Workflow suggestions for refurbishing MC2125

Also: is it OK to leave the leads in place if I should want to put amp back in service before completing testing?

Michael

Depends on how stable the amp is with little antennas left in place on the outputs. I have a 2105 here and it has ferrite beads on the wires going to the outputs to block RF and high frequency oscillation issues as best as I can figure.
So while I propose this test method leaving in the test leads is something I have never done on any amp so I can not in honesty recommend it. I have a special test clip set I use when i do this test, and I easily remove all of the test leads when I am done... So that is how I do it, and since I have no relevant history on your amps stability with extended leads left in place I just don't feel comfortable saying you can leave them in place
 
I thought about making a test harness too---wondering what you used for the clips down inside the sinks as it's pretty tight in there when folded back up.
 
In your case you only have to test clip the top two transistors. The sense lines connect to the bottom two outputs and you can read everything off the output assembly at the driver card and the output to transformer wire.

I use small test clips made for HP test equipment. They work fine for my testing purposes. Cheap clips don't clamp enough pressure to the connection to be reliable, especially on oxidized wiring. So chose clean connections to tap and then use clips that have teeth in them so they bite down on the connection. I have had mine for ever it seems so i can not recommend anywhere to go buy these nowadays. They weren't cheap when i got them so I doubt they got any lower priced since, if they still make them....
 
Thanks...I think just soldering the leads I need in place will be the most efficient way to work. Last thing I want is for the leads to come off and/or make an insufficient connection to get reliable numbers.

Aside: are these the clips you're using? They're HP #5090-4356.

AGILENT%20hp%205090-4356%20SMD%20GRABBERS%20IC%20CLIPS.jpg
 
Quick update: I'll be performing measurements on Thursday. Meantime, there's been a change in the amp...

I normally keep my meters set for "Watts" and yesterday when I powered the amp up, the left channel meter needle immediately pegged itself. The right meter remained at 0. If I change the meter from "Watts" to any of the "db" settings it seems to work just fine.

I've tried decoding the schematic but can't make out how to trace the problem back to a specific set of components. Any suggestions greatly appreciated.

Thanks!

Michael

ADDENDUM: This is weird but I guess I had forgotten that when I started this thread, the left channel meter didn't budge at all when put into "Watts" mode. It appears that during the process of recapping everything it started working and I failed to notice the improvement. And now, of course, it's not working again, but in a different manner. I'll visually inspect the meter and power PCBs on Thursday and see if there's an obvious problem with one of the components. Maybe the recap put stress on some other part that was marginal?
 
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Inspected and reseated both the meter and power supply PCBs---nothing visually obvious. The ribbon connector also looks good, no burned pins. Tomorrow when I open the amp up to take measurements on the output stage transistors and resistors I'll be able to inspect the individual power output PCBs and see of the problem may lie here.

Meantime: any thoughts/suggestions from folks on the list? ;->
 
Emitter Study

So, pulled the sinks, wired 'em up, put 'em back and took measurements. They are summarized in the table below. The only "odd" thing was the slight voltage measured across the emitter pairs on the left channel. Also you'll can see there's quite a bit of difference in the voltages between the two channels---I would have expected both channels, if operating correctly, to give the same numbers.

Also of note is the one of rectifiers on the right channel has a highly corroded lead. [See image at end of this post.] However it tests OK with my DMM thought it certainly seems like something that should be replaced regardless.

So: I've gotten the measurements. What are the next steps?

EDIT: The "Watts" problem seems to have solved itself---obviously there must be something loose someplace and all the jiggering around inside the amp (plus reseating the meter and power suppply PCBs again) must have improved the contact.

MC2125_EmitterVoltageDrops.jpg


IMG_2225_corrosion_sm.jpg
 
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That diode between the two sand resistors looks corroded. I'd replace that diode. Try that and see if things improve. It appears to be a critical part of the ground path.
 
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@KentTeffeteller: yes, that diode is nasty looking but measured OK with my DMM. It will be replaced no matter what along with all the other diodes down there. Just need to know what to replace them with. The original part was #070-031. I believe a current production replacement part would be 1N5394.

Meantime, what I really want to know is how to interpret the numbers for the individual emitter resistors and the resistor pairs. Should both channels be more or less the same? If so, which channel is "right"? Are the small voltages I measured across the pairs on the left channel indicative of a problem?

