[ACMP investiupgradifications] All things PREAMP

[hejsan]

I think that a 'repair' kit is required, consisting of :

REPAIR KIT
=========

- 4x BC441 transistors
- 4x BC461 transistors
- 2x inductor shields  (or replacement mid-lo & mid-hi inductors)

would just about get this to a reasonable state with about 3hrs work, consisting of :

i)  open up the unit (top, back, side screws and remove front panel selector knobs/nuts)
ii)  for each of 4 EQ boards
    iii) remove boards (screws, unplug connectors, unplug bus)
    iv) de-solder Q4/Q5 and resolder correct parts
    v)  install and afix shield OR desolder and replace inductor
    vi) replace board (screws, connectors,replug bus)
iii) modify resistor(s) in the gain selector switch to remove pop
iv) close up unit (ensure no push-switch binding)
v)  test

Don't forget that the kit should also have a switch assembly that works. I don't know if it can be retrofitted, or if the whole preamp pcb would have to be replaced.. maybe a switch with resistors soldered on it's tags could have leads running to the pcb.

 

[recordinghacks]

Sorry for the potentially obtuse question... I've been following this thread closely with the understanding that most of the mods discussed here address the ACMP-81: the bad transistors (zmix's fix), the poor grounding, the poor shielding.

The pop in the 7th position of the gain switch is common to all 3 preamps, as is the need to rotate the power supply to minimize the hum.

Do the other ACMP-81 fixes also address the ACMP-84?

Maxwall, is it known whether the Cinemag pieces you're pursuing for the -81 will also fit the -84?

Are there other shortcomings of the ACMP-84 that have not been investigated?

--
matt.

 

[Mylithra]

The preamp pop fix is supposedly the same for all units though the actual part label may be different from model to model.
The rotate transformer fix is for all 3

I dont believe either the 73 or the 84 have the transistor problem. (the 73 doesnt even have the transistors inside that were specified. I only have the 73 so dont take this as gospel.)

The 50/60 cycle hum in all units looks mainly to be caused by a combination of the crappy field the power transformer is putting out and the crappy inductors on the EQ board.
For me this was verified when I moved my transfomer from the one that buzzed to the one that didnt and the buzz went the transformer.
Countless others here and at another board I frequent have verifed the inductor problems. For me, it was when I had one unit on top of the other on my lap (cases all buttoned up), had them both hooked up and was flopping back and forth between their mute button on the console. One had a slight buzz to it till I turned the other on. The other buzzed regardless of which unit was on. Yes the buzz was a little more intensive on the one that did it when I powered on the other, but the change was insignificant. I then slid the units around on one another and the loudest buzz I ended up getting was when the transformer from one unit was right over the mid high inductor on the other. I did have the transformer over the mid low inductor but suprisingly , it did make it buzz but no where near what the mid-high inductor was doing.

For me,  the fixes will end up being.
1. Replace power trafo (or Power supply) on the unit that buzzes. ( probably end up doing both)
2. Replace inductors with properly shielded inductors.

I still havent decided which inductor to get yet. Check this out:
http://www.sowter.co.uk/eqinductors.php
Would the following entry be a part compatable replacement for the mid high inductor?
9810 Neve T1295 10/7/3/1.3 H 1000/800/490/310 ohms

If so, I might just order one and see what the differences are. Once Cinemag has a replacement, I might do that. Ive been watching the progress on that here and if a group order would be placed, Id be in for it.

 

[alexc]

The DIY 1081  inductors :

Mid-Hi : Carnhill Part No.  vtb9048 with inductance taps at 205mH, 296mH, 504mH, 767mH
Mid-Lo : Carnhill Part No. vtb9047 with inductance taps at 1.43H, 2.06H, 3.56H, 5.11H

Looking at the Sowter parts, there doesn't seem to be an off the shelf part to suit.

They have a custom service and would wind them specially, I guess.

 

[Mylithra]

Rats... I dont have the equipment needed to measure inductors either and unfortunately, the values arent listed in the schematic. Any ideas on what the values are to get someone to make them. Chance, if you're still checking in here, any way of getting the manufacturer to give us this information?

 

[rodabod]

Maybe some proper Mumetal shields might work with the original inductors.

 

[antichef]

Further investigations I'm doing are :
...
2. check 2N3055 bias
...

swap out the 2n3055 and the mic input and i bet the things sing.

I'm at the point where I need to stop messing with my units ("if it ain't broke, don't fix it" -- decidedly not the mantra on this board, I know ), but since there was some early attention to the 2N3055 in the 797 photo way back when, I've been wondering about it -- any opinions on what's there in what we got?

(and btw - I've got the 81s and the 73s, but not the 84s - I can confirm that the "transistor swap" with BC441[-6] and BC461[-6] doesn't apply to the 73s because transistors you want to replace aren't there to begin with -- all I've done to my 73s is shield/rotate the toroid and fix the gain switch pop)

 

[clintrubber]

Rats... I dont have the equipment needed to measure inductors either and unfortunately, the values arent listed in the schematic. Any ideas on what the values are to get someone to make them. Chance, if you're still checking in here, any way of getting the manufacturer to give us this information?

