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Old 9th October 2008, 09:32 PM   #11
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Ok Chris,

Thanks for the tips. Since I can't really do anything about the solution in the ultrasonic cleaner (they are in the medical industry and I'm not going to ask him to change their standard cleaner), I will tootbrush/simple green/rinse/repeat before giving them to him for the final multiple ionized water baths.

The OpAmps are corroded. I was planning on using: http://www.newark.com/59K8973/semico...evices-op97epz which is an Analog Devices OP97EPZ. I think that was mentioned (if not by manufacturer) in another post.

The zeners would be:
http://www.newark.com/26K3669/semico...equestid=63307
I think they are small signal diodes that are recommended.

Do these look ok to you? I apologize to any vendors that may use this forum. I only choose Newark, as they are who I know I can e-order from.

Last question; physically where are the ground look resistors we have talked about?

Once again, I thank you for sharing your knowledge and skills in this matter.
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Old 10th October 2008, 12:23 AM   #12
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so this was a specific cap formulation we're talking about here..... i wouldn't be in the audio business if there was nothing left to learn....

OT, but only for a second... anatech, it was 7.62x54R russian surplus ammo (1943 vintage) and some 8mm 1944 headstamped german ammo (both of them WWII surplus) which i bought for a couple of WWII surplus rifles i have.

ok, back on topic. it seems there were a lot of mistakes made over the years both with cap formulations and cap construction, even by the most reputable manufacturers. Sprague made a line of caps, that to this day i see the same failure on over and over. they're the Atom caps with the red epoxy seal, and both in the radial and axial types. the connection between the positive lead and the positive plate burns open right below the epoxy, leaving a black dot next to or a black circle around the positive lead. these caps usually also have a clear heat shrink sleeve around them. it also doesn't seem to matter whether it's wire leads or screw terminals, they still fail. i once saw a very expensive lab standard digital volt meter that had 20 of these in parallel in the power supply, and 18 of them had failed. i've seen other equipment with them in them (including audio amps). and i replace them as soon as i see them now. so i find it not at all unusual to hear that there were caps out there with a bad formulation, maybe even in production for years.
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Old 10th October 2008, 04:43 AM   #13
anatech is offline anatech  Canada
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Hi Numbdiver,
Those diodes are incorrect. I don't recall the exact op amp replacement, but somewhere there are a few references to them. I think Walt Jung commented on this.

The diodes you need will look like a transistor with two legs. They are a low noise 7 V breakdown device. You will probably see those numbers near where the op amp is mentioned.

Hi unclejed613,
Quote:
it was 7.62x54R russian surplus ammo (1943 vintage) and some 8mm 1944 headstamped german ammo (both of them WWII surplus) which i bought for a couple of WWII surplus rifles i have.
Are you going to try and fire them? Erk! You may be further ahead using the brass and reloading them with modern components. That is time consuming to work up to the right load. Be careful with that old stuff. I know what having a hang fire feels like. With rounds that size you stand a chance of flying brass, unlike a .22 round (not to say they aren't dangerous).

There are stories on this. The most popular is a factory that got a partial formula. They went on to sell it to many capacitor plants. Nasty stuff.

-Chris
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Old 10th October 2008, 06:02 AM   #14
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Single-channel opamps, right?

Analog Devices OP97, or AD820. Either does a nice job, but the OP97 seems to be tough to find.
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Old 11th October 2008, 01:01 AM   #15
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Quote:
Originally posted by anatech
Hi Numbdiver,
Those diodes are incorrect. I don't recall the exact op amp replacement, but somewhere there are a few references to them. I think Walt Jung commented on this.

The diodes you need will look like a transistor with two legs. They are a low noise 7 V breakdown device. You will probably see those numbers near where the op amp is mentioned.

Hi unclejed613,

Are you going to try and fire them? Erk! You may be further ahead using the brass and reloading them with modern components. That is time consuming to work up to the right load. Be careful with that old stuff. I know what having a hang fire feels like. With rounds that size you stand a chance of flying brass, unlike a .22 round (not to say they aren't dangerous).

There are stories on this. The most popular is a factory that got a partial formula. They went on to sell it to many capacitor plants. Nasty stuff.

-Chris
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Old 11th October 2008, 01:35 AM   #16
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oops, took too long to edit.......

if you want we can branch the vintage rifles to the "Everything Else" forum, as that's one of my hobbies. i like studying the innovations, technical quirks, and history in military small arms....

the zeners should be replaced with the proper type. most zeners are very noisy, which is why i'm surprised any manufacturers use them in audio amps these days. i've even seen a new 2007 model (made by American Audio) using zeners for the op amp and diff amp supplies in their amps. why? a couple of cents difference between 78xx and 79xx chips and a couple of 5 watt zeners and 2 huge 20 watt resistors? if it's oddball voltages, there's still no reason. there are 78xx and 79xx chips in single volt increments up to 15V, and even if there weren't, there is an app note from NSC that says you can put forward biased diodes between the ground pin and ground to fudge the voltage. LM317 type regulators are almost as simple to use, but have even more flexibility for voltage settings. the American Audio amp mentioned above also had a quirky way of muting the amp modules if the protection circuit tripped, a set of relay contacts would short the zener controlled rails to ground, shutting off the op amps and the diff amp.

as to cap formulations, i've seen equipment made from about 1920 to the present, and some cap manufacturers have tried some often bizarre formulations, and leave me asking "what were you guys thinking?????". one particularly nasty formulation broke down into raw ammonia (this was in a 1950's vintage TV set that i tried working on when i was about 12) after many years of sitting dormant in my grandfather's basement. boy did that clear out my grandfather's living room......
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Old 11th October 2008, 03:41 AM   #17
anatech is offline anatech  Canada
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Hi Glenn,
Without looking, I can't be sure of that. My gut tells me they are singles in that Adcom.