Michael
 
The meter that have been recommended more than once here for adjusting bias is a P3 "Kill a Watt." Gives you a direct digital reading of power draw at the input. Adjust the bias pot to give you the proper draw per Mac specs.
 
@jlovda Thanks for the response. If you read a few posts back you'll see that I have this meter and made a bias adjustment using it. The observed current draw was only half of the spec numbers given in the service manual. I made the adjustment anyway and the emitter numbers reported above are the result of that. My questions remain the same:

Why was the current draw half of what's listed in the SM and is that a problem?

How do I interpret the numbers for the individual emitter resistors and the resistor pairs?

Should both channels be more or less the same? If so, which channel is "right"?

Are the small voltages I measured across the pairs on the left channel indicative of a problem or acceptable?

Michael
 
The only "odd" thing was the slight voltage measured across the emitter pairs on the left channel. Also you'll can see there's quite a bit of difference in the voltages between the two channels---I would have expected both channels, if operating correctly, to give the same numbers.

So: I've gotten the measurements. What are the next steps?



MC2125_EmitterVoltageDrops.jpg


IMG_2225_corrosion_sm.jpg


Well now you can see that bias on left channel is LOW compared to right channel.

PLEASE DO NOT LISTEN TO PEOPLE RECOMMENDING THE WATT METER APPROACH TO SETTING UP A MAC OUTPUT STAGE INCORRECTLY. You have done this already and now we can actually see that your in left field on one channel bias at minimum... so pay no attention to folks that aren't current with this thread post, They mean well but are just a bit late to the dance so to speak...

I rest my case that now you can see that one channel is not bias'ed the same and the other. Please reset bias adjustment so left channel reads what right channel reads, approximately as a close as possible. Bias can be almost anything between 5.0 and 10.0 mvdc as per some of the more learned thread posters here have stated, and these number fall safety in typical ranges you would see with just about any home amp made by any manufacturer, except special high bias amps of course, and this is not one of this so no worries....

Then let the amp SOAK at this balanced bias setting for at least 1/2 hour. Before rechecking to see what thermal enhancement has done to the bias settings....I.E. let the amp idle with proper bias so you can retest to see if bias setting have even remotely stayed the same with even heat applied to both channels output stages ...:thmbsp:


I can see also that at lower bias you has some dissimilar outputs in the left channel, please bias them to about 5.0 mvdc and see what the numbers are after a soak period. Bias adjustments can effect how you perceive voltage drop across each emitter independently so both channel must appear to have similar bias levels then retake the reading after the warm up soak cycle...

Please re-post all bias number after you have balanced and soaked the amp at idle for at least 1/2 hour. please... we can try and see what you have then. For now though your right channel looks pretty darn good match wise < very livable IMHO > I'm just worried about left channel with its odd numbers at low bias, but it may just be low bias causing this so balance then recheck please... thanks I will be checking in on you thread posts to see what we end up with...
 
@1moreamp: Thank you! All understood. I'll try to make changes tomorrow and report back!

P.S what to do about that nasty diode? Replace, yes?
 
@1moreamp: Thank you! All understood. I'll try to make changes tomorrow and report back!

P.S what to do about that nasty diode? Replace, yes?

Clean the diode leads with solvent , it appears to have had some corrosive liquid come in contact with it. A soft brush and mild scrubbing would seem to work.
 
Well, now I've got a real puzzle because today I am seeing a difference between the pairs on both channels. The outputs tied to the B-- rail (NPN parts) are reading lower than those on the B++ rail (PNP). The difference is slight on the right channel but, more worrisome, is that the left channel gives me almost no reading on that half---close to 0mV. And the "Watts" meter pegging (full deflection to right) has returned.

Amp has been running for about 30 minutes so it's all warmed up. I tried cleaning the trimmers w/ a shot of deoxit on the odd chance that that's the problem. Supply voltages to both rails look good. Not sure what to make of this.

Am I having a bad (electronics) hair day or have things deteriorated?

