replacing a resistor on my ACMP-73

jasonbmerrill

the end of unreality
hello :)

this is the ONLY modification i plan to do to my ACMP73 because it seems to just be soldering. And i can "kinda" do soldering.

however, i am certainly a newb with it, so if anyone could help with a detailed, "for dummies" explanation of this, i would REALLY appreciate it.

first off, i need to know EXACTLY what the rating of THIS resistor is:

1231862900848-screenshot.png


isnt there some coding that tells you what it is? like the colour of the stripes?

basically, here is the context of the repair i need to do, with some pics supplied by ppl with non burnt resistors:

here is one look:

picture-one.jpg


closer:

picture-two.jpg


1231906390685-screenshot.png


(mine has wax on the screws that connect it to the board... how do i deal with this?)

some of the transistor:

IMG_0464CROP.jpg


1231862900848-screenshot.png


my burnt one:

IMG_0471.JPG


some info about it from this thread:

https://homerecording.com/bbs/showthread.php?t=180964&page=286

That's the emitter resistor for the output stage power transistor (2N3055)

i guess i could decipher it from this schematic but how? -- EDIT: wait, looks like it says 47 Ohms, .5 watts. duh. hehe

1231906356129-screenshot.png


???
 
I would think that you'll also have to figure out what caused the resistor to burn. Unless the resistor was seriously flawed in manufacturing (or the circuit design is wrong, but the others, pictured haven't burned), something drew too much current through it to cause the burn. The 2N3055 is shorted perhaps?
 
I would think that you'll also have to figure out what caused the resistor to burn. Unless the resistor was seriously flawed in manufacturing (or the circuit design is wrong, but the others, pictured haven't burned), something drew too much current through it to cause the burn. The 2N3055 is shorted perhaps?

This was uncovered in the group buy thread; an "in between" position on a gain switch causes the bias on that transistor to jump from 2V to 9V, exceeding the resistor's rating by more than 1W.

OP, cut off the burnt resistor, but leave some of the leads sticking up from the hole. Solder a new 47 ohm 2W resistor in its place by simply laying the resistor leads flat against the old resistor leads. Solder and trim.

I can send you 2x 100 ohm 1W resistors, which will get you 50 ohm 2W in parallel--you just need to stuff both of them on top of the old leads, and you need to email me your address . . . :)
 
This was uncovered in the group buy thread; an "in between" position on a gain switch causes the bias on that transistor to jump from 2V to 9V, exceeding the resistor's rating by more than 1W.

OP, cut off the burnt resistor, but leave some of the leads sticking up from the hole. Solder a new 47 ohm 2W resistor in its place by simply laying the resistor leads flat against the old resistor leads. Solder and trim.

I can send you 2x 100 ohm 1W resistors, which will get you 50 ohm 2W in parallel--you just need to stuff both of them on top of the old leads, and you need to email me your address . . . :)

i tried to send you a pm but it doesnt work....

hmm.

i was actually thinking of lowering the bias knob a bit and put the original resistor rating in...

if you had one of those instead (or additionally) that would be cool :D

anyway, how much do you want for em?

my address is

Jason B. Merrill
512 Church St. Apt 325
Honesdale PA 18431
USA

(its on my website anyway and anyone can find it so whatever)

thanks for the offer, and your help. I promise that this is the ONLY work i will do on my preamp. After this, im taking it to an amp guy in state college, PA ;) sometime next month after my valentines day gigs
 
OP, cut off the burnt resistor, but leave some of the leads sticking up from the hole. Solder a new 47 ohm 2W resistor in its place by simply laying the resistor leads flat against the old resistor leads. Solder and trim.

Eh. Why hack it? Judging from my experience desoldering other parts on these boards, a few seconds with a solder wick and the parts literally fall off the backside of the boards. They drilled the holes kind of big.

http://www.radioshack.com/product/index.jsp?productId=2062744

Just place a clean section of the wick over one contact and press it down over the contact. Hold it down with screwdriver a half inch away or so. Get a cheap, low wattage soldering iron that comes with a handful of tips. Use the tip with the big dimple in the end of it. Press the hot tip down on the connection and rock it around all four sides of the contact. Then, lift the wick and the iron together as a pair. The wick should lift off the board without any significant effort. If it sticks to the board at all, reheat the wick with the iron to melt the solder, then lift it off.

Repeat this for the second contact. The part should fall right off the back of the board. If it does not, lay a clean section of wick down on the contacts and press it down onto the board with a back-and-forth motion of the tip to soak up the last bit of solder and press the pin into the board slightly. The part should then fall loose or should pull easily out with pliers.
 
lol i wish i had 3.99 ... hehe

i only have an old soldering iron with one tip...

do you suggest a different type that isnt too expensive?
 
i tried to send you a pm but it doesnt work....

hmm.

i was actually thinking of lowering the bias knob a bit and put the original resistor rating in...

if you had one of those instead (or additionally) that would be cool :D

I would not change the bias knob, it's not the source of the problem anyway. The regular bias setting is fine, less than 0.1W idle. The problem is that pop on the gain knob sends waaay too much current through that part of the circuit. I do not know if changing the bias knob would affect that issue at all, but it will certainly degrade the performance of the output circuit (lower headroom).

