I just wanted to thank you for the information provided. I suspected Q601 (2SC2362) due to it testing out of tolerance and showing voltage where it shouldn’t (also comparing it to the good side). It was also not matched to Q603 at all (hFE of 59 VS 275 on Q603). I was able to locate a new old stock transistor of the same type and ordered it about a week ago, It took a little time to arrive, cost me $7.50 shipped, and tested at 222 hFE.
Once installed, I checked for DC and it’s now 0.01V. I hooked it up and it sounds great, been playing about 6 hours now, no hum, no DC. Just tested and it’s between 0.00 and 0.01 on what was the bad side. The good side is showing 0.10, should I try adjusting it at all, or am I good?
Once installed, I checked for DC and it’s now 0.01V. I hooked it up and it sounds great, been playing about 6 hours now, no hum, no DC. Just tested and it’s between 0.00 and 0.01 on what was the bad side. The good side is showing 0.10, should I try adjusting it at all, or am I good?
Adjustment had no noteable effect, so I decided to check a few transistors on the side with 100mv. Q602 was showing the voltage, so I pulled it and took a reading. It had a very low hFE of 84, vs 366 on the same transistor at Q606, and Q604 was slightly lower. I decided to replace both Q602 and Q604 with matching replacements I ordered previously. I ordered 29 of KSC1845FTA at $0.143 cents each which I found as a replacement for the 2SC2362. I found two at hFE 352, Ie 2.2, and 631mv. It was also close to the other transistors that were good. Put it all back together and voltage is now about 6mv (0.006v). I also added a Velleman K4700U 2-CHANNEL LOUDSPEAKER PROTECTION kit just to be safe should I ever have an issue again, my speakers will be much safer.
Thanks again for your replies!
Thanks again for your replies!
Good job, yes below spec beta is a sign of a failing transistor.
I'm a bit puzzled in that I don't remember that transistor number used in this amp,
perhaps in the very early production?
You probably don't want to touch the amp now that it is working but the drivers
really need to be upgraded if you plan to drive low z loads, say 2 ohms or less
or if you want protection against shorted outputs. There is discussion on here
about this factory update.
If your amp did not have a catastrophic (smoke or blown fuse) there is really no
reason to worry about the output power devices.
I'm a bit puzzled in that I don't remember that transistor number used in this amp,
perhaps in the very early production?
You probably don't want to touch the amp now that it is working but the drivers
really need to be upgraded if you plan to drive low z loads, say 2 ohms or less
or if you want protection against shorted outputs. There is discussion on here
about this factory update.
If your amp did not have a catastrophic (smoke or blown fuse) there is really no
reason to worry about the output power devices.
anatech (authorized service for this amp) wrote this from here:
http://www.diyaudio.com/forums/solid-state/157682-dc-adcom-gfa-555-a-22.html#post2116059
"Hi Fred,
Too much advice from all quarters again. Infinia and Pete (PB2) are giving you the most accurate advice so far. Time to slow down and think about things a little.
What Infinia said in post #213 is correct on all counts. And yes, those 2SD525 and 2SB595 drivers were hand selected - believe it or not! These days there are a better selection of driver transistors that do not require much in the way of hand selection with a curve tracer. MJE15032 and MJE15033 are what I use in these amps. I would worry about using the MJE15034 and MJE15035 as drivers. Into low resistance loads and high power, they may fail due to excessive current. This has been a problem Adcom has had in the past with different driver transistors.
Remember to test the resistors in areas where you suspect damage. Resistors often change value or open up without leaving much evidence. Often, the resistor may exhibit a burn mark on the surface closest to the PCB - under the part. So never assume a resistor (or any other part) is fine as the result of a visual check. I've had open fuses with an intact element. That cost me time.
What do we know right now about what is going on with this amplifier?
We know that the positive drive is limited, and that it breaks the feedback loop at this point. Your DC offset is the average voltage caused by the missing positive portions of the sine wave. This is expected with a clipped wave form.
Have you tested Q9 and Q10, plus associated parts? You can pull these transistors and run the amp during testing safely. Don't forget to put them back in when you have the amp running normally. R17 may have gone up in value, and all the emitter resistors may have opened on the positive bank. A base-emitter short or leakage in any one of Q17 ~ Q20 can cause similar behavior.
Relax, write out what you are going to test, then mark the results of each test as you do them. Those will be your working facts, or things we know for sure to be correct.
Hi wahab,
Quote:
Check the voltage through the diodes D5 and D6...
