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 Post subject: Re: Cary Audio CAD211FE and AES Six Pac Amplifier Repairs
PostPosted: Jan Fri 04, 2019 4:12 am 
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I spoke with Andrew of Vintage Tube Services today. My topic was the 12BZ7, the input gain/phase inverter tube used in the AES Six Pac amplifier.

He said that he performed tests not long ago on 30-40 12BZ7 tubes. He said they were all horrible. Noise of every kind.

The user's manual for the Six Pac amplifier states that a 12AX7 can be substituted.

Andy recommended and is shipping me two NOS pairs of his tested RCA 5751's :)


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 Post subject: Re: Cary Audio CAD211FE and AES Six Pac Amplifier Repairs
PostPosted: Jan Fri 04, 2019 5:11 am 
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Keep up the good work Smoky, you are relentless!

It's good to see the step by step progress.

Tim


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 Post subject: Re: Cary Audio CAD211FE and AES Six Pac Amplifier Repairs
PostPosted: Jan Fri 04, 2019 7:24 pm 
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Thanks Tim!

Fixing these tube amplifiers is so much fun.

I'm thinking of turning my detached garage into a little tech space :)

Unfortunately, it's crammed with other projects that I need to get back to working on!


Last edited by Smoky on Jan Sun 06, 2019 12:04 am, edited 2 times in total.

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 Post subject: Re: Cary Audio CAD211FE and AES Six Pac Amplifier Repairs
PostPosted: Jan Sat 05, 2019 2:17 am 
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The new 1uf capacitor test showed no improvement, in fact, the 1uf capacitor actually started to affect the top of the waveform. It began to put a dip in the center of it.

The best results were obtained at .91uf and with the bias set at 160ma. This was while using two Kemet .47uf axial capacitors in parallel. The .91uf was the actual reading on the Sencore Z meter for the pair.

Here are the numbers and the view from the Sencore SC61. I love that thing :)

.91uf
Bias set to 160ma
B+ 433v
12BZ7 heater voltage 6.86v
No more choke Buzz!


Attachments:
FinalResult.jpg
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 Post subject: Re: Cary Audio CAD211FE and AES Six Pac Amplifier Repairs
PostPosted: Feb Sun 03, 2019 11:48 pm 
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When I first started to explore how I could visually see the ringing associated to this amplifier, someone chimed-in and said "well, attach your probe to the amp."

At the time, my TDS420 was all that I had along with an A622 current probe. Since then, I've acquired three P6138A probes, be even so, they are rated to just 300 volts, much lower than the 600+ volts running around in the Six Pac amps.

Recently, I downloaded the TDS 420 and 460 service manual available online. Paging through it, I came across a section that listed probes that can be used in these oscilloscopes. One of them is part number P6009, a 100x probe.

I searched Ebay and found one for $25 plus shipping. Again, it looks brand new and can handle up to 1500 Volts DC.
Attachment:
P6009Probe.jpg
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The TDS420, since I went through recapping and repairing endless traces, continues to blow my socks off every time I use it!


Attachments:
TDS420Ringing.jpg
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Last edited by Smoky on Feb Mon 04, 2019 4:55 am, edited 2 times in total.
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 Post subject: Re: Cary Audio CAD211FE and AES Six Pac Amplifier Repairs
PostPosted: Feb Mon 04, 2019 12:11 am 
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Now let's compare the TDS420 to a 50 year old Tektronix 547 hooked up to the same amplifier :)


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Tektronix547.jpg
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 Post subject: Re: Cary Audio CAD211FE and AES Six Pac Amplifier Repairs
PostPosted: Feb Mon 04, 2019 12:35 am 
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The bridge rectifiers in the Six Pac amplifiers were cobbled-up using 1N5408 diodes soldered to terminal tie strips.

The difficulty that I had was adding or removing wires since all of the parts shared the same eylet. The same eylets that the Kemet capacitors need to attach to too!

Since the Vishay diodes were so inexpensive and the Sprague terminal tie strips stood so much taller than the factory tie strips, I rebuilt them.

Now, the power supply wires from the transformer and the output wires going to the choke and Kemet capacitors are alone on each eyelet!

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Bridge1.jpg
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Now the new vs the old:

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And installed :) ...the Kemet capacitors will be attached to the remaining output eylet.

