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Fender Princeton 112 Plus Heatsink installed Wrong?!

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  • Fender Princeton 112 Plus Heatsink installed Wrong?!

    Hello Everyone,
    My first post but have visited quite a few times. Hope someone can shed some light onto this discovery I made while repairing a Fender Princeton 112 Plus amp.

    Received only the amp chassis to repair. It was on Craigslist. Owner had told me it just needed replacement of 2 of the potentiometers that had broken off - otherwise he said amp worked fine.
    Received in mail and put on test bench... well everything looked good so I hooked it up to test and was dismayed to see it blew the fuse within 10 seconds or so. Obviously NOT working fine!
    Anyway did the troubleshooting and found both outputs bad. The TIP 142 (Q1) was badly shorted as well as 2 diodes (CR9 and CR10) in that immediate part of the circuit. Also the other output TIP147 (Q2) tested bad also but associated diodes tested good. Just for good measure I replaced the emitter resistor (.47 5w) for Q2 even though it tested ok.

    Thatís the ďback storyĒ on this guy. When I got around to installing the outputs, I noticed they were different than the old bad outputs. They are same size but are different in the fact the old ones didnít have the thick plastic casing over the metal tab that has the hole for the bolt/screw to connect it to the heat sink. That really didnít matter to me until I noticed odd marks on the secondary heat sink that is mounted between the actual output heatsink and the chassis. I may have been in an alternate universe at the time but, when I removed the PCB from the chassis, I made sure I didnít disturb the secondary heatsink.

    Ok, I know this long but Iím getting to the observation that prompted my QUESTION. The secondary heatsink had bolt marks dug into them that were caused by the mounting bolts for the output transistors. Due to the length of the bolt, they were forcing themselves into the secondary heatsink. I thought that was a pretty poor design and could cause problems with the effectiveness of the entire heatsink system used on this amp.
    With the new thicker output transistors, it wouldnít be an issue because the bolts wouldnít protrude that far. So I started applying thermal grease to the top and bottoms of the heat sinks. To my surprise, when I turned over the secondary heatsink, low and behold there were machined holes in the heatsink exactly where the protruding bolts would have gone! Iím pretty sure this amp had never been serviced so I had to assume it came from the factory this way. So I have to guess this was a mistake done at factory and wondered how many amps were sent out this way and what effect it really had on the longevity of the amp due to the impaired efficiency of the heatsink (IMHO).

    Has anyone run across this issue? I imagine that this chassis design was pretty popular with all the different Princeton solid state amps Fender made. Could have been a large production run for all we know!
    Anyways I havenít completed the amp repair yet, still waiting on some new potentiometers and bushings. Keeping my fingers crossed that Iíve found all the defective parts and idiosyncrasies of this amp (there were some erroneous values on some of the resistors - values in parts list didnít match what was installed).

    BTW, if anyone has any device on finding or building a cabinet for this amp, Iíd be very appreciative.
    Thanks for ďlisteningĒ and enduring the long drawn out commentary.
    Voxman

  • #2
    Welcome to the place!

    I'd bet my 1/4" nut driver that someone has taken the amp apart and reassembled it improperly. It's not likely Fender would have done that from the factory.
    "Yeah, well, you know, that's just, like, your opinion, man."

    Comment


    • #3
      It is extremely unlikely Fender put it together wrong. I agree with Dude. I don't know how we could say no one had ever taken it apart.

      I call this the Fender Wedge Chassis. Perhaps no one else calls it that, but I do. They used this basic construction for MANY MANY models of solid state.


      The output transistors come in a couple versions, some have the fully insulated tab and cab be mounted directly to a heat sink. The originals were the uninsulated type and they require the insulating mica wafer and insulating washer on the screw.

      Buying a cab for this chassis will be tough IMHO. The whole amp working is selling for $170-250 online
      Education is what you're left with after you have forgotten what you have learned.

      Comment


      • #4
        Also, when you replace parts and fire the amp up for the first time, make sure your speaker(s) are disconnected in case you missed something and the amp is putting out DC voltage. Check the output for DC before connecting the speakers.
        "Yeah, well, you know, that's just, like, your opinion, man."

        Comment


        • #5
          Agree with The Dude. It must have had the board pulled at least for a solder touch up or something.
          Yes the little 'bar' between heatsink and chassis can be installed wrong. But in working on hundreds of those, I never saw one backwards.
          It's not impossible that one could have left the factory that way. But if there had been a run of them, there would have been a service bulletin that went out to the warranty stations, and I never saw or heard of any.

          Like Enzo said, the full plastic encapsulated transistors are a variant with a different suffix.
          "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

          Comment


          • #6
            Here is the SM.
            Princeton_112_plus.pdf
            The actual cabinet dimensions are on the last page.