Michael
 
Well, now I've got a real puzzle because today I am seeing a difference between the pairs on both channels. The outputs tied to the B-- rail (NPN parts) are reading lower than those on the B++ rail (PNP). The difference is slight on the right channel but, more worrisome, is that the left channel gives me almost no reading on that half---close to 0mV. And the "Watts" meter pegging (full deflection to right) has returned.

Amp has been running for about 30 minutes so it's all warmed up. I tried cleaning the trimmers w/ a shot of deoxit on the odd chance that that's the problem. Supply voltages to both rails look good. Not sure what to make of this.

Am I having a bad (electronics) hair day or have things deteriorated?

Michael

I am not sure what is causing your meter to peg. I would disconnect it until we can get it sorted later on, but its likely in the meter drive board if your not seeing errant DC voltage on the output of the amp stage without the autotransformer in circuit.

As for your readings being bizarre well that is why i wanted you to test this way. You see the simple AC Killa-watt meter BS method of setting up your Mac is almost totally bogus on what your amp is really doing inside as you now can see...

Remember I am only interested in the voltage drop across each emitter resistors for the outputs only nothing else at this point. Your posted diagram only shows four output resistors for four outputs. If those four voltage drop readings are way off then your outputs are turned on unevenly which means unequal or poor current sharing. The amp may work like this and it may fail prematurely all depending on how badly the transistors have aged with use and how weakened they may be... All a big bunch of maybes unfortunately. So please post the resistor voltage drop numbers only for each channels output four resistors.
The rest we will look at later on, including that 12.5 ohm driver resistor, which seems to have stretch a bit in value. Right now I am only concerned with seeing how each output transistor turns on thru each emitter with a common cold bias setting of 5.0 MVDC with a soak of 30 minutes to allow for heat enhancement to your original 5.0mvdc bias setup...I am sure its going to rise off value, just how much off is of interest to me. Plus once it has risen then you will have to readjust it back on target and soak it again to see what it does...

I am not fond of the thermal tracking on any of this series of Mac amps but it has survived for decades so my main goal is just to get it setup properly and not to redesign an antique design idea to modern standards. So its a live with it situation once its setup correctly IMHO....unless you want to redesign the entire bias drive of the amp which will alter it enough to upset all of the folks that want vintage gear left vintage..I try not to upset the natives if possible....
 
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I'll have back at the amp on Thursday. Maybe there's a bad solder joint on one of the test leads.

Left meter behaves just fine when not in "Watts" mode which makes me suspect of portion of meter PCB circuit responsible for that calculation. I did recap that board recently so maybe it's a bad 'lytic. The diagrams use the labels "dB," "LOG," and "RECT" to identify different operation modes. Assuming "dB" is "Decibels" leaves "LOG" and "RECT" for "Watts" and "HOLD." Which is which?

Michael
 
As for your readings being bizarre well that is why i wanted you to test this way. You see the simple AC Killa-watt meter BS method of setting up your Mac is almost totally bogus on what your amp is really doing inside as you now can see...
This is the method described in Step 8 of the MC2125 Service Manual Notes. In the end, what is the difference in results between your method and Mcintosh's?
 
This is the method described in Step 8 of the MC2125 Service Manual Notes. In the end, what is the difference in results between your method and Mcintosh's?

First off , please take no offense from what i am going to say in the thread post, thank you.


The ancient Mac/kill-watt laymen method is just all of that. Its blind and un-enlighting about what else is going on INSIDE the amp. Plus I don't know of any amp maker using such an ancient way of seeing if a amp is idling correctly. Yes I measure AC power draw and DC power draw on amps as a baseline "DUH it's turning on test"
But AC or DC power draw figures vary just as much as AC line voltages and DC feed voltages do to any amp made by any maker so its at best a vague and arbitrary measurement that does not really look deep enough to see if everything inside the amp is balanced and operating properly at all...

I been doing this too long to base all my workmanship on a simple power draw test. Its a lot more involved inside of any amp then some simple idle power draw test can tell about.

