It's not a 1270, it's a 1207. The typo is on the schematic. 1207 is what's installed and on the parts list.
I guess Adcom should've hired a better technical editor! 🙂
Well THAT'S a horse of a different color. I saw your 2SA1207 and looked at my schematic which said 2SA1270 but I didn't check the parts list and I didn't pop the top on my amp. The Zetex ZTX796 at $1.85 (DigiKey) would indeed work fine there.
The gear I designed for DAV was done on the computer so the parts list, schematic and board layouts always agreed. It's the only way to do designs.
G²
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Yes I understood output stage drivers (Q11 & Q12) were in place but the differential pair (diff pair Q1 and Q2) were not, is that right? Without Q1 and Q2 there is no global feedback so the oscillation is hard to explain.
If C5, the 4.7uF across the bias pot went high on ESR it might do some peculiar things.
As I emailed to Fred last night off list, I had a ceramic cap go very leaky last year which caused some bizarre symptoms in a video tape machine.
Nothing ELSE seems to be bringing this amp back to life and capacitors are my number 2 failure mode behind connections/connectors.
G²
Oh, then yes, that's correct. Q1 and Q2 were NOT in place when that trace was obtained.
Suddenly, that makes a lot more sense. I had not understood that feedback was taken out since I didn't have it clear which pair we were talking about.
I'm gonna sleep on it and try to come up with a fresh strategy.
cheers, all
Unfortunately, C5 was replaced back at post #190 and the oscillation was still present.
This dog is tough.
The main global feedback is the voltage divider comprised of R6 and R5. Note that all the voltage gain of the amp is there at the collector of Q7, with the output stage (drivers and outputs) providing current gain since they are in the emitter follower configuration. The output stage is slow, so C4 provides a high freq feedback path from the VAS output bypassing the output stage. C2 is normally used for dominant pole compensation as negative feedback right around the VAS.
In some old Sony BetaCam (based on BetaMax sized cassettes) broadcast VCRs they used a glue on the boards to hold re-work wires stretched across the board which became conductive and caused all sorts of anomalies. These failures didn't show up for many (15+) years.
ALSO, electrolytic capacitor acid leaks can cause resistive paths where none was wanted. These look like a brown mist around 'lytic caps and often a small puddle under the cap is visible after the old cap is removed. Even if the cap measures 'OK' on ESR and value, it must be replaced and the board thoroughly cleaned before the new cap is installed. The acid WILL eventually destroy the copper traces if not cleaned off.
Is there ANYTHING on the top or bottom of the board that might be causing this?
G²
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Out of curiosity I checked my amp it is the newer production with the nicer silk screen on the PC board and it has the much larger 2SD525/comp drivers. It is interesting that opening the T1 circuit also disables the diff amp so I could run all these same tests by pulling one wire. The larger drivers probably explain why my amps have survived shorting the output a few times.
The bias arrangement looks unusual on this amp and I happen to also have the service manual for the 555-II. They went for a triple emitter follower output stage and it is known to drive low Z loads better. The drivers are 12A TO3P Sanyo devices with excellent performance; as large as they seem to be they will only do 1.5A at 70V before temp derating. There are 10 ohm base stoppers on the outputs. The bias circuit is replaced with a more conventional VBE multiplier with the transistor on the heat sink. The rest is very similar.
The bias arrangement looks unusual on this amp and I happen to also have the service manual for the 555-II. They went for a triple emitter follower output stage and it is known to drive low Z loads better. The drivers are 12A TO3P Sanyo devices with excellent performance; as large as they seem to be they will only do 1.5A at 70V before temp derating. There are 10 ohm base stoppers on the outputs. The bias circuit is replaced with a more conventional VBE multiplier with the transistor on the heat sink. The rest is very similar.
There's nothing that I can see. I have flushed the board with contact cleaner (deoxit) and whenever I've had a part out I've cleaned the area thoroughly with high-grade iso alcohol. I've not noticed anything, though it might be hard to tell the difference between a puddle from electrolyte acid and the marks from evaporated contact cleaner.
Similarly, I don't see any burned spots or discoloration that I would consider out of the ordinary for a board of this vintage that has seen so much service. The traces seem okay, I don't see any lifting or bubbling or delamination.
At this point, anything's possible, and I am inexperienced, but I don't see anything obvious.
Here are my thoughts on how to proceed, given the known facts.
1. Since I have at this point tested and/or replaced about 70% of all the components on the input board, including all but two of the transistors, I may as well go ahead and shot-gun in the rest. I would of course keep testing everything that goes in and comes off the board.
