Hello,
I picked up this GFA-555 with a problem on the left channel. The guy demoed it and it sounded good and worked, but the left channel had an audible hum and gets very hot, right is dead silent and runs cool.
When I got home I replaced all 4 electrolytic caps that are known to leak, there was no sign of leakage and they (old removed caps) all tested in spec with a MK-328 TR/LCR/ESR tester. I also disconnected all 4 filter caps and they also tested good and in spec. I also checked, cleaned, and soldered the ground connectors to the wires. Next I hooked it back up and had the same result as before.
I tested the DC voltage on the speaker outputs, (should have done this first) right is very low, around 25mV but the left has about 12 volts DC on it. I connected a physically damaged speaker and it goes full out and the heat sync on the far left gets hot fast. The transistors on the far left get warm fast but only with a load connected. I pulled all the components off the far left output NPN board, diode checked the 2SD424 Q625,Q629, Q633, & Q637- ok and the tester shows good with hFE of 75.1 to 83.8, Ie is .66 to .72mA.
Q621 - 2SC2344 also tests good with hFE of 220, Ie is 6.3mA. The diode and 4 ceramic resistors that are very close to .5 ohm are also good, temp veristor seems good too. Bias adjustment does little, a few mV.
I’m a noob at this type of repair and could use guidance on what to check next? Inside the amp is very clean, no discoloration of any boards even under a magnification glass. No visually damaged or replaced components, no buldged caps, or other visual clues. As I mentioned, it does power on and work, I’ve got to guess the crossover network was somewhat protecting his speakers with that much DV voltage. I assume the second bank of PNP transistors is a push pull design? And guidance would be great, should I be testing transistors on the input board? Anything to target first? I have no problem desoldering components and testing them.
I picked up this GFA-555 with a problem on the left channel. The guy demoed it and it sounded good and worked, but the left channel had an audible hum and gets very hot, right is dead silent and runs cool.
When I got home I replaced all 4 electrolytic caps that are known to leak, there was no sign of leakage and they (old removed caps) all tested in spec with a MK-328 TR/LCR/ESR tester. I also disconnected all 4 filter caps and they also tested good and in spec. I also checked, cleaned, and soldered the ground connectors to the wires. Next I hooked it back up and had the same result as before.
I tested the DC voltage on the speaker outputs, (should have done this first) right is very low, around 25mV but the left has about 12 volts DC on it. I connected a physically damaged speaker and it goes full out and the heat sync on the far left gets hot fast. The transistors on the far left get warm fast but only with a load connected. I pulled all the components off the far left output NPN board, diode checked the 2SD424 Q625,Q629, Q633, & Q637- ok and the tester shows good with hFE of 75.1 to 83.8, Ie is .66 to .72mA.
Q621 - 2SC2344 also tests good with hFE of 220, Ie is 6.3mA. The diode and 4 ceramic resistors that are very close to .5 ohm are also good, temp veristor seems good too. Bias adjustment does little, a few mV.
I’m a noob at this type of repair and could use guidance on what to check next? Inside the amp is very clean, no discoloration of any boards even under a magnification glass. No visually damaged or replaced components, no buldged caps, or other visual clues. As I mentioned, it does power on and work, I’ve got to guess the crossover network was somewhat protecting his speakers with that much DV voltage. I assume the second bank of PNP transistors is a push pull design? And guidance would be great, should I be testing transistors on the input board? Anything to target first? I have no problem desoldering components and testing them.
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Please don't shot gun anything and start replacing a lot of parts, this might hide
the source of the problem.
Is it easy enough to provide C-B-E voltages for all of the transistors on the circuit board?
No signal, no load.
We only need the bad channel voltages.
Do you have the repair manual?
If you remove transistors, it would be good to test them for C-E leakage at the rail
to rail voltage.
the source of the problem.
Is it easy enough to provide C-B-E voltages for all of the transistors on the circuit board?
No signal, no load.
We only need the bad channel voltages.
Do you have the repair manual?
If you remove transistors, it would be good to test them for C-E leakage at the rail
to rail voltage.
I have not tested it powered on, but I did pull all the transistors and tested them with the MK-328 tester. I was thinking it would be good to start with consistant parts in any case? All the power transistors were consistent and so were the other components on the heatsync boards. When I moved to the input board things were much less consistent, but did function without short or opens passing diode tests. The 2SC2362’s varied on hFE from 59.9 to 373, a big difference in gain. Ie also varied from 2.2 mA to 6.1. I think there is a problem with some of these.
Is there a good replacement for the 2SC2362, I think I want to replace all 6.
