Wanted: Cheap receiver/amp

[entropy]

I have been using a 2x50w receiver for years for downstairs, and it burned out. I tried replacing its internal fuse, but it immediately blows again. I'm guessing some circuitry or something is fried, and I don't think it's worth paying to get fixed.

What I need is a cheap 2x100w (or 50, but I have 100w speakers) receiver. 2 or 3 inputs would be optimal, but I can easily get a splitter to add more.

A/V or surround is not needed, or even wanted really. Just 2 (or 2x2, I guess) channels) - there's already a 5.1 system down there)

Would it be smarter to try to fix it myself? Or maybe just sell the speakers (I probably can't get much for them anymore, though... I paid $75/ea. originally)?

Edit: Actually, does anyone have any suggestions to fix it? I think I'd much rather try that...

[madmat]

You could go and replace the amp IC's (the ones mounted to the heatsinks) but that might not be the problem.

If it is you've just fixed it for <$20 and if not you've just invested that much more to a dead piece of equipment.

[entropy]

There are no heatsinks inside...

[madmat]

Got a pic of the guts? I can't believe that there's not a sink of some sort as those IC's will burn up without some sort of thermal management.

[entropy]

Here it is. This is the highest resolution with the highest settings (that I can figure out...) taken with my mom's camera. Mine couldn't get near this close ;D

Here's a link to the YouSendIt page, since it was too big for Imageshack.

http://s42.yousendit.com/d.aspx?id=33UXFA49ZLKIC0OUSJJY8MR88

[madmat]

Ok, I'm downloading it now...I'll point out the amp ic's after I get it opened.

[madmat]

OK, I've circled the IC's (your amp uses FET's) and the sink they're mounted to and put an arrow to each one...

http://img.photobucket.com/albums/v474/madmat_2001/ICs.jpg
Replace those 4 FET's (field effect transistor) and you might have a working amp again.

Make sure to keep the little plastic pieces to put in between the transistors and the sink if there are any and use cheap white thermal paste not AS3 or AS5 as it could short them together.

Good luck!

[profdlp]

madmat, I wish I'd have thought to ask this same question before I tossed my JVC receiver in the trash last year... :bawling:

Good luck, entropy. :)

[entropy]

So this would (or might) cause the fuse to instantly blow? Also, how do I get them out? They look like they're connected to those three "wires" and go down beneath the board... or will they pop off the wires?

One last question: where would I get these? ;D Should I just take them into Radioshack and go, "Uh... here. I need some of these, um, things."

Should I take the whole board out and then take them off, or no? I know my around a computer, but that doesn't mean I understand what every little trace does :(

Edit: So basically I need a babystep-by-babystep guide from you... if you're willing. Otherwise if you know of a site, that'd be great. Or I could just trash it :P

[madmat]

You unscrew the PCB from the chasis, disconnect all the wires (they're in push-fit connectors) then desolder the transistors. Those wires are the legs of the transistors.

Before pulling them out make a note on the sink (such as 1, 2, 3, 4) over each FET and a coresponding note on the FET so that they go back in the proper location. The FET's have the model number on them, try rat-shack for them and if not look for a good electronics supply house such as ECS (no not that ECS).

[entropy]

Ok, board removed, heatsink removed. I've got just the board sitting on my bed right now.


How should I go about desoldering those things? I don't want to poke around where they were originally solders, as I'm terrible at it. I know I'll melt it too much and it'll spill over some other traces and ruin it all anyway. Can I just snip those legs, then snip bits of the new transistor's legs off and solder them there? Or what?

Also: that white pad. What should I do with it? Why do I even need it? And you say use some cheap thermal paste? Why not AS%? I've also got some Radioshack-branded "Silicone-based Heat Sink Compound." Cheap enough?

Oh, is there any test I can do with a multimeter or something to test if it is in fact these transistors?

Uhhh... it looks like there might be burn mark on the underside... but I don't know.

