Winter Project - 3 MC-40's to restore / Update?

Thanks Guys -

This has been a real learning experience, especially with the help and guidance of the amps' owner. Plus the comprehensive library he gave me of how to test, how to use test gear, maintaining audio gear

I have learned that very few DMM's can accurately measure AC voltage over the typical audio range of 20hz-20Khz. Most, including the Fluke #77, are limited to a range of around 40-400hz. I believe.

I did some guided research and selected the Fluke #87-V, with an ac range of accuracy from 20-20Khz. I have the 177, and thought the "True RMS" meant it would be accurate for audio use. I am still puzzled by this. Its pretty clear if you drill down into the specs on the Fluke website. (Look in Selecting a DMM Look for "Wide Bandwidth"

I believe that old VTVM's were made to have this wide band range of accuracy, but I have so far resisted the urge to restore the ones I have.

I verified this last night by comparing the .5vac out from my sig. generator (a General Radio tube unit that, once warmed up, is remarkably stable and the dial indicator is spot-on,) The .5v is the input needed to make rated output with the MC-40's. The #87-V held steady right out to beyond 20Khz. while the #77 and the C.C.C.* DVM/DMM's fell off between 400-4000hz. and became hopelessly inaccurate.

Having said that, I ran some tests on the rebuilt amp #2499 and at .5v in with sine and square waves, it was putting out 43-45 watts with NO sign of clipping and really nice square waves with a tiny (to me) bit of overshoot or ringing. On the low end, something is up either with the Sig Generator or my scope so that the square waves from 20-100hz. or more are tilted / distorted at the input, so I have little idea of the true shape at the output.

I am also not seeing the 80+/- vac between the chassis and house ground that I was before. I have NOT yet installed a 3-prong modern grounded cord to the unit, though some reading here and elsewhere strongly suggests I should - Any thoughts on this?

I also ran some initial tests on one of the two remaining to-be-rebuilt amps, #2500, with the tubes it has installed (tests on #2499 above were done with all new or NOS tubes.)

It seems to perform very well. It puts out 47-48 watts with pretty clean, though not as clean output traces as the rebuilt amp.

All tests were done with the voltage selector on the amp at the 117v setting, into 8.2 Ohm non-inductive 100W resistor with bolted on heat-sink (still got plenty hot!) at .5vac measured input with dual trace scope on input and output.

Pictures of 'scope traces coming.

Since taking pictures of the scope with a post-it note showing the frequency is a little....20th century, I am interested in what PC-based scopes / audio analyzers folks use or recommend. I really want to be one of the cool kids in this science fair. I snagged the circuit board for Pete Millets' sound card interface and will build it eventually Pete Millet Soundcard interface He uses AudioTester Web Site... but I don't have enough knowledge to judge its specs or features.

Would this program be a likely candidate to measure frequency response, distortion, THD, IMD, plot an output curve, and automate what I have been doing with much scope - sig. generator - meter - knob twiddling and sketchy digital camera work?

Anything would be better than what I am doing now.....


Edit - I read through the features description of the Audiotester and it seemed to answer one question I had - How to use the programs signal generator OUT function while simultaneously monitoring it on the programs scope or FFT analyzer - "Separate sound cards for in- and output possible." how does one implement this for the PC - gurus?
Thanks to all!



*Cheap Chinese Crap
 
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Now that is impressive work! Nice Job.
Regards,
Jim
 
Crude Square wave plots on Re-built amp

In all - Sig. Generator output / input to the amp at top / output from 8ohm taps at bottom, .5vac input, input control on amp turned all the way clockwise but not switched, load = 8.2 ohm 100w non-ind resistor


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Hi all,
I'd PM'd dcgillespie and asked him to provide some analysis on the waveforms. I confess that I'd like some additional education on amp testing. He provided it, but he provided it in a PM. I've posted his analysis here for enquiring minds.

From dcgillespie,
Assuming I am, the shots for the "Rebuilt amp" are about as ideal as they come. At low frequencies, the source should not have any tilt with it -- if it does, it's either a poor source, or there is significant LF phase shift in the vertical amplifier inside the scope. At any rate, normally, the source will show little if any tilt, while at low frequencies, the output of the amplifier will have significant tilt, indicating the LF roll off necessary to achieve good LF stability. The LF traces all appear normal on this amp. At 15 kHz, the slight hump and lack of ringing following the hump is ideal. It indicates a smooth HF rolloff, lack of circuit resonance, and typically produces very good HF stability. Like I said, the traces for that amp are all ideal.

