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

On your questions:

1) Wide-band audio (for an RF engineer, this is an oxymoron) signals can be read with most any of the better digital volt meters (DVM) these days. For instance, my Fluke 117 specs a 5Hz-50KHz bandwidth for AC volt measurements. Most all but the cheapest of the DVMs I've seen sport input impedance specs that are like what a VTVM used to carry. So, just getting a decent DVM is about all you're going to need unless you have very specific testing requirements. BTW, the Fluke 117 also has a frequency counter, too. Really reduces the number of instruments you need on the bench.

2) PC-based measuring devices usually have some sort of 'front end' box that does the A/D work needed so the software package can do its job. Since you can get a really functional DVM for less, that's the way I'd recommend mainly because you can take the DVM where you'll not likely get a laptop and the dongle/outboard box setup. One thing I do usually have, however, is a small USB or Firewire device so I can use the PC as an audio signal generator. Room EQ Wizard has a host of nice utilities for sweep frequency and other things that get interesting for a work bench.

3) How fried is the board? If it was due to the larger resistors being mounted close to the board, just mount new ones standing away from it and you're set.

Cheers,

David

Hi David - Thanks again for your responses -

1) Looked up the specs of the 'best' DVM I have, and the AC response is 40-400Hz. Looks like I will be looking for a better one soon. The Fluke 117 you mentioned looks good - what should I be looking for in terms of input impedance, or is it ohms/volt? As I recall , VTVM's have a very high impedance. On my cheap DVM's this is not even listed. Is there a recommended minimum figure for accurate readings with tube circuits?

2) I think you are right - I feel I am spending too much time learning and obsessing about test instruments (right now), distracting me from the task at hand of re-building the amps. I will get a better DVM and look into PC based test-ware later.

3) Once stripped of all components, the board does not look half bad, just a lot of soot. Seems perfectly re-usable, even after ham-fisted de-soldering. Man they really wrapped the leads around the terminals in a death-grip! I found a purpose-built solder-iron with integral solder-sucker - $7.95 from MP Jones, I think - seems to work pretty well.

Thanks Again!
 
Wgrr -

I feel your pain, I have rental properties and no free time. I am now looking at an old office building for my wife's law firm. It was built in the 60's with almost no updates. Hope no water pipes froze in your building. To be honest I don't know how old the steam piping is but I suspect a re-plumb may be necessary.

The MC40's are coming along nicely though.:yes:

Wgrr -

Thanks - a re-plumb WOULD be a good idea, but a logistic and financial nightmare. The pipes above the condensate line seem healthy enough. The jacking and shoring went well enough, now I have some monster cracks in plaster and doors that worked fine in tilted frames, now cannot be closed......

Next weekend I have all to myself, will get MC-40 #1 back together - more progress pix soon!
 
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The Horror......

More deconstructive progress.


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1) I was thinking about re-painting the two transformers, but A) Its a pain, there is that nice logo plate on the top and the paint was not THAT bad, and B) I tried a little wet metal polish on one side & it looks great, and I think I'll go with the "cleaned up, but with the real patina of age" look rather than totally pimp it out. C) It will save time on the other two. Plus its too cold here to paint outside. Plus lets face it, these deserve better than a spray can job.

2) I included the last photo to show that the serial number is stamped not only on one lip on the bottom of the chrome chassis, but also twice on the top, one under each transformer. I wonder why?

3) Not sure what to do about the rust, will try some gentle polish / wax I guess unless anyone has any suggestions. Would like to minimize it and stabilize it if possible. That ugly strip of tape on the top mostly came off with a overnight soak in Goo-Gone, will have to try more or some more nasty solvents.

I have the whole weekend free, will strive to get this one wired up and running.
 

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A little more progress

Sockets wired for filaments & several misc. wires that might as well be done now:
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Re-swadgeing the eyelets that had loosened up:
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Soak in Goo-Gone to get rid of masking tape residue & soot...
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Cleaned & ready for solder:
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Delicate little arms - must make de-soldering less traumatic!
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...3) Not sure what to do about the rust, will try some gentle polish / wax I guess unless anyone has any suggestions. Would like to minimize it and stabilize it if possible. That ugly strip of tape on the top mostly came off with a overnight soak in Goo-Gone, will have to try more or some more nasty solvents...

For cosmetics, I use naphta spirits which can be bought in bulk as golf club grip solvent or lighter fluid. Naphta is a gentle solvent which in my experience works on metal, wood, and clear plastic dial covers. Naphta will dissolve rust, tape residue of all sorts, candle wax soaked into wood veneer, and sometimes Magic Marker. It is not caustic and will eventually evaporate within an hour or two leaving behind almost no residue. And it is fast working.
 
