Workflow suggestions for refurbishing MC2125

I guess this thread is coming to an end?

Certainly well off topic now. ;->

Yes, the Glendronach line uses 100% oloroso cask for finishing. I myself like the fruitcake finish so it's my jam when I want something for desert. Haven't explored the Glenrothes (I think that's what you meant) very much but I know they are well regarded and are definitely more restrained when it comes to finish.

If you want to chat more about whiskey, send me a PM.
 
Well, that was my inclination as well but I'm also looking to collect as much information as possible during this 'pass' as well as get it recorded in a thread for future generations. ;->)


Back on thread track::thmbsp:


Yeah I wanted to do a MC2205 total resto thread a while back but some of the folks her became openly hostile towards me about my technical feelings about some of MAC's designs and considerations like auto transformers, and Power guard. So I folded the idea, since my parts choices would also have some them pulling their hair out also.

Like I found all of these 35 year old MAC amps have some DC offset issues that are masked by the auto transformers. Some folks here think because the auto transformer blocks it from the speaker that it has no ill sonic effects. and I tend to disagree. Amps back from this era had high DC offset issues by design mostly due to a lack of better made silicon parts back then. Mismatch silicon devices cause DC offset issues internally in the main amp channel, and to me this causes possible distortion related issues.
Mac has lots of decoupling caps in each channel which help reduce this issue but, I would like to tinker around and get the amp to not have the issue to begin with if time and money allowed... Also reduce the caps count internal to each amp channel also. Sometimes less is more you know?

So I left all the open threads other did here alone and did not add my 2 cents worth. Didn't want anymore grumblings lol...Plus you gotta pull open and link back to at least 6 or more old threads to pull all the device replacement data together for any of these Mac amps. I know that is what it took for me to gather all the info together...Hours of read time and like you I agree it needs to be condensed to a single thread with links back out to those that deserve honors for all of their efforts also... Hope your thread goes well when you do post it... You have my support already:thmbsp:
 
At this point I have a reasonable distillation of Step 1 which is to replace the small electrolytics. Options for parts are Nichicon (combination of KZ, FG, and ES, depending on value and polar/bipolar), Elna (polar only), or WIMA. (I have a spreadsheet with both Mc and mfg part numbers.) Leave the film and poly caps alone unless I suspect a problem, which I don't. Same with the ceramics.

I'm also going to replace the vactrol on the PG circuit just to eliminate it as the cause of power difference w/ 2105, as measured using SPL---admittedly an unsophisticated method. Also because Mc has a replacement for it so get while the getting is good and support the home team.

Oh, and before I replace anything, I'll measure a bunch of voltages to and compare w/ my schematics establish a baseline. That's like Step 0 I guess.

I'll continue to post to this thread as I move forward.
 
The input gain stage is considerably different between a MC2105 and MC2125. Tilt at any windmill you wish but I doubt the LDR is leaking signal to ground......they fail towards open....switch the MC 2125 sensitivity to .75 to increase the gain. With the switch set to 2.5 we were clipping almost all midfi preamps to get higher volume levels. Most could barely put out a volt when we tested them.

I have a MC 2205 on the bench.....with another on deck. One driver board had 3 cracked molex pin solder joints, the other had two. I swear one i could wiggle one around. This has to be a top priority.
 
I forgot to mention, if you check the schematic, you can see that flipping the rear panel switch opens the LDRs path to ground. This essentially takes them out of the circuit.
 
@c_dk: Changing out the LDR won't be one of the first things I do, if at all. I've already done a simple listening test w/ the PG circuit bypassed using the rear chassis switch and didn't notice any change. The only parts that will get replaced at this point are the small 'lytics.

I will pay very close attention to the Molex connectors on the channel output PCBs when I take them out. I know it can take some work getting these things off the boards after they've been left undisturbed for so long. Easy to imagine solder joints on both the male and female ends of the connector being crystallized after so long and in need of reflow. I just hope I don't break any wires. I have to imagine they're pretty brittle by now.
 
@c_dk: Changing out the LDR won't be one of the first things I do, if at all. I've already done a simple listening test w/ the PG circuit bypassed using the rear chassis switch and didn't notice any change. The only parts that will get replaced at this point are the small 'lytics.

