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Acoustic 370 troubleshooting help needed

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  • #16
    Again, what do you hope to find on a preamp parts list that we don't see on the schematic?
    Education is what you're left with after you have forgotten what you have learned.

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    • #17
      Exact transistor numbers ... I see the the 2N4248 on Q102 but all the rest simply read 48 - 23' .... so are they all 2N4248 or something else ... Sorry but I go by I've always used parts lists to be sure when doing other repairs (not electronics necessarily) and I'm not an expert in amp repair ... but I have do some minor electronics repairs ... have a good set of low wattage irons and am a clean solder person ... never lifted a trace .... not that that means anything.

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      • #18
        Enzo ... sorry and thanks ... didn't see your earlier response on house numbered parts and no, I'm not considering wholesale parts replacement ... that would be ... building a new amp and costly. I just want to be sure I get the correct replacements ...

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        • #19
          BTW ... I probably would never have gleaned that information from the schematic .... not an expert you see ... and the sourcing of parts was my way of confirming what I thought and that was that the parts are available ... but I do believe it may have gotten expensive to pay for the repair. I don't know the tech as this is my first shot at a repair. This music shop was recommended to me by a friend ... and it was the same shop I visited some 15 years ago with was having trouble finding techs ... I just got the impression from the shop owner who was nice but a little overly confident and cocky ... that his techs were the best .... and their combined experience would find a solution. So much for that.... my initial discernment was correct ... overly confident and cocky.

          Anyway ... not to be disrespectful or anything but you are obviously more well versed in this area and I am not ... I am well versed in my area of work and sometimes make assumptions on things that I think others should know ... that is sometimes a wrong assumption.

          I do appreciate the help ....

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          • #20
            I am just being direct, no criticism intended.

            I don't have my Acoustic lookup handy, but the part number is easily translated. No, the 2N4248 and 48-23 are not the same, they would use the same indication for all same parts on the drawing. Also, looking at them, one is NPN and the other PNP.

            Apparently 2N2484
            Education is what you're left with after you have forgotten what you have learned.

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            • #21
              And I'm being overly sensitive. I should have stuck to the electronics as a hobby when I changed career paths but a lot of life got in the way.

              Thanks for the additional info.

              Luck may be on my side. My wife's mom has a new boyfriend who was a cape engineer for 40 years (as was her late husband), has done amp repair and is a ham radio hobbyist ... has build a few base stations and antenna arrays. He has a couple of o-scopes, oscillator and other good diagnostic tools. He says he has lots of parts as well ...

              So we are going to spend some time over the holidays going through my 370 and see if we can track down where the signal goes bad. In general, it just doesn't make sense to me for an amp like this to be put on the shelf or sold for parts ... it has a legacy that newer amps can't match and I've not played a quieter more thumping (if not deadly) combination. So it is a bit of a refusal to surrender just because someone said it is hopeless or can't be done.

              I've spent most of my career in software and database development doing what others said couldn't be done but I tend to look at things differently ... anything is possible ... just depends on how determined you are.

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              • #22
                Enzo ... is the acoustic lookup something you could share ... once we get into this thing I'd like to be able to put together ... and here is that dirty word ... a "list" of what we will need if Lee doesn't have it on hand. More concerned over the transistors and not having biasing issues (stay original) ... and all that fun comparing of data sheets.

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                • #23
                  I have an entire career of technical manuals and collected notes... in my storage locker. Pretty buried, and winter happened and we won't do much deep digging until warm returns to Michigan.
                  Education is what you're left with after you have forgotten what you have learned.

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                  • #24
                    Winter! What is that? Here in Florida we blink and it is over!

                    You are forcing me to remember things long forgotten (that hobby I spoke of which almost became a career). This is the first (maybe second) schematic I've looked at in over 35 years ... maybe more.... and the first time I've had a need to try troubleshooting an issue. So I'm going to build myself a little audio probe to start with, check voltages, and then if I find nothing take a trip to the cape and visit an old NASA engineer! Should be fun ... and warm!

                    OK ... stay warm ... I have good friends up in the Grass Lake area (west of Ann arbor). I love their pictures of them trying to stay warm! They just moved back to their home in Grass Lake after selling their home here in Florida. I grew up in St Louis area so I have an idea of what winter is really all about.

                    Thanks .....

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                    • #25
                      Originally posted by mkreutz View Post
                      And I'm being overly sensitive. I should have stuck to the electronics as a hobby when I changed career paths but a lot of life got in the way.