Also the 2SA1208 varied more then they should, HFE 141 to 209,
I have yet to test the good side to compare to, or find what the spec should be. I logged all the data though, and should be able to test many of the other components with out removing them with the transistors out.
I’d love to find good replacement PN’s for thes Transistors from Mouser or Digital Key:
2 x 2SC2910
6 x 2SA2362
4 x 2SA1208 - are 2 of these for fan control?
4 x 2SA608
2x 2SC1016
2x 2SC536
Is there a good replacement for the 2SC2362, I think I want to replace all 6.
Also the 2SA1208 varied more then they should, HFE 141 to 209,
I have yet to test the good side to compare to, or find what the spec should be. I logged all the data though, and should be able to test many of the other components with out removing them with the transistors out.
I’d love to find good replacement PN’s for thes Transistors from Mouser or Digital Key:
2 x 2SC2910
6 x 2SA2362
4 x 2SA1208 - are 2 of these for fan control?
4 x 2SA608
2x 2SC1016
2x 2SC536
Are my tests at low voltage inconclusive due to the much lower voltage and not able to reveal failures that would occur at rail voltage? Should I proceed on the input board first, since there does seem to be issues there? I assume I would need to reassemble the input board before I can collect any C-B-E voltage data on the output boards? What’s the best way to test for leakage, just apply rail voltage and see if I get anything where I should not see anything?
You can't properly test power transistors with the tiny couple of milliamps supplied by a 328 type tester. They are fine for signal level parts and can verify that you indeed have a transistor but will tell you anything you want to believe about their measurements when their hFE is specified in the range of several amps of collector current. To a lesser degree, that applies also to medium power devices like the driver transistors.
Generally, don't try to measure or match power transistors without using a decent size heatsink and an appropriate regulated supply test current of at least 0.5 A.
Generally, don't try to measure or match power transistors without using a decent size heatsink and an appropriate regulated supply test current of at least 0.5 A.
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Did you notice this thread by any chance?
Another high DC Adcom GFA-555
I wrote this:
"I'm not sure if this is clear but the original in use failure was due to a shorted output.
The design should, hopefully survive a shorted output just with a fuse blowing but
the early units were shipped with lower rated driver transistors that are a weak link
and there is a factory authorized upgraded driver. My unit has the upgraded drivers
from the factory and has simply blown fuses when the output was accidentally shorted.
Once we were able to test the outputs we found at least one bad output device,
probably due to a failed driver.
It is important to treat a failed amp like a crime scene, try to make as few changes
when you do your tests. I suggested a way to remove a few transistors and test
the output devices for leakage, the transistors were removed, but some new
counterfeits were also put in with far to low breakdown voltage. Once that part
was identified testing of the output stage went smoothly.
My guess is that with the shorted output a driver failed which then led to an output
and perhaps the VAS to also fail. The design is slightly marginal under short
circuit conditions and even warm temps."
Another high DC Adcom GFA-555
I wrote this:
"I'm not sure if this is clear but the original in use failure was due to a shorted output.
The design should, hopefully survive a shorted output just with a fuse blowing but
the early units were shipped with lower rated driver transistors that are a weak link
and there is a factory authorized upgraded driver. My unit has the upgraded drivers
from the factory and has simply blown fuses when the output was accidentally shorted.
Once we were able to test the outputs we found at least one bad output device,
probably due to a failed driver.
It is important to treat a failed amp like a crime scene, try to make as few changes
when you do your tests. I suggested a way to remove a few transistors and test
the output devices for leakage, the transistors were removed, but some new
counterfeits were also put in with far to low breakdown voltage. Once that part
was identified testing of the output stage went smoothly.
My guess is that with the shorted output a driver failed which then led to an output
and perhaps the VAS to also fail. The design is slightly marginal under short
circuit conditions and even warm temps."
Fred in that thread started replacing parts and put in one or more counterfeit ones.
Keep in mind that there was an update to replace the marginal driver transistors.
Also, an issue with a big amp like this is that if you slip with a probe and short
something there are a LOT of output devices that you'd need to replace and they
are no longer available. Do you have a dim bulb tester?
Can you post a link to the schematic that you're using so that I can find the transistor
reference numbers that you mention?
You wrote: "I’d love to find good replacement PN’s for thes Transistors from Mouser or Digital Key:"
Keep in mind that there are many counterfeits out there so find the defective parts
and replace only what is needed, confirm that it works, then if you want replace more
one at a time after careful testing of the replacement parts. Yes, for breakdown voltage also.
Keep in mind that there was an update to replace the marginal driver transistors.