2. I'll re-connect the board, running new wire, to the output boards. The output boards I will leave with only one output transistor connected on each, and with the temporary drivers (30/31) still installed.
3. I'll run new ground wires and new wire to the speaker jacks.
4. Turn it on and run tests...
5. If it fails the tests, I will take G2 up on his offer to check my semis on his curve tracer, because at that point I can only conclude that either (a) there is an issue with a replacement part that is too subtle for my crude test suite or (b) there is an error of reassembly or defect on the board that I'm just not seeing.
6. If G2 finds no issue with the semis, there will probably be a parts 555 going up on ebay real soon (and a truly humbled noob in Portland, OR).
1. Since I have at this point tested and/or replaced about 70% of all the components on the input board, including all but two of the transistors, I may as well go ahead and shot-gun in the rest. I would of course keep testing everything that goes in and comes off the board.
2. I'll re-connect the board, running new wire, to the output boards. The output boards I will leave with only one output transistor connected on each, and with the temporary drivers (30/31) still installed.
3. I'll run new ground wires and new wire to the speaker jacks.
4. Turn it on and run tests...
5. If it fails the tests, I will take G2 up on his offer to check my semis on his curve tracer, because at that point I can only conclude that either (a) there is an issue with a replacement part that is too subtle for my crude test suite or (b) there is an error of reassembly or defect on the board that I'm just not seeing.
6. If G2 finds no issue with the semis, there will probably be a parts 555 going up on ebay real soon (and a truly humbled noob in Portland, OR).
Just as a reality check, today I tested the good channel by actually making it play music. It did, just fine, as clear as music can be if your source is an ipod and your speaker an 8" cardboard cone in an old guitar amp box. I didn't turn it up much, of course.
So it's unlikely that I did something in my cleaning process or otherwise damaged anything globally.
So it's unlikely that I did something in my cleaning process or otherwise damaged anything globally.
Let me offer one more suggestion that I would have done when I had a lot less experience, especially since you seem almost ready to junk it. The output stage is wired in using large wire wrap to the circuit board, at least in my amp, the posts look like standard molex posts. You could unwrap them and put connectors on them. Then after you do the tests that I suggested on the output stage and confirm it as good or bad, you could connect it to the other input stage keeping the small fuses there to protect things and bringing it up slowly on the Variac. You've got one good side that does you no good if you can't fix the other side so risking it is not a major issue.
When I remove or install devices, I like to use the amps power supply to leakage test the devices at full voltage. This is particularly important in a high voltage amp. Removing the drivers allows you to do the output stage in place. Since opening T1 seems to fix things you should be able to do the outputs and drivers in place.
Another very simple way to further test the output stage once you confirm the DC tests. Is to use the good channel to drive the input of the failed channel's output stage with a load.
We all have stories of those repairs that seemed nearly impossible. I etched my own PC boards as a kid and once had a hairline open in one of the lands. I gave up on that amp, after spending weeks on it, only to find the fault years later when I was going to use the driver board for another amp.
I had gotten used to both screws on a TO-3s connecting the case, and getting lazy I would pull devices and during test only put one screw back in until it passed basic low power tests. One amp only used the other screw for electrical connection and I spent days on it.
I think right now there might be two issues with the amp, you might have fixed the output stage by removing parts but are not aware because Q4 was causing it to oscillate and masked the correction. As I said, I would start putting the output stage back together with T1 open. You might want to take a few days off.
Having G2 test those parts is a good idea also.
When I remove or install devices, I like to use the amps power supply to leakage test the devices at full voltage. This is particularly important in a high voltage amp. Removing the drivers allows you to do the output stage in place. Since opening T1 seems to fix things you should be able to do the outputs and drivers in place.
Another very simple way to further test the output stage once you confirm the DC tests. Is to use the good channel to drive the input of the failed channel's output stage with a load.
We all have stories of those repairs that seemed nearly impossible. I etched my own PC boards as a kid and once had a hairline open in one of the lands. I gave up on that amp, after spending weeks on it, only to find the fault years later when I was going to use the driver board for another amp.
I had gotten used to both screws on a TO-3s connecting the case, and getting lazy I would pull devices and during test only put one screw back in until it passed basic low power tests. One amp only used the other screw for electrical connection and I spent days on it.
I think right now there might be two issues with the amp, you might have fixed the output stage by removing parts but are not aware because Q4 was causing it to oscillate and masked the correction. As I said, I would start putting the output stage back together with T1 open. You might want to take a few days off.
Having G2 test those parts is a good idea also.
