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MC2505 catastrophe

Unless there is a lead marked - then the (shape) lead to the can is what you should be measuring: half-circle to can = 200MFD, triangle to can = 500MFD. If you measure half-circle to triangle you are measuring both sections in series, to dummy lead should read open-circuit.

Was there no wire to the can?

Anyway, triangle to half-circle (both in series) should measure around 143MFD.
 
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SaturationPt said:
Unless there is a lead marked - then the (shape) lead to the can is what you should be measuring: half-circle to can = 200MFD, triangle to can = 500MFD. If you measure half-circle to triangle you are measuring both sections in series, to dummy lead should read open-circuit.

Was there no wire to the can?

Anyway, triangle to half-circle (both in series) should measure around 143MFD.
Click to expand...

This photo shows how the factory wired Benz's two-section C34 can. There are no wires going to the common can twist lugs. The can engraving appears to indicate the "dash" symbol is common positive. I interpret that to mean the lug without a marking is can common -- instead of the can itself being can common like on most FP cans.

c34.jpg

common.jpg
 
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That does confuse me then, and if the can is therefore not electrically connected, why would they use the FRS isolated mounting?

Either way, it appears that the series capacitance is in range measuring across both.
 
Agree it’s confusing, and MAC wired the later serial C79 500 uF FP can differently by making the can common negative. Benz’s earlier model throws us a curveball.

I’m still suspicious that Benz’s C34 can has “left the building” to meet Elvis.
 
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I have the tools, and the skills, and it's not much money so if it were mine I'd be replacing all of those electrolytic capacitors and not wasting the time to test them. I realize this isn't everyone's situation.

However, no commonly-available electrolytic capacitor has a life/rating beyond 10 years. Any electrolytic capacitor therefore (whether stored or used) is beyond it's "freshness date" if it's over a decade old, just replace them.

The other justification is that there are failure modes of power-supply electrolytic caps (even more in high-voltage tube amps) that will cause collateral damage and cost much more than routine replacement.

On the other side is that infant-mortality is real, you buy cheap capacitors (they are all over evilbay) you might put your nice equipment at more risk than the 20+ year-old caps you're replacing. Buy quality. It's not that much money compared to the value of these amps/preamps, and it's a lot cheaper than having a catastrophic faiure that lets the smoke out of the amplifier (and finding all of the damage). I will spend the money for correct-value Authenticap over cheap and/or much higher value JJ caps (for example), when it's buttoned up and playing all evening without worry it is SO WORTH IT.
 
SaturationPt said:
I have the tools, and the skills, and it's not much money so if it were mine I'd be replacing all of those electrolytic capacitors and not wasting the time to test them. I realize this isn't everyone's situation.
Click to expand...

I agree all the 55-year-old electrolytics should be replaced with high quality replacement caps. But there is nothing wrong with @Benz's approach of diagnosing the problem.

The question is: does the Atlas ESR meter confirm C34 is faulty? It appears to me that Benz measured C34 correctly and this FP can has a fault. @Celadon also owns a circa 1968 2505, like Benz, and the work DeWick performed on his unit indicates C34 was faulty as well.
 
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monkboughtlunch said:
Looks like your Half-Moon (200 uf) and Triangle (500 uf) sections of C34 both test bad!
Click to expand...

I agree it looks like these are open to the common negative lead. The interesting thing is you get what looks like a good measurement for the two in series. It's as if they two sections are internally connected, but the connection to the exterior terminal is open somehow. Make sure when you measure the two sections that your connections to the common lead are solid (no old flux or crud keeping it from making a good connection). But if it is a repeatable open measurement then yeah that can should go.

I've lost track of which cap that is, this thread is getting long! But if the caps are open, you could have a lot of AC going places it shouldn't be going, could be the cause of your issues.
 
monkboughtlunch said:
I agree all the 55-year-old electrolytics should be replaced with high quality replacement caps. But there is nothing wrong with @Benz's approach of diagnosing the problem.<<snip>>
Click to expand...