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InstalledBridge.jpg
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 Post subject: Re: Cary Audio CAD211FE and AES Six Pac Amplifier Repairs
PostPosted: Feb Mon 04, 2019 10:55 pm 
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Location: Montvale NJ, 07645
Coming along nicely.


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 Post subject: Re: Cary Audio CAD211FE and AES Six Pac Amplifier Repairs
PostPosted: Feb Tue 05, 2019 2:37 am 
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Thank you Scott!

Now what's next on the list are the main filter capacitors.

Even though I have the noise floor down to ~800 microvolts, there is plenty of raggedness on the waveform.

When Paul Carlson started the restoration on his Six Pac amps, changing out the main filter capacitors was the first thing he did. It knocked the noise off drastically. I waited until almost the end :)

The original capacitors are of the screw type measuring 1-3/8" in diameter and standing 3-1/4" tall.

Attachment:
FilterCaps.jpg
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Determining the capacitance was the challenge for me. There is no stamping of any part numbers on the caps.

Things I do know:

Two filter capacitors run in series in each amplifier.
Paul Carlson measured 500uf on one of his and I measured as high as 520uf on mine.
Each must be rated for at least 400volts.
The amplifiers are 16 years old.

After reading about 30 spec sheets from Cornell Dubilier, Nichicon, Panasonic,and Kemet, I firgured that this is a 560uf capacitor with a tolerance of +/- 20%, and which is also based on my 6 month electronic's career that every capacitor I have bought so far is -10% of its value printed on the label :)

So! After reading numbers for days until my eyes burned, I came down to one that seems to be head and shoulders above the rest, the Kemet ALC10A561DL500.

It has the highest ripple current capacity, voltage capacity, and longevity than all of the others that I researched and it's all crammed into the same size can that I have. The only exception is that it's a snap-in versus screw attachment capacitor. I prefer a solder joint anyway.

The only company who has this capacitor in stock is Digi-key and they refused my request to match four for me.

So! I will buy 9 and return 5 :)


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 Post subject: Re: Cary Audio CAD211FE and AES Six Pac Amplifier Repairs
PostPosted: Feb Wed 13, 2019 4:28 am 
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The Fedex truck made a stop at my house today!

The new Kemet capacitors arrived :)
Attachment:
Kemet560.jpg
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By 9pm, the Sencore Z Meter was fired up and ready to go.

Since each amplifier uses two capacitors in series, I bought eight in hopes to have two matched pairs with similar leakage and value, and let me tell you, Kemet didn't let me down!

First off, I needed capacitors that read a value between 500-520uf, the choice to buy 560uf capacitors was right! They all averaged around 515uf :)

Here is how I tested the caps. First I did an initial value test. Next, I did the leakage test with the Sencore set to an applied voltage of 500 volts right out of the gate. My Fluke 8024B read 525 at the terminals. Immediately following the leakage test, another value test was done to detect dielectric absorption.

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KemetLeakage.jpg
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Here are the numbers! The top number in black is the initial value. The second number in red is the leakage in micro-amps. The bottom number in blue is the value taken immediately after. I see three near perfect pairs!

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KemetNumbers.jpg
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These are some quality capacitors. They aren't cheap (~$18 each). Funny thing is, the specs on Digi-key didn't match Kemet's spec sheet. These caps have a useful life of 15,000 hours and a shelf-life of 30,000 :shock:

And with a little help from 3M's plastic compound and plastic polish, the caps are gleaming without the print!

The last few pictures shows the prepped caps, with larger 18awg silver/teflon wire, being installed.

Anyway, feeling good on a rainy night in Raleigh :)


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Cap3-1.jpg
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 Post subject: Re: Cary Audio CAD211FE and AES Six Pac Amplifier Repairs
PostPosted: Feb Sat 16, 2019 6:43 am 
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The purpose of replacing the original filter capacitors is an attempt to reduce the noise on the waveform as much as possible.


Last edited by Smoky on Mar Fri 08, 2019 9:15 am, edited 5 times in total.

