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Thread: Acoustic 370 Repair Low Output

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    Acoustic 370 Repair Low Output

    Hey ya'll,

    I'm working on an acoustic 370 that isn't sounding too hot. I haven't worked on these before, and in general, have a much easier time troubleshooting tube amps! It has very low output into a 4ohm load, and seems to mainly be passing predominantly midrage frequencies. Can someone who's more familiar with these offer troubleshooting suggestions? Thanks!

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    Supporting Member The Dude's Avatar
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    Here's the schematic.

    Click image for larger version. 

Name:	acoustic-370-schematic.jpg 
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Size:	1.61 MB 
ID:	52398

    Do the line outs work normally? That will tell you if it's a preamp or power amp problem.

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    Lifetime Member Enzo's Avatar
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    I am NOT suggesting you just change them all, but my immediate suspects are those little 1uf caps strewn through the preamp.

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    Old Timer J M Fahey's Avatar
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    I would inject 1kHz 18V RMS or 50mVpp at high gain input, J401, set all tone controls to 5 and follow voltages along the path. Just as suggested in the schematic.
    Yes, we can find missing voltage and it may be caused by those capacitors, in which case the test will show that.

    Check all voltages; those enclosed in triangles are Audio RMS voltages; if using a scope Vpp will be "almost" 3 times as high (actually 2.83) but as a rough check itīs close enough.

    You should reach 250mV RMS or 700mVpp at preamp output jacks.

    Also check power supply, you should have some +90V DC, then half that, some 45VDC at speaker out rail, on the positive lug of C404 and if all is well, some 20VAC at speaker out jacks.

    Take those measurements and report any abnormality.

    Being within 20% of expected value is fine.
    "Low output" by itself tells me nothing, "numbers" are way more precise.

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    Juan Manuel Fahey

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    Thanks for the schematic link. They aren’t working at all!

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    Quote Originally Posted by The Dude View Post
    Here's the schematic.

    Click image for larger version. 

Name:	acoustic-370-schematic.jpg 
Views:	47 
Size:	1.61 MB 
ID:	52398

    Do the line outs work normally? That will tell you if it's a preamp or power amp problem.
    Quote Originally Posted by J M Fahey View Post
    I would inject 1kHz 18V RMS or 50mVpp at high gain input, J401, set all tone controls to 5 and follow voltages along the path. Just as suggested in the schematic.
    Yes, we can find missing voltage and it may be caused by those capacitors, in which case the test will show that.

    Check all voltages; those enclosed in triangles are Audio RMS voltages; if using a scope Vpp will be "almost" 3 times as high (actually 2.83) but as a rough check itīs close enough.

    You should reach 250mV RMS or 700mVpp at preamp output jacks.

    Also check power supply, you should have some +90V DC, then half that, some 45VDC at speaker out rail, on the positive lug of C404 and if all is well, some 20VAC at speaker out jacks.

    Take those measurements and report any abnormality.

    Being within 20% of expected value is fine.
    "Low output" by itself tells me nothing, "numbers" are way more precise.
    I took some measurements in the suggested test points on the schematic. I injected a 1k 18mv RMS signal and on the positive leg of C107, I only read 139mV. 161mv on line outputs, and 104mv on speaker outs. All DC Voltages I took looked normal, and power supply is supplying all the correct voltage

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    Lifetime Member Enzo's Avatar
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    OK, you put signal to the input, and checked the output (of the preamp) Now go through the preamp stage by stage still using that input signal. See where the signal path seems to lose it.

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by Steelwitch View Post
    I took some measurements in the suggested test points on the schematic. I injected a 1k 18mv RMS signal and on the positive leg of C107, I only read 139mV. 161mv on line outputs, and 104mv on speaker outs. All DC Voltages I took looked normal, and power supply is supplying all the correct voltage
    Cool, now we are working

    Keeping all tone/EQ controls to "5", please rise volume to "10" ... do you now have expected 250mVAC on C107? (both legs should show the same.

    If so, continue with other indicated test points.

    You should have about same VAC value on C116 and C127 , since those controls set to 5 are unity gain.
    And same on line outputs.

    Which I already pre-confirm as "good" , since:
    read 139mV. 161mv on line outputs
    , showing about unity gain.
    Small discrepancies can come from pots not being *exactly* on 5 when set to that number, but in any case, I donīt see any significant problem with the preamp, letīs concentrate on following stages.

    That power amp has about 80X gain so even 160mV at its input should put out some 15V RMS at the speaker out..

    It does not, we must find why.

    1) rise volume to 10, check now you have 250mVAC at preamp out.
    If somewhat less, rise input signal slightly until it does.

    2) follow preamp signal path, measure at *both* ends of C130 - R301 - C301
    Be careful with the latter, measuring is safe but if meter slips and touchd what it shouldnīt it may damage the power amp.

    How are pre out and power amp in connected?

    If by a couple connectors and wire, some of them might be open or corroded.