Also, an issue with a big amp like this is that if you slip with a probe and short
something there are a LOT of output devices that you'd need to replace and they
are no longer available. Do you have a dim bulb tester?
Can you post a link to the schematic that you're using so that I can find the transistor
reference numbers that you mention?
You wrote: "I’d love to find good replacement PN’s for thes Transistors from Mouser or Digital Key:"
Keep in mind that there are many counterfeits out there so find the defective parts
and replace only what is needed, confirm that it works, then if you want replace more
one at a time after careful testing of the replacement parts. Yes, for breakdown voltage also.
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I got the manual/schematic here: Adcom GFA-555 - Manual - Stereo Power Amplifier - HiFi Engine
Some of the threads I read were confusing because they kept talking about Part numbers not in my 555 and Q numbers that must be dropping all the numbers in the middle or from a different schematic or amp.
I will need to reassemble the input board I’m assuming first. There was one 2SA2362 that was way off from the rest, I was hoping to at least replace that one.
When you say a dim bulb tester, you mean a test light? Something that can be used with around 100 volts? I do have one if thats what’s needed. I’m unsure of what I should be testing or at what points or am I using it as the load on the speaker wires?
The power transistors that were weak, are they the same Part number as the better replacements? Can they be identified somehow so I know which I have?
I’m also getting I should remove some of the power transistors on the suspected bank and run it like that to see if the DC voltage / hum / heat goes away? A process of elimination...
Some of the threads I read were confusing because they kept talking about Part numbers not in my 555 and Q numbers that must be dropping all the numbers in the middle or from a different schematic or amp.
I will need to reassemble the input board I’m assuming first. There was one 2SA2362 that was way off from the rest, I was hoping to at least replace that one.
When you say a dim bulb tester, you mean a test light? Something that can be used with around 100 volts? I do have one if thats what’s needed. I’m unsure of what I should be testing or at what points or am I using it as the load on the speaker wires?
The power transistors that were weak, are they the same Part number as the better replacements? Can they be identified somehow so I know which I have?
I’m also getting I should remove some of the power transistors on the suspected bank and run it like that to see if the DC voltage / hum / heat goes away? A process of elimination...
I'm not sure what to tell you, beta can vary A LOT from lot to lot of parts, is it in spec?
Transistors that have been overstressed will often have high leakage and lower Vce
rating.
If you are going to put it back together give us a list of voltages.
Your part numbers do not look to familiar to me but I've not looked at the design in a while.
Transistors that have been overstressed will often have high leakage and lower Vce
rating.
If you are going to put it back together give us a list of voltages.
Your part numbers do not look to familiar to me but I've not looked at the design in a while.
How can I find out what spec is? So far I’ve not found anything on this part or any suitable replacement (2Sc2362).
I’ll put together a Dim light tester, I have the parts laying around. The part numbers I posted are off the schematic, but they also match up to the numbers on the parts, except the first couple digits. For example the part I listed as 2SC2362 has C2362 G6F printed on the part.
I guess I’ll put it back together with the same parts, should I put it back as it was or move the weak transistor to another spot? The one I suspected has the hFE as 59.9 when the others were between 275 and 373 and Ic 6.1 vs 2.2-2.5 so that one seems pretty far off from the others.
The main power transistors are Toshiba 2SB554 and Toshiba 2SD424, the other list is from the input board.
Where should I be pulling voltages from?
I’ll put together a Dim light tester, I have the parts laying around. The part numbers I posted are off the schematic, but they also match up to the numbers on the parts, except the first couple digits. For example the part I listed as 2SC2362 has C2362 G6F printed on the part.
I guess I’ll put it back together with the same parts, should I put it back as it was or move the weak transistor to another spot? The one I suspected has the hFE as 59.9 when the others were between 275 and 373 and Ic 6.1 vs 2.2-2.5 so that one seems pretty far off from the others.
The main power transistors are Toshiba 2SB554 and Toshiba 2SD424, the other list is from the input board.
Where should I be pulling voltages from?
I now have a Dim Bulb Tester.
I’ve removed half the transistors on the output boards selecting the ones that showed any higher voltage or heat when a load was across the speaker connectors, this has had little effect. The three wire connectors going from the input board the the output boards have the save voltage on them as the pins they are connected to as expected. The good side does not have this voltage. I suspect the input board, and Q601 to be specific as it is one with the big variance when I tested them.
For the power output boards:
On the NPN 2SD424 far left I have 7.9 volts on the emitter, 8.5v on the base, 74.3 on the collector.
With the input board disconnected I have 0.02 on the emitter, 0.08 on the base, 75 on the collector.