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 Post subject: Re: Cary Audio CAD211FE and AES Six Pac Amplifier Repairs
PostPosted: Feb Sat 16, 2019 11:18 pm 
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**Update** :)


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KemetEffect.jpg
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 Post subject: Re: Cary Audio CAD211FE and AES Six Pac Amplifier Repairs
PostPosted: Feb Sat 16, 2019 11:20 pm 
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I ramped up the amp on the Tenma variac and let it run-in for about 45 minutes. After that, I plugged it directly into the wall outlet

The level of noise while idling has dropped in half! It's averaging 420 micro-volts :)

The fuzziness is still present on the waveform though. I then swapped-out the old 12BZ7 phase inverter and the EL84 current synch tubes with brand new TAD 12AX7A-C and EL84-STR tubes. The noise floor dropped into the 380 micro-volt level but the fuzzieness remained. Six new EL34 output tubes are on the way and we'll see if the old Electro Harmonix EL34-EH's are the culprits.

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TAD.jpg
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The Sencore tells another story:

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Top1.jpg
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This is the top of the waveform and reading where the .91uf capacitors are attached. The peak-to-peak voltage has dropped 15 volts.

What I think this is telling me is that the .91uf capacitors to stop the "ringing" with the old power supply filter capacitors was fine. But now with the new filter capacitors, .91uf of capacitance is too much with the new Kemets installed. Notice the dip in the top of the waveform (and what is probably accounting for the loss in peak-to-peak voltage).

Guess what :)

...I'm going to try the Epcos/TDK .68uf capacitor again to tame the "ringing."

Attachment:
EpcosTDK-1.jpg
EpcosTDK-1.jpg [ 229.25 KiB | Viewed 227 times ]


...off to Six Pac #2 :)
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Last edited by Smoky on Mar Fri 01, 2019 3:12 am, edited 5 times in total.

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 Post subject: Re: Cary Audio CAD211FE and AES Six Pac Amplifier Repairs
PostPosted: Feb Thu 28, 2019 4:50 am 
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I have reached a point where the Six Pac amplifiers are near completion and ready for independent voltage tests.

Both amplifiers have the same installed components and are using the same set of reference tubes in the same socket positions.

The capacitors to reduce the "ringing" were not installed for this test.

Here are the numbers :)

Attachment:
VoltageCheck.jpg
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The voltages were very close between the two amps.

**Update**

I took additional voltage measurements while the amplifiers were idling. These voltages were taken at the EL34 tube socket. Pin #3 (Plate) and pin #4 (Screen grid). This is where I installed the 2K ohm 3 watt wirewound resistors between the two pins and on every other EL34 socket. I recall that this size resistor was recommended when using the Electro Harmonix EL34-EH tube. The screen grids measured 6-7 volts lower than the plates.

Carry on... :)

The only tube in the chassis where the heaters are run on DC is the 12AX7 phase inverter tube. The voltage to the socket seemed a little bit too high (~7.65).

I installed a Dale 1 Ohm resistor between the diode and the capacitor that feeds it and that brought the voltage down nicely!

Attachment:
Adjustment1.jpg
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Attachment:
Heater1.jpg
Heater1.jpg [ 248.14 KiB | Viewed 152 times ]


Last edited by Smoky on Mar Mon 04, 2019 9:11 pm, edited 9 times in total.

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 Post subject: Re: Cary Audio CAD211FE and AES Six Pac Amplifier Repairs
PostPosted: Feb Thu 28, 2019 5:34 am 
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On a side note, while doing the noise tests I slipped the bottom covers under the amplifiers thinking that the sorbothane feet would be the most quiet. Not!

Placing the amps on two studs on top of two rolled sections of bubble wrap was :)

The oscilloscope proved it! ...l recall the improvement was well over 100 microvolts quieter.


Attachments:
SoftSpot.jpg
SoftSpot.jpg [ 239.96 KiB | Viewed 152 times ]


Last edited by Smoky on Feb Thu 28, 2019 9:32 am, edited 1 time in total.
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 Post subject: Re: Cary Audio CAD211FE and AES Six Pac Amplifier Repairs
PostPosted: Feb Thu 28, 2019 5:48 am 
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The ringing in the Six Pac amplifiers is gone. The Epcos/TDK 0.68uf capacitors easily take care of that issue and they mount tight to the chassis with the heavy-duty attachment tape. Physically, these Epcos capacitors are very durable along with being very accurate.

