Help A Newb Calculate Capacitors?

Lee Miro

New Member
Hi all,

I've got a Macintosh MA 5100 stereo receiver that powers on but plays only the faintest, scratchiest sound. A voltage meter shows the current dropping after one of the capacitors, so I'm replacing that first.

The capacitor is a Spague axial 40 MFD, 200 VDC. A website on vintage radios informs me that I can increase the µF by up to 80%. Is this correct? Can I also increase the voltage, or should that stay the same?

Thanks in advance!
 
Voltage can be the same or up. Generally not more than 2 ratings though,
In this case not more than say 250v
While modern caps tend to be smaller than older. Size will become a factor
 
Great! So 40 to 47 µf and 200 to 250v. Thanks so much!
. . . I think I took the measurements of the original to make sure replacement will fit.
 
I'd be looking for something else like maybe a problem in the power supply area. Check the fuses.

Could be a capacitor but my experience suggests something else first.
 
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I didn't want to get into diagnosing.
A few thoughts. I'm guessing you cleaned the controls. If it were a PS issue , it likely would effect both channels.
If that has separates jacks (pre out , amp in) you might try swapping the channels. See if problem stays with amp sections or preamp sections. at least you know where to look.
 
Thanks, cademan! The capacitor didn't give a meter reading, so I'm confident that's at least *part* of the problem. I'll check the power supply and fuses, too.
 
You didn't mention a specific channel so it is both channels right?

Just curious, did you just get this?

And......not being familiar with Mac stereo stuff, does it have pre-out and main-in jacks?

Does it have a separate or internal switch?
 
Nothing's cleaned yet, but I plan to deoxit it prior to putting in the new capacitor. It seemed like the pre-amp wasn't getting powered because of that capacitor. It's been a few months since I did basic testing, but I believe that both channels gave the same extremely faint, scratchy sound. I'll definitely try swapping on the next go-round. It's good to know where to head next.

I got it some years back---a curb find!

It doesn't have a pre-out or main-in jack that I see. Could you clarify separate vs. internal switch?
Here is a pic from the manual.
 
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I don't know why but for some reason i had it in my head that was a one channel issue.
After reading the comments after mine , I reread the beginning and see the errors in my thought process:whip:.
That being said, while it probably still needs a good cleaning regardless, you probably should check all the supply voltages.
Hopefully it's just that cap.
I'm in favor of making it work before you start a restoration but it's probably due for a refresh anyway.
I hope that cap solves the issue
 
Hey, I can use all the help I can get! I wish I'd posted sooner after testing it so I could remember more details. I'll take your advice and save the deoxit for last. This is my first foray into vintage audio equipment, so I appreciate the tips.
 
Thanks, cademan! The capacitor didn't give a meter reading, so I'm confident that's at least *part* of the problem. I'll check the power supply and fuses, too.
What did you measure with? Ohm meter will give no reading.
 
Hmm. Honestly, I can't remember the setting. I followed some guidelines online for using a multi-meter to test capacitors.

Edit add-on- I tested multiple capacitors this way, and it showed others working.
 
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Lee,
First we all started somewhere so why not here.
Just so we can possibly help you better. Could you tell history if any you know about the Mac and briefly, any understanding about electronics, troubleshooting, reading schematics (you may have).
Lastly, if you don't mind me asking , where are you?
Not looking for specific address or even town.
But area. The reason I ask is if you end up needing guidance and someone who's game is nearby, it might save you some time and trouble.
We can cross that bridge when we get there if it becomes necessary.
 
Are you sure it's MA5100? Are you sure it's a receiver? Because an MA5100 is not a receiver. What is the serial number as there several different Service Manuals depending on the serial #? Can you explain "current dropping after one of the capacitors"? I couldn't find a 40UF/200V capacitor in any of the service manuals.

Craig
 
Craig,
(GAS Thaedra I, Thaedra II, Thalia I, 2- Ampzilla IIa, Son of Ampzilla, Grandson of Ampzilla, 2- SUMO NINE, NINE+, THE POWER, SAE 1B, Mark IVCM, 5000A)
Impressive GAS collection (SUMO too):thumbsup:
 
The back panel drawing matches the owners manual in digital docs.

I also am not seeing a 40uF cap in the power supply.
I'll dig through the manual a bit. It might be time to back up and start from the beginning.:idea:
 
Just so we can possibly help you better. Could you tell history if any you know about the Mac and briefly, any understanding about electronics, troubleshooting, reading schematics (you may have).

I can read a schematic, just not going to be able to tell you what anything is specifically doing or why very easily. I'm used to troubleshooting computer hardware, but just an audio novice.

Found it for free on the street, so don't really know anything about it.

We're in the Hartford, CT area, but really probably only interested in fixing it ourselves. If we can't get it to work we'll probably just sell it.

Thanks for all your help here though!
 
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