Components placed and wrapped for fit:
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"Almost Done" -or so I thought -Also noticed that the two electrolytics are 400v when they should be 450.. Will change before applying any juice. I have had a few issues with miss-labeled parts in recent orders, as well as a bunch of 68 ohm resistors that were marked perfectly, but ALL measured at 45ohms..... guess thats why they were surplus!
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I tried to keep the original color-coded wire scheme - note to self - SOLID WIRE, not stranded. And 20ga. rather than 18 or 16.... Sheesh.
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Input pot and bits...
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Power supply assembly - I used two caps to make the 150uf / 250v units for the voltage doubler & increased the voltage rating to 400v. I may re-stuff the salvaged cans...
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Top side of power supply unit:

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Part of OCD re-building - cleaned all the screws and nuts etc in a tumbler with walnut shell media and a bit of case cleaner - Scrubbed the output terminals with a old toothbrush....(I am somewhat in the nuts & bolts biz, so its sort of a must do for me..)
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Starting assembly and fit- nothing soldered yet.
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I am realizing the virtues of pure point-to point rather than this crazy hybrid!
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An opposing point of view:
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I had hoped to get this one running, or at least tested for basic voltages this weekend, it was not to be.

Also, in poking and tracing wires on the amp in back, #02500, I had lifted several caps and connections to test and view better, naturally, when I buttoned it up to play / test it, it has massive distortion, and the amp meter on my variac bounces with the beat a LOT more than normal - I am sure that this suggests something specific to experts - if you know what I may have missed, please don't be shy!
 

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looking really good. I wish I had time for large audio projects. It seems like all I do is work on my house when I have free time. That usually breaks down to posting here when the weather is bad because I am lazy and don't like cold weather.:D

We got a massive 1 inch snow storm yesterday morning that melted off by evening. Funny thing is about this time last year we got well over two feet of snow and temps at -18 *F. This Winter I don't think I have scrapped the windshield on my service truck even one time. We are back in the upper 50's by tomorrow. Fine with me.

I meant to add that AES Tubesandmore has the can caps for these amps. I believe that is what Terry posted. It looks like they are all new on the amps he rebuilt for me. Some folks say the CPE caps made on the original Mallory equipment are not that good. Terry has posted he has had no trouble with them. Another thing. The can cap closest to to front of the amp is a can common positive I believe. The original had a paper insulator cap and the one Terry installed in my old amp is covered with black duct tape. He had to make a minor adjustment on the mounting to get it to fit into the case. I no longer have my MC40's to look at or I would.
 
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For cosmetics, I use naphta spirits which can be bought in bulk as golf club grip solvent or lighter fluid. Naphta is a gentle solvent which in my experience works on metal, wood, and clear plastic dial covers. Naphta will dissolve rust, tape residue of all sorts, candle wax soaked into wood veneer, and sometimes Magic Marker. It is not caustic and will eventually evaporate within an hour or two leaving behind almost no residue. And it is fast working.

Great Suggestion - I will try some - never heard that Naptha would remove rust, and I am in the decorative metal industry - good info!

Thanks
 
looking really good. I wish I had time for large audio projects.

I meant to add that AES Tubesandmore has the can caps for these amps. I believe that is what Terry posted. It looks like they are all new on the amps he rebuilt for me. Some folks say the CPE caps made on the original Mallory equipment are not that good. Terry has posted he has had no trouble with them. Another thing. The can cap closest to to front of the amp is a can common positive I believe. The original had a paper insulator cap and the one Terry installed in my old amp is covered with black duct tape. He had to make a minor adjustment on the mounting to get it to fit into the case. I no longer have my MC40's to look at or I would.

Thanks - By any reasonable measure, I don't have time for this, just a singular opportunity that came up to help a friend and get to work on some gear I might never otherwise have a chance to experience - also to try and do the very highest quality job I can, regardless of time input!

Your recall is correct - the second 150uf cap in the voltage doubler is positive to ground - and that is why it is on the lone phenolic washer - I used an AES can cap for the quad, I did not see that they still have single units in cans - I could have used the 50/50/50/50 µF @ 350 VDC @ $35.00, also I am trying to keep the $$ outlay for my friend reasonable.

Parenthetically, or O/T - I have seen Many pictures of Mac amps with the little studs to hold / locate a tube cover / cage (sim to the one on the Marantz 8B,) but I have never seen the cages themselves - were they an accessory that was never bought? (seems unlikely) - were they lost? - Aliens? .gov conspiracy?
 
Cages would have been added onto a customer's order if the amp was to be used on a shelf or somewhere out in the open. I believe many of the well-heeled who purchased Mac gear did so with them built into a console of some sort. In this application the power amps would have been located underneath or in the back and out of sight and from prying fingers, hence no cage as we find them.

This was the case with the MX-110/MC-225 combo that I received from my step-mother-in-law. In this case, the MC-225 was inside the console along with all the docs and purchase receipt. The receipt had the cage added on then removed as a line item. BTW, total purchase in 1963 for an MX-110, MC-225, Barzilay console and speaker cabinets, JBL D130 woofers, JBL 075 horns and crossovers was $963.00. Have no idea how much that is in today's $$.

