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Thread: SUNN SR4150 running hot.

  1. #1
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    SUNN SR4150 running hot.

    I have a Sunn (Fender) SR4150 that I bought from a relative, it was used for years with their band and finally developed a problem with the volume cutting out on it on all four channels, they no longer tour with the band anymore and sold it to me along with the speakers that they toured with that go with it.

    I resoldered a bunch of stuff on both circuit boards that looked iffy but the volume was still dropping off, it would start out ok and then suddenly the volume would get really quiet but if you banged on the microphone hard enough the volume would come back just to drop out again several seconds later, this was happening on all four channels and with different mics and mic cords.

    All TO-92 transistors were pulled from the amp board and tested fine and were reinstalled, all three TIP102s, all three TIP107s, and the TIP29s and TIP30 tested ok but were all replaced at the same time anyway, I had to tweak the little balance adjuster on the amplifier circuit board to get it to amplify again and it no longer has the problem with the volume cutting out although honestly I don't really know where the adjuster is exactly supposed to be and couldn't find anything online about how to correctly adjust it.

    The TIP29 and TIP30 that make up the power supply voltage regulators are both running really hot and do not have heat sinks which I am planning on getting for both of them.

    And on the back of the case where the giant heat sink is it is running REALLY hot hot enough that I am concerned about it, I asked the previous owner if this was normal or not but so far I have not heard back from them.

    The sticker on the back by the output jacks says "Minimum Load 4 ohms; 4 ohms one jack or 8 ohms both jacks." one speaker is reading as 8 ohms, the other at 7.2 ohms, which comes to 3.79 ohms but I don't think that 0.21 ohms missing is going to cause it to run this hot, I also checked the voltage at the TIP102/TIP107 transistors it is either reading +43 volts or -43 volts depending on the transistor and these are all rated for either 80 or 100 volts.

    So my questions to everyone are...

    1. Is it normal for the back of this thing to be REALLY hot and should I not be concerned about it or is this abnormal and something to worry about?

    2. Does anyone have the adjustment procedure on how to properly trim the balance adjuster?

  2. #2
    Supporting Member The Dude's Avatar
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    My best guess is that the amp is running hot because you messed with the bias adjustment. As a general rule, you shouldn't go twiddling pots in gear unless you know what they are for and how to adjust them. You could do serious damage. Here's the manual with bias procedure on the last page.

    http://music-electronics-forum.com/a...0-sx-4150-.pdf

    Start by adjusting it according to procedure and then let us know what works and what doesn't. I suspect the original problem was likely switching jacks or a connection issue.

    P.S. Welcome to the place!
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  3. #3
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    Quote Originally Posted by The Dude View Post
    My best guess is that the amp is running hot because you messed with the bias adjustment. As a general rule, you shouldn't go twiddling pots in gear unless you know what they are for and how to adjust them. You could do serious damage. Here's the manual with bias procedure on the last page.

    http://music-electronics-forum.com/a...0-sx-4150-.pdf

    Start by adjusting it according to procedure and then let us know what works and what doesn't. I suspect the original problem was likely switching jacks or a connection issue.

    P.S. Welcome to the place!
    Thank you for the schematic, what I didn't realize is that I have been looking at the old version of the schematic where the Q1 and Q2 resistors were 150 ohms, this amp has apparently already been through the "resistor conversion" and those are now 330 ohms, and the old schematic does not have that little note at the bottom telling you how to bias it at 10mV DC.

    Normally I never touch trim pots they are touchy and good luck getting it back where you had it but its the only one that this has so I figured it was the problem with it not amplifying after changing out all the parts and I tweaked it not far from where it had been and the amp started working again but then it had the getting really hot problem, and yes I agree that serious damage can occur.

    So... I clipped some alligator wires onto the two resistors and set my DMM for a reading of 0.00 and where the trim pot was at had the bias at 0.50 instead of 0.10, this trim pot is really touchy and was only really happy fluctuating between 0.10 and 0.11, eventually it settled at 0.10 and the heat sink is now running cold at room temp.

    The only odd thing is that when I turn it on the bias is at 0.08 and the speakers buzz but after about 8-10 seconds the buzz goes away and the bias returns to 0.10 and the amp runs fine, its kind of like waiting for a tube amp or old TV to warm up and doesn't really bother me in one aspect but if I were one of my customers I would be complaining about it, so I'm wondering if I have a bad capacitor somewhere, like C9 or C20 that is pulling the circuit down initially.

