acmp73/81 needs fixing, trying myself, any help?

[jasonbmerrill]

i have an acmp81 and an acmp73.

if i dont have to pay a DIME (that means, shipping both ways, parts, etc)...

i will GIVE you my acmp81 (which you also may need to fix - the eq seems to hum, like they all do), if you FIX my acmp73 (has major hum issues even without eq engaged).

but only if i can trust you obviously. I really only need one, for sure. Depending on who you are, i would need some decent thumbs up from the good people here or otherwise before i even send it.

I don't even know if this is a good deal, or worth the work, im just putting my feelers out there!!!!

I am in northeast PA.

willing to do the trade when school starts up, in September. Until then im dealing with using the acmp81, which works great! And is only powered on when i need it.

contact me @ jasonbrianmerrill@gmail.com

 

[antichef]

that's an interesting offer (which I can't attempt to take you up on due to time considerations) -- with regard to the 73, I'd be specific about how to define success, e.g.,

1) how much hum reduction is enough?
2) any other issues (the pop?) to be addressed?
3) generally how far down the path of the various proposed remedial programs does it need to go?
4) what happens if the attempt fails?

(have you tried just twisting the toroidal transformer around some?)

 

[jasonbmerrill]

if someone could give me or point me to a tutorial on some basic ways to fix this, i would try to do it myself...

however, i wouldnt assume anyone would want to spend their time spelling it out for me, i would feel bad.

1. I would just like it at a reasonable level - i dont know exactly what to put it at - at least as quiet as my dmp3 - this unit seems to make no noise.

2. i dont care about the eq at ALL, tho that would be nice. The pop, isnt that big of a deal, unless there is proof it can lead to more problems.

3. i am not sure what you mean by this...

4. If it fails... that is rough. I dont know what to say about this. I would pay to ship my 81 back? I am not sure how to deal with this situation, or what is reasonable.

twisting the transformer?

 

[dgatwood]

It will always be noisier than the DMP3, at least if you are talking about hiss. It's a discrete transistor-based preamp and the DMP3 uses a finely tuned single-IC solution. But hum... it shouldn't do that. Probably a defective transistor somewhere. Apparently, they used really dodgy parts in these things.

I've gotta admit I'm curious why a few of the 73s hum. Both of mine were fine out of the box. Would you consider shipping just the boards from the unit for diagnosis? The metal case and the power supply add about $20 to the cost of shipping these things.

If you want to diagnose it yourself, the first thing I'd do is put together a jumper cable. Get yourself three of those three-pin fan Y cables for inside a computer (or one three and one four-pin if you can find a four-pin---you need 7 pins). Make sure each of the cables has two male headers. Use these to skip boards. Unplug the jumper cards on either side of the board of your choice and then plug the cables between the boards on either side. Makes sure wire should goes straight across to the same pin three connectors over.

Once you have isolated it to a single board or a pair of boards, start swapping out transistors with replacements, starting with the ones that various folks on these boards have identified as problematic.

 

[jasonbmerrill]

yeah, im only concerned about the hum, hiss is fine

I could probably just ship the boards, sure...

maybe ill try and do some stuff myself ?

 

[antichef]

there's a lot of discussion here and especially on prodigypro that spells out the steps to do the simple things pretty well, as well as a lot of diagnosis, planning, and speculation about how to make bigger changes.

There are multiple potential sources of hum, but on the 73 the main source (and frequently the only source) has been stray EM from the donut shaped power transformer hitting other components in the system (normally, this toroidal design of transformer is very nice to have, but some diagnostic surgery over at prodigy pro revealed that the innards of these ones are "suboptimal") -- an easy and free way to try and fix this is to

1) unplug the unit
2) take off the top and identify the donut shaped thing with the power cables going into it (back right, can't miss it)
3) loosen the bolt through its center slightly and simply twist it around some in either direction - there's enough slack in the wires that attach to it to allow for this
4) tighten the bolt, replace the cover, plug in, turn on, and see if it's better
5) repeat until you find the best orientation

if you're able to take sufficient safety precautions, like rubber gloves, proper insulation for yourself, etc., and you're sure you know what your doing, you can adjust the orientation with the power on and the device hooked up to something that lets you hear the hum (something cheap - not your prized A/D converter, in case you massively screw up and spill pepsi on everything) - obviously that's much faster because you can use your ears to just twist to the best spot, but I can't recommend anyone do that, because it's potentially dangerous for you and the equipment. I don't want to overstate the danger, but I definitely don't want to understate it.

