hi i have a problem in the left channel in this amp i get sound but distorted.I have changed all the outputs all the resistors all the transistors in the amplifier board changed the power resistors in the power board i have done everything i can think of to get this fixed i also changed the capacitors any help surely i would be greatfull thank you
Hi changsecho,
Now you know what doesn't work as a repair tactic.
What was the amp doing before you changed all those parts?
Now for the questions.
Do you have a good DVM?
An oscilloscope?
Clean, well lit work area?
Another channel that works fine?
-Chris
Edit: a small question. Do you have the schematic or the servcie manual?
Now you know what doesn't work as a repair tactic.
What was the amp doing before you changed all those parts?
Now for the questions.
Do you have a good DVM?
An oscilloscope?
Clean, well lit work area?
Another channel that works fine?
-Chris
Edit: a small question. Do you have the schematic or the servcie manual?
hi thank you for your reply the left channel is distorted the right one is fine i have no service manual i would be greatfull if you could put me in the right direction where to look i don;t have all that equitment you mention thats why i changed all those parts i'm not a pro at this but i do love to try to fix them however i do know a little about amps thank you
Hi changsecho,
With out so much as a DVM, I can't see you completing this project. You really need this basic instrument, without which you are dead in the water.
Did you install the exact new parts, or did you use something else? If so, what did you use?
Also, appropriate capitals and some punctuation would go a long way to helping with your posts.
-Chris
With out so much as a DVM, I can't see you completing this project. You really need this basic instrument, without which you are dead in the water.
Did you install the exact new parts, or did you use something else? If so, what did you use?
Also, appropriate capitals and some punctuation would go a long way to helping with your posts.
-Chris
Hi i was in a hurry my last post yes i do have a dvm in fact 3 as far as my typeing go's lol i;m not the best.i'll try to do better.I got most of my parts at mouser i got the mj15024/25 and mostly nte transistors xicon for capacitors.Anyway my service manual the writing is so small i can;t read it.It's useless.i even changed the triac same thing distortion in the left channel.there is a carbon resistor on the ground of the speaker termal to the case it keeps toasting it maybe this will tell you something.Do you think it;s in the power supply maybe one of the filter capacitors at this point i don't have a clue. You probably think i should hang it up and take the thing in i don;t give up easy.I would like to know your views here point me in the direction where the problem is.Thank you for helping me.
Hi changsecho,
Reinstall the old semiconductors unless they are defective.
Did you buy and use fresh thermal compound and mica insulators? If not, you must at your earliest opportunity.
Once you have the correct parts installed, we can solve your problem.
-Chris
Reinstall the old semiconductors unless they are defective.
The On Semi parts are okay. The NTE parts are expensive garbage, never buy those again. They need to be used for experimenting or binned.
Did you buy and use fresh thermal compound and mica insulators? If not, you must at your earliest opportunity.
Once you have the correct parts installed, we can solve your problem.
I suspect it's in series with a capacitor, 0.01 uF ~ 0.47 uF. If this is the case, it tells us that your amplifier is oscillating. This is a stability fault. You should check that you supply decoupling caps are okay and the solder connections on your compensation caps is okay. Use a magnifying glass to read your manual. I use reading glasses.
-Chris
changsecho said:
If the resistor is from the ground to the case, then you have a large voltage between chassis ground and signal ground, check your grounding, especially screws on the chasis and large caps.
Would you be willing to scan the manual at a high res and mail it?
I happen to have one of these sitting around also in need of repair.
Pete B.
As I said, I have one here that was given to me for parts, I've not touched it in many years but just decided to have another look.
I do remember all the connectors, low quality connectors will corrode over time, they are often the largest contributor to failures in electronic systems. Too many in this design, IMO.
This amp worked well for several years, then blew outputs in one channel. Was repaired by the dealer, same channel blew within months. Might have been repaired one more time, blew.
One channel, the one that has held up, has 8 and 5 outputs from memory, the other has Toshiba replacements.
The transformer is huge!
Would make a nice chassis for a more reliable design. I would still like to see the repair manual for this amp.
Pete B.
I do remember all the connectors, low quality connectors will corrode over time, they are often the largest contributor to failures in electronic systems. Too many in this design, IMO.
This amp worked well for several years, then blew outputs in one channel. Was repaired by the dealer, same channel blew within months. Might have been repaired one more time, blew.
One channel, the one that has held up, has 8 and 5 outputs from memory, the other has Toshiba replacements.
The transformer is huge!
Would make a nice chassis for a more reliable design. I would still like to see the repair manual for this amp.