For the non rebuilt 2500 unit, frankly the HF response looks too good, but since there is a lack of consistency in the wave size of each display, it is hard to tell for absolute certain. What is concerning however is the peak at the top of the hump on 15 kHz square waves. Note on the rebuilt amp that this is slightly rounded without a pronounced peak, where as on the non rebuilt unit, the peak of the hump is very pronounced. This usually indicates excessive HF response (although not always), along with an accompanying loss of HF stability. Other than that, the displays for that amplifier at lower frequencies appear normal as well.

If you are looking for anything more specific, let me know, but that is my first reaction to the displays.

I hope this helps!

Dave
 
Excellent, extensively executed work, China! Congratulations on all of the carefully, professionally managed, hard work.

There is one major concern that I have regarding a question you asked early on:

-Should I wire in a grounded 3-wire power cord or leave the 2-wire configuration (I plan to use a new, non-crusty one)

>> Dshoaf: No. Don't recommend it. Keep the grounding scheme both inside and out as found. Inspect the AC cord, however, for breaks and any dangerous situations. Replace with a similar cord. You may need to file down one of the blades as they are all polarized these days. I've not had to replace one of these.

It seems that dshoaf decided to make the seriously flawed claim that I've read before, both here and elsewhere. There is no safe or 'good' reason to keep the dangerously inadequate, ancient lack of grounding configuration. This is a major safety issue, as in life, serious injury or potential death and I presume it stems from a drastic misunderstanding of basic electrical safety concepts.

What was acceptable electrical 'design' and configuration in the '30s thru the '60s no longer applies in this century, not to mention several previous decades. There is _no sane reason_ not to use a three prong ground plug and provide a safety net of protection to an exposed, metal chassis that has a crying need for true ground potential. Dshoaf is welcome to elaborate (or not) as to why the safe, sensible approach of a true, grounded metal chassis (life or death safety potential) seems to challenge his and yes, others' arguably unsubstantiated opinions.

China, you might recall the restoration I completed on a couple of MC-60 monoblocks that you read about a few months ago. I definitely brought those two amps up to current, truely grounded chassis using three prong safety ground plug standards, long before they left my bench.
 
Perhaps you too, Stereo type, could elaborate a bit on how you wired the 3-conductor ac cord for your MC60's. I have two MC60's I have rebuilt this past Summer and would be very interested (and most appreciative) in learning how to properly wire that amp for safer (and better quality) operation with a three wire AC cord. I don't want to add a ground loop (or noise) to those amps so please do explain how it is done properly. Thanks, Steve
 
Updates

Thanks to Don and Dave G for their comments - Its good to know that I am on the right track.

I have almost finished dis-assembling amp #2 and will be putting it back together shortly -

At some risk of censure, and NOT to open something raw, I'd like to re-post or cross-post the following regarding plugs and sockets from a thread in "Tubes" that unfortunately was closed. I have NO interest is stirring any pot, just would like to relate this to the Mac-centric crowd.

Not knowing how to cite or reference a single post (it was #59 in the other discussion) i am just copying the text and adding the pix back in. If this violates rule #4 please let me know and this post is gone.


Quote:
Originally Posted by loomis View Post
Hmmm maybe I should just add a polarized plug and not worry about the 3-prong. Interesting.
Quote:
Originally Posted by gadget73 View Post
Those polarized plugs only work if your house is actually wired properly. I recently dumped all the 2 prong plugs in my home, 2/3 of them were wired backwards, so what should have been hot was neutral, etc. Amazingly every single one of those boxes has a ground wire in it, and the grounds are functional.



Interesting discussion. I hope it stays alive and that all can remain civil. Obviously quite a charged topic....

I posed the same question in the thread below, where I am refurbishing some McIntosh tube amps from the early 1960s. I asked their owner what he would like me to do with or to the plugs. He was professionally involved in pro audio, as I understand it, selling, importing, consulting and setting up top name recording / mastering studios and the like for 40 years. He read through this thread and the one below, and reached out to another source - the service department of Audio Classics - as they, like many members here, professionally restore older gear for continued and safe use.

I have to assume that they are concerned with both audio performance and legal liability. I understand that they specialize in McIntosh gear, and the following applies only with regard to McIntosh model MC-40's, but may well be of interest in this discussion.

