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Thread: Acoustic 370 repair

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    Acoustic 370 repair

    I have an Acoustic 370 bass amp that seems to be a little different from the schematic I have.
    My schematic shows 100 ohm resistors at R339 and R340 yet my 370 does not have them. It also does not have double diode CR310.
    It only has 30 volts on its output even though the rails are at 90 volts. I also cannot get more than 8 volts on the diff. bases unless I disconnect R314. Any thoughts ?

    Thomas Robert

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    Please post the exact schematic you are referring to, or a link to it.

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    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

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    Here's the schematic I have. It should work without the missing parts. As the output is sitting between the rails I'd suspect the LTP transistors. Will R304 adjust the output voltage?

    Click image for larger version. 

Name:	Acoustic 370-Schematic.jpg 
Views:	50 
Size:	1.87 MB 
ID:	53993

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    That is the schematic I have. R304 will move the rail but it never gets above 30 volts. I have observed that the output voltage drops within a few seconds of turning it on.

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    I suspected the differential pair. They showed a slight (probably normal) leak on my tracer. I changed them but no change on the output.

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    Are the base voltages the same on the differential pair? The output voltage should be roughly 4 x the voltage on Q301 base. Can R304 be adjusted to set 11V on C304 + (and Q301 base)? If it won't go above 8V check for a leaky capacitor (C301, C302, C304)

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    Dave, thanks for helping me. Sorry for the long delay. I subbed a pair of 2N5400 for Q301 and Q302. I can adjust the base of Q301 to 11v but the base of Q302 is low (below 8 volts) as are their emitters. I disconnected c301, 302 and 304 with no change. I have the FET out Q315) but I don’t think it’s needed to operate. I also pulled the drivers 306 and 311 out. 306 was hot and 311 was cold. All the transistors test good. I also noticed I have 2.2 volt difference at Q-304 even though the voltage there is low (about 28v). I’ll go farther into it tomorrow. Again, thanks for help. Thomas

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    I'm going to sound like a broken record, but how old are the electrolytic caps? If they're not less than 10 years old, replace them all before going any further. A little leakage in C301, 304, 305, 306 and/or 307 can hose up the bias points of this circuit.

    If I were working on it, I'd replace all the electros in the power amp (at least) including the output capacitor. Then I'd start debugging the DC voltages on transistors.

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    Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

    Oh, wait! That sounds familiar, somehow.

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    Quote Originally Posted by R.G. View Post
    If I were working on it, I'd replace all the electros in the power amp (at least) including the output capacitor. Then I'd start debugging the DC voltages on transistors.
    Good plan, I'd also check C308 for leakage. With the base voltages you have on the LTP Q301 should be off and Q302 should be on. Is the collector of Q301 at 0V and the collector of Q302 at 8V? That should put 0V on the base of Q303 to turn it off (unless C308 is leaky). Is Q303 base at 0V?

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    Next step.
    Once I was sure that the caps were not leaking me into problems, I'd remove Q306,7,8,9,11,12,13,and 14 temporarily. If a solid state amp is not loaded it should be stable, and since the output stage amounts to a gigantic emitter follower, it doesn't have much to do with the overall gain and stability of the amp as long as the loading on the output doesn't require high currents. So with no external load, SS amps will generally be stable and can be diagnosed without their gain-of-one outputs. Leaving Q305 and Q310 in ought to leave the amp with much the same pin voltages on the input transistors, gain stages, and drivers, and it ought to pass a signal to the now-open-circuited output suitable for testing with a signal. You may and probably will have crossover distortion, but we know how to deal with crossover distortion when everything gets put back together.

    If everything looks great without the output follower-buffer transistors, you have isolated your problem as something to do with the output devices.

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    Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

    Oh, wait! That sounds familiar, somehow.

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    Since you have rotated everything except the screw, before you proceed to further repairs, restore the previously disassembled components.
    In order for the differential pair to operate properly, the R314 must exist because the offset voltage of the output stage is maintained through it.

    The amplifier never fails by itself. Usually somebody helps him, the owner, the bad main voltage, the bad contacts or the cables, or the defective loudspeaker.

    When a fault occurs, it is usually possible to damage output transistors (Q307-309; Q312-314) driver transistors (Q305,306; Q310,311) and transistor for idle current Q304.
    If an accident occurs in the "output" output stage, the offset voltage is outside of regulation and (or) can not be regulated.

    Question
    1. How much is voltage on + UC403
    2. In which limits the voltage is changed to + UC404
    When the amplifier is correct + UC404 is approximately equal to half the voltage at UC403 (the order is + 45V +/- 5%)

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    Acoustic 370 repair

    To avoid unnecessary financial costs that may arise during repairs, install a protection in the + power supply, which consists of a resistor limiter 220-470 ohms / 50W and more and a 1 ohm resistor sensor for monitoring all current changes.

    Repair Mode
    - How much is the voltage drop on the sensor resistor 1 ohm. (1 mV = 1mA)
    - How much change the voltage to + UC404 with the change of the R304
    If the amplifier can not be adjusted, disconnect the transistor bases (Q307-309; Q312-314) from the driver emitter (Q306, 311)
    - Measure how much voltage is at + UC404.
    - Monitor voltage changes (current) on the sensor resistor 1 ohm. (1 mV = 1mA)

    Click image for larger version. 