[madmat]

Where's the mark? yes you need the pads as they electrically isolate the transistors from the sink (which is why you avoid Arctic Silver BTW) and to desolder I use solder wick which is woven copper that you lay over the solder to be removed then you apply heat to it with your iron. As the solder melts it "wicks" into the mesh and leaves nice clean traces.

I'm not familiar enough with testing the current flow through transistors to tell you how to test them but I do know they're basically a tri-mode diode, a base leg, a collector leg and an emitter leg and are available in n-p-n or p-n-p (n being negative and p being positive) and generally in a system such as that with a push pull amp circuit they're arranged with one n-p-n and one p-n-p on each channel.

But as to what I asked at first, where's the discoloration?

[entropy]

I'm working on a Photoshop of it. It's kind of hard to see, though, since the camera isn't very good at macro, but I tried.

Sigh. All that work and it turns out I could just scrape away whatever it was :( I'm guessing some of that glue just got burned, but here it is anyway:

http://s53.yousendit.com/d.aspx?id=1LO5O58J0HYRR1222S6SIRLW8P

By "macro" I just meant little tiny things. I ended up doing exactly what you said, though :p

[madmat]

Hold the camera away from it a few feet, zoom in on it (if it has optical zoom) and click away at the highest resolution setting.

Works like a charm. Macro? We dun need no stinkin' Macro... ;D

[profdlp]

...I'm guessing some of that glue just got burned...
That looks like some of the rosin from the solder. Normally it wouldn't be a big deal, but the fact that it appears to be burned might indicate that what madmat said about replacing those parts is right on target. The big question remaining is did the parts themselves burn out, or did something else cause them to fry?

[madmat]

That's normal flux, the burning comes from standard soldering and isn't anything to be concerned about.

The transistors are to the right of the image and the top one is n-p-n.

If I had my handy-dandy compendium of arcane knowledge I could tell you a replacement part number but I lost all my parts catalogs.

Try Parts Express for replacement FET's, a pair of 50W transitors per side are likely what you're needing.

Yes I know your amp outputs 50w total and a pair of 50w transitors will net a total of 100w per channel but they build them like that to allow for peak output (50w is continuous rating) and to allow the transitors to supply the needed wattage for 4 speakers if using parallel wiring to 2 pairs of speakers.

At any rate, give replacing those 4 transistors a shot. If it fixes it you're in business and if not at least you've tried and learned someting for a few bucks invested.

[drasnor]

A burned-out FET shouldn't blow your fuse, it should just stop making sound. Typically the fuse goes when you have a short somewhere. Good luck finidng it though.

If I were you I'd get on eBay and start shopping for power amps. You ought to be able to get a Crown D75 for a reasonable price.

-drasnor :fold:

[GHoosdum]

Just for kicks, what's the resistance rating of your speakers? If the speakers are 4-ohm and the amp is rated for 8-ohm, that could cause the amp to burn out.

[madmat]

A burned-out FET shouldn't blow your fuse, it should just stop making sound. Typically the fuse goes when you have a short somewhere. Good luck finidng it though.

If I were you I'd get on eBay and start shopping for power amps. You ought to be able to get a Crown D75 for a reasonable price.

-drasnor :fold:
No but one with an internal short will...and that does happen.

[madmat]

Easy way to see if it is the FET's is to desolder all of them then plug in the reciever after reassembling it. If it blows the fuse then forego replacing the FET's since the fault lies elsewhere (like the transformer) but if it doesn't blow the fuse then you've found the culprit.

[entropy]

Speakers: 8 ohm nominal
Amp: 8 ohm min

I'd already checked on that. So when I desolder the FETs, if they're faulty, it should still work? It won't like, ruin other junk in there? I'm going to try to find some of that copper mesh stuff at work tonight (I work at a big hardware store)

[madmat]

If the FET's are the problem then when they're removed the receiver will power up since there's no short to blow the fuse and if not the fuse will blow.

