I am in need of of a good replacement for an opamp IC labeled Adcom 2A that is used in a GFA-565. It is a single opamp and appears to be a low power piece. Would a TL061 work, or could any one suggest other suitable replacements? There was an old thread about the orginal poster never did reply with what IC worked.
Thanks,
James
Thanks,
James
It has about 2 volts DC offset on the speaker terminals. The IC is part of the servo circuit. The cap next to it leaked electrolyte all over it, after replacing the cap and many other bad caps and cleaning the PCB very well, the IC is bad. It is very temperature sensitive, heating or cooling it a little will make the output voltage go crazy. I am placing an order with Mouser tomorrow and they don't stock the AD820, I was trying to find something equivalent that they stocked, they carry ALD, Fairchild, Intersil, NJR, ON Semi, ST Micro, and TI brand op amps.
Hi James,
The IC is an AD 820 as EchoWars said. Watch the supplies as well!
If it is very temperature sensitive, you may not have cleaned the board and parts well enough. I used an ultrasonic cleaner and a degreaser (aqueous) to clean both the board and parts. I removed everything in the area, anything "saveable" went into the bath.
You really have to scrub these things.
-Chris
The IC is an AD 820 as EchoWars said. Watch the supplies as well!
If it is very temperature sensitive, you may not have cleaned the board and parts well enough. I used an ultrasonic cleaner and a degreaser (aqueous) to clean both the board and parts. I removed everything in the area, anything "saveable" went into the bath.
You really have to scrub these things.
-Chris
I am going to order an OPA27 and try that as the specs look good on that chip. I have been a consumer electronics tech for >20 years and have seen plenty of leaky caps and damaged PCBs. This cap leaked but didn't corrode anything except the IC. The board is now nice and clean and without it in the cirucit I get only about .1V offset.
Thanks,
James
Thanks,
James
Hi James,
I am not doubting your abilities as a tech. Take it from me, the board may still have residue on it. This stuff is very difficult to remove. If you don't believe me, ask the folks at Adcom. I was a warranty shop for them.
-Chris
Edit: It could just as easily be on the chip you have too.
I am not doubting your abilities as a tech. Take it from me, the board may still have residue on it. This stuff is very difficult to remove. If you don't believe me, ask the folks at Adcom. I was a warranty shop for them.
-Chris
Edit: It could just as easily be on the chip you have too.
The perfect chip for the job is the AD820, and I'm pretty sure that anatech is the one who suggested it to me a few years ago. The specs needed for a good audio IC are a lot different than those needed for a good DC servo.CompNut said:
For instance, the typical input bias current for the AD820 is 2pA. For the OPA27 it is 7,500 times that much. Input offset current is 20x higher on the OPA27 than the AD820.
It's just a matter of using the best part for the job.
RE: Your current offset. On the 555 II, the IC supplies the bias current for the input differential stage...the amp won't even operate without it. If your amp is similar (and I admit that I do not know), any measurement you take with the chip removed will be meaningless. Besides, with a good IC in place offset should remain well under 1mV.
OK, I am going to get the AD820, I just hate ordering parts from 3 places to get what I need. I was already placing an order with Mouser and Antique Electronic Supply to get some parts to fix a customers MC275 and also build some knockoff QuietLineII noise filters. I guess I will also place an order with Newark and get the AD820.
I checked with Adcom a few minutes ago and they want $32 for that IC, that is a little steep!!
Thanks,
James
I checked with Adcom a few minutes ago and they want $32 for that IC, that is a little steep!!
Thanks,
James
Hi James,
Get more than one IC then. I hate ordering from multiple suppliers and I usually end up with Newark, Digikey (great shipping!) and Mouser (horrible shipping!). Since I'm in Canada, things are a little tougher for me. Even Canadian suppliers charge me in USD, I lose on the conversion.
While you are at it, grab some other useful IC's and bits. I can't afford to order without looking at what I need and what I might need.
As far as Adcom goes, remember they are dealing with old skews. The originals where from LT. Adcom was bought and moved, stock included. I have seen much worse markups than that in my time. The markup probably isn't that bad when you look at their original pricing and the fact that these had to go into the system. On top of custom marking and a likely reorder.
I'm not trying to make excuses. This is the way the industry works.
-Chris
Get more than one IC then. I hate ordering from multiple suppliers and I usually end up with Newark, Digikey (great shipping!) and Mouser (horrible shipping!). Since I'm in Canada, things are a little tougher for me. Even Canadian suppliers charge me in USD, I lose on the conversion.
While you are at it, grab some other useful IC's and bits. I can't afford to order without looking at what I need and what I might need.
As far as Adcom goes, remember they are dealing with old skews. The originals where from LT. Adcom was bought and moved, stock included. I have seen much worse markups than that in my time. The markup probably isn't that bad when you look at their original pricing and the fact that these had to go into the system. On top of custom marking and a likely reorder.
I'm not trying to make excuses. This is the way the industry works.
-Chris
I always order more than one when I need something. Also I always end up adding a bunch of other stuff to the order. I like to have a good stock of stuff on hand for doing these side jobs so I don't have to haul things into work. I have been doing TV/VCR/Audio and various other electronic repairs for so long I am starting to get tired of it. Oh well, it pays the bills!!
The 820 is not the right chip for the 565. The 820 is best for a monopolar supply like the 555 mkII. Use a good quality OP 97 as per Walt Jungs suggestion.