I will pay very close attention to the Molex connectors on the channel output PCBs when I take them out. I know it can take some work getting these things off the boards after they've been left undisturbed for so long. Easy to imagine solder joints on both the male and female ends of the connector being crystallized after so long and in need of reflow. I just hope I don't break any wires. I have to imagine they're pretty brittle by now.


The plastic wire covering will be stiff to work with with heat and age, but the fun part will be how the wiring to the spade terminals will just pull out of the terminals due to copper cold flowing with age. You will likely need to load new terminals and I suggest soldering them for added reliability rather then just crimping them as originally done by Mac. Age takes its toll on crimped copper wire terminals with the soft copper just cold flowing loose in the terminals I replaced all of them in my rebuild...and I soldered them for good measure....
 
The plastic wire covering will be stiff to work with with heat and age, but the fun part will be how the wiring to the spade terminals will just pull out of the terminals due to copper cold flowing with age. You will likely need to load new terminals and I suggest soldering them for added reliability rather then just crimping them as originally done by Mac. Age takes its toll on crimped copper wire terminals with the soft copper just cold flowing loose in the terminals I replaced all of them in my rebuild...and I soldered them for good measure....

Sounds like I'll need to have some terminals on hand as well as a crimper. Did you run into any issues with the wires themselves, i.e. need to replace them? If I recall, they are trimmed pretty tightly.
 
Sounds like I'll need to have some terminals on hand as well as a crimper. Did you run into any issues with the wires themselves, i.e. need to replace them? If I recall, they are trimmed pretty tightly.

Ahh not really, I found ample wire length from Mac's build, but I did have to snip off ends to get good quality copper wire due to oxidation and other corrosion related issues.
I just re-laced them with zip ties so everything was tight and neat again, and not subject to shock and vibration related future issues.

Plus I like to separate wiring a bit pulling away wiring with signals away from wiring with AC or DC voltages thus limiting as much as possible any possible induced voltage pickup issues. Something I picked up along the way of working on everything and car audio wiring runs which have such issues regularly...

I found that vintage gear like this rarely was assembled with these thoughts in mind, and I even ran all new AC wiring to the switch and transformer that was shielded 100% to prevent AC induce hum pickup, Again vintage gear tends to overlook these finer details of assembly, so I add lib as my engineering tells me to. Oh I also added a RC snubber across the power switch when I did my rebuild, Mac left this out on the one I worked on and my engineering said to add it too extend switch contacts lifespan. Mac did this on some other of their gear so I again add-libed this one also as I felt it worthwhile and not a hindrance of Mac's performance in any way...
 
What I found

Here are some things I found today while recapping the left channel PCB.

1- I was a little surprised to find that the electolytics on the channel cards were Nichicon. (See photo.) Are these original parts? There were also two ROE bipolar caps as well, which I expected. [BTW: I decided to replace the electrolytics with Elna Silmic II and the bipolars with WIMA.]

2- The .01uF poly caps (ERO KP1834) on each card look different: one set is yellowish-white but the other set is silvery grey. (See photos below.) Is this normal or does it indicate a problem? Also, the tolerance on the ERO parts is 5% while the service manual indicates that 064-142 should be a 2% part.

3- If the connectors for the cards are Molex, it's not a type I've ever seen before. The pins are rectangular, not round. (See photo.)

4- Two connections are made to each PCB using a short pin soldered to the board and a crimp-on Molex-like terminal. These carry Headphone Out and Input Limiter. Dunno if the terminals are normally soldered to the pins but they were on my PCBs. It was a complete PITA separating them and I had to replace the terminals which were destroyed in the process. [EDIT: now that I've worked on the second card, I don't actually think these connectors were soldered together---they were just very very tightly mated.]

IMG_2118_sm_oldcaps.jpg


IMG_2124_sm_silverycaps.jpg


IMG_2120_sm_femaleconn.jpg


IMG_2123_sm_singlepin.jpg
 
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Someone has been there before, most likely chasing an intermittant.

I sent the day troubling shooting a 7 keypad, 2 CR12, 2 Mc7108 whole house system that all the local Mac dealers ran away from.
 
@c_dk: So the Nichicons are not original then! I see them on the inputs too----which is odd because A) very difficult to get to and B) they are electrolyics when the SM specifies bipolar. Benign or should I replace?

What about the connectors: originals or replacements?
 
I see them on the inputs too----which is odd because A) very difficult to get to and B) they are electrolytic when the SM specifies bipolar. Benign or should I replace?