The pre-amp won't care if there's no load as it is low voltage and current.

[entropy]

Okay, found some of that "soldering wick" (they call it desoldering braid... whatever). I'm going to try to take them off tonight, stick the whole shindig back in the case and try it. I assume I need to stick it all in and screw it all back down, right, so that everything's grounded properly?

[EMT]

It's probably not necessary to screw the thing back in to use it (just like a motherboard), so I wouldn't bother if it's much work...

In case you haven't heard btw... be careful messing with circuits like this. I like to just use one hand so shocks can't go across the heart.

[madmat]

I wouldn't Suggest running it with the case open and just a couple of screws should do the job to make certain that the board can't shift around and short out.

After you're done testing it, unplug the power cord and if it hasn't blown the fuse turn the power switch on and leave it set for a couple of days to help discharge the caps although caps can hold a charge for as much as a month.

EMT is right on the mark with the one handed advice. I was taught to work with one hand in my back pocket. High voltage can cause your muscles to clamp down and the idea behind having the hand in the pocket is that when you're swinging your arm around to knock the other loose you'll gain enough momentum to jar the hand loose when the free hand hits it.

[entropy]

MADMAT! I LOVE YOU!

It freakin' worked! I took my time, desoldered everything, set it back in and TA-FRICKIN'-DA, it doesn't blow! I let it sit for a few seconds to make sure and yep, nothing wrong with it! I'm so excited right now you have no idea. I've never done any circuit board work with ANYTHING, so this is a great feeling. Thank you so, so much.

I just need to find some replacements for those things, and then I should be good to go. Will generic radioshack thermal paste be good enough, or is there a specific, non-conductive kind I should buy?

Edit: It doesn't have a "switch" per say. More like a button that brings it out of standby. Should I do that, and then unplug it for a few days/a week?

[EMT]

That's pretty sweet, man. I hope to hear you get it fixed. Good call, madmat.

[madmat]

I'm thrilled for you, just let it set for about a week then, use caution around caps and yes the generic white goop is dead as a doornail electrically.

It's fun working on something like that, I remember how I felt after my first tube amp mod (and those babies use 600v plus inside) I was just estatic when it all worked :)

[entropy]

So I can't find these things anywhere. Went to Radioshack, and they had some smaller ones, but not the size I need. The manager told me to go to Bytehead (I thought it was a stupid idea, but I went anyway). The guy didn't even know what they were, or how to get them.

There's absolutely nothing here... is there a place I can order some from?

[madmat]

Look at Parts Express or Rat Shack.com

You'll need parts numbers. I take it that theres no real electronics supply stores there...

[entropy]

Haha, no. In a city of 6,000 there's not likely to be anything. Even in Sheboygan (probably 50,000+ or something) there's nothing. I'd have to go to Milwaukee or something, and honestly I don't feel like driving an hour just for these... I could maybe talk to some family down there. But my grandpa just went into the hospital again... I don't want to bother anyone :(

[profdlp]

Send me as much detail as you can provide and I'll help you look for them. I'm sure we could probably find a place online selling them at a cheap price. If not, I'd imagine that I could find them in Cleveland. :)

[entropy]

Awesome, thanks prof. I've been looking at Parts Express' website, but I can't find anything.

I need 2 "c4466" and 2 "a1693". They look kind of like these (http://www.partsexpress.com/webpage.cfm?webpage_id=3&SO=2&DID=7&CATID=41&ObjectGroup_ID=355), but I'm not sure that's exactly what they are. At the bottom of all 4 is says "9DP"... don't know if that's necessary or not.

[ryko]

i know you would rather fix your old reciever, but if it doesn't work out i have a RCA 2x100w stereo reciever that i would be willing to part with. $85 shipped. i would ask like $65, but this thing is heavy so shipping is going to be a lot. PM me if you want some pics or more info...