On the PNP 2SB554 center left I have 7.8v on the base, 8.4 on the emitter, 74.3 on the collector.
With the input board disconnected I have 0.11 one the emitter, 0.03 on the base, 75 on the collector.
Not sure why, but voltage on speaker output is down to about 8v after reassembly.
There is no heat when a speaker load is not connected, if I connect a speaker to the bad side there is heat and the dim bulb comes on, speaker moves to the extreme position (trash damaged speaker). I only connected it for a few seconds.
What should I check next?
I’ve removed half the transistors on the output boards selecting the ones that showed any higher voltage or heat when a load was across the speaker connectors, this has had little effect. The three wire connectors going from the input board the the output boards have the save voltage on them as the pins they are connected to as expected. The good side does not have this voltage. I suspect the input board, and Q601 to be specific as it is one with the big variance when I tested them.
For the power output boards:
On the NPN 2SD424 far left I have 7.9 volts on the emitter, 8.5v on the base, 74.3 on the collector.
With the input board disconnected I have 0.02 on the emitter, 0.08 on the base, 75 on the collector.
On the PNP 2SB554 center left I have 7.8v on the base, 8.4 on the emitter, 74.3 on the collector.
With the input board disconnected I have 0.11 one the emitter, 0.03 on the base, 75 on the collector.
Not sure why, but voltage on speaker output is down to about 8v after reassembly.
There is no heat when a speaker load is not connected, if I connect a speaker to the bad side there is heat and the dim bulb comes on, speaker moves to the extreme position (trash damaged speaker). I only connected it for a few seconds.
What should I check next?
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Testing voltages on Q601 (2SC2362) shows 7.09v on the emitter, 72.3v on the collector, 6.51v on the base, while Q602 on the good side shows 0.28v on the emitter, 72.3v one the collector, and 0.87v on the base.
It looks like the hFE min rating on the 2SC2362 is 160, so my part is out of tolerance.
Also connecting just the three wire input board to output board on the PNP inner left board (2SB554) does not cause any voltage issue, but when you connect the three wire connector to the NPN far left board does cause the DC voltage on the speaker outputs.
Still hoping someone can point me to a replacement part for the 2SC2362 labeled as C2362 G6F.
It looks like the hFE min rating on the 2SC2362 is 160, so my part is out of tolerance.
Also connecting just the three wire input board to output board on the PNP inner left board (2SB554) does not cause any voltage issue, but when you connect the three wire connector to the NPN far left board does cause the DC voltage on the speaker outputs.
Still hoping someone can point me to a replacement part for the 2SC2362 labeled as C2362 G6F.
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You are pulling parts so fast that it is hard to keep up with what you are doing.
Are you an engineer?
Why would you put a speaker load on an amp that is putting out 12V DC?
The output transistors get hot for this reason:
If you put an 8 ohm speaker on there then I = V/R = 12/8 = 1.5A
The rail voltage is about 80V, with 12V on the load there is 68V across the output transistors,
and Power = I*V = 1.5 * 68 = 102W THIS IS WHY THE HEATSINK GETS HOT.
With a positive output voltage only the transistors on the pos rail are conducting then
those are the ones that get hot, the others are off so they do not get hot.
You need to figure out what is the problem.
In all of your writing you've not answered what the part numbers are on the actual driver
transistors.
Are you an engineer?
Why would you put a speaker load on an amp that is putting out 12V DC?
The output transistors get hot for this reason:
If you put an 8 ohm speaker on there then I = V/R = 12/8 = 1.5A
The rail voltage is about 80V, with 12V on the load there is 68V across the output transistors,
and Power = I*V = 1.5 * 68 = 102W THIS IS WHY THE HEATSINK GETS HOT.
With a positive output voltage only the transistors on the pos rail are conducting then
those are the ones that get hot, the others are off so they do not get hot.
You need to figure out what is the problem.
In all of your writing you've not answered what the part numbers are on the actual driver
transistors.
For the power output boards:
On the NPN 2SD424 far left I have 7.9 volts on the emitter, 8.5v on the base, 74.3 on the collector.
With the input board disconnected I have 0.02 on the emitter, 0.08 on the base, 75 on the collector.
On the PNP 2SB554 center left I have 7.8v on the base, 8.4 on the emitter, 74.3 on the collector.
With the input board disconnected I have 0.11 one the emitter, 0.03 on the base, 75 on the collector.
They are Toshiba 2SD424 O 6F & Toshiba 2SB554 O 6F
The Schematic was downloaded at the site I gave you, but If you have somewhere I can upload it to I’d be happy to do so.
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