Don't take this as bad mouthing Mac and their conception of everyone else's limited abilities < the only reason I know of they would tell you to test such a way for >
But McIntosh did not Invent amplifier technology, They just build and sell it. Just because they profess a laymen's way of basic testing in a service manual meant for some handyman's repair desk, it does not in any way really get you inside of any amp made to see if its really working balanced and aligned properly.
Plus I think they use that weird way of testing to check see if the amp is passing huge DC offset into those beautiful auto-transformers which HIDE errant DC offset voltages and odd behavior any amp is capable of...You see if the amp is passing DC there is no way it will be anywhere near their base current draw expectations.
I prefer to test with the transformer out of circuit to get my numbers and to test each and every emitter voltage drop for even and balanced current sharing of the output stages etc, etc, etc....BUT this requires that you have some rudimentary engineering understanding of the design in the first place. So what I do is not for the faint of heart or the untrained and uneducated. I drive deeply inside of the circuitry to make sure its all intact and functioning as expected.

There are lot of want-a-be type tech's and mechanics out there. Hell I have seen hack doctors in my lifetime. So please allow me my grace to grail chase what I know and understand to be the best repair methods I found in 35 years of doing this sort of work on the side while I engineered my way thru Silicon valley for 30 years of that. And in Silicon Valley WE made all the Silicon parts your listening to now, so I like to think I worked with some of the best in my lifetime and they taught me well along the way... So I do things infinity deeper then most Service manuals talk about.... And remember service manuals were intended for 9th graders in most cases. I know this because I have written a few in my lifetime and I was told to condense to 9th grade level language and technology limitations... I worked for GM, 3M, Kodak, Verbatim, Intel, AMD, National Semi, and some nice folks that do Military that I can not speak of, and everywhere I went I have had to condense everything to levels way below the average 2 year college level minimum requirements to be called a "Tech".... So why would you expect McIntosh to be doing otherwise in their service manuals?

I hope I have not offended you in any way but I had to elaborate in depth for you to see my point of view concerning service manuals in general and McIntosh's very vague and unrealistic testing methodology they infer in their service manuals.

I also can share Alpine, Kenwood, Adcom, Sony, SoundStream, Phoenix Gold, etc service manuals and none of them test in such a manner, and I do mean NONE of them... They require Voltage drop measurement inside of the amps circuitry to validate the amps proper operating condition, I even have some brand names rebuild line tech data and requirements, and they even require the same in-depth technical measurements to verify a amps proper internal operations.....
I have AC and DC power measurement equipment on my bench at my home even. I use it to do efficiency measurements mostly < I.E. power in versus power out testing methods for amplifier efficiency ratings etc.... But I never harness my buggy to this measurement as the sole test for any amps proper operations and anyone that does is either untrained or unknowing and therefore unqualified to be servicing amps IMHO.... The test just isn't good enough in the real world I live in and understand ...


Sorry, I am not trying to be mean, just knowledgeable in a true engineering sense. I don't open up gear that I don't completely understand how to work on and what I am looking at. It would be Un-professional from my background.
Plus I don't like looking stupid when I do what I do for anyone and I have had the honest pleasure of working for people that looked down on that sort of workmanship from their PHD level of education... So its keep up or give up,,, but don't get in the way where I came from... and I have seen a many fall to the wayside in my time... Working for well educated people beyond masters level is not the easy way of making a living...But I feel smarter for having done so in my lifetime. If it has made me hard well you should see some of the mess's I have had to clean up from want-a be's out there....That made me feel more like a janitor, then the engineer I was all my life....sad but true...

So no hard feelings I hope, I just plain Know better then what any Mac service manual preaches... plain and simple... They <Mac> build audio gear they did not invent the technology of silicon, They just use it to make money and Wow do they make a lot of money with Silicon...I actually know what building a amp costs, even a Mac amp, and Mac got huge margins in what they build and sell, why do you think three other companies have bought and owned them... They aren't going broke anytime soon... heck didn't one of the founders retire and buy a whole mountain in Arizona. Like DUH ! You don't do that losing money in business...

PS I also have a FCC license and I can work on RF amplifiers and generators for broadcast stations also. I can't begin to tell you how many DC power supplies and RF generators I repaired and rebuilt in my lifetime here and aboard in Japan used to manufacture silicon and electronics related products mind you..... It's been a long road and I am not about to change what I do now, since it has worked so well in my lifetime...:thmbsp:


It's all about the little details, and it always has been about the fine details...or none of this stuff would work at all...There is a mathematical reason for everything inside of any amp made... Its all about the numbers...or you can't design a amp from the ground up. Simple as that... again I hope no one is offended.
 
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