If it's not known (perhaps some original N***-documentation ?) it could be calculated from the capacitance & intended frequency
(  freq = 1/(6.28*sqrt(L*C))  ), but the circuit-topology might be a clouding factor here.

Bye,

  Peter

 

[rodabod]

Values are listed in the '81 schematic. Otherwise, take them from the originals.

 

[maxwall]

Cinemag quote $25 per 500 piece order.

Mu cans available. Numbers must hit target and order must be prepaid but are refundable
if target is not met.

Cinemag feels the China inductors are inferior with certainty.

81 and 84 inductors will be different.

Modeling the replacement inductors after the China ones may not be the correct path.
I have a good LCR analyzer so I can measure the 81's

 

[clintrubber]

I'd love to have those inferior crappy inductors here, Here, HERE ! 

But no green yet...


Sorry..... 

 

[recordinghacks]

81 and 84 inductors will be different.

Is there any benefit to setting up a single group buy for both models? (or possible 3 models, if the ACMP-73 uses a third variety?)

 

[Mylithra]

If the Mid Hi board in the 84 has the following frequencies:

.36
.7
1.6
3.2
4.8
7.2

Then the inductor for the 84 and the 73 will be the same. I believe it is. Is there someone here who has both a 73 and an 84 who can verify? If not, Ill take a hi res photo of my Mid Hi board for someone to compare with an 84.

Of the 2 '73s I have, one buzzes much worse than the other and I had been focusing on the worse of the 2. Today, I looked at the one that doesnt buzz so badly. I ended up making a small shield out of some cardboard and several layers of copper tape for the mid hi inductor. That got rid of almost all but the faintest of buzzes and that was with the output gain maxxed. With the gain at normal levels, I couldnt hear a buzz anymore.

On the one that buzzes badly, the shield made no difference. Im pretty certain that the cause of the buzz on this one is the power transformer but I wont be 100% sure till I replace it.

 

[okgb]

Just for reference ,
there are references to Alan Hyatt saying they had inductor resonance
problems and that they may have to be changed
this at least as far back as june last year .

Anyone have a link handy to those i was just looking but i can see my own
post saying , i hope it's a coomon inductor to change


got my 81's today and definitely an unusable amount of  Hum  / buzz
just by switching in the eq in without any gain

thanks for the legwork Maxwell , obviously if there were any common
inductor values among the chineve eqs it would be easier to hit 500

 

[alexc]

Thanks Maxwall for the Cinemag input.

> Cinemag feels the China inductors are inferior with certainty. 

500 target per piece sounds pretty unachievable?

I guess that leaves talking to Sowter or using Roddy's excellent suggestion of
OEP mu-metal shielding cans (didn't think of that!) or other mumetal solution.

Or awaiting some action from the GB organisers (Step 1 - wait. Step 2 - don't hold breath)

For us cheaper seats, sounds, like the Carnhill + OEP can.

 

[maxwall]

I could handle the grp buy on the 81 inductors and leave the 73'a and 84's to other members to setup, that is if the inductors have different henry values. Physically I think they are the same footprint and lousy core material.

I don't have any of the 73'and 84's just he 81's. Glad I did'nt get any more crap units from the same source.

The 73's and 84's should be easy to get as they are already available. Not sure on the 81's

I can 't see why we all could not wind our own inductors on high quality pot cores. it cant be any harder than winding a Pultec Toroid
inductor. But the carnhills and others are inexpensive enough just to buy outright.

Well 500 would be the cheapest price, but as the target number goes down the price goes up. So were probably looking at
$60 each at least, which is more than Carnhill. Cinemag just will not negotiate with me, I don't have the political pull and contact friendship like others may. And it only takes Cinemag a few minutes to wind one on a computerized winding machine. How easy is that.

Cinemag should be a little more price friendly, I agree , but I'm not a big player so the small guys take the hit. And they know it.

If I was Fender , Neve , Harman etc , or some other big company then I'd be getting samples thrown at me for free.

Either way this mess gets fixed, its gonna cost extra $100 or more to do something about it. Which is'nt welcoming news. But if its done right its not a show stopper either.

 

[alexc]

OK - I thought the 500 was the target to wind any at all. That's better.

At higher prices, it still the best option.

And no doubt a fully sealed mumetal unit will shield better than a DIY solution.

 

[okgb]

Maxwell did you explain the story to them ? they have been good to us in the past ,
for grp buys on the api outputs
Either David or Tom  , usually David

 

[rodabod]

Sowter might be worth speaking too. They'd be guaranteed to be very high quality at least.

It would be nice if we could find a mumetal can source which is close to the industor size.

 

[okgb]

Right Chae here at prodigy distributes Sowter correct ?
they may accomedate a grp buy if numbers were good
it would be responsable shopping to get three quotes  but

I just emailed David at Cinemag with a little more info
and he'll work up some numbers for us