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rf 325.1 bad outputs? repairable?


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just4today 
Member - Posts: 10
Member spacespace
Joined: June 20, 2009
Location: Ohio, United States
Posted: June 20, 2009 at 7:46 PM / IP Logged  
just4today wrote:
Lots of different settings on DMM . I used ohms . I know . . . you said diode right . Well , i`m just not seeing it . Seems like a nice enough meter , surprised it`s not on there .
Q120 - 28n15 = .0 ohms
Q123 - 28n15 = 11.5 ohms
Q118 - 36p15 = 8.15 m ohms
Q122 - 36p15 = 8.02 m ohms
Was this any help to you ? Thanks again for your help tonight . You seem to have been around a few amps .
i am an idiot 
Platinum - Posts: 13,719
Platinum spaceThis member consistently provides reliable informationspace
Joined: September 21, 2006
Location: Louisiana, United States
Posted: June 20, 2009 at 8:01 PM / IP Logged  
It appears as though you used the center leg and the left leg, instead of center and right.  If you did, the transistor that read 0 is the shorted one.  If you have some very small diagonal cutting pliers, Radio Shack sell a pair that they call or used to call Nippy Cutters.  They work great.  cut all 3 legs of that transistor.  Cut as close as you can to the board, and then flip the pliers over and cut as close as you can to the base of the transistor.  You want to cut a section out so that there is no chance of any contact.   After you do this, power the amp up and check for DC voltage.  If it is above (.)2 volts, do not connect a speaker.  If it is below that, connect the speaker again and listen to the amp.  You are to be listening for distorted sound at low volume.  Do not turn it up loud.  If it sounds good at low volume, the only defective part is the shorted output.  However you really need to replace both of them.  The 36Ps should be OK.  The reason you need to replace both of the 28s is, they both went through the same abuse.  One of them failed, but the other one can be and probably is damaged.  Yet another reason that you should not play it loud.  Let me know what it sounds like when you play it.
just4today 
Member - Posts: 10
Member spacespace
Joined: June 20, 2009
Location: Ohio, United States
Posted: June 20, 2009 at 8:33 PM / IP Logged  
Stock speaker probably never sounded so good . Thanks alot ! I went A+/B- because this is mono amp . Anyway the meter read .0002 +/- DCV . I only cut the one transistor 28N15 location Q120 that read 0 ohms . So , now I buy a couple transistors . I found them online at Fairchild semiconductors . They`re like $1.50 each however it`s 11 dollars shipping . Is there any cheaper place or an equivalent part number to source locally . I`m only about 20-30 minutes from MCM electronics .
About the actual replacement - desolder with braid and/or pump and then solder in place finally applying thermal compound . You mentioned torching something . What`s up with that ? All I have is propane torch for plumbing . This could be turned down I guess but what is it that needs torched ?
Thanks for helping this rookie out . Someday I hope to be as helpful to someone .
just4today 
Member - Posts: 10
Member spacespace
Joined: June 20, 2009
Location: Ohio, United States
Posted: June 20, 2009 at 8:53 PM / IP Logged  
I just read this note - rookies can not edit . Ok then , that`s why the double post earlier .
Once this amp is stable and installed I plan on using a 10" kicker comp sub , any opinion on sealed vs. ported or box size ?
i am an idiot 
Platinum - Posts: 13,719
Platinum spaceThis member consistently provides reliable informationspace
Joined: September 21, 2006
Location: Louisiana, United States
Posted: June 20, 2009 at 9:23 PM / IP Logged  

I don't think MCM sells those.  You can check though.  I would recommend buying a butane torch from Radio Shack.  20 bucks.  I would not use a plumbing torch.  The transistors are soldered to that thermal strip.  This is not going to be an easy task.  The amp needs to be unmounted from the heat sink, all components that are mounted to the strip that contains the output need to be unsoldered from the circuit board.  You need to go to Office Depot and purchase some medium size binder clips.  Spring clips to hold a stack of papers together. Whatever other devices are mounted to that strip need to be clamped down with the clips. If ther are 3 metal legs that connect to the strip, they need to be retained also.  A pair of Hemostats works really well for this.  Once you have all other devices clamped down, place the strip in a vice.  Have the strip horizontal and the end away from the parts you are working with is to be clamped in the vise.  With the radio Shack torch turned up all the way, start heating area below the transistors to be removed.  Place a small screwdriver in the hole at the top of the transistor.  While heating it, keep a little pressure on the transistor so when it comes off the strip, you can move it off of the strip with the screwdriver.  There are a couple tiny surface mount components near the transistors, you need to make sure that you do not disturb them.  The solder will heat up on them, but they will remain in place if they are not disturbed.  Continue heating the other one and remove it the same way. 

Before you start the above procedure you need to scuff the back side of the new transistors, 400 grit sand paper or a very fine file will work,  just scuff till the shine is gone.  If you do get the Fairchild parts, you may not need to scuff them.  You need to heat the back of the transistors and apply solder to the entire back of the transistor. 

After removing the transistors from the strip while the strip is still hot, you need to add a little solder to the pads where the transistors came off of.   Use the gold dot on the strip as a sight along with the hole in the tranny.  A pair of Vice Grip brand C Clamp works really well for the reinstallation of the new transistors.  They make several sizes.  The small ones work great.  With the transistor placed over the gold dot, start heating the bottom of the strip, with the c clamp pliers opened all the way up, have the top of the clamp resting on top of the transistor to apply gentle downward pressure to the tranny.  When the solder begins to heat up and the transistor sits down on the strip, close the pliers and hold in place untill the solder hardens up,  20 to 30 seconds.  Put a clamp on that one too.  Then install the other one. 

just4today 
Member - Posts: 10
Member spacespace
Joined: June 20, 2009
Location: Ohio, United States
Posted: June 20, 2009 at 11:04 PM / IP Logged  
Sounds good - will do . I will order parts and install as directed . I had no idea those transistors are soldered to thermal strip . Excellent idea on clamping the parts down . I expect on tues. or wed. I should be able to post results .
Meanwhile , I`m going to build a box for that sub after more research on this site . You and this site both have proven invaluable on this repair/install .
i am an idiot 
Platinum - Posts: 13,719
Platinum spaceThis member consistently provides reliable informationspace
Joined: September 21, 2006
Location: Louisiana, United States
Posted: June 21, 2009 at 7:29 AM / IP Logged  

Hey if you have a digital camera, before you remove the amp from the heat sink, take a few pictures of the strip and the output transistors.  Pay close attention to the tiny components mounted next to the outputs.  You may have to put your camera in the Macro setting.  That is the picture of a flower.  This will allow the camera to focus in an extreme close up shot. 

Also on the 3 metal legs that connect the circuit board to the strip, there is a fiberglass rectangle with a hole in the middle, the screw goes through this part.  When attempting to keep the metal legs in place while torching, put this piece over the legs when attaching the hemostats.  The hemostats alone will not keep all 3 legs in place.  As far as repair difficulty, on a scale of 1 to 10, this repair because of the mounting system, comes in at an easy 12. 

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