            Comment


            • #7
              Thanks everyone for your input and comments. I apologize for not getting back to this thread but have been busy working on a toilet flange issue that broke.
              Anyways I finished the repair and had my meters on critical points to monitor the amp as I powered it up. I have an old Sencore Powerite that can see the current as I increase the voltage. It came up fine with no problems. Played guitar through it using headphones and everything worked fine. Checked for any thermal issues but everything cool to touch. The seller was able to find the reverb tank so that was a bonus I hadn’t counted on. Since I couldn’t find ANY new or used knobs for the broken potentiometer treble and master volume, I located a suitable substitute that had a 6mm shaft and was able to make it work. Since the new knobs were different (and blue), I put them on the volume controls. Works for me.
              I haven't decided if I’ll build a combo cabinet or just a chassis cabinet head...depends on my wood working skills these days. Thanks for the dimensions info btw.
              Since I’m just now starting to get back into electronics repair, I find I don’t have all the necessary gear to properly test amps and other gear. I have an old Sencore oscilloscope to use but it is soooooo big I don’t really have room on my test bench for this beast. I love it and have used one for many years servicing musical equipment so I’m very familiar with its operation and features. I especially liked the fact I can check DC/AC voltages, frequency, and other parameters without connecting/reconnecting various different meters...all done with a push of a button and it reads out on an LCD display. Saves me lots of time and fumbling with different probes. BUT I’d like to replace it with a new compact oscope but don’t know anything about the new stuff that’s available and if they have similar way to measure what the Sencore does. I know certain brands like Tektronix and Leader but the new stuff I’ve never heard of so I don’t know if they are comparable and worth the money. If anyone can offer some advice on what they use or have experience with some of the newer Asian oscopes I’d appreciate hearing from you.
              Along that same line, I’m going to need some load resistors and see a lot of really cheap 8ohm non inductive 100 watt resistors from Asian sources. I’m used to using Dale resistors that cost quite a bit of money compared to these Asian counterparts. Are the ones I see on eBay for less than $7-8 really ok to use or are they going to blow up in my face �� if I push them hard? Just seems too cheap to be reliable but it’s a different world we live in now since I was a tech so I guess it’s possible that they can make them so cheap. If anybody has any comments on these cheap load resistors please advise.
              That’s about it for now... thanks everyone for your thoughts and advice.
              This is a great forum and glad I found it.
              Gary

              Comment


              • #8
                I think the load resistors you are referring to require substantial heatsinks to achieve their power rating.
                "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

                Comment


                • #9
                  Originally posted by Voxman View Post
                  Thanks everyone for your input and comments. I apologize for not getting back to this thread but have been busy working on a toilet flange issue that broke.
                  Anyways I finished the repair and had my meters on critical points to monitor the amp as I powered it up. I have an old Sencore Powerite that can see the current as I increase the voltage. It came up fine with no problems. Played guitar through it using headphones and everything worked fine. Checked for any thermal issues but everything cool to touch. The seller was able to find the reverb tank so that was a bonus I hadnít counted on. Since I couldnít find ANY new or used knobs for the broken potentiometer treble and master volume, I located a suitable substitute that had a 6mm shaft and was able to make it work. Since the new knobs were different (and blue), I put them on the volume controls. Works for me.
                  I haven't decided if Iíll build a combo cabinet or just a chassis cabinet head...depends on my wood working skills these days. Thanks for the dimensions info btw.
                  Since Iím just now starting to get back into electronics repair, I find I donít have all the necessary gear to properly test amps and other gear. I have an old Sencore oscilloscope to use but it is soooooo big I donít really have room on my test bench for this beast. I love it and have used one for many years servicing musical equipment so Iím very familiar with its operation and features. I especially liked the fact I can check DC/AC voltages, frequency, and other parameters without connecting/reconnecting various different meters...all done with a push of a button and it reads out on an LCD display. Saves me lots of time and fumbling with different probes. BUT Iíd like to replace it with a new compact oscope but donít know anything about the new stuff thatís available and if they have similar way to measure what the Sencore does. I know certain brands like Tektronix and Leader but the new stuff Iíve never heard of so I donít know if they are comparable and worth the money. If anyone can offer some advice on what they use or have experience with some of the newer Asian oscopes Iíd appreciate hearing from you.
                  Along that same line, Iím going to need some load resistors and see a lot of really cheap 8ohm non inductive 100 watt resistors from Asian sources. Iím used to using Dale resistors that cost quite a bit of money compared to these Asian counterparts. Are the ones I see on eBay for less than $7-8 really ok to use or are they going to blow up in my face �� if I push them hard? Just seems too cheap to be reliable but itís a different world we live in now since I was a tech so I guess itís possible that they can make them so cheap. If anybody has any comments on these cheap load resistors please advise.
                  Thatís about it for now... thanks everyone for your thoughts and advice.
                  This is a great forum and glad I found it.
                  Gary
                  Well thats a crappy job.

                  nosaj
                  Binkie McFartnuggets‏:If we really wanted to know the meaning of life we would have fed Stephen Hawking shrooms a long time ago.

                  Comment


                  • #10
                    Originally posted by g1 View Post
                    I think the load resistors you are referring to require substantial heatsinks to achieve their power rating.
                    Well the resistors look almost identical to the ones I’ve used before (Dale) and are rated for 100 watts. I’ve seen a few tech benches on YouTube that have the same physical characteristics but I couldn’t tell if they were Asian or US made. Just seems like there is such a wide price gap between the Asian and US/European manufacturers. I see some odd looking ones that Parts Express sells. They look more like a rectangular gold brick and are priced somewhat affordable to me. I know Parts Express well as I’ve used them a lot in my earlier tech days. MCM was another big electronics supply house...looks like they merged with another a favorite of mine, Newark Electronics. I use Mouser a lot these days and sometimes Digi-Key.

                    Comment


                    • #11
                      If you were an Asian manufacturer wanting to make cheap knock-offs, would you make them look different? Or would you make them look like the expensive, high quality ones?


                      I have some really nice Dale 250 watt resistors. They are my bench loads. But were I to make one today, I can load up on 50watt resistors and series/parallel them into a 500 watt load a lot cheaper than buying one resistor at the full spec.
                      Education is what you're left with after you have forgotten what you have learned.

                      Comment


                      • #12
                        Originally posted by nosaj View Post
                        Well thats a crappy job.

                        nosaj
                        Took me awhile, but I guess you were referring to the toilet flange job.

                        Originally posted by Voxman View Post
                        I see some odd looking ones that Parts Express sells. They look more like a rectangular gold brick and are priced somewhat affordable to me.
                        The aluminum case ones are the ones I meant (that need to be mounted on a heatsink).
                        "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

                        Comment

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