Resistors are free, I have a bunch laying around I am not using. I do not have the original value though, and I wouldn't use it anyway.

Eh. Why hack it? Judging from my experience desoldering other parts on these boards, a few seconds with a solder wick and the parts literally fall off the backside of the boards. They drilled the holes kind of big.

Because I am lazy and didn't want to pull the board. Although if the holes are that big, you could cut out the part, heat the leads, and pull them out from the top. That will probably leave the hole clear enough for the new part . . .

If you pull the board and desolder, I'd still cut the old part out first, makes desoldering a lot easier, and it's not like this part is reuseable.
 
I would not change the bias knob, it's not the source of the problem anyway. The regular bias setting is fine, less than 0.1W idle. The problem is that pop on the gain knob sends waaay too much current through that part of the circuit. I do not know if changing the bias knob would affect that issue at all, but it will certainly degrade the performance of the output circuit (lower headroom).

Resistors are free, I have a bunch laying around I am not using. I do not have the original value though, and I wouldn't use it anyway.



Because I am lazy and didn't want to pull the board. Although if the holes are that big, you could cut out the part, heat the leads, and pull them out from the top. That will probably leave the hole clear enough for the new part . . .

If you pull the board and desolder, I'd still cut the old part out first, makes desoldering a lot easier, and it's not like this part is reuseable.

my email is jasonbrianmerrill AT gmail.com, ill send you postage via paypal (i have like 1.63 in the account....which is all i got -- i know its basically an insult but its all i gots man hehe... the preamp was my bread and butter for the month)

its not possible that using a higher rating will blow anything else out? i know nothing about this stuff... lol
 
I've been using a "vacuum desoldering tool" like this one at radio shack -- not the same one, but similar:

http://www.radioshack.com/product/index.jsp?productId=2062745

less than $10 (more than $3.99) - my soldering iron is was $12 and I think it's 40 watts, which is maybe high? Anyway, with the board out and upside down, I position the tip of sucker right next to the little dimple of solder, move in the [standard pointed] tip of the soldering iron and touch for a second or two until the solder liquifies, then push the button on the solder sucker. When I've done each lead, then yes, the stuff falls out of the board. When putting new components in, I usually have to bend the leads outward slightly to keep the new component from falling out also when I invert the board to solder it in place.

We're both newbs, so let me say this -- it may be tempting to skip the desoldering step and just yank out the old part once you've heated up the solder (once for each side, I guess), then heat up the solder again to put the new part in place. I've done this before, but have found that it very likely ( like greater than 50%) that I'll "lift the trace" on the back side of the board when I push in the new lead - you don't want to do this, of course, because lifting the trace is separating the conductive metal printed stuff from the rigid green fiberglass board that neatly holds it in place. You can repair it most of the time, but you wind up spending a lot more time that if you had desoldered in the first place (and not just on that solder joint, but rather overall, because of the high screw-up rate that I mentioned), and it looks embarrassingly bad even if you successfully fix it. (and if you were unable to successfully fix it, you'd either have to declare the unit "bricked" or bring in someone with more skills, who would laugh at you - me, I take the "bricked" option because I'm sensitive). Fortunately, I learned these lessons on my ART Tube MP :D
 
I've been using a "vacuum desoldering tool" like this one at radio shack -- not the same one, but similar:

http://www.radioshack.com/product/index.jsp?productId=2062745

less than $10 (more than $3.99) - my soldering iron is was $12 and I think it's 40 watts, which is maybe high?

It's high for leaded solder, and cheap irons that run on full power all the time tend to eat tips for breakfast, lunch, and dinner. I have to have a regulated tip myself; I use the Weller WES51. It's really surprising how much easier it is to use a decent iron . . .

Up from there, the prices get crazy expensive, but I'd at least spring for the Weller WLC100 or the Tenma equivalent, just to get the wattage control--those run about $40.
 
lol i wish i had 3.99 ... hehe

i only have an old soldering iron with one tip...

do you suggest a different type that isnt too expensive?

*shrugs* The tip I like for this purpose came on a Lenk 25W from Wal-Mart. I think they're about $10. :)
 
A big part of the story for me is I work with lead-free solder with no-clean flux. If you're used to leaded solder, the first time you use that, you'll cry like a little girl. I did :o But with the better irons, it's second nature now.

The other thing is I never have to change tips for different work, because of the regulation I can use the stock tip for everything from cables to 0603 surface mount. And a tip lasts about three months (that's about 300 hours).

Oh! The other great thing I just discovered: drying tip cleaning (steel wool thingies). I don't know why I never noticed that before, it's great!
 
I can send you 2x 100 ohm 1W resistors, which will get you 50 ohm 2W in parallel--you just need to stuff both of them on top of the old leads, and you need to email me your address . . . :)

hey man i hate to be a heel but have you sent this stuff yet by any chance? My address is earlier in the thread :D I really appreciate it.
 
hey man i hate to be a heel but have you sent this stuff yet by any chance? My address is earlier in the thread :D I really appreciate it.

No, sorry, as I posted in the big thread, I misplaced the bag . . . but antichef had a single resistor in the correct value, and you might want to hold on for the "off" position mod, as that might obviate the need for a 2W resistor there (although you will still want to replace your damaged resistor).
 
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