sgr100:
"So...I need to find a good non-NTE subsitute for a 2SC2240. Looks like the amp uses 4 per side, so I may just replace them all."
matelectronics.com has them at 25 cents a piece. Unfortunately they have a $25 minimum order. See if you have any of the following in your "parts bin" for quick test/replacements - 2SC2362, 2SC1845, 2sc2631, 2SC1708A or 2SC2389.
"Also, it safe to power up the amp with parts missing from one channel of the driver board?"
I would disconnect the output boards (just remove the fuse on each).
Good Luck
Mayank
"So...I need to find a good non-NTE subsitute for a 2SC2240. Looks like the amp uses 4 per side, so I may just replace them all."
matelectronics.com has them at 25 cents a piece. Unfortunately they have a $25 minimum order. See if you have any of the following in your "parts bin" for quick test/replacements - 2SC2362, 2SC1845, 2sc2631, 2SC1708A or 2SC2389.
"Also, it safe to power up the amp with parts missing from one channel of the driver board?"
I would disconnect the output boards (just remove the fuse on each).
Good Luck
Mayank
Mayank, I checked and it appears I don't have any of the semiconductors you mention as "testing" replacements. I may have one somewhere I can borrow.
The Adcom service manual shows a suffix of BL or GR. The installed 2SC2240's are all BL's. I assume this makes some sort of difference?
I may hit the local place and pickup a NTE equiv for testing. I just need to know if I have this figured out. If so, then I can park the GFA-555 and add the needed parts to my next order.
Thanks for the help.
The Adcom service manual shows a suffix of BL or GR. The installed 2SC2240's are all BL's. I assume this makes some sort of difference?
I may hit the local place and pickup a NTE equiv for testing. I just need to know if I have this figured out. If so, then I can park the GFA-555 and add the needed parts to my next order.
Thanks for the help.
Update: I ended up buying an NTE 199 which is supposed to replace the 2SC2240BL to further my tests. Ultimately I will replace the NTE with the proper device once I place my parts order. I don't trust NTE semiconductors.
With the NTE 199 in position Q10 and a different 2SC2240BL in Q1, the amp operates normally, output on both channels, DC offset of 28mV on the repaired channel and 45mV in the original channel, however I cannot get more than 7mV of bias on the repaired channel, so something is still not right.
I don't know enough theory to be able to look at the schematic and figure out the likely suspects in the bias circuit.
With the NTE 199 in position Q10 and a different 2SC2240BL in Q1, the amp operates normally, output on both channels, DC offset of 28mV on the repaired channel and 45mV in the original channel, however I cannot get more than 7mV of bias on the repaired channel, so something is still not right.
I don't know enough theory to be able to look at the schematic and figure out the likely suspects in the bias circuit.
Hi sgr100,
And on back to some previous remarks ...
I've said this many, many times before. Looks like I need to repeat myself.
Never, never, never swap parts from a working channel to a non-working channel! That's a very good way to create more bad parts. An expert can do this if they know the other circuit has no major faults. For the rest of the experimenters, just assume you can not do this.
No comment.
Right now you need to worry about hard knowledge. Don't even consider "upgrades" until you know how things work and you also know cold the characteristics of each part type. Messing around with different kinds of parts may lead to a technician induced failure. Forget this for now.
Why do people think that repairing an amplifier does not require a lot of education and proper test equipment? It's not that simple to do a decent job and there are too many hackers out there to add to that number. It's a lot more fun when you know what you are doing!
-Chris
Gain range. Some parts need to be matched, like the differential pair transistors.
Make sure the leads are in the correct order. US numbers are E-B-C and Japanese are E-C-B. The speed and gain range differences may cause some funniness.
Neither do I, so why are you depending on one in order to troubleshoot this amplifier?
And on back to some previous remarks ...
Wow. So trusting and hoping for the best.
I've said this many, many times before. Looks like I need to repeat myself.
Never, never, never swap parts from a working channel to a non-working channel! That's a very good way to create more bad parts. An expert can do this if they know the other circuit has no major faults. For the rest of the experimenters, just assume you can not do this.
This is the very worse and potentially most destructive thing you can do!!! Read my comment above again. Make a big sign that says this and put it over your bench.
