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Guitar processor parts

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gabeNC

Member
Howdy,

I had this old Digitech RP6 that I bought in '96 and I've been wanting to salvage some of the parts. Seems like a gold mine compared to my small amount of inventory. Spent several hours reading data sheets and trying to figure out how things are supposed to work. Here are some of the more interesting chips on the board.

PCM54HP 16Bit dac
Z80 (sweet!)
D41464 Dynamic nmos ram
5532D dual op amps (supposedly low noise high quality)
Some kind of DSP by Harman International


A couple of questions for the dumpster divers.

What is the best way to unsolder some of these surface mount chips? From what I've read, keep the soldering iron as low as possible. Sometimes the data sheet will have the maximum time for the soldering iron to be in contact with the component... and sometimes it doesn't. I've seen tips for the soldering iron that span multiple pins. I would really like to be able to use some of this stuff, that DAC is $30 on mouser. Well... that one is in a socket. :D

anyway,

thanks for spreading the knowledge.
 

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Sceadwian

Banned
You have to be a little more 'butch' when desoldering multi legged chips. My two favorite methods are a propane blow torch on low (seriously) on the back of the board, move the tip of the flame evenly but quickly across the back of the board from the components while keeping a steady pressure pulling the chip away from the other side. You'll feel it start to get loose and some point, a quick head count to 2 or 3 and a few more quick passses of the torch and the chip should let go pretty smoothly. I would practice this on junk boards though (I did)
Also using an iron to apply a thick block of solder to each leg quickly and then afterwards reheating all the blobs at once to get the whole thing lose for removal. You'll NEVER desolder those multi legs with a low temp settings, cause you have to get the solder on EVERY leg molten at the same time. You take your chances with what dies from over heating. You could use thin pieces of cut silicon (like a cooking mat) and a small block of aluminum on expensive or sensitive chips to guard the main body of the chip from heat. The silicion insulates without burning or transfering heat and the aluminum sucks up the heat the chip would be getting.

The actual best method is a high airflow temperature controlled hot air rework tool, which is like at he torch method but much more controlled. You really want high heat but for a short period of time. Things like the connectors will act like heatsinks, and so will larger IC's so it's high heat for controlled periods of time that's the real trick. If the legs aren't crimped and you're going after surface mount components I've also heard of people using a toaster oven to heat the entire board and just wait till the parts drop off. The important part is once the part is free is getting it out of the heat, too long in the heat can damage it, but basically if you don't melt the package itself it should be okay. Using a propane torch I often 'blow' surface mount parts off boards with the torch flame even on quick passes. I don't test every bit I get, but they're usually cool to the touch when I pick them up right afterwards. Watch out for via's under chips as it'll let the torch heat go right through the board to the chip.
 
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gabeNC

Member
Wow... good stuff thank you!

Couple of questions... VIA? That is wires or components under a IC to save real estate space? Also if the chips are crimped can I use a pair of needle nose pliers and gently bend them straight? Or maybe use a Dremel and cut them off smooth?

I'm sure the misses would LOVE for me to slap this in the toaster oven! hahaha.

I would be curious of what people (audioguru) thought of these op amps.

Also scattered all over of the board are other types of opamps, I suppose to keep the signal high across the board?

Thanks again, I've been learning so much from this site, so much I don't understand but it is fascinating. The experience and knowledge from the regulars and experts are incredible! Maybe someday I'll be able to contribute. Even if it's a reply "look it up on google!" :rolleyes:
 

duffy

Well-Known Member
Most propane soldering irons have a "heat gun" tip which will work like Sceadwian's propane torch, only without the bare flame (it uses one of those ceramic catalyst things) and is a little more precise.

A "via" is just a plated hole in the PCB. It's used to connect from one layer to the next.

Hate to say it, but this kind of thing is rarely worth the effort. Look up the chip prices online first - you can get many of them brand new for such a low price that it just isn't worth the effort to remove these parts that might work, or even close to being worth the potential domestic strife that will ensue when the missus finds you put the PCB in the toaster oven.
 
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Mike_2545

Super Moderator
Wow... good stuff thank you!

Also if the chips are crimped can I use a pair of needle nose pliers and gently bend them straight? Or maybe use a Dremel and cut them off smooth?:
If the chips are crimped your screwed, but if it's just the leads then by all means straighten them out with the pliers.
 
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Sceadwian

Banned
I'll have to look into one of those Duffy, I've seen them and I have a crate full of boards here that need stripping. The torch was just used cause that's what I had around and it burned the back of the boards pretty bad. Then again I used a full sized utility torch (again it's what I had around) so the flame wasn't really controllable. I find that with a high temp device like a torch it's best to apply heat for a count of 2-3 then remove it and let it sink in a couple tips before you apply heat again. That way you heat the board all the way through instead of just torching the outside.

Typically if an IC leg is bent it's just the corner pins to hold it in place for soldering, though I have seen some boards where whole through hole IC's were bent to the side, it's not really practical to remove those once except by brute force and they may not survive. A chip puller is a good thing to have for bulk desoldering IC's like this. Surface mounts are easier but you do have to be careful not to burn them. the silicon mat + aluminum plate should insulate the body of the chip more than enough to desolder without damaging it though.
I think it's worth the effort Duffy, for two reasons. You can't beat free, and it's interesting to see the variety of IC's you end up pulling off boards. But I don't have much of a budget for parts and if you're taking apart 'quality' devices you tend to get quality IC's.
 

3v0

Coop Build Coordinator
Forum Supporter
SMD board are a bit easier to get working parts off. I used an electric heat gun to harvest a few MBs just to show how it can be done. So far I have only used a battery holder, there is other good stuff there like 3.3 regulators and mosfets.
 

Sceadwian

Banned
I sneezed once while desoldering a board, lost a dozen SMD components, slippery buggers.
 

3v0

Coop Build Coordinator
Forum Supporter
LOL. I took apart an old bathroom scale with a large 7 segment readout. The actual LEDs were single units set into a mask. Several per segment. I ran my finger over the PCB and a shower of tiny leds fell off the board. Man was I suprised. I wanted to use that display too. Oh well.
 

Mikebits

Well-Known Member
Make sure you pull that battery off before applying heat...
 

gabeNC

Member
Thanks Mike... I'll make sure to pull that battery.

Thanks to all for the advice and funny anecdotes.
The flip side of this board are a couple of dozen led's, three seven segment displays and about 15 buttons/switches. But like duffy said, might not be worth the effort and I'm not too worried about recovering those.
 

duffy

Well-Known Member
I sneezed once while desoldering a board, lost a dozen SMD components, slippery buggers.
I stood up from the bench, felt something gritty on my hands, idly brushed it off as I went to get some coffee. Lost almost my entire stock of high-voltage SOT23's.
 
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