If the power transformer orientation is a (or the) source of hum, there's no way to know what the best twist position is other than to try and see -- this is because of the internal flaws I mentioned that make for an uneven and unpredictable (and wholly undesirable) pattern of EM emission.

 

[jasonbmerrill]

interesting, ill try some things myself first and see what happens, thanks for the info.

if anyone has any links offhand to these posts where they talk about fixing the units, that would be cool too. I figure if i cant sort it out myself by the school year, ill have someone else do it

 

[jasonbmerrill]

ok, since i have some money again, im finally getting serious about this again.

here is the DI portion of my acmp73:

http://www.jasonbrianmerrill.com/shupFiles/Di-clean-left.flac

its a bit loud, so it might be distorting, but the DI seems clean..

here are some other clips from that:

http://www.jasonbrianmerrill.com/shupFiles/Di-amped-Stereo.flac

http://www.jasonbrianmerrill.com/shupFiles/Di-clean-Stereo.flac

here is the noise in question, from the max of 80db (output also maxed) one notch down each:

http://www.jasonbrianmerrill.com/shupFiles/80db.flac

http://www.jasonbrianmerrill.com/shupFiles/80db-1.flac

http://www.jasonbrianmerrill.com/shupFiles/80db-2.flac

http://www.jasonbrianmerrill.com/shupFiles/80db-3.flac

http://www.jasonbrianmerrill.com/shupFiles/80db-4.flac

and here are my experiments with the transformer:

http://www.jasonbrianmerrill.com/shupFiles/right.flac

http://www.jasonbrianmerrill.com/shupFiles/right_1.flac

http://www.jasonbrianmerrill.com/shupFiles/left.flac

http://www.jasonbrianmerrill.com/shupFiles/left_1.flac

It seemed to have no effect

anyway, im looking around to get both fixed now, for as cheaply as possible, without even touching the eq. I dont care about the eq, i just want the preamp/DI on both to work.

 

[jasonbmerrill]

this might be a thread for a different forum now

if someone could change the title to:

"acmp73/81 needs fixing, trying myself, any help?"



and move to the app. forum i would appreciate it.



anyway, here is the acmp73 frequency noise

 

[dgatwood]

The changes that made the biggest difference for me were the transistor swaps and moving the power cord to the other end of the stack. The boards all get power from a cable that runs from the supply board down to the right end of the stack.

The most obvious architectural difference between the pres that hum and the ones that don't is that the pres that don't hum don't have a full-depth board in the rightmost slot. Because of that, their power cable plugs in much farther towards the front of the unit, and the DC cable doesn't run right past that noisy transformer.

So to make things more consistent, I drilled holes in the second board from the left and put in a right angle connector there, then moved the cable down to the opposite end of the unit so that the power flowed towards that rightmost board instead. I also added larger filter capacitors on many of the boards.

 

[thajeremy]

The changes that made the biggest difference for me were the transistor swaps and moving the power cord to the other end of the stack. The boards all get power from a cable that runs from the supply board down to the right end of the stack.

The most obvious architectural difference between the pres that hum and the ones that don't is that the pres that don't hum don't have a full-depth board in the rightmost slot. Because of that, their power cable plugs in much farther towards the front of the unit, and the DC cable doesn't run right past that noisy transformer.

So to make things more consistent, I drilled holes in the second board from the left and put in a right angle connector there, then moved the cable down to the opposite end of the unit so that the power flowed towards that rightmost board instead. I also added larger filter capacitors on many of the boards.

ALRIGHT!!!!!! YEA!!!!

um....some of us will be needing pix...

I still have 3 73 and 2 81's that havent even been turned on....

 

[jasonbmerrill]

The changes that made the biggest difference for me were the transistor swaps and moving the power cord to the other end of the stack. The boards all get power from a cable that runs from the supply board down to the right end of the stack.

The most obvious architectural difference between the pres that hum and the ones that don't is that the pres that don't hum don't have a full-depth board in the rightmost slot. Because of that, their power cable plugs in much farther towards the front of the unit, and the DC cable doesn't run right past that noisy transformer.