There is a lot of confusion here between the 555 II and the 565/585 - they are not the same amplifier! The 565/585 have dual differential darlington inputs that are biased with the same +/- 13v supply as the dc servo - it is a very complex circuit to troubleshoot. The 555 II uses a + 13V supply for the dc servo and is a standard differential input it is a complex circuit as well, but fairly straightforward for an experienced tech.
There is a lot of confusion here between the 555 II and the 565/585 - they are not the same amplifier! The 565/585 have dual differential darlington inputs that are biased with the same +/- 13v supply as the dc servo - it is a very complex circuit to troubleshoot. The 555 II uses a + 13V supply for the dc servo and is a standard differential input it is a complex circuit as well, but fairly straightforward for an experienced tech.
A very helpful gentleman who responded to my post over at AudioAsylum stated that:
You can read his entire post either at AudioAsylum or here at THIS thread (near the bottom of the page).
@woodman: Yes I read Walt Jung's post where he made that suggestion. I don't doubt Walt or you, however, CompNut was able to use the 820 and Charlie S (the gentleman from the AudioAsylum post) was able to use a LT1012. Could you clarify what type of supply the 565 has (monopolar, bi-polar, etc...) and explain why a certain type of op-amp is better for the 565 versus others? This would help cement the correct choice in all of our minds. Thanks!
@anatech: How is the DC servo circuit in the 565 different from the 555 II? There is not much information on the workings of this circuit in the 565, could you shed some light on how it works exactly? The input stage PCB is VERY complicated. Do you agree with Charlie's description (from AudioAsylum) of the 565 DC servo nulling circuit? Thank you!
You can read his entire post either at AudioAsylum or here at THIS thread (near the bottom of the page).
@woodman: Yes I read Walt Jung's post where he made that suggestion. I don't doubt Walt or you, however, CompNut was able to use the 820 and Charlie S (the gentleman from the AudioAsylum post) was able to use a LT1012. Could you clarify what type of supply the 565 has (monopolar, bi-polar, etc...) and explain why a certain type of op-amp is better for the 565 versus others? This would help cement the correct choice in all of our minds. Thanks!
@anatech: How is the DC servo circuit in the 565 different from the 555 II? There is not much information on the workings of this circuit in the 565, could you shed some light on how it works exactly? The input stage PCB is VERY complicated. Do you agree with Charlie's description (from AudioAsylum) of the 565 DC servo nulling circuit? Thank you!
mjraudio said:
The LT1012 is the Adcom 2A (though, as I recall, they were selected for minumum offset from the manufacturer) - it will work. If you look at the 565 schematic, you will notice the network hanging off J101 & 102. This is a shunt regulator that drops your rails from +/- 80v to +/- 13.8V to power your DC servo & bias the current mirrors.
Let me re-quote what Walt said regarding the opamp choice:
"1) The servo op amp must be a low power part, as the current it draws is supplied from a low power zener string (the series LM329 diodes, +/-13.8V). This fact alone rules out most std parts like 741s, 711s, etc. The LT1012 was the original Adcom 2A part, but has suffered excessive failures (not due to any circuit stress, for sure). A quality OP97 will substitute directly, and would be preferred from a no-other-changes-required point of view. Drop it in, and if the PCB is clean and nothing else is broken, it will work. "
Why is a certain type preferrable? Because Walt (the designer) said so.
He has been the opamp master for many years, so I will defer to his explaination. Some parts will draw too much current, some are too noisy, some may have too much drift, and yet others may work better with a single supply as in the 555II.Charlie S is exactly correct in his reply.
Good luck,
Hi mjraudio,
James is right on here. Many op amps draw too much current for this particular circuit. There is also the DC offset and noise to consider.
Walt Jung helped design the input stages and is the master of op amps as far as I am concerned. Whatever anyone says, listen to Walt. End of story. To the best of my knowledge, LT1012 is the original device.
The DC servos are similar in operation in these amps. I can't remember exact details on each and would have to look them up to tell you for sure. They don't usually present a problem for me to troubleshoot, but you never know .... Seems there is always an amplifier that has a fault caused by an odd component failure mode. They pop up every now and again.
-Chris
James is right on here. Many op amps draw too much current for this particular circuit. There is also the DC offset and noise to consider.
Walt Jung helped design the input stages and is the master of op amps as far as I am concerned. Whatever anyone says, listen to Walt. End of story. To the best of my knowledge, LT1012 is the original device.
The DC servos are similar in operation in these amps. I can't remember exact details on each and would have to look them up to tell you for sure. They don't usually present a problem for me to troubleshoot, but you never know .... Seems there is always an amplifier that has a fault caused by an odd component failure mode. They pop up every now and again.
-Chris
Hi I have an GFA 565,
some final transistor are fault, and now turn on with PIN 4 desconnect, and without final transistor,
I read this square wave 200 Vpep .. (on PIN 4) without input signal. Someone know this problem ??
On PIN 4 there is an incremental DC value ( at the mains on ) from 4 V until over 10 V
Thank you
some final transistor are fault, and now turn on with PIN 4 desconnect, and without final transistor,
I read this square wave 200 Vpep .. (on PIN 4) without input signal. Someone know this problem ??
On PIN 4 there is an incremental DC value ( at the mains on ) from 4 V until over 10 V
Thank you
Attachments
Last edited:
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.