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    Quote Originally Posted by J M Fahey View Post
    Cool, now we are working

    Keeping all tone/EQ controls to "5", please rise volume to "10" ... do you now have expected 250mVAC on C107? (both legs should show the same.

    If so, continue with other indicated test points.

    You should have about same VAC value on C116 and C127 , since those controls set to 5 are unity gain.
    And same on line outputs.

    Which I already pre-confirm as "good" , since:
    , showing about unity gain.
    Small discrepancies can come from pots not being *exactly* on 5 when set to that number, but in any case, I donīt see any significant problem with the preamp, letīs concentrate on following stages.

    That power amp has about 80X gain so even 160mV at its input should put out some 15V RMS at the speaker out..

    It does not, we must find why.

    1) rise volume to 10, check now you have 250mVAC at preamp out.
    If somewhat less, rise input signal slightly until it does.

    2) follow preamp signal path, measure at *both* ends of C130 - R301 - C301
    Be careful with the latter, measuring is safe but if meter slips and touchd what it shouldnīt it may damage the power amp.

    How are pre out and power amp in connected?

    If by a couple connectors and wire, some of them might be open or corroded.
    Ok, so we have 250mv on the preamp out. and 250mv going into the power amp. There is a significant decrease in ac voltage going across R301 and C301. Across them, - to + for C301, it decreases to approximately 50ma. I may test them out of circuit to see if they are functioning properly, I could not get a reading on R301 in circuit at all.

    Preamp and power amp are connected via plastic terminal strips and held onto the board with zip ties... which I really don't love. The wiring does look a bit corroded, but conclusively there are no bad solder connections. But it I guess that doesn't mean on of them couldn't be shorted.

    Wow!! Thanks for all your help! I'm not as familiar with solid state circuitry as I should be, and the layout of this amp has been difficult for me to lock down what's going on here. Anyway, the decrease in signal across them is abnormal, correct?

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    Old Timer J M Fahey's Avatar
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    You are doing fine

    Sorry maybe I was not clear enough with instruction details.

    Now we checked preamp is doing fine (may have other, minor problems such as hiss, hum, buzz, non working controls, but at least itīs amplifying and passing signal as expected) but now I must expand my
    2) follow preamp signal path, measure at *both* ends of C130 - R301 - C301
    to mean "measure at each end (left/right or top/bottom) of C130 - R301 - C301 relative to ground"
    So 6 values in total.

    Signal voltage should be same at each side of a coupling capacitor (or both sides would not be "coupled" and somewhat less, say 20%, after going through R301, but going from 250mV to 50mV is too much loss.

    But letīs have the proper measurements first .

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    Here’s a list of all voltages I’ve gotten per your suggestion, some of them have changed. Anyway:

    Input - 17.8mv
    Line out - 1.5 v
    Speaker out 968mv
    C130- 280mv, 1.04V
    R301- 40mv, 1.2 mv
    C301- 40mv, 0v

    It seems that I have no signal past this coupling cap?

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    Supporting Member The Dude's Avatar
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    Yep. Try changing the C301 cap.

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    Quote Originally Posted by Steelwitch View Post
    Here’s a list of all voltages I’ve gotten per your suggestion, some of them have changed. Anyway:

    Input - 17.8mv
    Line out - 1.5 v
    Speaker out 968mv
    C130- 280mv, 1.04V
    R301- 40mv, 1.2 mv
    C301- 40mv, 0v

    It seems that I have no signal past this coupling cap?
    Ok, fine, letīs pass an even finer comb.

    Please post the VAC to ground at the following points.

    Click image for larger version. 

Name:	acoustic-370-signal loss.jpg 
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ID:	52432

    Notice that *many* are in Theory the same, since they are directly connected by a solid line ... but such line in the Real World is a piece of wire, may include a connector, a soldered connection or a PCB track ... any of which may be damaged.

    And some of your measurements are inconsistent, such as having 4 times larger signal exiting C130 than entering it, or having R301 getting 1.2mV, exiting 40mV , or having 280mV or 1V at C130 (which is already weird) but onlt 40mV reaching R301 ... although both are joined by a solid line (a piece of wire).
    Such as 4 and 6 or 7 and 8 ... "the same point" ... but on opposite sides of a connector.

    Sorry for the annoyingly OCD way of measuring things, but it usually leads me *straight* to the problem, without random parts replacement.

    You do have old parts in poor state, no doubt about that, but letīs find the real problem first.

    Which often doesnīt even lie on a bad part, but bad wiring or connection.

    And my "bakerīs dozen" list of tests, looks long but is what I would do on my own bench, and goes "tick ... tick ... tick ..." end to end, just 5 or 10 seconds each ... until I find something WEIRD that is.

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    Last edited by J M Fahey; 02-09-2019 at 12:43 AM. Reason: forgot image
    Juan Manuel Fahey

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    Thank you for showing me where ac signal exists! my results are as follows:

    input 18mv
    1. 1.5v
    2. 1.5v
    3. 1v
    4. .97v
    5. 370mv
    6. 1v
    7. 1v
    8. 1v
    9. 9mv
    10. .1mv
    11. 36mv
    12. .7mv
    13. 2.5mv
    14 .1mv

    So it looks like we have significant signal loss after R301. This is what we're looking for! It seems though that there is some cumulative signal loss across some of these 1uf caps. Should those be considered for replacement?