Pete B.
anatech said:
I can see the challenge in it, and if I wanted a Class H amp I might consider it, but there seem to be several reports on the web of these failing. Who knows? One channel worked fine ... Why, I don't know, could come up with lots of ideas but don't really want to invest the time. I have seen the schematics, but it's been a long time, might be interesting to take another look.
I need a nice chassis for my next project .... so this will probably be it.
I understand where you're coming from Chris, but I don't think this amp has much historical value so I don't have a problem using it as a donor.
Pete B.
hi i have been reading you peoples post.You know i think you may be right maybe i should just junk it.The service manual i got and i might add was a pain to fine took long time and the one i got isn;t worth bothering with.The printing of the one i got is so small an ant couldn't read it lol.There is hardly any information on it as well not to many have one working anyway.I can use lots of the parts maybe for something else the outputs is the same as adcom 555 maybe i should look for another project.Would anyone here have a good surgestion on an amp for a good project.I enjoy working on them.And as i bet you all know the more you learn the more ya don;t know thats me anyway.Thanks for all of your inputs.
changsecho said:
If you like the amp, and can confirm that it will hold up if properly repaired then go ahead and finish it. Odd that one channel in these amps seems to hold up.
Let me ask, does one channel work? This would indicate that the power supply is good since I believe it's shared.
You've changed everything? Have you cleaned the connectors?
Checked for shorts to heat sinks, chassis, etc? Check for corrosion at any screw terminals, especially ground since you say that resistor overheats. Check for cracks in the PC board traces. Seems you've put a lot into the amp, might as well try to finish it.
I dug out my schematics, OK what I didn't like about them, was that they're not layed out very well, hard to follow. Power supply has a triac and an SCR, a bit complicated if you ask me.
One think to keep in mind, if you rebuild is that there's really not enough heat sink area for a full 250-350W /ch since obviously the Class H design ran cooler. A thermally switched fan could be used I suppose. The GFA-555 is a nice, proven design I agree and it is simple. The main circuit board is *very* small. 555-II might be better.
Let me scan these schematics, and I'll post them, some troubleshooting hints are included.
Pete B.
Hi changsecho,
This amplifier isn't right for me, and I have more than a few amps myself already. So my personal choice would be to do something else with it. I'm pretty sure that Pete is in the same boat that I am.
There are plenty of people who would love this amp for years. You may well be one of them, not to mention that you repaired it yourself. If you were to get anything out of what I had said, I'm hoping it would be that if this amp were repaired properly, it should last many years.
You decide what is right for you. I'd rather you see if you can repair it.
-Chris
No, that's not what we were saying at all.
This amplifier isn't right for me, and I have more than a few amps myself already. So my personal choice would be to do something else with it. I'm pretty sure that Pete is in the same boat that I am.
There are plenty of people who would love this amp for years. You may well be one of them, not to mention that you repaired it yourself. If you were to get anything out of what I had said, I'm hoping it would be that if this amp were repaired properly, it should last many years.
You decide what is right for you. I'd rather you see if you can repair it.
-Chris
Hi again i forgot to add on my last post and some of you good peoples help.By the way i;m very happy i join this form the people in it are more than helpfull.Anyway like some of your input i have found an adcom 555ll it has a problem blowing the main fuse.Would anyone have an idea what it could be before i pick it up.Another project for this winter. Thank you all very much
Took another look at the A5002 schematics, it is fairly standard with additional circuitry to work from two rails, +/-62V and +/-93V.
However, it does not have a CDom cap, and it has a 4 stage emitter follower output. A bit complex. We could probably figure it out, but I'm not sure if it's worth the time.
Pete B.
However, it does not have a CDom cap, and it has a 4 stage emitter follower output. A bit complex. We could probably figure it out, but I'm not sure if it's worth the time.
Pete B.
Hi changsecho,
Hi Pete,
Thank you. A lot of that silicon appears to be concerned with commutating the voltage up and down, and perhaps some protection. I didn't look at it that long. Beyond that it doesn't seem to be that bad to figure out. Keep in mind I am use to what Bob Carver dished out too. I think the Cdom is C4 at 47pF, returning to the inverting input instead of Q5's base.
-Chris
That's a new thread for later on.
Hi Pete,
Thank you. A lot of that silicon appears to be concerned with commutating the voltage up and down, and perhaps some protection. I didn't look at it that long. Beyond that it doesn't seem to be that bad to figure out. Keep in mind I am use to what Bob Carver dished out too.
I think the Cdom is C4 at 47pF, returning to the inverting input instead of Q5's base.-Chris
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