[ACMP investiupgradifications] All things PREAMP

[Mylithra]

Are these the correct parts to fix the 60 cycle hum ?

http://export.farnell.com/magnatec/bc461/transistor-pnp-to-39/dp/1208598
http://export.farnell.com/magnatec/bc441/transistor-npn-to-39/dp/1208596

 

[BradM]

Brad,

Loosen the single retaining screw holding your power supply toroid and move it around  a inch or two towards the back of the chassis while you have the mid band engaged with the hum present and report back your results here.

you won't be able to move the inductor since its soldered to the board.

There's a user on Gearslutz that has done just this.  It's definitely an inductor shielding / proximity issue:

http://www.gearslutz.com/board/low-end-theory/187097-acmp-73-neve-73-clone-45.html#post3706676

Brad

 

[MATTI]

well, I would change the transistors before any other conclutions. -Still waiting my units

Matti

 

[BradM]

I was under the impression that the transistors were only in error on the 81's. 

Brad

 

[okgb]

It won't hurt but
i think maxwell had the experience of changing the transistors and still having the problem
until moving the power transformer [ which won't hurt either  ]

 

[maxwall]

Thats correct okgb, I'm confident the transistor change may have fixed another issue far more subtle than the EMI/RFI interference I still have with my 81's.

Brad, If you conclude its the inductors without moving PSU transformer then your just speculating.

Pot core inductors don't need shielding, even Cinemag pot core inductors for a pultec are not sold with shielding unless specified. It just matters how close you put them to EMI/RFI devices like a power transformer. Like anything.

You need to get your hands dirty to understand the problem.

Besides part of the shielding is seperately tapped inside the winding , and that would require more than just throwing a mu metal jacket over them. I tried the so called shielding trick by using a vacuum tube shield with  nickel coating and no change at all. I even grounded the shield to chassis. still no change. The location of the PSU transformer is the culprit.

 

[MATTI]

I wasn´t following glose enough, it seems. It´s kind of good you sort this out before I
get mine...

Thanks

Matti

 

[audioforge]

since i received my acpm81 (thanks again Franck).i did a quick test.
here my report
First i ve changed the 8 filters boards transistors with 2n5321 and 2n5323 as i suggested to maxwall for a close matched replacement for the bc 441 bc461.
it works great.
troubles i ve had
first a dead line mic switch. replaced with a toggle one.
btw thoses china "alps copy" switches are real poor quality. if a led fails and you ll have to open the switch  good luck to re mount it and keep it working.
i doubt they will do long use.
So it will be a good idea if "chance "can stock an amount of thoses switches cos illuminated switches like thoses could be hard to find.
btw there s no traces for the led on the line mic switch .it could have been implanted with some mods.
I have the pop (well it s not a pop but an uggly shortcut) between 6 and 7 position.
as stated before it s the 73/84 mic off position which have been ommited. bad design.
no way to sort out easily without mod the board and loose one position..
switch to line before change 6 to 7 position is the simplest thing to do.
the 81 preamp is not a 81 preamp but a 73.(see ba284 sections)..
the 39 ohm resistor (line switch) was dead too. quite tricky to find as it was the lead which was broken at the resistor body junction.... replaced it and everything was ok..
regarding the HUM thing
well i didn t notice it on the oscilloscope
1) i m a lucky guy
2) i run the unit within 220V 50hz (all the hum troubles reports seems to come from people with 115v 60hz)
definitively the psu traffo seems a bit short to feed the unit..
somes mods to improve the psu board can be done (adding pot to fine tune the 24v and 48v adding some diodes for the 24v regulators etc...) but it work as is....
3) i did not do the "good" eq combinations setting to have the hum appears on the oscillo...
please elaborate
i did nt test the 81 in real so i can t comment about the audio
when my 1084 (with carnhill tx and inductors) will be finished (almost done) i will compare them with an sca1272 as reference...( maybe a bit too much gearslutz? no??)
audioforge

 

[okgb]

Thanks for the update , good to see you got your's
i think clint would be curious what you paid in duties & shipping .

More & more this would seem to be the chinese
selling us a design [ rejected ] that they already had
but i still think it is easier to mod than build one from scratch

 

[CHANCE]

For some strange reason I stopped getting new post notification E-mails. I received a strange box from China with 10 ext PSU's and 210 pots. I got no E-mail from them but am waiting for a reply to the E-mail I just sent them

 

[okgb]

That's funny , i was thinking serious power supplies
likely not expect any help from them [ they got the money ]

 

[CHANCE]

I just received a reply. "It was a mistake, please return"

 

[Mylithra]

i will compare them with an sca1272 as reference...( maybe a bit too much gearslutz? no??)
audioforge

This is what I have been comparing them with as well. While I think the SCA n72 has a bit more depth, the 73 does hold its own. Its bit hissier than that n72 but thats not too hard to control. I like the way the EQ sounds, If only I could get the buzz out of it. I tried making a shield around the mains transformer but that didnt make but a really small difference.

