W
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guitar zero
c = b#
Alas, I've run out of patience. Sold my 81s. But I still have my 310s and 6802. Looking forward to mod possibilities on those. Thanks to dgatwood and others for slogging through the circuit boards and sharing their findings.
crazydoc
Master Baiter
I've got Mumetal shields around the toroid and both inductors now. Hum from board 4 is inaudible, but I can still hear some at 120 Hz with board 5's switch engaged and output gain cranked all the way up. However, it is well below the preamp circuit noise at the last 3 positions of the mic input gain switch, and is certainly inaudible at recording levels with a condenser mic connected.
So now I've installed sockets for the mic preamp transistors 1Q1 and 1Q2 and am awaiting arrival (not until Monday, unfortunately), of 100 each of 3 types of low noise transistors, (among them more BC184C's), so I can go through them and cherry pick the lowest noise ones for the circuit to see if that improves the high gain hash.
So to summarize, on this ACMP 81, my worst case of 3 units, I've swapped the output transistors on boards 4 and 5, rotated the PS toroidal transformer to a position of least hum, and shielded it and the inductors. It is now usable, but there is too much circuit noise at high gain (though I may never need more than 62 dB, up to which point it doesn't seem too bothersome), but I going to try to improve it.
So now I've installed sockets for the mic preamp transistors 1Q1 and 1Q2 and am awaiting arrival (not until Monday, unfortunately), of 100 each of 3 types of low noise transistors, (among them more BC184C's), so I can go through them and cherry pick the lowest noise ones for the circuit to see if that improves the high gain hash.
So to summarize, on this ACMP 81, my worst case of 3 units, I've swapped the output transistors on boards 4 and 5, rotated the PS toroidal transformer to a position of least hum, and shielded it and the inductors. It is now usable, but there is too much circuit noise at high gain (though I may never need more than 62 dB, up to which point it doesn't seem too bothersome), but I going to try to improve it.
dgatwood
is out. Leave a message.
Crazydoc, if you've done the transistor swap and shielded the inductors and you still have hum, I suspect your remaining hum is caused by poor grounding, particularly in boards 3, 4, and 5.
On each board, there's a filter cap between the 24V rail (labeled 24v+)and the 24V ground (labeled 24vdi). Solder a heavy gauge wire to the - side of that cap (being careful not to short it to the other side) and put a crimp-on wire spade on the other end. Do this for each board. Unscrew one screw for each board and shove the wire spade under it and screw it back down. Odds are, adding a ground wire to boards 3 and 4 would be sufficient by itself, but it won't hurt to do this to all of the boards.
For additional ground improvement, if needed, you can wire up a series of short wires with crimp-on wire spades on both ends to go between each board. Finally, add a longer one to the back right screw on the power supply. That said, I doubt that last part is really necessary.
It's an easy hack, so it is certainly worth trying to see if it finishes off your hum.
On each board, there's a filter cap between the 24V rail (labeled 24v+)and the 24V ground (labeled 24vdi). Solder a heavy gauge wire to the - side of that cap (being careful not to short it to the other side) and put a crimp-on wire spade on the other end. Do this for each board. Unscrew one screw for each board and shove the wire spade under it and screw it back down. Odds are, adding a ground wire to boards 3 and 4 would be sufficient by itself, but it won't hurt to do this to all of the boards.
For additional ground improvement, if needed, you can wire up a series of short wires with crimp-on wire spades on both ends to go between each board. Finally, add a longer one to the back right screw on the power supply. That said, I doubt that last part is really necessary.
It's an easy hack, so it is certainly worth trying to see if it finishes off your hum.
dgatwood
is out. Leave a message.
Ugh. Make that "the hum is greatly diminished" or "the ground potential is greatly diminished". Must have been typing late at night.
antichef
pornk rock
Thanks for the clarification -- I actually knew what you meantUgh. Make that "the hum is greatly diminished" or "the ground potential is greatly diminished". Must have been typing late at night.
-- was the mixture of 1000 and 2,200 values because that's what you had on hand? Would all 1000s have worked?
I
iownrocknroll
New member
Super Excited! my 81's are on their way. Should be here sometime next week.
What is everyones thoughts compared to their originals????
What is everyones thoughts compared to their originals????
crazydoc
Master Baiter
Crazydoc, if you've done the transistor swap and shielded the inductors and you still have hum, I suspect your remaining hum is caused by poor grounding, particularly in boards 3, 4, and 5.
On each board, there's a filter cap between the 24V rail (labeled 24v+)and the 24V ground (labeled 24vdi). Solder a heavy gauge wire to the - side of that cap (being careful not to short it to the other side) and put a crimp-on wire spade on the other end. Do this for each board. Unscrew one screw for each board and shove the wire spade under it and screw it back down. Odds are, adding a ground wire to boards 3 and 4 would be sufficient by itself, but it won't hurt to do this to all of the boards.
For additional ground improvement, if needed, you can wire up a series of short wires with crimp-on wire spades on both ends to go between each board. Finally, add a longer one to the back right screw on the power supply. That said, I doubt that last part is really necessary.
It's an easy hack, so it is certainly worth trying to see if it finishes off your hum.
Thanks - I'll give that a try this weekend while I'm waiting for the transistors, though I still suspect it's from EMI that I can't shield from the toroid.
antichef
pornk rock
[subscribe...]