[madmat]

Here's one.. http://www.bluestar-online.com/2SC4466_p/2sc4466.htm

And the other.. http://www.matelectronics.com/acatalog/Mat_Electronics_2SA1693_5116.html

[entropy]

"The specified product (reference 2SA1693) has just been removed from the catalog. Please return to the catalog and continue shopping. If you press the Refresh or Reload button on your browser, the removed products will no longer be visible in the catalog. We apologize for the inconvenience."

I'll probably order those other ones tomorrow, though. thanks!

[madmat]

That sucks, they were there last night and this morning.

[madmat]

http://www.bdent.com/search/part.jsp?partnum=2SA1693

http://www.bdent.com/search/part.jsp?partnum=2SC4466

Both in stock and both from the same place...

BTW, I can cross reference for an alternate part too should these be gone when you go to order them. God bless Google :)

[profdlp]

madmat, you are awesome! :rockon:

[madmat]

madmat, you are awesome! :rockon:

Thanks...I have no life so I've got a ton of free time ;)

[profdlp]

No life? What's a "life"? :scratch: ;D

[entropy]

Thanks madmat! I guess I'll be ordering 4 of each, since you need a $15 minimum. If I ever need to do this again, I'll be set, haha.

[madmat]

Or order 2 each and some solder and canned air and...

[entropy]

Where'd you find all that? All I can see is different capacitors and diodes and whatnot.

[madmat]

My bad, I thought they carried a full range of electronics parts and supplies...

Go figure, you'd think they'd carry basic supplies as well.

[madmat]

Remember, the 2SC4466 is NPN and the 2SA1693 is PNP so look on the PCB for the p-n-p and n-p-n markings for the legs of each IC that you're placing in there as they won't take kindly to being put in the wrong polarity...not at all.

[entropy]

Well, I've got the originals marked 1, 2, 3, 4 like you said, as well as the heatsink. So I should just be able to match up the two 4466 with the original two's places... correct?

[madmat]

Yep!!

[madmat]

Any luck finding the chips yet boss? I'm really interested to know how it goes.

[entropy]

Not other than that one site. Look like I'll just have to cough up the extra money.

[entropy]

WELL... after a long time of not bothering to order them, I'm finally going to. How does this site look? It seems kind of sketchy to me, but they have both parts in stock, and, well, no minimum charge.

http://www.cpcares.com/

[madmat]

Looks like it should work although I'd call and make certain they have it in stock first...

[edcentric]

You mean that you never built a power amp from scratch?????????
It is a right of passage.
A beautiful class AB tube amp.
Finding a transformer that was wound for +/-150V was a pain,
But tubes at 500W is pure bliss. I had an Ampzilla, wish that I still had it.
Of course then I would need the old AR speaker to go with it.........

If you need parts picked up in Milwaukee and sent to you let me know.
ed

[entropy]

FRICK EFF MOTHER OF GODDAMNIT!

All the wasted time! Sonuva f*ck.

I ordered them, and they finally came a week or three late. So now I was all excited to get them in and try this sucker out. WELL - it doesn't work. I got them soldered on perfectly (believe me, I took my time), got everything hooked up, did everything perfectly and it MOTHERTRUCKIN' BLEW. The fuse blew, when it should not have! Replacing these things should've fixed it.

Excuse me while I BEAT UP A KITTEN. So... frustrating...

Oh man. Ohhh-hoho. We have a five-day weekend that started today. Tonight we're supposed to get a huge blizzard. I'm going to set this thing outside overnight, let it soak in and blow itself to pieces (remember, the capacitors should technically be charged now, since I tried turning it on), then take a bat to this f*cker. If only Parnell Tower wasn't so far away, I think this would make a nice explosion of itty bitty useless goddamned electronics.

[ryko]

please don't waste it....that enclosure would make a killer htpc case. what are the dimensions of it? it's weight?

seriously if you are going to trash it i might just take it off your hands. i would pay for shipping only since you are going to toss it though.