Name:	Acoustic 370 repair.jpg 
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ID:	54046

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    That's reasonable. Another approach would be to make and use a light bulb limiter, which is a simpler way from keeping the whole amplifier on a power budget.

    Removing the outputs to eliminate the power transistors as a source of both problems and chain failure, ("disconnecting Q307-9, Q312-314") is also reasonable I recommend (and recommended) it. With those disconnected, the amplifier will still work from a voltage gain standpoint, but cannot drive much current at all. That's OK for getting the voltages running correctly.

    Another trick one can use with power amps that are misbehaving is to simply short the bias structure temporarily. The amp will have gross crossover distortion, but will behave OK from a DC voltages standpoint, and you can easily remove th biasing short when you get everything else working.

    The temporary patches (disconnect output buffers, short biasing, etc.) are designed to simplify whatever else may be mysterious.

    I'm eager to hear what capacitor replacement does.

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    Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

    Oh, wait! That sounds familiar, somehow.

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    Thanks RG and you are so right. I will replace them before I call it done.

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    Thanks Dave, I removed C302, C304, C305 today with no change. I removed Q-303 and my new Q-301 seems to work.
    I replaced R309 as it was out of spec. Adjusted base voltage to 11 volts gets 11.6 on the emitter and I was reading about 4 volts on the collector. This is with Q303 out. You know this unit does not have a CR310, R341, R339 or R340.
    Q310 has enough leak that I’ll sub both pre-drivers with ECG-128 and 129 which I have. The pre-drivers in it were labeled
    128 and 129 so they have been changed. They have the added heat sinks. Also the board had lost some of the traces around these so it was sort of patched using long leads. The one leaking looked discolored.
    It’s a good idea to change all the paper caps. Tomorrow I would like to pull the drivers and maybe oall the outputs as I test my new predrivers with no load. What do you think?
    Thomas.

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    Thanks. I plan on removing all the TO-3 tabs tomorrow.

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    Quote Originally Posted by Thomas Robert View Post
    Adjusted base voltage to 11 volts gets 11.6 on the emitter and I was reading about 4 volts on the collector. This is with Q303 out.
    I think there's 4V on the collector because it's trying to turn on the non existing Q303 to bring the output voltage down. Where is the output sitting with Q303 out? It should be high (~90V) if it's not the bootstrap capacitor (C307) could be leaky. Disconnect one end of it to check (I'd replace it anyway).

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    Quote Originally Posted by Thomas Robert View Post
    My schematic shows 100 ohm resistors at R339 and R340 yet my 370 does not have them. It also does not have double diode CR310.
    http://mikesgig.com/wp-content/uploads/2013/11/Acoustic-370-manual-410056.pdf
    Acoustic 370 instruction manual

    Question
    Instead of resistors R339 and R340 whether there are jumpers or not. From the layout of the printed board, there must be traces of the intervention of the soldering iron.

    If 370 does not have a resistor R339 and R340 (100 ohm) it indicates that someone for some reason removed them, and separated the driver and power transistors from the driver part of the power amp.

    Without the resistor R339 and R340 power amp can not work, it just raises the question of whether the malfunction is in the driver part of the output stage, or power part (transistors Q305 - Q314)

    In order to repair the amplifier, you must first bring it into an operational state, that is, you must return it to the factory schematics and by measuring to determine which component is defective.
    Return R339, R340, CR310 and all the missing components of the power amp.

    To start repair first disconnect preamplifier (resistor R145) from power supply (+ 87V) and embed limiter protection (resistor 220 - 470 ohm / 50W and more) as described on schematics.

    https://music-electronics-forum.com/attachment.php?attachmentid=54046&d=1560978663

    Limiter protection (resistor 220 - 470 ohm / 50W) prevents the power amp from "pulling" more than 0.5A current, and also enables it to comfortably perform all the necessary tests.
    Sensor resistor (1 ohm / 1W) makes it possible measure current through power amp with DVM.

    Check all driver and power transistors correctness with ohm meter (range x1000). Replace faulty transistors with the same type.
    When replacing the transistor, it is desirable to be matched hfe in the limits +/- 10% and tested on the breakthrough voltage (min 120 V).

    Visually and measurement check R320 - R327 whether due to a malfunction did not change the value.
    Reliable operation of the power amp depends on their correctness.
    Be sure to check C308, 311, 312 to not leak.
    Preventively change (due to age of the amplifier) C306, C307
    All suspicious and defective components replace with new same values.

    Turn on power amp. Limiter protection is in repair mode.
    Measure voltage at + UC404. Whether and how much change by changing the resistor R304.
    Using the R304 set the voltage at UC404 to +45V (half voltage UC403)

    Measure voltage at sensor resistor. By measuring the voltage on the sensor resistor, indirectly measure the current of power amp.
    Edit: 1mV = 1mA
    Question How much is your measured values?

    On the schematics 370 are given some voltage measuring points.
    Question How much is your measured values?

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    Last edited by vintagekiki; 06-22-2019 at 07:35 PM.

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