Understand what this means to you then. You have no idea whether a transistor is leaky, has low gain or is intermittent. In other words, you only now that it's not a piece or wire or open, that's all. You absolutely need to build or buy a good transistor tester. Given that you say you work on other equipment, I can't believe you don't have this very basic ability. Note: the beta test on some meters will not cut it. Hmmm, your meter doesn't have this? What are you using for a meter??
Ahhhh, yup! I'd certainly say so! Throwing parts at something is not a good way to attempt a repair.
Nothing wrong with these really.
No comment.
Right now you need to worry about hard knowledge. Don't even consider "upgrades" until you know how things work and you also know cold the characteristics of each part type. Messing around with different kinds of parts may lead to a technician induced failure. Forget this for now.
These are really good amplifiers. They did have some problems in driver selection, but there is no problem with that these days. Given the high quality this amp could be again, it would be a giant shame to lose it because it wasn't important to service properly.
Okay, so then why don't you study the theory? Now's the time to learn and you absolutely can not properly service anything without knowing how they work. Every job you do until you learn is a hack job (no offense, effort has nothing to do with this). There is a ton of info available here on this site, and some really good books on the subject. Douglas Self has a really good book that covers some basics, it's called "Audio Power Amplifier Design Book". If you need to start with something more basic, then try some high school electronics books. Your local library should have some you can borrow.
Why do people think that repairing an amplifier does not require a lot of education and proper test equipment? It's not that simple to do a decent job and there are too many hackers out there to add to that number. It's a lot more fun when you know what you are doing!
-Chris
karl walter
Hello,
If you are still offering the schematic and parts list for GFA-555, I'd appreciate a copy. I looked thru all the pages in this thread and don't find a link for schematic download.
thanks,
Hello,
If you are still offering the schematic and parts list for GFA-555, I'd appreciate a copy. I looked thru all the pages in this thread and don't find a link for schematic download.
thanks,
Hello guys,
this thread is very interesting and it caught my attention as I am currently working on my 1st ver of GFA-555.
Left channel is dead. One of the 2SD424 is shorted, the rest are good.
All driver Qs (2SA1011 & 2SC2344) are shorted as well, but all 2SB554 are good.
All 0.82 ohms emitter resistors of the NPN output Qs are open and most of the component in the bias circuit (2SC2240, 2SA970, 1K trimmer, 4.7uF, 682 ohms, 4.75k ohms and the 100 ohms) are burnt, even the pcb and the D1 & D2 (DSC30) in the output stage are broken into pieces.
I've checked all the remaining resistors, capacitors and transistors and found them to be good.
I cleaned the board, replaced 2SC2240, 2SA970, 1K trimmer, 4.7uF and the 100 ohms.
For the 682 ohm, I replaced it with the 680 ohms 1% tol which is the only available here.
For the 4.75 K ohms, is it okay if I just replace it with a 1% tolerance 4.7K ohms?
Or should I need to series connect a 47 ohms with the 4.7K ohms?
By the way, I'm changing all the output Qs to MJ15024 and MJ15025.
Also, is it just fine if I change all the emitter resistors from 0.82 ohms 3W to 0.75 ohms 5W?
Please help me decide for these changes and I will follow all the safety procedures on powering amps and proper biasing, posted in this site.
Big thanks and appreciate all suggestions and help.
Joel
this thread is very interesting and it caught my attention as I am currently working on my 1st ver of GFA-555.
Left channel is dead. One of the 2SD424 is shorted, the rest are good.
All driver Qs (2SA1011 & 2SC2344) are shorted as well, but all 2SB554 are good.
All 0.82 ohms emitter resistors of the NPN output Qs are open and most of the component in the bias circuit (2SC2240, 2SA970, 1K trimmer, 4.7uF, 682 ohms, 4.75k ohms and the 100 ohms) are burnt, even the pcb and the D1 & D2 (DSC30) in the output stage are broken into pieces.
I've checked all the remaining resistors, capacitors and transistors and found them to be good.
I cleaned the board, replaced 2SC2240, 2SA970, 1K trimmer, 4.7uF and the 100 ohms.
For the 682 ohm, I replaced it with the 680 ohms 1% tol which is the only available here.