You are absolutely right, I lost track of the fact that he is diagnosing a problem, not just testing caps for replacement.

Definitely diagnose, repair, then restore.
 
SaturationPt said:
I have the tools, and the skills, and it's not much money so if it were mine I'd be replacing all of those electrolytic capacitors and not wasting the time to test them. I realize this isn't everyone's situation.

However, no commonly-available electrolytic capacitor has a life/rating beyond 10 years. Any electrolytic capacitor therefore (whether stored or used) is beyond it's "freshness date" if it's over a decade old, just replace them.

The other justification is that there are failure modes of power-supply electrolytic caps (even more in high-voltage tube amps) that will cause collateral damage and cost much more than routine replacement.

On the other side is that infant-mortality is real, you buy cheap capacitors (they are all over evilbay) you might put your nice equipment at more risk than the 20+ year-old caps you're replacing. Buy quality. It's not that much money compared to the value of these amps/preamps, and it's a lot cheaper than having a catastrophic faiure that lets the smoke out of the amplifier (and finding all of the damage). I will spend the money for correct-value Authenticap over cheap and/or much higher value JJ caps (for example), when it's buttoned up and playing all evening without worry it is SO WORTH IT.
Click to expand...
I generally agree with you that given the age of the amp and caps, it's a good idea to refresh them while I'm in there, regardless. My hesitation isn't so much cost, it's my skill level with the soldering iron. That said after thinking it over I feel I can pull it off if I'm careful so I'm going to see if I can replace C33, C34 (consensus seems to be that it's bad, anyway), and perhaps also C29 and C30 if I can find the OEM Mac caps Monk has referred to early on in the thread.
 
Bob the Tech said:
I agree it looks like these are open to the common negative lead. The interesting thing is you get what looks like a good measurement for the two in series. It's as if they two sections are internally connected, but the connection to the exterior terminal is open somehow. Make sure when you measure the two sections that your connections to the common lead are solid (no old flux or crud keeping it from making a good connection). But if it is a repeatable open measurement then yeah that can should go.

I've lost track of which cap that is, this thread is getting long! But if the caps are open, you could have a lot of AC going places it shouldn't be going, could be the cause of your issues.
Click to expand...
I sanded the three terminals and then treated them with contact cleaner. I'm still getting the same reading, "open circuit/low capacitance" on both triangle and half moon terminals when connected to the common lead. I still get 153uF / 0.19ohm when I connect the triangle and half moon terminals.
 
I really appreciate all the input, it certainly appears that C34 is faulty. I'm going to start looking for replacements for both C33 and C34 as mentioned a few posts ago. Monk gave me some links to investigate for C34, and this would appear to be the correct part to replace C33: https://www.cedist.com/products/capacitor-ce-mfg-250-v-150808050-f , or this: https://hayseedhamfest.com/products/mcintosh-amp (but the physical dimensions aren't posted). Now I haven't yet been able to find OEM Mac replacements for the 9300uF caps but am looking into that right now. EDIT: found some references to this cap being a modern C29-C30 equivalent, appears to be a factory special order: https://www.mouser.com/ProductDetail/Cornell-Dubilier-CDE/DCM103U050CB2B?qs=yQ3nQrcaeVta3yNjbNyqvg==
 
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The McIntosh parts dept is your source for 9300uF replacement cans. New cans will be blue and 15,000uF.

If it was me, I would repair the issue at hand first, starting with C34. Once the amp is working again, I would then go back and replace the remaining old, but still working, capacitors section-by-section. If you replace a whole bunch of caps / parts all at once and then power it up on a dim bulb tester and it shows you have a short, you might have a harder time finding the fault/s.

For preventative maintenance, I would also recommend replacing the four caps that were installed in 1996 as they are close to 30 years old, as well as the original factory white Mallory caps and Tropical Fish films. Might as well knock all that out while you have it on a bench for long term reliability.
 
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That's a good suggestion, it might take me a bit longer this way but it will allow me to stay focused on the task at hand and solve the issue first. I'll head over to mouser and check out the links you posted yesterday.
 
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