So to make things more consistent, I drilled holes in the second board from the left and put in a right angle connector there, then moved the cable down to the opposite end of the unit so that the power flowed towards that rightmost board instead. I also added larger filter capacitors on many of the boards.

i wish i knew enough to do this. Could i just give these instructions to someone to do for me as a possible solution? I really need to find someone to do this. I have the money now...

 

[thajeremy]

yea...I know there are several threads about fixing these...Is there somewhere that I could get a list of things that need to be fixed and take these to a local electronics repair shop to take care of it for me??? I know of several places in the area that will take care of replacing the parts as long as I can tell them what to replace...

 

[jasonbmerrill]

I also know a guy who might work on it, but he doesnt want a crapshoot... but if i had some directions that at least had a chance of working, i could just tell him to do that...

 

[Chili]

Hi Jason, I changed the title as you requested and moved the thread to the DIY, Mod, Homebrew forum. In the future, you can PM a mod with your requests and it'll get done quicker.

I still have 3 73 and 2 81's that havent even been turned on....

I'd be interested in taking one of those 73's off your hands if the price is right.

 

[dgatwood]

The problem is that it's a crapshoot. There are several flaws in these devices, and there's no way to know which one or two or three of those flaws are manifest in your particular unit. The flaws are, in no particular order:

1. Oscillation caused by poorly chosen and/or outright defective transistors. There are certain pairs that are more problematic, and there's a thread about that over at GS, IIRC.



2. Hum in the DC lines.

a. The power cable from the power supply board (center top) to the rightmost board (six pins, IIRC) has to run right past the AC transformer. This makes the power very dirty if the transformer is highly emissive (poorly made core). Thus, it creates substantial noise in the first board that then gets amplified further by both being part of the audio signal and part of the DC rails, creating something that looks very similar to parasitic oscillation but really isn't.

My fix for that was to buy an 8-pin right angle Molex connector, cut off two pins to get a 6-pin connector, and drill holes through the power traces on one of the leftmost jumper boards or circuit boards.



b. The boards should have larger decoupling capacitors. These are supposed to prevent variations in the voltage caused by the amplification process itself from propagating from board to board. Those are the big cans that go between two of the horizontal traces near the top.



3. Hum induced into the toroidal inductors. This is usually the last little bit of hum. Eliminating this hum is important, but it's not going to help until you get rid of oscillation and other similar problems. I shielded them somewhat by adding Metglas shielding film covered by self-stick copper foil, which I then grounded.



4. Because the boards are not grounded individually (there are no metal ground pads against the screws), there's significant resistance between the boards' grounds and the main voltage ground. You can improve this by tying the ground sides of the decoupling capacitors (the ones you beefed up for #2) to the case screws.



5. The boards do not have anything approaching ground planes at the board level, with the exception of the leftmost board. This is one of the reasons the ground resistance is as high as it is. There's not much you can do about this problem, though, except to try to work around it in your fixes.

6. The power supply section is completely unshielded from the rest of the unit. This is not a particularly critical problem, but I did a half assed job of shielding it anyway with some aluminum I had lying around. Not a great shield, but better than nothing, I suppose.



7. There's a resistor in the main amplifier stage that runs way beyond its wattage rating and burns up if you put the volume knob in a certain halfway position between two actual positions. To prevent this meltdown, there are three possible fixes:

a. You can modify the switch to break the contacts completely before it makes the new one.
b. You can remove one particular resistor in the leftmost front panel board to disable one position on the volume knob entirely.
c. You can replace the resistor in the main amplifier stage with a wire-wound resistor so that it won't burn out. I chose this one.



I'm probably forgetting some others....

 

[jasonbmerrill]

Hi Jason, I changed the title as you requested and moved the thread to the DIY, Mod, Homebrew forum. In the future, you can PM a mod with your requests and it'll get done quicker.

thanks! And next time ill remember this I just felt like it would be a bother... hehe

 

[jasonbmerrill]

i cant find anyone locally to do this - im gonna check in scranton again tomorrow but is there anyone here with credentials/experience that could do this at a cheaply reasonable rate?

 

[timthetortoise]

Would love to help, as I'm always working on stuff like this, but it looks like this would be quite a bit of work to track down with no guarantee of actually finding the issue :| Good luck in your search!

 

[jasonbmerrill]

Would love to help, as I'm always working on stuff like this, but it looks like this would be quite a bit of work to track down with no guarantee of actually finding the issue :| Good luck in your search!

well, i would only want someone to be able to follow the instructions above really... try out that stuff