Their practice, as it was related to me by my friend, is to install a polarized 2-prong (NEMA 1-15,) Plug, wire the Hot side to the fuse, then to the black lead of the transformer, the voltage selector switch (thanks Bobabode - see below,) returning to the Neutral side (Black / Yellow (115vac) or Black / Red (125vac)) I hope that is correct.

I believe this is what I will do. I have already tested the outlets in my home, and verified that they are wired correctly. As has been pointed out, if your house wiring is suspect, best to get it checked and set to rights for all kinds of good reasons.

I welcome any comments, suggestions or corrections. !



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China, That is at least a start in the correct direction, although there is no form of a safety net in the event of an unexpected fault condition. That's better than nothing, yet not good enough: Why? They may think they are performing an upgrade of sorts, yet that is neither safe, nor adequate protection. This is the point where the third prong ground, bonded directly to the chassis is the key for some reasonable factor of safety.

It's surprising to see how much rampant lack of understanding permeates this aspect of the electronics industry. Electronics techs should know better, although I suppose that those who really don't, might end up learning the hard way when they least expect it, whether they want to or not. :thmbsp: :yes: :sigh:

Don't worry, I won't even try to fight the blatantly dangerous ignorance quite so thoroughly, as so far it's been mostly lost, at least on those "unarmed" individuals who attempt to refute the message I'm trying to convey. They can stop reading right now.

Back to your good questions: Whether you add the third prong grounded plug or not, at the very least, be sure to insist upon GFI receptacles for these amps as one of the best outside safety measures in any event.

For those who care and want to know more: What's wrong with the early, old school picture, anyway and why? The early engineers had no basis for safety before the polarized configuration was a minimum. Since the engineers were left with the roughly 50/50 chance of hot/neutral, the power switch ended up in one side of the incoming AC line and the fuse was sometimes installed in...(gulp) the other line.(!) It might seem as though it didn't/doesn't matter if both the fuse and the power switch were in the same line or not (it does to a degree), although I'm giving a real world example of the original, factory wiring I constantly see in all sorts of early equipment on a daily basis, from guitar amps to all the other forms of early equipment.

Now, reverse that non-polarized plug, which will reverse the scenario; it makes no safety sense either way. Here's why: If the fuse happens to end up in the neutral side and blows, guess what? You could still get killed! Get this: Shocked, injured or killed while the fuse is blown open, by the hot lead that is still most certainly live! The same applies when the switch is switching neutral instead of hot. The switch is off, you're safe, right? Dead wrong; there's the 50/50 roulette chance that the other incoming line is live while that switch is off (open), which has lifted the neutral.

Those are just a few examples; there are several other scenarios that are equally deadly, too. This is how the potential killer of the deadly roulette wheel spins out of control in many cases. I kind of wonder how many around here actually understand the concepts regarding old equipment and maybe just as important, how to address and reconfigure these pieces for some form of safety...:scratch2:

I just finished upgrading an old Fender amp. As an aside, these amps 'feature' a death cap. Does anyone know why they call it a "death cap"? Back to this subject: It had no polarized plug, as is typical from the mid '60s and backward in time. It had a polarized receptacle that was built in, but that was worse than none. The neutral side was wired to the power switch. The fuse was in the other line. See my point? Yes, I totally reconfigured it by running the switch and the fuse in the hot side only and installed a three prong, polarized, grounded plug.
 
Back to my earlier request for info on how to wire the "green" ground wire in a 3-wire AC cord to my MC60's: Do I simply run it to the chassis somewhere convenient like a ground post on, say, the unused preamp socket, a transformer chassis bolt or somewhere else? No 0.01 ceramic cap or anything else required, too? I'd really like to know since I am using a 3-conductor hospital grade male AC plug with the green wire capped where it enters the chassis. It would be great if Stereo type or someone else could answer this. Thanks!!
 
tuber:
Back to my earlier request for info on how to wire the "green" ground wire in a 3-wire AC cord to my MC60's: Do I simply run it to the chassis somewhere convenient like a ground post on, say, the unused preamp socket, a transformer chassis bolt or somewhere else?
A direct contact bolt/lockwasher/nut that is physically through a hole in the chassis is best; generally close to the area where the AC cord enters the chassis. That unused preamp socket will work if you have mounting bolts instead of rivets. Those bolts are probably #4 or so, which is very small and I hope you have a small lug at the end of the green ground wire.

