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

  • Thread starter Thread starter jasonbmerrill
  • Start date Start date
thajeremy said:
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....
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Did you try turning them on? Odds are good that the 73s will be usable without any modifications.

Jason - I only have two 73s and two 81s - not a big sample set, but my guess is that with your 73, the problem is 65% likely to be solved by a transformer twist. The 81's another story. But on mine, I really only had to swap the objectionable transistors and twist the transformer for one of them to get totally (by that I mean easily tolerably) quiet both with the the EQ on and off. The other is quiet with the EQ off, and I'm, um, still working on it for the EQ, but it's not bad. I'm the group that pulled the resistor in order to prevent the big pop (and potential fire -- who said these weren't "firewire" units -- haha -- get it... um...) when turning the volume knob. That's easy to do, and I don't miss the switch position at all.

That said, dgatwood's right - you don't know what you're looking at until you start looking
 
ive already tried twisting the transformer... are we talking really subtle twists?
 
no, more likely a pretty big twist, but possibly a small one - picture a bunch of rays of electromagnetic grunk radiating out with a few null spots - you want the rays pointing at things that don't pick them up, like the walls.
 
what about encasing the transformers in copper or something? isnt there a material that is good for this? And how would you do it?
 
I too am following this thread with interest... got two 1073s, would like to get the most out of them.
 
I dunno - I tried wrapping mine in mu metal foil (and cut the crap out myself on the edge of the foil - careful with that stuff!), but it didn't seem to make much difference. I've seen folks wrap them in copper foil, but I don't know if it's something they tried that didn't work, and they just left it, or if it really helped. Somebody even built a metal enclosure - was that you dgatwood?

If the twisting thing doesn't work, then you gotta experiment, I guess. It took me a while to get the twist right.
 
antichef said:
I dunno - I tried wrapping mine in mu metal foil (and cut the crap out myself on the edge of the foil - careful with that stuff!), but it didn't seem to make much difference. I've seen folks wrap them in copper foil, but I don't know if it's something they tried that didn't work, and they just left it, or if it really helped. Somebody even built a metal enclosure - was that you dgatwood?
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Custom can made out of MagnetShield (tm) with a secondary aluminum cover over it and the power supply boards. It might have made a little difference, but it wasn't huge. You really can't block a strong 60 Hz magnetic field like this one. I mean you can, but we're probably talking about several inches of ferrous metal to do it usefully. You'd have an easier time unwinding and rewinding the transformer on a core that isn't made of several pieces of scrap iron welded together....

What did make a difference was getting the DC power cable and the volume control's audio cables as far away from that field as possible. :)

Something you might try is flipping the entire transformer upside down with the wires up on top.
 
alot seems to focus around this transformer. I think im going to revisit trying to re-orient it. thanks so much guys.
 
jasonbmerrill said:
alot seems to focus around this transformer. I think im going to revisit trying to re-orient it. thanks so much guys.
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Good luck with it. I'm tempted to volunteer to help you out, especially if I could get a free pre out of it... ;) Ah, but I haven't done this kind of stuff in ages.

Let us know how you make out.
 
jasonbmerrill said:
alot seems to focus around this transformer. I think im going to revisit trying to re-orient it. thanks so much guys.
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Certainly worth a shot. However, usually that's only one of several hum sources that all work together to wreck things. IIRC, I moved the transformer outside the box and that didn't get rid of the worst of the hum. Chances are you aren't even hearing the transformer-generated hum until you've fixed several of the other problems. :D
 
dgatwood said:
Certainly worth a shot. However, usually that's only one of several hum sources that all work together to wreck things. IIRC, I moved the transformer outside the box and that didn't get rid of the worst of the hum. Chances are you aren't even hearing the transformer-generated hum until you've fixed several of the other problems. :D
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if only there was a tutorial somewhere...

hehe, i have plenty of info for now - i just need to really put some time into it after finals in a week or so.
 
Someone's selling me an 81 but I don't know. I'm thinking really hard before maybe sending $200 down the toilet. The 81's were some of the most problematic, right?
 
Marcadiablo said:
Someone's selling me an 81 but I don't know. I'm thinking really hard before maybe sending $200 down the toilet. The 81's were some of the most problematic, right?
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The 81's have a terrible noise in the eq circuits, making the eq unusable. This is easily fixed by swapping out 8 cheap transistors.

Then you are left with the same problems as the 73 and the 84 - a resonant hum in the eq inductors at certain settings. This can be resolved or improved in most cases by rotating the power transformer on its axis. Further improvement and resolution can be had from shielding the transformer and the inductors with mumetal, or replacing the stock inductors.

You are then left with a loud oscillation ("pop") when switching the gain in the mic setting between the 6th and 7th positions on all the pres. This can blow speakers if you're not careful, and may over time blow components in the preamp also. This can be fixed by reconnecting a resistor in the gain switching circuit, leaving you with an "off" position there, or modifying the switch itself.

It's all fixable. Then you are left with a generally excellent sounding pre that you really can't depend on because some capacitors are underrated in their voltage specs, and some resistors seem to have been slobbered on by alien acid saliva (though you could replace these and the caps too.)

Your call.
 
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