It must be about 650mV across each diodes.
The precise voltage drop across these diodes is dependent on ambient temperature. They roughly track the Vbe changes in Q3 and Q4. The actual voltage drop may be a bit different from what you are citing as the voltage to expect.
We already know these diodes are working correctly. The diff amp is operating. Failure of this part of the amplifier will shut it down, look at where the thermal cutout is located. Also, the existing compensation is not a concern here at all. Never mess with compensation components in a successful commercial amplifier unless you have a very good reason to do so. This situation hardly qualifies.
There are times when output transistors are changed to a different type / part number and oscillation then occurs. Since the high frequency may have changed, the compensation may also need to be adjusted. This is also true when replacing an STK power IC with a newer part numbered the same.
Worrying about absolute beta as stated on the data sheet is counter productive unless you are currently looking for a replacement. Keep in mind that figures quoted in a data sheet are very rough and taken under conditions that may be different in this amplifier. Yes, details can matter - but don't sweat them to this degree at this point in a repair.
Hi infinia,
Generally speaking, testing an amplifier during service should use either all the output transistors, or none of them. Pete is right when he says the compensation will not be correct any more. This may possibly cause oscillation to occur. This depends on the amplifier of course. Running fewer outputs and causing oscillation will only cause lost time a you try to figure that out, and it doesn't happen every time as you know.
Happy hunting! -Chris"
http://www.diyaudio.com/forums/solid-state/157682-dc-adcom-gfa-555-a-22.html#post2116059
"Hi Fred,
Too much advice from all quarters again. Infinia and Pete (PB2) are giving you the most accurate advice so far. Time to slow down and think about things a little.
What Infinia said in post #213 is correct on all counts. And yes, those 2SD525 and 2SB595 drivers were hand selected - believe it or not! These days there are a better selection of driver transistors that do not require much in the way of hand selection with a curve tracer. MJE15032 and MJE15033 are what I use in these amps. I would worry about using the MJE15034 and MJE15035 as drivers. Into low resistance loads and high power, they may fail due to excessive current. This has been a problem Adcom has had in the past with different driver transistors.
Remember to test the resistors in areas where you suspect damage. Resistors often change value or open up without leaving much evidence. Often, the resistor may exhibit a burn mark on the surface closest to the PCB - under the part. So never assume a resistor (or any other part) is fine as the result of a visual check. I've had open fuses with an intact element. That cost me time.
What do we know right now about what is going on with this amplifier?
We know that the positive drive is limited, and that it breaks the feedback loop at this point. Your DC offset is the average voltage caused by the missing positive portions of the sine wave. This is expected with a clipped wave form.
Have you tested Q9 and Q10, plus associated parts? You can pull these transistors and run the amp during testing safely. Don't forget to put them back in when you have the amp running normally. R17 may have gone up in value, and all the emitter resistors may have opened on the positive bank. A base-emitter short or leakage in any one of Q17 ~ Q20 can cause similar behavior.
Relax, write out what you are going to test, then mark the results of each test as you do them. Those will be your working facts, or things we know for sure to be correct.
Hi wahab,
Quote:
Check the voltage through the diodes D5 and D6...
It must be about 650mV across each diodes.
The precise voltage drop across these diodes is dependent on ambient temperature. They roughly track the Vbe changes in Q3 and Q4. The actual voltage drop may be a bit different from what you are citing as the voltage to expect.
We already know these diodes are working correctly. The diff amp is operating. Failure of this part of the amplifier will shut it down, look at where the thermal cutout is located. Also, the existing compensation is not a concern here at all. Never mess with compensation components in a successful commercial amplifier unless you have a very good reason to do so. This situation hardly qualifies.
There are times when output transistors are changed to a different type / part number and oscillation then occurs. Since the high frequency may have changed, the compensation may also need to be adjusted. This is also true when replacing an STK power IC with a newer part numbered the same.
Worrying about absolute beta as stated on the data sheet is counter productive unless you are currently looking for a replacement. Keep in mind that figures quoted in a data sheet are very rough and taken under conditions that may be different in this amplifier. Yes, details can matter - but don't sweat them to this degree at this point in a repair.
Hi infinia,
Generally speaking, testing an amplifier during service should use either all the output transistors, or none of them. Pete is right when he says the compensation will not be correct any more. This may possibly cause oscillation to occur. This depends on the amplifier of course. Running fewer outputs and causing oscillation will only cause lost time a you try to figure that out, and it doesn't happen every time as you know.
Happy hunting! -Chris"
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