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PostPosted: Sat Oct 15, 2016 6:37 pm 
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Bottle Rocket

Joined: Sat Oct 15, 2016 6:01 pm
Posts: 3
Hi All,

I'm going to give my 26 year old Mesa 50/50 Power Amp a capacitor change. I thought it would be easy enough until I settled down tonight to do the job and saw that the printed circuit board is hard-wired to various points on the chassis. It looks like there is no way to get to get to the bottom of the PCB to desolder the old caps and solder in the new ones without first unsoldering 14 wires that go from the PCB to the power valve sockets. This would allow me to tilt the PCB up at an angle to get to the underside, but even then it might be an awkward job.

Before I jump in at the deep end, can anybody tell me if I'm being stupid and missing an easy way of doing this cap change? I guess I could leave the PCB in place and snip out the old capacitors and solder the new ones on to the legs of the old caps, but this wouldn't be a neat job I'd be proud of.

Thanks,
Daz.


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PostPosted: Sun Oct 16, 2016 8:31 am 
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Bottle Rocket

Joined: Sat Oct 15, 2016 6:01 pm
Posts: 3
I've found a bit more info on an old thread from 2008, where somebody asked how to get to the underside of the PCB to solder it.
'disassembled' (Greg) responded:

"You have to remove the power tranny and faceplate. Once you do that you can lift the board enough to solder underneath."
(viewtopic.php?f=5&t=23758)

I never thought of this! The PCB will still be wired to the valve sockets but I guess it's OK to bend all those wires to get to the underside. I'll have a go at this soon, but if anyone can chime in with more detail to reassure me I'd really appreciate it. Thanks.


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PostPosted: Mon Nov 07, 2016 7:19 pm 
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Bottle Rocket

Joined: Sat Oct 15, 2016 6:01 pm
Posts: 3
Even though this thread had no responses I thought I'd post an update on what I learned. Maybe this will help out other Mesa 50/50 owners in the future.

First, check with a multimeter that the Mesa 50/50 is fully discharged and safe to work on. The amp normally discharges to a safe level within 5 minutes, but NEVER EVER take this for granted. SAFETY FIRST.

I changed the electrolytic capacitors by removing the faceplate of the 50/50 and tilting the PCB vertically to desolder the old caps and solder in the new caps from the underside of the board. This was quite time consuming: in order to get the faceplate off, I had to remove the volume and presence pots because they were gripping the bottom of the chassis and preventing the faceplate from being removed. I didn't have to remove the power transformer at all... I just had to undo the nuts holding the faceplate to the transformer, and then disconnect the power transformer multi-plug to the PCB. I also removed the screws from the power valve sockets on the chassis to give them a bit of movement when lifting the PCB vertically (because of the short stiff wires from PCB to valve sockets). There were also cable ties to snip to take the stress of off some of the wires from the PCB to the chassis.

Because the electrolytic caps are stuck to the PCB with acid-free silicone adhesive, you need to snip the cap legs and use a craft knife to slice through the plastic outer sleeve of the capacitor. You can then remove the cap then peel the cap sleeve off of the PCB and remove the silicone adhesive.

The soldering on the PCB underside was not easy to do with the PCB in a vertical position with the heavy front plate still hard-wired to it. All the time I was thinking there had to be an easier way to do it, and a way that didn't put so much stress on the flexible PCB and all the stiff wires connecting the PCB to the chassis. There was an easier way... READ ON!

Anyway, I put everything back together and the amp worked fine.

Two days later, Channel A developed a fault... I heard an increasing loud hum in Ch. A and noticed one of the power valves was red-plating. Paranoia kicked in and I thought the problem might be related to either my capacitor change work or my new power valves. Turns out it was the Orange Drop capacitor C14, connected from the plate of the 12AX7 phase inverter to the 220K resistor going in to Pin 5 of the red-plating 6L6 - it had gone fully short circuit. The amp had developed an intermittent noise 8 years ago before it went into storage... it might have been this Orange Drop cap on its way out.

I wasn't going to go through the hassle of removing the front plate again and lifting the PCB up to work on the underside, putting the PCB and all those connecting wires under more stress. So this time I snipped out the faulty Orange Drop C14, solder-sucked the holes (removing the remains of the capacitor legs) and bent the legs of the new capacitor so that they would not go any deeper than the underside of the PCB... I didn't want them touching the metal chassis! I soldered the cap legs from above... the holes are through-plated, so it makes no difference if you solder from above or below. On the top of the PCB, component-side, I left the cap legs a bit longer, maybe 10mm above the PCB, to allow me to get the soldering iron in.

I then put acid-free silicone adhesive between C14 and C15 and the job was done. (I also used this adhesive to stick the electrolytics to the PCB, as is done in the factory).

The amp is now up and working, and lots learned. This might all be obvious to regular amp tinkerers, but hopefully it'll help out other Mesa 50/50 owners.

So to recap (and to re-cap!), my advice for replacing components on the Mesa 50/50 PCB is not to remove the PCB and front plate at all... just MAKE SURE THE AMP IS DISCHARGED, snip out the old components, de-solder & remove the remains of the old component legs. Then bend the new component legs so they only go as deep as the thickness of the PCB. Leave enough leg-length on the cap on the component side of the PCB so that you can get your solding iron in without doing any damage to surrounding components (don't leave the legs too long... 10mm should be fine). Solder the component legs on the top-side of the PCB. Use acid-free silicone adhesive to secure the components as they were originally.

Cheers,
Daz.


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PostPosted: Wed Nov 09, 2016 11:54 am 
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Mark IV

Joined: Sat Jul 07, 2007 9:13 pm
Posts: 505
I don't have anything to add as I don't own a Fifty/Fifty, but you're a brave man to take on that work. I attempted a cap and LDR job on my Mark III recently and ended up needing to send it to Mike B. at Mesa to fix all the mistakes I made.


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