Hi unclejed613,
Some zeners are not that noisy, especially the ones used in this Adcom. Another device that can be used are the adjustable shunt regulators.

The 78xx and 79xx have poor noise rejection at the higher frequencies. I haven't used those in audio circuits for at least 10 years and probably longer. The LM317 and LM337 chips are supposed to be better for noise. They can easily be set for any odd voltage. A nifty shut down can be made by shorting the lower voltage set resistor to ground. This drops your output voltages to about 1.2 VDC. Pretty neat eh? Revox was fond of doing this.

I tend to make my own regulators using a CCS and zener, or shunt regulator. You can improve the high frequency performance that way. You can also use an RC filter on the output of your regulator for low level stages as the change in current draw isn't much.

Back to this amplifier. Use the original high quality, low noise shunt regulators, or a direct sub. If you can clean up the old parts, go ahead and use them.

-Chris
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Old 12th October 2008, 06:03 AM   #18
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i just recently read an article that shed an interesting light on why 3 terminal regulators behave badly at high frequencies and what to do about it. the article was by Bob Pease (the senior analog engineer at NSC) and he described the operation of 3 terminal regulators as "active inductors" meaning that if you just indescriminately place caps at the inputs and outputs, you get a resonant circuit (with gain, maybe even enough to oscillate). i plan on reading the article in depth this week, but the idea that regulators act like an inductor actually would explain why one person might have them work fine most of the time, and another person might have them misbehave most of the time. it's all in the application and external components that one would prefer. i've seen many instances where somebody sees 10uf caps used at the input and output in a circuit, and it works well for whatever application (i've done this myself too...) and says "oh, 10uf caps work well here, so i'll use 10uf caps too...", and get in the habit of almost always using 10uf caps with 3 terminal regulators. eventually, doing this will come back to bite you, since chances are pretty good that eventually you may happen on an application where this is simply THE wrong application of this particular combination of components. Bob Pease designed 3 terminal regulators, so i think he's not only had a lot of experience with them, but has had to field a lot of questions about them over the years. there are also different processes for making 3 terminal regulators, so an NSC LM7812 might work well in a circuit, but a KIA or a hitachi (does hitachi even make them anymore?) might oscillate. there are also direct replacements for the original 78xx and 79xx devices with improvements (such as also being a Low Dropout Regulator or having the reverse bias bypass diode built in), and so might behave a lot better than the originals.

the article also shows ways of testing their high frequency behavior that could probably be a big help in designing circuits that don't misbehave at high frequencies.

another approach would to use discrete transistor regulators, using fast enough devices to maintain good regulating and filtering action well above the frequency limits of most 3 terminal regulators. i don't really like using resistor-zener regulators because they tend to be high rate-of-failure circuits, and actually have a relatively high output impedance. noise is easily bypassed with a capacitor, so unless the zener is getting ready to fail, or the cap dries out, it's usually not very noisy. there's also a simple (1 transistor, 1 or 2 resistors, and one cap) circuit that acts as a capacitance multiplier on power supply rails, and so can filter a power supply rail extremely well without huge caps.
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Old 12th October 2008, 06:04 PM   #19
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The original Adcom 2A op-amp part is made by Linear Technologies LT1012, however Walt Jung recommended an Analog Devices OP97 IC, which has worked flawlessly in the 3 565 rebuilds I've done.

You'll need Analog Devices part # OP97FPZ in the 8-pin DIP package.

And please don't get me started on firearms and ammunition, I could drone on for days.
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Old 13th October 2008, 05:00 PM   #20
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Well, I've decided to give it a try. They certainly could of made it easier for me to get the board out though! I labeled everything, and decided for safety's sake, I'm doing one at a time, so I have a reference, just in case. Caps were absolutely leaking. Not a lot of corrosion elsewhere, which is a good sign.

I emailed Adcom about a manual, but haven't gotten a response. I'll try calling, but does anyone have a scan if I can't get one?

MjrAudio: Thanks for verifying the correct AD op amp number!

You guys went off on a diode tangent that while interesting reading, kind of got away from anatech's statement that I was looking at the wrong type. He mentioned a transistor-looking style with 2 legs. I do see some Adcom J2/8934M 3-legged thingys that have a diode symbol printed on the board. Are those them?

Lastly I still need the physical location of the these resistors so I can test them:
"The 10 (or 100) ohm resistors break a possible ground loop. They can be burned out. Just test them, replace if you want."

Sorry for all the questions. I want to do this right, and until I can find a service manual, you guys are my only source of info.

Thanks once again!
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