. I am in Australia and live nowhere near anyone who can look at the amp. Just hoping for some thoughts on what might be the problem before going down the expensive road of posting it off. It is hard because I would have to spend lots of money before knowing how much it would cost and whether it is worth it. It sounds so good through the Crown I wonder if it is possible to disconect power to the main amp board and just use it as a preamp ?

 

[bottomzone]

I'm sure agedhorse will chime in soon.

 

[agedhorse]

Sounds like you damaged the power amp with the shorted load. You will need to contact the Mesa authorized service agent for AU, they will have access to the necessary service information.

A couple of things… first try bypassing the effects loop with a 1/4” patch cord just in case it might be something that simple (unlikely but possible), assuming you don’t have the bypass switch mod installed already).

Some of the parts are becoming difficult to source as the original manufacturers of these parts discontinued them years ago. Some of these parts are only available as fake parts that do not work or don’t work reliably, beware.

 

[bass_69]

I did patching a 1/4 cord between the send and return. I was just trying to guage what might be involved in fixing it. Would it still produce sound if a mosfet was damaged ? If this is several hundred dollars to repair it is not worth it. Is there a way of unplugging power to the power amp board and just using it as a preamp ?

 

[agedhorse]

I did patching a 1/4 cord between the send and return. I was just trying to guage what might be involved in fixing it. Would it still produce sound if a mosfet was damaged ? If this is several hundred dollars to repair it is not worth it. Is there a way of unplugging power to the power amp board and just using it as a preamp ?

Bad MOSFETs can cause distortion but do can many other defects.

A QUALIFIED tech should be able to disconnect the supply to the power amp, but there may be more to it depending on the actual fault.

 

[bass_69]

I know it might sound crazy to the class D hating mob, but I think this sounds better running the Walkabout preamp through the class D Crown amp. It is just as full but faster and tighter. Andy - Am I right in suspecting that the preamp cannot be run on its own as it is not just the gain but the main volume controlling output to the send output ? Another thought - I would love to be able to put an Icepower module in this and take its power ratings up.

 

[agedhorse]

I know it might sound crazy to the class D hating mob, but I think this sounds better running the Walkabout preamp through the class D Crown amp. It is just as full but faster and tighter. Andy - Am I right in suspecting that the preamp cannot be run on its own as it is not just the gain but the main volume controlling output to the send output ? Another thought - I would love to be able to put an Icepower module in this and take its power ratings up.

Putting an IcePower module into a Walkabout requires a LOT of work in addition to a high voltage power supply and a bunch of interface parts.

 

[bass_69]

OK.. Ta.. I am just trying to assess options without having to send it somewhere first. Postage to a service centre and back is about $100. Plus another $200 for them to look it and diagnose the problem. Then maybe $200 in parts and $200 in labour to fix it. It starts costing as much as the amp is worth to repair it. Is there anyway I can tell the complexity of being able to turn this into a stand alone preamp now without it draining power to the amp section all of the time ?

 

[agedhorse]

OK.. Ta.. I am just trying to assess options without having to send it somewhere first. Postage to a service centre and back is about $100. Plus another $200 for them to look it and diagnose the problem. Then maybe $200 in parts and $200 in labour to fix it. It starts costing as much as the amp is worth to repair it. Is there anyway I can tell the complexity of being able to turn this into a stand alone preamp now without it draining power to the amp section all of the time ?

Nope, that’s not a DIY job. There’s a lot going on and no way to know what’s damaged. That’s why experience and knowledge are part of the answer.

As a point of reference, with my experience and familiarity with the WA, it would take me about 10 hours to work out the details and do the conversion and I work with this technology every day. It’s really not practical from purely a cost basis to change a single amp. Then there’s about $300 (USD) for the parts.

 

[bass_69]

I never said I wanted to DIY anything. I was just hoping for some thoughts on options before sending it somewhere. If parts are in short supply then this is something where I may end up with a bill for hundreds of dollars and be in the same situation I am in now. If the power amp cant be fixed then how hard is it to turn this into a sand alone preamp (not DIY) ?

 

[agedhorse]

The problem is that the power supply, which may be part (or all) of the problem, is also required for a stand alone preamp.

Without knowing what the problem is, everything is a guess. There are a lot of possible causes, but unfortunately they can’t be identified with a crystal ball.

 

[bass_69]

OK thanks Andy.. Looks like it is too much of a gamble.. She is destined for land fill..

 

[bass_69]

How much power is the amp using if no speaker load is present and can the fan then be disconnected due to noise ?

 

[agedhorse]

How much power is the amp using if no speaker load is present and can the fan then be disconnected due to noise ?

It depends on the fault, it could be almost nothing or there may be unexpectedly high. The fan shouldn’t be disconnected because the amp can enter thermal runaway.

Maybe it has value as a donor unit to repair other Walkabouts.

 

[HotelX]

I had a similar problem with distortion. Turns out the B side of the AB amp wasn’t working. This was quite some time ago so I don’t remember the cause or repair cost. I don’t remember it being incredibly expensive. I would get it fixed though. I’m still using the amp for band practice and it still sounds amazing.

 

[bass_69]

Interesting.. I have no idea what the B side is but thank you..

 

[B-string]

I have no idea of what a "B side" is either? I have heard of people calling the positive going voltage swing of a Class AB amp "A" and the negative going voltage swing "B", but this is completely incorrect. A, B, C, AB and D are all operating classifications of amplifiers, Class A, Class AB and Class D (may still be unofficial) Class G and H (I am not sure of the acceptance personally) lend themselves well to audio. How they amplify the signal to output to a transducer (speaker) determines operating "class".

 

[Boneless]

The discrete parts may be hard to source but older amps can usually be serviced just fine (that's the conundrum with modern amps: servicing a class D amp generally amounts to simply replacing the power module, low on labour, relatively high on parts cost - until they are available, which won't be a problem in the near future but may be thirty years from now). Also, sometimes there are replacements that, while not being 1:1 to the original part, can be made to work identically (for example switching in higher power transistors). There will be some good techs in Australia that can have a look at it, even not official Mesa techs. It's not like the amp is under warranty anyway. I mean, unless you live out in the outback, there must be a repair centre somewhere "close" to you.

 

[bass_69]

The discrete parts may be hard to source but older amps can usually be serviced just fine (that's the conundrum with modern amps: servicing a class D amp generally amounts to simply replacing the power module, low on labour, relatively high on parts cost - until they are available, which won't be a problem in the near future but may be thirty years from now). Also, sometimes there are replacements that, while not being 1:1 to the original part, can be made to work identically (for example switching in higher power transistors). There will be some good techs in Australia that can have a look at it, even not official Mesa techs. It's not like the amp is under warranty anyway. I mean, unless you live out in the outback, there must be a repair centre somewhere "close" to you.

Thanks for the reply. Would non Mesa service centres be able to get schematics to the WA ?

 

[eniac]