The concept here is to keep the length of the green ground wire as short as possible within reason, instead of in and around the perimeter of the chassis any distance away from the entry point. A point near the entry of the AC cord and around the area of the power transformer and the PS electrolytic filter caps is the best location, or that AC cord entry side of the chassis is best.

No 0.01 ceramic cap or anything else required, too?
No, definitely not for this app. That cap you sometimes see is for a different use/configuration that is distantly related to a type of ground reference of sorts.

I'd really like to know since I am using a 3-conductor hospital grade male AC plug with the green wire capped where it enters the chassis. It would be great if Stereo type or someone else could answer this. Thanks!!
Glad you asked.
 
2nd Amp Update

Thanks StereoType for the info. Really appreciate the time you put into it!


Moving along smartly (for me) on re-assembling amp #2

Chassis and transformers (& base) polished with Flitz;

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Pre-assembly:

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Clever / sleazy repair - FP multi-section can cap was replaced - for speed, I guess, the associated resistors were clipped with the top of the cap lugs and tack-soldered onto the new can..Certainly makes removal a breeze!

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Wanted to remove the chrome "McIntosh-40" emblems from the base to polish / clean underneath. I deal with these push-nuts a lot at work, they are a royal pain. The most useful tool in my shop came to the rescue; a thin pointy dental pick. It not only un-tensioned the grips on the spring clips with ease, it is an excellent de-soldering tool, as melted solder simply sticks to it and solidifies - looks like mild steel, may be some stainless steel?

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Crud under the emblem, now vastly cleaned up.

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Gonna have to find the manual for my camera- if I WANTED the fuzzy/ soft look my pictures have, I bet I could not do it, but it seems that is the default mode for the damn thing.
 

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2 (almost) down, 1 to go...

Finished and tested amp #2 tonight.

It passed smell / smoke tests without tubes, then run with the old set of tubes to test. Playing "Take Five" right now. Sounds good, but square wave tests not the same as before or as good as the first completed amp. Voltages are within about 7% or so of nominal. Hope the new set of tubes puts that right. It is definitely distorted. :scratch2:

I'll post initial scope pix in the AM. This POS shop computer & my camera really don't get along.
 
The newly re-assembled #2500 shown at bottom, #2499 at top:


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Square wave at 10KHZ - input at top / 8 ohm tap into 100w non-inductive resistor at bottom:

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Square wave at 20KHZ - input at top / 8 ohm tap into 100w non-inductive resistor at bottom:

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Square wave at 200HZ - input at top / 8 ohm tap into 100w non-inductive resistor at bottom:

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More detailed / controlled tests to follow. These were done rather late at night and I was lacking the brain power to do more specific work - input voltage about 1/2v, input pot about 1/2-way....

From these traces, showing overshoot & slight ringing (I think those are the right descriptors..) what does this suggest is at issue here? I am guessing some missed connection or crossed connection where it should not be. I am thinking that signal tracing from input towards output will help pinpoint the location of the problem.

Also, these were taken with the "as-found" tubes installed. I will also swap in an all new / set, hoping that might help.....

Any suggestions on how to go about troubleshooting this would be greatly appreciated!
 

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China -- (got your PM) -- Square wave testing should always be done with any level controls on the amplifier fully advanced, while the test itself should be conducted at low levels, of no more than a watt or two.

Keeping the level controls at max will eliminate any response reduction they might contribute by way of the Miller Effect on the input stage, and performing the test at low levels will ensure that potential clipping is not inadvertently cutting off any peaks in the display, which can actually make a square wave look better than it really should if clipping is occurring.

You might rerun your tests with these thoughts in mind to see if they change any. As is, if the level control and output levels used are not otherwise contributing to the shots significantly, they potentially show that the high frequency stability networks are over compensating the CL response of the design, as indicated by the kink noted on the sides of the 20 kHz shot.

I hope this helps!

Dave
 
@ DCgillespe - Thanks for the tips - I will re-test tonight. Also found that I *may* have installed two incorrect value coupling caps - between V2 & V3 - at least the value I noted on the new schematic was off...

@ AudioDon - You will note that I did not say winter of which year, or listed the geographic location of said winter.....

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Recnt Progress

So, I fixed the issues with the second Amp. #02500, as seen below and have started dis-assembly on the last one, #01397.

Testing after touching up suspect soldier joints:

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First attempt at re-stuffing caps - technically OK, looks like crap.

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