For the 4.75 K ohms, is it okay if I just replace it with a 1% tolerance 4.7K ohms?
Or should I need to series connect a 47 ohms with the 4.7K ohms?
By the way, I'm changing all the output Qs to MJ15024 and MJ15025.
Also, is it just fine if I change all the emitter resistors from 0.82 ohms 3W to 0.75 ohms 5W?
Please help me decide for these changes and I will follow all the safety procedures on powering amps and proper biasing, posted in this site.
Big thanks and appreciate all suggestions and help.
Joel
I know this thread has not been touched in a while but I'm currently in the process of trying to fix an Adcom GFA-555(I). Finding parts has been very difficult!
Is it possible for some one to post any substitutions for the components or perhaps even upgrades to the originals? Transistors and the Silver Mica capacitors seem to be my biggest problems.
Thanks very much in advance!
Is it possible for some one to post any substitutions for the components or perhaps even upgrades to the originals? Transistors and the Silver Mica capacitors seem to be my biggest problems.
Thanks very much in advance!
Just purchased 2x Adcom GFA 555se. Listening and owned a lot of amplifiers I always thought my Adcom GFA 545 mkII sounded more precise,detailed and with much tighter and better low end than my i.e Cambridge Audio CXA80 (2016) design.
With this criteria i purchased two 2x Adcom GFA 555se, intending to use them in vertical passive bi-amping configuration for my B&W 805s. Mainly I am using The Audio Cambridge CXN as a digital source for TIDAL, Spotify, and FLACK from NAS drives. using one of the new Adcoms on stero mode after few days of burning inn, i notice great improvement in all areas apart than I would have to say the bass was a bit bloated compared to the 545 MKII,
Also i noticed the heat-sinks temperatures had a really big difference working under the same conditions. Left Chanel with right Chanel unit 1 ( L slightly warm, R Almost cold). Unit 2 ( L warm to Hot , R Almost warm).
1. Can anybody explain why, and if Bias , dc offset has something to do with it.
2. Any ideas why the bass sounds bit bloated in comparison with the rest of my amps, and if any cure.
With this criteria i purchased two 2x Adcom GFA 555se, intending to use them in vertical passive bi-amping configuration for my B&W 805s. Mainly I am using The Audio Cambridge CXN as a digital source for TIDAL, Spotify, and FLACK from NAS drives. using one of the new Adcoms on stero mode after few days of burning inn, i notice great improvement in all areas apart than I would have to say the bass was a bit bloated compared to the 545 MKII,
Also i noticed the heat-sinks temperatures had a really big difference working under the same conditions. Left Chanel with right Chanel unit 1 ( L slightly warm, R Almost cold). Unit 2 ( L warm to Hot , R Almost warm).
1. Can anybody explain why, and if Bias , dc offset has something to do with it.
2. Any ideas why the bass sounds bit bloated in comparison with the rest of my amps, and if any cure.
"1. Can anybody explain why, and if Bias , dc offset has something to do with it."
Set the bias, check for oscillations.
"2. Any ideas why the bass sounds bit bloated in comparison with the rest of my amps, and if any cure"
Needs bypass caps across the main filter caps, try around 22µF~47µF.
The SE should have them, just replace the old ones.
Set the bias, check for oscillations.
"2. Any ideas why the bass sounds bit bloated in comparison with the rest of my amps, and if any cure"
Needs bypass caps across the main filter caps, try around 22µF~47µF.
The SE should have them, just replace the old ones.
djk and Chris
1. Can anybody explain why, and if Bias , dc offset has something to do with it."
Set the bias, check for oscillations
1A. I do not know how to set the bias. I need a pictorial diagram with arrows to pin point where exactly each adjustment is and some instructions ?
2. Any ideas why the bass sounds bit bloated in comparison with the rest of my amps, and if any cure"
Needs bypass caps across the main filter caps, try around 22µF~47µF.
2A Do you need pictures so i can understand what do YOU actually mean.
Please be patience due to the fact that not any specialized electricians in my area.
1. Can anybody explain why, and if Bias , dc offset has something to do with it."
Set the bias, check for oscillations
1A. I do not know how to set the bias. I need a pictorial diagram with arrows to pin point where exactly each adjustment is and some instructions ?
