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MC2505 catastrophe

If you have a schematic that isn't in our https://audiokarma.org/forums/index.php?forums/digital-docs.141/ Digital Docs Database it is always appreciated if you submit it to help others.

Electrolytic caps don't have to leak externally to fail. Think of an old-fashioned gum wrapper that is paper with an aluminum flash-coating, if it's left out the aluminum will corrode, or the waxed-paper can even become compromised that separates the aluminum from the gum, the paper in an electrolytic capacitor like this is much thinner and there are several internal failure modes that will not be evident. If you disconnect the output from the main DC bus / isolate it, so that turning on the power only powers the main rectifiers and capacitors and still get that BUZZ when you power it up it is the main caps or diodes. Remove the caps from that circuit one at a time and you will see which one (potentially). Just another method, be sure the original (3.2A?) fuse or smaller is installed (should be able to do this with a 1A I believe and create less risk to the transformer windings).

If the BUZZ occurs with only the input to the transformer connected and all outputs disconnected you have a transformer problem (much much less likely).
 
Since it wasn't available, I created a pdf of the service manual for the MC2505-02 (starting with serial no. 26K01). Not sure how to add to the AK database however, as I can't seem to be able to upload it here due to file size (2.6MB).

Cheers

EDIT: just submitted it to ak.pdfs@gmail.com as per instructions provided in the sticky. In the meantime if anyone needs the manual just let me know, and I'll happily send it to you separately.
 
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SaturationPt said:
If you have a schematic that isn't in our https://audiokarma.org/forums/index.php?forums/digital-docs.141/ Digital Docs Database it is always appreciated if you submit it to help others.

Electrolytic caps don't have to leak externally to fail. Think of an old-fashioned gum wrapper that is paper with an aluminum flash-coating, if it's left out the aluminum will corrode, or the waxed-paper can even become compromised that separates the aluminum from the gum, the paper in an electrolytic capacitor like this is much thinner and there are several internal failure modes that will not be evident. If you disconnect the output from the main DC bus / isolate it, so that turning on the power only powers the main rectifiers and capacitors and still get that BUZZ when you power it up it is the main caps or diodes. Remove the caps from that circuit one at a time and you will see which one (potentially). Just another method, be sure the original (3.2A?) fuse or smaller is installed (should be able to do this with a 1A I believe and create less risk to the transformer windings).

If the BUZZ occurs with only the input to the transformer connected and all outputs disconnected you have a transformer problem (much much less likely).
Click to expand...
Thanks for the link, I just finished creating a pdf version of the SM as you posted the above.

I will note down the steps you suggest and add to my list of troubleshooting actions, again, really appreciate all the input. Also note that there is no fuse in the amp, only a circuit breaker (2.5A).
 
Benz said:
I went back through the entire thread to extract all of the troubleshooting steps in a separate document, so I can go through them in an organized fashion and take notes as I go. As I was rereading everything I noted you mentioned the use of an ESR meter a few times already... apologies for the redundant questions, and thanks again for the suggestion to use the ESR70. I can get one for $200CAD on AMZ. So far my troubleshooting shopping list includes components for a DBT, a variac and an ESR meter. I will start with the DBT and ESR, as they both appear essential to most TS steps. I already own a couple of DMMs.
Click to expand...

Check prices at Newark (Electronics)- would think they sell to Canada - my Peak ESR was around $90 USD ....
 
Hi @Benz , what does the Service Manual say the FP can with the phenolic wafer is in your 1968 unit? How many sections does the can have and what is the capacitance of each section and rated voltage?

I’m curious as my later 2505-2 from 1972 is a 500uf 50v single section FP can. Looks to me like MAC might have used a different can cap in the earlier units like yours.

In later units like mine, the FP cap is numbered C79 on the schematic…
 
monkboughtlunch said:
Hi @Benz , what does the Service Manual say the FP can with the phenolic wafer is in your 1968 unit? How many sections does the can have and what is the capacitance of each section and rated voltage?