                      Thanks for the additional info.

                      Luck may be on my side. My wife's mom has a new boyfriend who was a cape engineer for 40 years (as was her late husband), has done amp repair and is a ham radio hobbyist ... has build a few base stations and antenna arrays. He has a couple of o-scopes, oscillator and other good diagnostic tools. He says he has lots of parts as well ...

                      So we are going to spend some time over the holidays going through my 370 and see if we can track down where the signal goes bad. In general, it just doesn't make sense to me for an amp like this to be put on the shelf or sold for parts ... it has a legacy that newer amps can't match and I've not played a quieter more thumping (if not deadly) combination. So it is a bit of a refusal to surrender just because someone said it is hopeless or can't be done.

                      I've spent most of my career in software and database development doing what others said couldn't be done but I tend to look at things differently ... anything is possible ... just depends on how determined you are.
                      May I ask how did the amp fail?

                      You should tap lightly on all pots to make sure it's not just a broken solder joint or something silly like that.

                      PCB is mounted on a chassis via pots only which are soldered directly to the board; generally not a good idea with amps that reproduce low frequencies.

                      Also, I had to bypass preamp out to power amp in because shorting jacks just weren't making contact anymore; I see you already tried that.

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                      • #26
                        The 48-23 being a 2N2484 is something that is noted on the power amp parts list. That Enzo mentioned it does not necessarily mean there is a preamp parts list, or Acoustic parts lookup.
                        That being said, you may have more luck looking for the pre parts list by board number, rather than model, in case the same board is used in some other model.
                        Originally posted by Enzo
                        I have a sign in my shop that says, "Never think up reasons not to check something."


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                        • #27
                          Worked one weekend ... didn't the next ... amp wasn't moved to my knowledge in between ....

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                          • #28
                            Yup .. .see that in a lot of ACC schematics as well ... just opened the amp up last night and removed the pre-amp board for a visual/mechanical check and don't really see anything glaring. Hope to get back to it this weekend sometime to start checking voltages and then apply a signal and trace that. I did power it back up and the distortion/crackling gets louder as the volume is increased ...

                            I thought it would be nice to have a component layout so I've been working on that using the digikey.com Schem-it tool. It is not a necessity but has been educational ... having to recall the resister color coding has been fun but there are great tools for that on the internet. It will just help me step through the signal chain and process things in my brain as I move from schematic to board... maybe ... read that as a double check I guess. Visual person.

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                            • #29
                              Checking voltages revealed low voltage at q101 (.432) and q102 (7.4) ... from there it was the same throughout the preamp. Didn't find a single voltage other than the source and the voltages at R144 and R145 to be at spec. Using a simple audio probe the signal fell apart at Q107 ... was clean up to and at Q106, quite a bit of an increase coming out of Q102. Need to go back and check Q107 again but the signal was very dirty, broken up from Q107 on. Not settled in it being the issue and it being something in between ... bias perhaps ... drifting resistors and bad caps ... current drain somewhere (leaky caps) effecting all the voltages ....

                              Did notice the Mylar cap C111 at the midrange pot to be ... burnt orange? Like almost brown ... suspect. I have almost a full list of caps from the 1mfd on up put together from mouser and digikey (comparing prices) ... will start there and check biasing resistors along the way for drift.... and pick up a few replacements for the 2N2484 just in case. Has anyone replaced these using the TO-18 case? I'm wondering about the need to heat sink the case and room for that ...

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                              • #30
                                Enzo,

                                I applied a 1khz signal and traced it all the way to the pre-amp output and never lost the signal. Keep in mind I'm just tracing the audio at this point and trying to find the breakdown point without a scope. When I applied a varying signal (brought up itunes and a song with a heavy bass line), it appears the signal goes to crap at Q107 ... base, emitter provide the same trashed signal unless I have the pin out wrong but it leads me to believe it is not really Q107. Output from Q105 and Q106 is good which doesn't leave a lot in between. I've been staring at the schematic (and know I need to go back through everything again) ... are Q107, Q108 & Q109 part of the EQ circuitry? The tech mentioned bypassing the EQ, pulling the signal off after C116 from Q105 emitter ... or is that a dangerous idea. I didn't really like the idea ... why have EQ if it can't be used kind of thing. I figure the power amp input would need to be disconnected from C130 and relocated to C116 do this..... ???

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