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  #31  
Old 02-15-2012, 11:03 AM
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That high voltage buzz
 
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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
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  #32  
Old 02-24-2012, 09:48 AM
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Old unit fixed and some glacially-paced progress

Quote:
Originally Posted by chinacave View Post

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











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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  #33  
Old 02-28-2012, 10:40 AM
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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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  #34  
Old 02-28-2012, 04:35 PM
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Everything is looking great. I look forward to more updates.
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  #35  
Old 03-01-2012, 01:11 PM
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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!
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  #36  
Old 03-01-2012, 06:24 PM
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Great job!
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  #37  
Old 03-21-2012, 03:11 PM
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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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  #38  
Old 03-21-2012, 03:15 PM
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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


20Khz
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  #39  
Old 03-27-2012, 04:03 PM
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audiodon audiodon is offline
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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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  #40  
Old 03-27-2012, 05:31 PM
Rodeodave Rodeodave is offline
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Check this link for square wave response interpretation:
http://users.tpg.com.au/users/ldbutler/Waveforms.htm

Lovely work! Thanks for the updates!
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  #41  
Old 03-28-2012, 11:02 AM
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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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Last edited by chinacave; 03-28-2012 at 11:32 AM.
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  #42  
Old 03-28-2012, 12:18 PM
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Now that is impressive work! Nice Job.
Regards,
Jim
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  #43  
Old 03-28-2012, 03:15 PM
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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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Last edited by chinacave; 04-02-2012 at 01:23 PM.
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  #44  
Old 03-28-2012, 03:22 PM
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Un-re-Built amp #2500 / as-found

Same conditions as above....















Interesting pictorials on interpreting what the squiggles mean: Square waves and also here; http://users.tpg.com.au/users/ldbutler/Waveforms.htm [Whoops! Just noticed that that URL seems familiar because Rodeodave posted it a few posts back - Thanks Dave]
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Last edited by chinacave; 03-28-2012 at 06:41 PM.
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  #45  
Old 04-02-2012, 08:10 AM
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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
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