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Thread: Acoustic 370 troubleshooting help needed

  1. #1
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    Acoustic 370 troubleshooting help needed

    I own an Acoustic 370 (with original 301 cabinet) which failed a few weeks back.. The output to any speaker cab and from the monitor outputs to a power amp produces weak and broken up (crackling) output. I've already taken it to one repair shop which evaluated it as not repairable due to availability (or the lack thereof) of old transistors and diodes. I've asked for a list of what they think is the failed components so I can begin my own search.

    I have a little electronics knowledge but no test equipment other than a digital and analog dvm. I'm not willing to make this a "for display only" amp as it has been my main amp since originally purchased back in 1970 while playing in high school bands (yes I just aged myself).

    I would appreciate any help, advice, direction, information that will steer towards restoring my 370 to full operation.

  2. #2
    Senior Member Enzo's Avatar
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    "Can't get transistors" is just silly.

    The line outs are from the preamp. So it would be the problem so far. Now the LAST thing I am is a replace all the caps all the time guy, but in an amp this age (over 45 years) all those little 1uf caps in the signal path are instantly suspect. But before changing them all, the schematic has DC voltages all over it, so go through the preamp and compare yours.

    I would also apply a steady strong input to the amp and follow it through teh preamp to see where it goes away.
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    Schematic

    acoustic-370-schematic.jpg
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    As far as the transistors go, yes some of the exact replacements are obsolete. But there are perfectly usable substitutes available.
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    Senior Member Enzo's Avatar
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    Exactly, most any good small signal transistor of the right polarity should work just fine in the preamp so long as voltage spec is met.
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    Supporting Member The Dude's Avatar
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    And, since this is a single supply output section with an output coupling cap, don't forget to check C404.
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    Does the sound get loud during that crackling or stays weak?

    I had a similar problem with my Acoustic 230 a couple of months ago. The crackling would stop if I pushed the volume knob at a certain angle. After reflowing all the pots and cleaning them the amp came back to life.

    I'm sure the guys at the repair shop would check for something like that, but given their response, anything is possible.

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    Senior Member Sowhat's Avatar
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    Got one of those as well sitting in my shop collecting dust, the customer abandoned it when I told him it was beyond economical repair. That's a pretty harsh assessment but that's probably what the tech you took it to was trying to say. It's not that the parts are unavailable or even expensive, it's the time involved in making things right that eat right into the economics of this repair. You might get lucky with some De-oxit but Enzo is probably right as usual when he suspects the 50 year old caps, the silicon stuff is probably just fine.
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    Check all those heavy ass inductors and orange drops in the preamp board to see if one of the legs has broken the solder connection.

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    Quote Originally Posted by Dave H View Post
    Schematic

    Click image for larger version. 

Name:	Acoustic 370-Schematic.jpg 
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    Dave, Thanks for the schematic ... I have a couple other copies of this but yours is a bit clearer .... I also have a parts list for the power amp section but do not have that for the pre-amp section .... if anyone has that I would greatly appreciate it as well. Not giving up on this just yet!

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    I do understand the response in a way .... and the economics of it. I can easily pick up a PF500 and sell my 301 bottom ... or perhaps a used Carvin which will run down to 2 ohm loads, but it isn't my 370. .... I spent a good part of Friday sourcing parts for the power amp section ... mostly the transistors ... and found pretty much all of them ... not having a parts list for the pre-amp ... I haven't gotten to that yet and am hunting that parts list (if anyone has one would appreciate it).

    It was cleaned and recapped about 15 years ago and a few transistors were replaced ... not sure which ones ... the entire job cost me a whopping $80 but it was done by a tech I was in the Navy with ... he moved to California or I would take it back to him.

    Right now I'm just sourcing parts ... and then will decide if it is economical ... and then decide if I can part with it. I've not used any other amp for any performance in all of my over 50 years of playing ... except recently here where I've been playing lead and acoustic ... but that is a different story ... it is just hard to part with something that has performed so well and has been the basis of my bass sound .....

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    That is my thought as well ... nice thump still from the power amp section on power up ... once I locate a parts list for the preamp section I'll start sourcing parts ... based on the schematic it looks like some of the pre-amp transistors are the same as some used in the power amp section.

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    Yup .. that was my thought as well ... and I never really dug into the electronics in all my years in the service ... had all the training ... not that it means anything ... took a lot of college classes ... not that it means anything either ... don't have the test equipment other than a few good dvm's ... ... that slows me down a bit. I know I can generate a steady tone and feed that to the input but tracing it may be fun without a scope.... not exactly what I would consider ideal trouble shooting.

    I don't think I can make it any worse unless I do something really stupid ... short short zap ... but the preamp section doesn't really rely on matched output transistors ... more of the power amp section where that comes into play. If I had to work there I'd definitely be at a disadvantage without the proper test equipment to find matching power trans.

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    Senior Member Enzo's Avatar
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    I am a little concerned over this parts list thing. Are you planning wholesale parts replacement?

    The parts are mostly identified on the schematic. The 49-23 transistors are house numbered, also called 480023, the lower left notes tell you about component sorts.

    What parts do you need to identify that a list would help?
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    Brought my 370 home from the shop last night ... I'll definitely check on all the recommendations everyone has offered so far ... and continue searching for parts. My wife's stepfather was an engineer at the cape ... had a very nice scope and signal generator ... he passed away a few years back and I'm not sure my mother in law kept all that or sold it but going to check that out ... having a decent scope will help trace the signal path ...

    Thanks for all the input ...

    Needs ... which I haven't found yet ...: Pre-Amp parts list if anyone can point me to that
    Common sense?

    I'm not playing bass right now and it was one of our other base players that was playing when my 370 gave it up .... the guy actually feels bad that it happened while he was playing. I understand the age issue (not me the amp!) ... he though maybe he did something. I did my best to explain to him that it was a tank but stuff breaks ... especially old stuff.

  16. #16
    Senior Member Enzo's Avatar
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    Again, what do you hope to find on a preamp parts list that we don't see on the schematic?
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    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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    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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    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 ....

  20. #20
    Senior Member Enzo's Avatar
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    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
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    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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    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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    Senior Member Enzo's Avatar
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    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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    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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    Quote 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.

  26. #26
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    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.
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    Worked one weekend ... didn't the next ... amp wasn't moved to my knowledge in between ....

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    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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    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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    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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