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    Also, as OCD as it seems, I agree with you it's ultimately the quickest way

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    Old Timer J M Fahey's Avatar
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    Ok, the GROSS signal loss is across R301

    It *might* be open , not too likely but to rule that out for good measure resistance across it, with amp off .
    Use the 200k scale. I bet itīs fine (around 27k) but letīs check anyway.

    After that, between it and C301, we have a couple grounded elements, which might be dumping signal voltage to ground.

    Main suspect is Q315 Mute FET ; notice it is a P Channel Fet (most common ones are N Channel) , which needs positive voltage on its gate to open, schematic specifies almost +10V ; I suspect it has 0V or at least insufficient gate voltage, in which case it becomes a 100 to 300 ohm resistor shunting signal to ground..

    So letīs start by measuring DC voltage referred to ground at point 15 .
    Points 16 and 17 should also be around +10V and point 18 is the source voltage feeding them, some 80/90V (actually +V).

    Click image for larger version. 

Name:	acoustic-370-mute.jpg 
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    I suspect C315 is shorted or lossy , which coupled to verybhigh 6M8 resistior feeding it, amounts to about same in practice.

    If significantly less than +10V is found where it should be, junk C317 and replace it by a 10uF x 25V or 35V , whichever you have available.
    I bet when you recover proper Fet bias, youīll recover full signal into power amp

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    Last edited by J M Fahey; 02-10-2019 at 12:01 AM. Reason: as usual,forgot image :(
    Juan Manuel Fahey

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    Quote Originally Posted by J M Fahey View Post
    Ok, the GROSS signal loss is across R301

    It *might* be open , not too likely but to rule that out for good measure resistance across it, with amp off .
    Use the 200k scale. I bet itīs fine (around 27k) but letīs check anyway.

    After that, between it and C301, we have a couple grounded elements, which might be dumping signal voltage to ground.

    Main suspect is Q315 Mute FET ; notice it is a P Channel Fet (most common ones are N Channel) , which needs positive voltage on its gate to open, schematic specifies almost +10V ; I suspect it has 0V or at least insufficient gate voltage, in which case it becomes a 100 to 300 ohm resistor shunting signal to ground..

    So letīs start by measuring DC voltage referred to ground at point 15 .
    Points 16 and 17 should also be around +10V and point 18 is the source voltage feeding them, some 80/90V (actually +V).

    Click image for larger version. 

Name:	acoustic-370-mute.jpg 
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    I suspect C315 is shorted or lossy , which coupled to verybhigh 6M8 resistior feeding it, amounts to about same in practice.

    If significantly less than +10V is found where it should be, junk C317 and replace it by a 10uF x 25V or 35V , whichever you have available.
    I bet when you recover proper Fet bias, youīll recover full signal into power amp
    It was open! we now have 80v supplying, and 23v going to the gate of Q315, but no voltage on the drain or source. Let me know if that is an indication that it should be replaced, maybe I'm missing something. C315 has almost 10v on the positive leg, and 0v on the negative leg. points 16 and 17 also have 10v

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    Lifetime Member Enzo's Avatar
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    The bottom end of C315 is grounded, teh top end is wired to three resistors, and hte note says 9.8v there. Isn't 10v close enough? I'd sure think so.


    But you didn't report if that restored the sound...

    Well???

    TP16 and 17 are wired together, so we would expect them to measure the same. The schematic says 10.4v there. 10v sounds close enough to me.

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    Quote Originally Posted by Enzo View Post
    The bottom end of C315 is grounded, teh top end is wired to three resistors, and hte note says 9.8v there. Isn't 10v close enough? I'd sure think so.


    But you didn't report if that restored the sound...

    Well???

    TP16 and 17 are wired together, so we would expect them to measure the same. The schematic says 10.4v there. 10v sounds close enough to me.
    It unforunately did not.. but we have 1.2v on the speaker out which is a step in the right direction. one thing I did notice is that the line out has 1.2v now as well

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    Old Timer J M Fahey's Avatar
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    Yes, it is a step in the right direction: now signal finally *reaches* the power amp , goes through it, and appears at speaker out.

    Remaining problem is that power amp now has "gain of 1" or "unity gain" meaning: 1.2V in ... 1.2V out .

    No voltage amplification

    While it should show, as written on diagram: 20VAC (at speaker)/0.25VAC (at input)=40X

    I suspect C306 (which is part of the Negative Feedback Network which determines gain) is dry/open so plain replace it.

    Not very critical value, if you donīt have the exact one you may use up to 4.7uF or even 10uF there, by 35V or 50V , which you might already have in your parts box.

    I am quite certain youīll recover proper gain and if not yet, we are very very close.

    Ok, go and try it.

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