Help me bring my Dad's MC240 back to life

[Bill Ferris]

There are Bumble Bee caps in there. It's well documented that they were defective from manufacture and are most likely cracked, leaky and unreliable. This has a significant effect on SQ and reliability. Replace them. This is where Jim McShane can certainly help with a carefully chosen selection of film & PIO caps.

- Clean then tighten up the tube sockets using a dental pick or large safety pin to coax the socket tabs a little closer together.

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[chibrew]

Thanks for all of the replies and advice. The kit I bought has replacements for all of the capacitors. Overall I'm really pleased with the kit from C-Tech. The biggest question is whether or not to replace the Sprague non-bumble bee caps.

I've gone ahead and ordered all new resistors. There were enough out of spec that it makes sense. Now it's just finding more time to put in.

Here are some pics of what is done so far. Yes I need to clean things up a bit...too much flux.

Keep the suggestions and feedback coming, AK has been an awesome resource in this journey!

 

[x3workshop]

The 160P Sprague caps are generally not considered defective, but... I've tested enough on my cap tester to know that many of them are indeed leaky and out of spec. I used K40-Y Russian Paper In Oil caps with no regrets.

 

[edlipman]

Hi:
You may be using too much flux , but you are also not heating the joints up enough to get a smooth surface on the solder joints. Try just reheating the joints you have taken pix of and let the solder flow into the joint , not enough heat to burn things but you will see the solder flow out evenly. Try this on some other joints first then the joints in the amp.
If using flux core solder the the flux is usually a little too much but not really too much.
Hope this helps
Ed

 

[chibrew]

Thanks Ed. I added additional flux to try and get the old solder to flow before using the solder sucker. Not sure if this is a good or bad idea. It seems to help on the older joints. If anything I may have the iron too hot? I am running about 650 and was a little higher on the big capacitors to get things to flow. Any suggestions on a good flux remover?

The order of new resistors should be here today so I will be starting to change them with the capacitors at the same time. Any recommendations of hook up wire or tubing/sleeving for leads? I have heatshrink but am not sure if there is something better. I'd like to make this look as good as possible.

Thanks,
Mike

 

[PakProtector]

don't forget the electrolytic coupling caps on the vertical board.
cheers,
Douglas

 

[chibrew]

So I've made a little progress today. Not done yet but here is the progress. Main board resistors and capacitors are done. Still have the two bumblebees left as well as some resistors but getting close.

 

[chibrew]

Well we are back on line! I think I am still going to schedule a bench check at Absolute Sound Labs just to make sure all is good, but it sounds great. I need to get some better hookups on it, but all and all I think I have a working MC240. Looking forward to many hours of listening.

 

[PakProtector]

Now that you have it running, there are improvements that the circuit deserves.
-Get rid of the 12AU7's; the 6CG7 only needs a heater circuit mod to plug and play
-Use a CCS instead of the 18k tail resister in the 12AU7 phase splitter. Run matched plate loads, now that the tail load is high and the 27k/30k deliberate imbalance not needed.
-Do away with the positive feedback applied to the 12BH7. The PFB is a side-effect of the need for a *HUGE* voltage output needed for the 50% cathode FB delivered by the OPT

With PFB gone, the global loop of NFB can be removed.

The fixed, un-adjustable, fixed bias should also go. You will be able to manage each tube's idle current and the OPT will perform better across the board.
cheers,
Douglas