    C1 and C2 were already pulled and tested about a month ago one of them was reading low but it was within 10% and I reinstalled it but now I'm wondering if its going bad under load? Guess I'll start pulling all of the capacitors and testing them again.

  4. #4
    Senior Member Enzo's Avatar
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    Please don't pull all the parts and test them. Your meter will not put real world voltages on the parts and so cannot see if a cap is leaky for example. A meter might tell you a transistor can transist, but not if it can do it at current. Your cap meter will not put 40v on the caps. The value of the cap is rarely a problem, leakage is the issue when they fail.

    I doubt twiddling the bias brought back the sound. It should amplify at any setting. On the cold end, crossover distortion is excessive, at the other end, the amp gets blazing hot, but still amplifies. Intermittent loss of sound is most likely a bad patch jack, especially if hitting the thing with a brief really loud signal brings it back. Try cleaning out the Graphic EQ input jack.

    Speakers are rated at impedance, not resistance. It is normal for an 8 ohm speaker to measure about 7 ohms DC resistance.

    Bias reading: 0.1 and 0.11 are either one close enough, don't sweat 10%.

    Adding heat sinks to the transistors is a great idea.
    Education is what you're left with after you have forgotten what you have learned.

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    Quote Originally Posted by Enzo View Post
    Please don't pull all the parts and test them. Your meter will not put real world voltages on the parts and so cannot see if a cap is leaky for example. A meter might tell you a transistor can transist, but not if it can do it at current. Your cap meter will not put 40v on the caps. The value of the cap is rarely a problem, leakage is the issue when they fail.

    I doubt twiddling the bias brought back the sound. It should amplify at any setting. On the cold end, crossover distortion is excessive, at the other end, the amp gets blazing hot, but still amplifies. Intermittent loss of sound is most likely a bad patch jack, especially if hitting the thing with a brief really loud signal brings it back. Try cleaning out the Graphic EQ input jack.

    Speakers are rated at impedance, not resistance. It is normal for an 8 ohm speaker to measure about 7 ohms DC resistance.

    Bias reading: 0.1 and 0.11 are either one close enough, don't sweat 10%.

    Adding heat sinks to the transistors is a great idea.
    I think that one side of the power supply is coming up to voltage before the other which is causing the bias to be at 0.08 and then slowly come up to 0.10 and make the speakers buzz, so what I am going to do before anything else is hook two DMMs up to it and see what the separate - and + voltages are doing right when I turn it on.

  6. #6
    Supporting Member Jazz P Bass's Avatar
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    I would not focus so acutely on the bias voltage.

    That is not going to make the speakers 'buzz'.

    You can check the capacitors for ripple.
    Set your meter to read Volts ac.
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  7. #7
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    Quote Originally Posted by Jazz P Bass View Post
    I would not focus so acutely on the bias voltage.

    That is not going to make the speakers 'buzz'.

    You can check the capacitors for ripple.
    Set your meter to read Volts ac.
    Ok, I'll see if I can figure that out, its only buzzing for about 10 seconds and then it goes away.

    Here is what I figured out earlier this evening...

    I hooked up two DMMs to the outputs of Q1 and Q2 the voltages fly right up to -about- where they should be and stop at about 14.5 volts, however the circuits have two zener 15 volt diodes and both positive and negative sides slooooowly climb over about 15 minutes from +14.5/-14.7 up to +14.98/-15.05 volts and then stay there, but when I turn the power off the both plummet to about 0.xx at the same time but the negative side drops to 0.0x but the positive side is retaining 0.5 volts and never drops to 0.0x.

    Meanwhile... the +44.x/-44.x volt supplies instantly fly up to their respective 44.x volts and stay there, but again when I power it off the negative side drops to 0.xx but the positive side is retaining 1.xx volts and never drops to 0.xx, so that leaves me two questions...

    1. Why is the 15 volt side taking so long to completely come up to 15 volts?

    2. Why are both positive and negative sides not equalizing to 0.xx equally when the power is turned off?

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    I did check for A/C voltage on C1 and C2, from ground I'm getting 0 volts on C2 but 95 volts on C1.

    On the DC side from ground I'm getting +44/-44 so it makes no sense why 95 volts A/C is leaking through.

    Edit: I also pulled all four rectifier diodes out they are testing at 457, 457, 483, and 499, getting infinity the other direction on all four, this is probably it for now until after Thanksgiving.
    Last edited by LandonRobinson; 11-22-2017 at 07:55 AM.