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Hope that your hassle will end soon and in a happy way,
as there s considerable work provided by there by yourself
and Pb2 whom i agree with when he says that there s sometimes
amps that acts as mules.
as there s considerable work provided by there by yourself
and Pb2 whom i agree with when he says that there s sometimes
amps that acts as mules.
Went for a good hard ride and cleared my head a little. Pete and Wahab correctly sensed a little frustration (thanks guys!). But I mean really, with the brain trust assembled here, and over 350 posts (I won't say how many of my hours), and no musical noises. Jeez.
Anyway, what I thought was this.
1. First I take Pete's advice, and rebuild/conclusively test the output boards, using the good channel of the input board as needed.
2. Shot-gun all the rest of the components on the bad channel input board (or very nearly all) and re-wire it all. EXCEPT, pull and test Q4 for leakage using full PS voltage, as Pete suggests.
3. IF Q4 passes, plug it all back in, hook at all up and run tests, including with a load as NP suggests.
4. If it fails those tests, send the input board to G2 for his analysis of the semis.
5. If G2 finds nothing, don't give up forever. Use it as a one channel sub amp until that channel dies too and then put it in a closet until I'm smart enough to fix it. 🙂
Hey Guys,
I don't have nearly the experience or background in amplifier theory as the other contibutors here, but I would like to make a suggestion. In many of the posts, there was discussion around caps that are used to control oscillation. Since this amp is oscillating, is it possible that one of those caps is bad? I know that electrolytic caps go bad much more often then other types but they do occasionally go bad. Also, with NP caps, shouldn't you test them under their rated load with a bench cap tester, not just with an ESR/capacitance meter? So, does it make sense to shotgun those caps first?
I don't have nearly the experience or background in amplifier theory as the other contibutors here, but I would like to make a suggestion. In many of the posts, there was discussion around caps that are used to control oscillation. Since this amp is oscillating, is it possible that one of those caps is bad? I know that electrolytic caps go bad much more often then other types but they do occasionally go bad. Also, with NP caps, shouldn't you test them under their rated load with a bench cap tester, not just with an ESR/capacitance meter? So, does it make sense to shotgun those caps first?
I've had a look at a few GFA-555s with this problem. I don't recall definitiviely finding any bad parts. One time I replaced the input devices, washed the boards and dried them with a hair dryer, and got one running. There is the slight possibility that the PCB material itself in a run or two of those amps has some abnormally low resistance. Someone at Adcom surely knows exactly what the problem is. After all the technicians around having run up against this one time or another you'd think it would be common knowledge by now.
Sjdemark, Most of the caps, including both 'lytics, were replaced to no effect. I'll replace the last couple when I shot-gun the remaining components on the input board.
NP, that's correct. It's not in a testable state currently. I'm waiting for parts to arrive so I can put it all back together. Then I will surely do tests with a load, I promise!
Andrew, what did you use to wash the boards? I've flushed the input board with contact cleaner, but it could be shinier. I haven't flushed the output boards because I don't want to wash out all the thermal grease. Like I said, I don't see any problems with the PCB, but I haven't continuity tested every trace or anything.
This amp *might* have a congenital problem with the PCB material. It has clearly had work done in the past, including what looks like a full replacement of one set of output transistors on the good channel. Needless to say, the original owner did not see fit to mention this. That said, it did give me 8 years of flawless operation.
I'm headed to the woods for a few days of R and R, so I'll be going dark until next week. Parts should arrive next week so I can reassemble the input board and get testing again. Thanks everyone! Keep the ideas coming...
I will say this, btw, my experience on this board has inspired me to learn more and to, eventually, build some amps of my own...
NP, that's correct. It's not in a testable state currently. I'm waiting for parts to arrive so I can put it all back together. Then I will surely do tests with a load, I promise!
Andrew, what did you use to wash the boards? I've flushed the input board with contact cleaner, but it could be shinier. I haven't flushed the output boards because I don't want to wash out all the thermal grease. Like I said, I don't see any problems with the PCB, but I haven't continuity tested every trace or anything.
This amp *might* have a congenital problem with the PCB material. It has clearly had work done in the past, including what looks like a full replacement of one set of output transistors on the good channel. Needless to say, the original owner did not see fit to mention this. That said, it did give me 8 years of flawless operation.
I'm headed to the woods for a few days of R and R, so I'll be going dark until next week. Parts should arrive next week so I can reassemble the input board and get testing again. Thanks everyone! Keep the ideas coming...
I will say this, btw, my experience on this board has inspired me to learn more and to, eventually, build some amps of my own...
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