I’m curious as my later 2505-2 from 1972 is a 500uf 50v single section FP can. Looks to me like MAC might have used a different can cap in the earlier units like yours.

In later units like mine, the FP cap is numbered C79 on the schematic…
Click to expand...

I can't quite decipher, but I believe the below will provide the answer (from the manual I pdf'ed this morning):

8.jpg 10.jpg
 
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Thanks Monk, this helped me better understand the diagrams. I've been studying them and comparing with what I see in the amp and I can follow most of it. I should get the ESR70 in the coming days and I can then start testing the caps properly, as well as diodes. All the resistors I checked so far look good. I checked voltage on the big cans (9300uF), one had about 500mV, and the other one had about 15V. I discharged both, then noticed they slowly creep back up to to about 240mV and 1V repectively.
 
Benz said:
Thanks Monk, this helped me better understand the diagrams. I've been studying them and comparing with what I see in the amp and I can follow most of it. I should get the ESR70 in the coming days and I can then start testing the caps properly, as well as diodes. All the resistors I checked so far look good. I checked voltage on the big cans (9300uF), one had about 500mV, and the other one had about 15V. I discharged both, then noticed they slowly creep back up to to about 240mV and 1V repectively.
Click to expand...

Benz --
Sounds like you are headed down the right path; this has become an invaluable thread for me.

Awesome that you found the best version Peak, at a great price! -- It really is a cool lil piece of test equipment.

So very nice that you have all this original documentation that was included with your amp. Provenance counts :) LOL

Again my very best --

Celadon
 
I've learned a lot on AK over the years from reading well documented threads with great insights, pictures, and references. Trying to do the same here and post as much info as I can as I go through the process.

I may or may not be successful in fixing my amp myself but am intent on trying. I've done some low/medium skilled stuff in the past but this is a next level challenge for me. I'm reading, researching and again learning from the great suggestions and insights that thave generously been provided so far. Somewhat slow process but I'm doing this as a hobby, and being cautious as I don't want to make things worse.

I've owned many pieces of equipment these past 15 years but this MC2505/MX112 combo is special to me; looks great, sounds great, with all of the original (mint) paperwork. So these are definitely keepers.

Cheers
 
I received the ESR70 today, so starting to test caps. C11, C12, C35, C36 test good, as expected since they have been replaced.

Here are the results for C29-C30, the big caps (9300uF):

20230502_174157.jpg 20230502_174225.jpg
 
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Now C34, the 200/500uF can... not sure if I'm doing this correctly, but I results don't seem right. Perhaps because I'm testing them in-circuit?

20230502_174354.jpg 20230502_174456.jpg 20230502_174523.jpg 20230502_174529.jpg
 
C34’s ESR looks suspect, but I’m not sure if you are measuring correctly. MAC wired that can differently on the early units.

Both sections of C34 should be less than 1 ohm. So 6 ohms is high. Just for grins, what reading do you get on C34 with one lead clamped to a can twist lug and the other to a terminal “shape”?

How does c33, the 4 section cap, measure?
 
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monkboughtlunch said:
C34’s ESR looks suspect, but I’m not sure if you are measuring correctly. Both sections of C34 should be less than 1 ohm. How does the 4 section cap measure?
Click to expand...
I don't think I'm measuring the multi caps correctly; I get different values depending on which 2 terminals I connect to the ESR70. On C34, should the probes be connected the way I have them in the picture (D + gnd)(triangle + gnd)?

I've searched online but couldn't find anything on where to connect the probes in order to properly test multi caps. I can get different values on C33 depending on how I connect the probes. If I connect D, square and triangle to ground I get 45.63uF (1.06 ohm); 48.52uF (1.00 ohm) and 47.64uF (1.08 ohm) respectively. If I connect the post with no symbol (ground?)to the nut that holds the cap, I get 67.04uF (0.77 ohm).
 
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