Here's what's left to be done on the Six Pacs and where some may cringe :)

I placed an order for four 0.22uf Mundorf MCap Supreme Classic capacitors to replace the Jensen coupling capacitors.

The Jensens test good on the Sencore Z meter but I think they are getting old (~14years) and one of the amps looked to have seen some rough days along the way.

As it stands right now, one Six Pac idles with 580 microvolts of noise floor and the other at 820 microvolts.

We'll see if the new Mundorf caps level-out the noise that remains.

Sonic Craft is matching the four Mundorf caps to within 1% for an extra $1 each. They'll be here Friday!

Stay tuned :)


Last edited by Smoky on Mar Fri 01, 2019 2:25 am, edited 1 time in total.

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 Post subject: Re: Cary Audio CAD211FE and AES Six Pac Amplifier Repairs
PostPosted: Mar Fri 01, 2019 1:42 am 
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Here is a picture of the Jensen capacitors that will be replaced.

I originally thought that these were maybe made in 2011 by looking at the numbers at the bottom of the caps.

But then I'm told that the year is actually the first two numbers and the month is the last two numbers.

That would mean that these caps were indeed made in 2004.

This makes sense since all of the Nichicon capacitors that I replaced are dated around 2003.

Who knows?

Anyway, I'm hoping that the new Mundorfs remove the slight noise and the roughness riding on the waveform :)

Attachment:
JensenCaps.jpg
JensenCaps.jpg [ 247.9 KiB | Viewed 122 times ]


**Update** Just moments ago, Bjoern Jensen of Jensen Capacitors, replied to me to say that these capacitors were made in 2011 :)

Nowww... If the Mundorf caps remove the noise that I see clinging to the waveform, they will stay. If not, I'll keep the Jensens out of the round-file!


Last edited by Smoky on Mar Fri 01, 2019 9:30 am, edited 4 times in total.

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 Post subject: Re: Cary Audio CAD211FE and AES Six Pac Amplifier Repairs
PostPosted: Mar Fri 01, 2019 2:52 am 
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Here's a glimpse of what's coming up on the bench this Spring :)

On Tuesday, and after my coffee and Better Bacon Cheddar bagel at Bruegger's, I swung over to Cary Audio and picked up a new faceplate and knobs for my SLP98P preamplifier!

The old knobs are faded, the faceplate is worn and scratched, and the cranberry paint is chipped around the edges.

I'll be spraying some bc/cc, upgrading the capacitors, and bringing this baby back with a different look!

The cursive silkscreened letters and tube numbers are gone too.


Attachments:
SLP98P.jpg
SLP98P.jpg [ 239.36 KiB | Viewed 116 times ]
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 Post subject: Re: Cary Audio CAD211FE and AES Six Pac Amplifier Repairs
PostPosted: Mar Fri 01, 2019 9:54 pm 
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Ok, I recall being decent in math in high school but am I seeing things???

I ordered capacitors that would be within 1% of each other.

These Mundorfs appear to be less than 1/10 of 1% apart!!!!

Thank you Sonic Craft :)

Attachment:
MundorfSupremeCapacitors.jpg
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Attachment:
MCaps.jpg
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I used Kimber TCSS lead-in wires to attach the new capacitors. This way, I'll never need to desolder any wires on the tube sockets for future capacitor changes.

Attachment:
MundorfInstall.jpg
MundorfInstall.jpg [ 241.11 KiB | Viewed 94 times ]


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 Post subject: Re: Cary Audio CAD211FE and AES Six Pac Amplifier Repairs
PostPosted: Mar Sat 02, 2019 3:53 am 
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***LOOK AT WHAT THE MUNDORF CAPACITORS DID*** :)

Amplifier "A"

Attachment:
AmpA.jpg
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Amplifier "B"

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AmpB.jpg
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There is a lot to notice here.

#1- The super-matched Mundorf capacitors leveled the output for both amplifiers.

#2- This tells me that the Jensen capacitors were slightly imbalanced with one amplifier at 580 microvolts and the other at 820 microvolts output before the swap.

#3- Look how amplifier "B" has a flatter waveform than "A". There is "something" in amplifier "B" that is better.

...We'll need a scientist to determine the difference. I'm extremely happy :)

This Winter has been an absolute blast!

...and thanks again Sonic Craft!


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