For those voltage doubler caps, I'm sure those circuit boards will be fine as long as the cap values and the operating voltages are not under spec. I've used some voltage doubler caps from Jim McShane to good effect. In this case, I also used new cap clamps so I could remount them securely as the old ones were.

On the cap that floats above ground with all that voltage on it, I put a few extra wraps of electricians tape where the clamp secures it to ensure there was plenty of insulation in case there was a burr on the metal clamp that might pop through the plastic wrap that comes on the cap. As it is now, however, you can get the amps up and running to verify their operation then go back and redo if time/$$ permit. It just makes for a nicely finished job, that's all.

Still, quite a project that we get to share as you go along. Keep up the great work!

Hope that helps.

Cheers,

David
 
Old unit fixed and some glacially-paced progress

Also, in poking and tracing wires on the amp in back, #02500, I had lifted several caps and connections to test and view better, naturally, when I buttoned it up to play / test it, it has massive distortion, and the amp meter on my Variac bounces with the beat a LOT more than normal - I am sure that this suggests something specific to experts - if you know what I may have missed, please don't be shy!


Well - had some time last night to attack this & somehow fixed whatever was the issue with the other amp, I re-soldered the junctions I had lifted and re-checked everything, also re-seated the tubes. Now it works fine.

On the Soon-to be re-built amp, I tested the power supply assembly, by itself, so I expect these voltages to change with connection to the rest of the circuit and load - looks OK, but not enough voltage drop between the first & second section of the can cap:

C16A (Should be) 470v - is 454v
C16V (Should be) 380v - is 430v
C16C (Should be) 170V - is 213V
B- at C17 (Should be) -150v - is -161v

Filament is right at 6.0vac

As-found voltages, before any new parts & with existing tubes were:

C16A (Should be) 470 - was 429v
C16V (Should be) 380v - was 421v
C16C (Should be) 170V - was 175V
B- at C17 (Should be) -150v - was -161v

Should I increase the value of R26?

Started to install the rainbow-festooned tagboard


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I puzzled over the sequence of installation, I ended up keeping the flying wires on the tag-board and trimming to fit once in place. What a pain. Not made easier by the too-heavy gauge stranded wire I am using...
 

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Done - Almost - its working with no red-plating.

I'm pretty stoked about this - missed a connection tying together R24 & R25 in the V4 / V5 bias supply. Hidden from recent view by the two large blue caps on the left of the tagboard. Proven again, you just can't double-check too much, and systematically eye-balling each solder joint and connection with an eye on the schematic is priceless, at least in this case.

Still need to adjust the B-, and replace a few caps whose voltage is a little marginal for my liking.

Darn thing runs at .75A for hours and BARELY gets warm.

Stoked additionally as this is by far the most complex amp I have yet completely rebuilt, the most challenging in terms of layout and complexity of re-assembly (put the damn choke screws in FIRST before bolting on the transformer....) and also the pressure of it being a classic / $$$ / etc that belongs to a friend. Fundamentally glad that I did not manage to fry anything irreplaceable.

The amps I have completely re-done before are several Heathkit W4's and a pair of Fisher 30-A's. Several other stop-gap repairs along the way, and a really complex pre-amp that I started to re-do many years ago but have set aside till I gain experience. I also have some (to me) treasures that I am saving until I am Really on top of my game to re-do.

Also I fell that I did about the neatest job I am so far capable of (still need to neaten things up and tie together some of the transformer leads..). Learned a lot that will be of much use going forward. Hope this has not been boring for others.


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Wgrr and David - Thanks for your kind words -

Hopefully finishing up tonight on this one & on to the next two.

I do have 82vac from the chassis to ground (housing on grounded-to-the-house Variac), which I can't imagine is good, though I have not managed to get a shock. Something is still slightly amiss.

IE - more to learn!
 
Finally actually finished the first amp...

As in it now has a power cord, inrush limiter and the bottom plate installed and is not up-side down on the bench finished:

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Some Scope shots at 10Khz and 20Khz...

Not sure of what power these were at, maybe 1vac input, or 1/2 rate output.

Full report shortly.

Input trace @ top, output on 8.2 ohm non-inductive resistor / 100w rated at bottom

10Khz
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20Khz
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Chinacave,
I just use an old fluke 77 multimeter and a scope. I find the 77 averages what the scope shows multiplied by .707, so I assume it measures RMS AC voltage.
My other piece of test gear is a signal generator I downloaded for a Macbook. It does what I need. That's about it.
This stuff is looking very good.
I interpret the output scope trace to say there's very little overshoot and a slight lag, which you'd expect in any lead/lag circuit. Again. Looking good. I'll let someone else more educated on scopes chime in so I can learn too.
 
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