Anyone know what the TL072s are for? They're not listed on the only schematic I have for this.

 

[clintrubber]

This is what I have been comparing them with as well. While I think the SCA n72 has a bit more depth, the 73 does hold its own. Its bit hissier than that n72 but thats not too hard to control. I like the way the EQ sounds, If only I could get the buzz out of it. I tried making a shield around the mains transformer but that didnt make but a really small difference.

That's interesting to hear, cool that people have access to the 'other clones' as well for comparison.

Anyone know what the TL072s are for? They're not listed on the only schematic I have for this.

That's FAQ#7  , but here goes: they're not in the signalpath but used for the 'signaling-LEDs'. No need to upgrade these chips (some people might be inclined to).

Bye,

  Peter

 

[CHANCE]

i will compare them with an sca1272 as reference...( maybe a bit too much gearslutz? no??)
audioforge

This is what I have been comparing them with as well. While I think the SCA n72 has a bit more depth, the 73 does hold its own. Its bit hissier than that n72 but thats not too hard to control. I like the way the EQ sounds, If only I could get the buzz out of it. I tried making a shield around the mains transformer but that didnt make but a really small difference.

Anyone know what the TL072s are for? They're not listed on the only schematic I have for this.

Are you referring to the 73, 81, or 84? I have detailed schems for all three. I thought I already posted the 73, maybe not here.

 

[Mylithra]

The 73, The only one I have is a 1 page PNG file but it only has the signal path. There no power section or anything outside of the signal path.

http://goose-tracks.com/thegeekgoose/images/scem73wv9fi2.png

 

[okgb]

Could repost all three [ whoever has them  ]
some on the other forums are reporting increased
noise when engaging the eq [ on the 84 i believe ]
you get what you pay for  [ not a bad deal in this case ,
well haven't got it yet ] but was expecting to upgrade components
and still be ahead of the game .
the guys building real 73's & 81's were too serious for my brain

on another note , perhaps some lpf'iltering on the torriods may help
[ easier if you are moving these to an external box ] as the torriods
pass allot more Hi freq garbage than the traditional power transformers

 

[0dbfs]

Also, the suffix 6 or BC 441-6, 461-6 is not required to make these work. You'll have to buy a load of transistors and measure the hfe of each one to get selected parts. Newark sells the BC 441,461 but you won't find suffixes
available when you make the purchase. I suspect suffix 6 parts are really pricey. Kind of like selected vacuum tubes.

The non suffix 6 BC parts wont need a heatsink , they don't get hot in my ACMP 81.

Hi maxwall,

I've got a handful of each of the non-suffix-6-BC4x1's.

If you are using the non suffix 6 BC parts what type of hfe measurement would be ideal?

The BC4x1-6's are rated for hfe of 115-250
The BC4x1's are rated for hfe of 40-250
The supplied BC557 hfe of 125-900 and BC547B hfe of 200 -450

Thanks!
-jonathan

 

[maxwall]

Also, the suffix 6 or BC 441-6, 461-6 is not required to make these work. You'll have to buy a load of transistors and measure the hfe of each one to get selected parts. Newark sells the BC 441,461 but you won't find suffixes
available when you make the purchase. I suspect suffix 6 parts are really pricey. Kind of like selected vacuum tubes.

The non suffix 6 BC parts wont need a heatsink , they don't get hot in my ACMP 81.

Hi maxwall,

I've got a handful of each of the non-suffix-6-BC4x1's.

If you are using the non suffix 6 BC parts what type of hfe measurement would be ideal?

The BC4x1-6's are rated for hfe of 115-250
The BC4x1's are rated for hfe of 40-250
The supplied BC557 hfe of 125-900 and BC547B hfe of 200 -450

Thanks!
-jonathan

Jonathan

mine were hfe 90 , and they sound fine, no issues.



Chance

Whats the gameplan for fixing the gainswitch problem. I have read thru various forums and all I see is
comments but no solutions. Are the 81 owners going to have to live with this problem with no solutions ?
Tried emailing you directly thru the musicainsworkshop email address but 'zero' response. a simple reponse is appreciated either way.

 

[mitsos]

Hi maxwall,

is the switch the only remaining problem for you? Did you fix your EMI/RFI issue? How? I hope you didn't have to move the power trafo outside the box??

As far as the switch goes, it's strange that it would only cause problems in one spot. I mean if it were non-shorting it would pop all over the place right? Is it possible the switch could be wrong in another way? Comparing Chance's 1073 schematic to the original 1073 schematic the switch part seems pretty close to the original.. At least the 6 and 7 positions look the same. I hope the schemo is correct, that is, I hope that it reflects what is, in fact, in the box, and there aren't any surprises by our Chinese friends.
One thing I did notice comparing it to the 1073 schemo on Dan A's site is that the 2x 10K resistors (R31 and R32) are not in the switch section. Could this make a difference?

If there is something mechanically wrong with the switch, hopefully Chance can convince the manufacturer to send out a batch of new ones, although we probably have very little leverage at this point.