Please do provide some detailed reporting on this -- I've been eyeing that preamp board quite a bit, but have no idea what to do. Truth be told, I should probably stop messing with my units, because they're fine now, but if I could reduce hiss (the only noise left, and only noticeable when gain is way up) even further, that would be pretty cool.
dgatwood
is out. Leave a message.
Thanks for the clarification -- I actually knew what you meant
I looked at what was used in the 73 and figured that there was probably a reason they used the 2,200uF part. I figure it is one of two reasons. Either they used the 2,200uF part on the DI board because it has to handle signals at a lower level and thus needs to be cleaner or they used it because the 24V rail was getting too noisy by the time it made it that far over and they needed to do a little more damage. Since I had no way to determine which, I decided to throw in a 2,200uF part periodically. Chances are, 1,000uF is fine.
I
ittam
New member
There is some really interesting reading at prodigys
Matti
Matti
dgatwood
is out. Leave a message.
Moving to lower noise transistors is probably a good idea. That said, there are a lot of other things you might also consider trying that could improve the high frequency noise.
The first thing I'd suggest doing (because it is easy) is replacing the DC blocking caps (e.g. 5C7, 5C5) between the boards with something a little less non-linear than the electrolytic caps they used---poly film or silver mica, ideally. That alone will probably clean up the high end somewhat.
I'm guessing the resistors are all carbon-based, so moving to better resistors (e.g. wire-wound) would also help with noise.
The first thing I'd suggest doing (because it is easy) is replacing the DC blocking caps (e.g. 5C7, 5C5) between the boards with something a little less non-linear than the electrolytic caps they used---poly film or silver mica, ideally. That alone will probably clean up the high end somewhat.
I'm guessing the resistors are all carbon-based, so moving to better resistors (e.g. wire-wound) would also help with noise.
W
wreckd504
New member
Chance, can I have my 2 pre's sent to another member? Sent a few emails cc'ing drbill in
dgatwood
is out. Leave a message.
New ACMP-81 mod pics at https://homerecording.com/bbs/showthread.php?t=275506. Basically, by improving grounds alone, I've almost completely eliminated the hum. There are still lots of other lovely noise issues, but the hum is almost completely eliminated, and I didn't have to swap a single transistor.
Hopefully, these mods will prove useful to the people who did the board layout on these things in convincing them to improve the relatively poor grounding on the ACMP-81 boards. Assuming it behaves similarly with the existing small caps, this would be basically a zero-cost-per-unit fix, design time notwithstanding.
Hopefully, these mods will prove useful to the people who did the board layout on these things in convincing them to improve the relatively poor grounding on the ACMP-81 boards. Assuming it behaves similarly with the existing small caps, this would be basically a zero-cost-per-unit fix, design time notwithstanding.
crazydoc
Master Baiter
Nice pics, dgatwood - thanks for those. So far I've grounded boards 3,4,and 5, with no audible difference. However, ground potentials are within 1mV of chassis ground on those boards now (were 5 to 6mV prior.) My greatest problem is still the 120 Hz hum from the inductor on board 5. The inductor's pinouts are different from the schematic (all rotated one pin around), but seem to be connected appropriately to the board. Shielding has helped quite a bit - it resolved the hum on board 4 completely, but board 5 is being pretty pesky.
I haven't changed the cap values as you have - maybe that will help. Well, time to eat some lunch and then back to work. (I'm sure I'm having more fun trying to fix these than I ever will using them.)
I haven't changed the cap values as you have - maybe that will help. Well, time to eat some lunch and then back to work. (I'm sure I'm having more fun trying to fix these than I ever will using them.
)
yrdbrd
New member
(I'm sure I'm having more fun trying to fix these than I ever will using them.
many thx to you guys doing all this great detective work!!!
thou i will say i'm havin a blast with my ACMP73s
that said... two of my 73s dont have the dreaded POP on the gain switch
any ideas why that would be????
wish i had the electronics know how that you guys have...
crazydoc
Master Baiter
I posted some stuff over here https://homerecording.com/bbs/showthread.php?t=275506 which is where I guess we're posting preamp fixes. I think I've got a working ACMP 81 now (when I get everything cleaned up and screwed down, and a few more experiments.)
I think it's important to realize that the problems in these units are quite variable, and what fixes one may not fix another. Hopefully after awhile, it will boil down to a cookbook of possible fixes, with an some sort of simple algorithm to help go through it.
I think it's important to realize that the problems in these units are quite variable, and what fixes one may not fix another. Hopefully after awhile, it will boil down to a cookbook of possible fixes, with an some sort of simple algorithm to help go through it.
dgatwood
is out. Leave a message.
Could somebody with a better electronics background than I have take a look at my solution for the oscillation problem in one of my boards and tell me if that seems sane? Posted in the ACMP-81 thread in the DIY forum.
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nuitronic
New member
Chance - Are you still accepting virtual check payments? I tried to pay my shipping invoice on Thursday, 01/29/09, and after I submitted my virtual check info I got an error screen. But I also received a payment confirmation email immediately saying that my order will be processed, along with an order number.
My shipping invoice still shows "not paid" and "not shipped" in the user control panel and no money has been taken out of my bank account yet. Do I need to pay you again for the shipping?
My shipping invoice still shows "not paid" and "not shipped" in the user control panel and no money has been taken out of my bank account yet. Do I need to pay you again for the shipping?
Creamyapples1
www.murphycabs.com
I had the same problem. The Virtual Check System is kinda buggy seems like but it works even though no confirmation screen loads. Wait for Chance to respond (maybe even send him a PM) to confirm.
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