I saw blue Panasonic's on the Mac I did and they appeared to be stock to me, they were date code correct for the manufacture of the amp which was like 1979...
Replace them ! Your there, there older then your children, replace them.... so splurge and spend maybe two bucks in a good way. It can't hurt anything to be thorough and complete since your already there and in neck deep:thmbsp:

I see your 5% film caps and they don't match original in my 2205 I worked on. This circuitry is feedback related, and has only high precision devices located near by it. I would tend to try and find the 2% original style parts from any manufacture and duplicate for both channels as best as possible. I have only seen 2% parts where your questioning, and the resistors near by connected are similar tolerances so I think Mac knew what they were doing and I would try my best to mimic Mac's original design just in this specific area... but this is my opinion and YMMV and others my disagree..........


The itty bitty blue caps behind the RCAs are bi-polar and WIMA 2.2ufd fits nicely there but you will likley have to de-solder the RCA connections to the board to access the board clearly on both side to replace the electrolytic, and then resolder the RCAs back to the mini board, after you clean and de-oxit it the RCAs themselves including the inner tip sleeve.. Be aware of the hard wiring trying to break off while you handling the board like this. You may end up reattaching a wire or two due to embrittlement of the aging cabling....Just reattach as needed with fresh solder and you should be golden...

I had to do all this because the entire assembly on the last 2205 I worked was covered in soot left by a Mac service centers hasty repairs, and the soot was shorting out the RCAs to ground when no creating a hugely bad contact situation, and just plain fugly to look at.

If your replacing your vintage RCAs then this should come out anyway just to ease your install of new RCAs. and while there I replaced the electrolytic's with WIMA's since they too are bi-polar and being a film type cap they should last you a lifetime. You can 0.1 bypass them also if you think a 1.0 to 2.2 ufd cap value is effecting your high end signal bandwidth. They serve one single purpose being there and that is to block any errant DC offset from your preamp output, or as techies like to say they provide AC coupling, while blocking DC voltages...tada !


Oh take a few snap of your work are before you disconnect anything then if a wire breaks off you will have fall back to figure out where it went... Love my cheap bench camera... saves me loads of headaches ....
 
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Oh replace those orange ERO bi-polars either with WIMA's < PM me for link to correct size parts links > ~ $5. something each , not cheap but the best never is > Or Elna Silmac II's or Nichicon Muse, your choice, but the film caps I suggest will out last your lifetime most likely...and you can 0.1 ufd bypass them if you like here again they serve one sole purpose and that is to block DC offsets or they are AC coupling caps...all in the semantics I guess lol....


And there is 0.47 ufd wet cap up front before the input. Again you can WIMA, or Elna SIlmac II or Nichicon, your choice. And you can 0.1 bypass if you like also I believe.
There have been some posting here on the AK where at least one other person upped this part spec to a 1.0ufd device. They proclaimed it enhanced bass bottom end. Which is possible since at 0.47 ufd I think Mac may have meant to provide some sort of sub sonic roll off with this value along with providing AC coupling and DC blocking. Its a fairly odd value from just about every other amp I ever saw in my lifetime so the sub sonic roll off is just a guess without spice modeling the circuit and due to the age of the parts involve no Spice models exist for these transistor... so out on a limb I best guess its a roll off thing along with AC coupling, because you got 1.0 to 2.2 on the RCa's already,a nd two 10ufd bi-polars after this input circuitry so 0.47 ufd is no where near the rest of the circuits design ideas... best guessing though...YMMV and so will opinions...
 
Your connectors look stock, c-dk is right someone has been chasing intermittent connections by soldering on some of the connections it would appear at first glance.

OK hope this doesn't tee anyone off but here it goes.. Way back when this amp was built these connectors were state of the art. Now not so much. Times and tech change as does experience which is usually the thing that changes everything.
I kind of hate these connector plugs but unless you want to do drastic things your kind of stuck with them. I hate IC sockets also for similar reasons. but these conveniences make assembly and repair work on a monster like this a lot easier to perform.

So unless you want to hard wire these driver boards < which I am not even trying to convince you to do > Your kind SOL and need to perhaps if your in doubt rebuild these connectors for the next 39 years of use.

As I said your options are very limited IMHO, and if your into originality then rebuild these plugs as you see fit. I suggest you look long and hard for heavy gold plated contact inserts, they don't corrode. And gold contacts will cost you dearly now that gold is where it is price wise.