2. Any ideas why the bass sounds bit bloated in comparison with the rest of my amps, and if any cure"
Needs bypass caps across the main filter caps, try around 22µF~47µF.
2A Do you need pictures so i can understand what do YOU actually mean.
Please be patience due to the fact that not any specialized electricians in my area.
Hi APAPAS,
I agree with djk, i you don't know how to set the bias current, don't touch it! Now, since the bias set on an Adcom has a procedure, be absolutely sure whoever does this for you follows it to the letter. You have to get the heat sinks hot, then let them cool as you correct the bias current. Very experienced audio technicians with a lot of experience can shave some time off setting up an Adcom, but unless they did these under warranty and followed the manual at one time, they know less than nothing about them.
If they don't have, or search for a manual - run like the wind with your amp in tow!
The "technician" has to use a DVM that resolves mA measurements well, and accurately. Those no-name , or cheap meters, are worse than not having a meter. Look for a Fluke or HP / Agilent / Keysight handheld. A bench meter could be the same brands, or Kiethley and a few other more expensive meters. Someone who uses cheap equipment doesn't really know what they are doing, or what is important. They have to own decent tools, not the best necessarily. Just good, competent tools. They would also understand why bias is important and not second-guess the factory.
Bias current keeps the transistors turned on a little bit at idle. This reduces distortion. Too much bias and everything runs hot and may fail - or reduce the life of the amplifier even if it doesn't fail. Not enough bias and the distortion rises. A bias related "cross-over" distortion sounds particularly bad.
-Chris
I agree with djk, i you don't know how to set the bias current, don't touch it! Now, since the bias set on an Adcom has a procedure, be absolutely sure whoever does this for you follows it to the letter. You have to get the heat sinks hot, then let them cool as you correct the bias current. Very experienced audio technicians with a lot of experience can shave some time off setting up an Adcom, but unless they did these under warranty and followed the manual at one time, they know less than nothing about them.
If they don't have, or search for a manual - run like the wind with your amp in tow!
The "technician" has to use a DVM that resolves mA measurements well, and accurately. Those no-name , or cheap meters, are worse than not having a meter. Look for a Fluke or HP / Agilent / Keysight handheld. A bench meter could be the same brands, or Kiethley and a few other more expensive meters. Someone who uses cheap equipment doesn't really know what they are doing, or what is important. They have to own decent tools, not the best necessarily. Just good, competent tools. They would also understand why bias is important and not second-guess the factory.
Bias current keeps the transistors turned on a little bit at idle. This reduces distortion. Too much bias and everything runs hot and may fail - or reduce the life of the amplifier even if it doesn't fail. Not enough bias and the distortion rises. A bias related "cross-over" distortion sounds particularly bad.
-Chris
Regarding how to adjust the bias current idle.
Remove the cover, Look at the drive board to see VR 2 to the left side for a left bias adjustment. The right side is for adjusting the biasing right channel.
Adjustable bias by measuring the voltage drop each of the RE 0.22ohm applox 5mV.
Should adjust the bias for the two channels have a similar heat.
The reply i got from the factory but does not explain procedure. i.e.
Connected to the speakers or not.
Connected to the amp or not.
And any other parameters
Remove the cover, Look at the drive board to see VR 2 to the left side for a left bias adjustment. The right side is for adjusting the biasing right channel.
Adjustable bias by measuring the voltage drop each of the RE 0.22ohm applox 5mV.
Should adjust the bias for the two channels have a similar heat.
The reply i got from the factory but does not explain procedure. i.e.
Connected to the speakers or not.
Connected to the amp or not.
And any other parameters
Moolly,
Why would a brand new amp need the bias checking and adjusting ?
i noticed the heat-sinks temperatures had a really big difference working under the same conditions. Left Chanel and right Chanel unit 1 ( L Almost cold to warm, R Almost cold to warm). Unit 2 ( L warm to Hot 42C , R Almost cold to warm.
Why would a brand new amp need the bias checking and adjusting ?
i noticed the heat-sinks temperatures had a really big difference working under the same conditions. Left Chanel and right Chanel unit 1 ( L Almost cold to warm, R Almost cold to warm). Unit 2 ( L warm to Hot 42C , R Almost cold to warm.
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