  9. #9
    Senior Member Enzo's Avatar
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    Your high and low voltage rails come up together. At this point you can rule out uneven rails as the source of your hum. I appreciate interest in circuits, but since your hum problem is not on power down, it kinda doesn;t matter about the discharge of the one side. It does not indicate a problem.

    I also pulled all four rectifier diodes out they are testing at 457, 457, 483, and 499, getting infinity the other direction on all four,
    I think your meter might have a decimal point before those numbers? The diode test reads in voltage - the forward voltage drop of the diodes. By removing and replacing parts to test them, you are more likely to damage or weaken them. You know the diodes work, because your power supplies work.

    95vAC? What meter are you using? DO me a favor, measure 9vDC on a 9v battery with it. Now set the meter to AC volts and measure that battery. If it doesn't settle to zero volts after a couple seconds, your meter is not able to measure AC with DC present. If the battery measures 12vAC or something that cinches it. I suspect your meter just sticks a diode in series with the DC readings. One direction it blocks, the other direction it doesn't
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  11. #11
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    Quote Originally Posted by Enzo View Post
    Your high and low voltage rails come up together. At this point you can rule out uneven rails as the source of your hum. I appreciate interest in circuits, but since your hum problem is not on power down, it kinda doesn;t matter about the discharge of the one side. It does not indicate a problem.



    I think your meter might have a decimal point before those numbers? The diode test reads in voltage - the forward voltage drop of the diodes. By removing and replacing parts to test them, you are more likely to damage or weaken them. You know the diodes work, because your power supplies work.

    95vAC? What meter are you using? DO me a favor, measure 9vDC on a 9v battery with it. Now set the meter to AC volts and measure that battery. If it doesn't settle to zero volts after a couple seconds, your meter is not able to measure AC with DC present. If the battery measures 12vAC or something that cinches it. I suspect your meter just sticks a diode in series with the DC readings. One direction it blocks, the other direction it doesn't
    I tested a 9 volt battery its reading as 9.26 volts, on AC its reading as 19.2 volts so yeah this meter can't read A/C correctly with DC voltage present, its a cheap $7 Harbor Freight meter that I kept in the car for on-the-side-of-the-road troubleshooting but my really good meter that I have had for years died and it won't be replaced until I get my Xmas bonus from work, one of my roommates has an expensive Fluke I am going to retest with tomorrow.

    I was over at my Dad's place for Thanksgiving he has been fixing stuff since before I was even born, he suggested replacing C3, 4, 5, and 6, and well sure enough C3 is reading as 0.38uF, C4 was reading as 8.94uF, C5 and C6 were both reading as 10.xx uF they should have all been reading 10.xx, since I already had them out all four were replaced anyway, R1, 2, 3, and 4 are all testing within 10% tolerance, but the amp still buzzes for 10 seconds after you turn it on and the +/- 14.5 volts still takes forever to get up to 15 volts.

    C24, 25, 26, and 27 are going to be checked tomorrow.

  12. #12
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    Quote Originally Posted by LandonRobinson View Post
    I tested a 9 volt battery its reading as 9.26 volts, on AC its reading as 19.2 volts so yeah this meter can't read A/C correctly with DC voltage present, its a cheap $7 Harbor Freight meter that I kept in the car for on-the-side-of-the-road troubleshooting but my really good meter that I have had for years died and it won't be replaced until I get my Xmas bonus from work, one of my roommates has an expensive Fluke I am going to retest with tomorrow.

    I was over at my Dad's place for Thanksgiving he has been fixing stuff since before I was even born, he suggested replacing C3, 4, 5, and 6, and well sure enough C3 is reading as 0.38uF, C4 was reading as 8.94uF, C5 and C6 were both reading as 10.xx uF they should have all been reading 10.xx, since I already had them out all four were replaced anyway, R1, 2, 3, and 4 are all testing within 10% tolerance, but the amp still buzzes for 10 seconds after you turn it on and the +/- 14.5 volts still takes forever to get up to 15 volts.

    C24, 25, 26, and 27 are going to be checked tomorrow.
    C24, 25, 26, and 27 were checked and were fine but while I had them out I just replaced them, but long story short I am just going replace a majority of the capacitors on the board just to get them done and out of the way so that I don't have further problems with this thing down the road, chances are that it is still going to have that short buzz and have a slow build up on the +15/-15 power side but oh well...

    The input board has several chips and other things that all use the +15/-15 volts and its probably going to be a nightmare to find which one is causing the problem, so once the main board is operational again I'm just going to leave this thing alone until something dies.

    So thanks everyone that chimed in for helping with the original bias adjustment problem that was my biggest concern, please consider this issue closed.

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