Gold is best... if you got the moola...
 
All the electrolytics on both channel cards were replaced with Elna Silmic II and the bipolars w/ WIMA. It was pretty straightforward. I'll send a photo tomorrow. I also replaced most of the caps on the meter board because I had some extra parts. To finish that part of the job up I need a couple more electrolytics (22uF and 47uF---the latter is also a ROE ;->) The WATTS setting now seems to work correctly for the first time since purchasing the amp so obviously something was wonky.

I guess I'll also need to find a replacement for those ERO polyester film caps if in fact they are not the original parts. And kind of weird about the difference in color on the two boards. I did some googling but didn't see any mention of color differences either because of production or aging.

I also did some voltage measurements before and after. The recapping didn't change anything. However, I did find that B++,B--,+Drive, and -Drive are all a little low in both channels:

B++: 40.6V/40.4V (spec is 42V)
B-- : -40.4V/-40.4V (spec is -42V)
+Drive: 40V/40V (spec is 41.5V)
-Drive: -40V/-40V (spec is -41.5)

What might this be telling me?
 
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I also did some voltage measurements before and after. The recapping didn't change anything. However, I did find that B++,B--,+Drive, and -Drive are all a little low in both channels:

B++: 40.6V/40.4V (spec is 42V)
B-- : -40.4V/-40.4V (spec is -42V)
+Drive: 40V/40V (spec is 41.5V)
-Drive: -40V/-40V (spec is -41.5)

What might this be telling me?


My question is at what AC line voltage did you get these readings? Transformers cannot AC voltage regulate so whatever the incoming line voltage is will change the AC output voltages and thus the DC voltages your likely to be reading.

Also the thing I check for is Balance between the DC rails. If they are not balanced then you got some real problems IMHO. Balanced class AB Complimentary circuitry should load the DC rails in a balanced fashion. Voltage drops across current limits should be about the same withing a narrow margin of tolerance. If not your circuitry is not loading in a balanced fashion and you may a weak semiconductor, or some circuitry balance related issues you need to debug...
To me balance is the tell tale clue to watch out for. After all its supposed to be a balanced complimentary amplifier design. Being out of balance can cause DC offsets at the output of an amp. These problems are linked so while working to resolve DC offsets start with circuitry balance voltage drops and such.


Also I meant to tell you that when buying caps wet caps or film caps I buy way more then i need and then hand select the caps I am going to use in the amp by measuring their cap value in ufd and measuring their ESR spec.

This is grail chasing But I find it relaxing to make sure all the replacement caps I install are measuring exactly as a set of matched value components. Wet electrolytic caps can vary by -10% to +50% and film caps are usually +&-5 % tolerance ranges devices. I like to make sure they are more closely matched then that.
If a circuit calls for a wet cap value of say 10Ufd well I find a set of caps that read as dead nutz possible at just that value. While I am sure Mac engineers knew the possible variations in wet cap values and the possible interactions that variations might do inside the amp. I like to pull things in a lot tighter to what they intended in the first place.

You will end up tossing out of spec caps to the side a lot but your work will be rewarded by knowing the circuitry is tightly balanced and with original blue print spec as possible... Solves a lot of second guessing and haunting feelings later on also, as you know for sure all values in circuit are right on target value of spec.....Yes i grail chase, and wouldn't have it any other way...plus it lets me some use of all the shiny expensive tools i got clustered in front of me:thmbsp:
 
All my amps are getting 123VAC.

DC readings are:

B+ 15.8 on both channels
B- 15.8 on both channels

Those are your op-amp bi-polar rails and they are balanced very well also..


If all rails are balanced then I think your all good to go, or as best as your likely going to get things.
There are other tests where you measure voltage drop across current limits resistors and such just to make sure everything is copo.
But it would take a few more pages of instructions to get you thru all of that, and for the most part even though you feel you have lower then normal rails I think your amp will do its rated power into load just fine at this point.

And if in doubt please by all means test your work afterwards into test loads with a single sine wave since this is how the FTC does it.
Then if your not making Mac's rated power spec then yes i would say the rails supply might be an issue...

here is a link a basic ohms law calculator if you wish to try the math yourself. Just pick the line with the two parts of info you have and plug in the numbers and click the equal mark and VIOLA ! basic ohms law done for you...

http://www.the12volt.com/ohm/ohmslawcalculators.asp
 
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