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Thread: Marshall JTM30 bias mod problem & IC3 (LM348) problem

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    Supporting Member Tone Meister's Avatar
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    Marshall JTM30 bias mod problem & IC3 (LM348) problem

    JTM30 has two issues. 1) excessive hum and 2) bias mod problem

    Problem 1: The amp powers on but has substantial hum that overrides any amplified audio signal. I can unplug CON3 and that noise is eliminated - of course a slight hum is introduced with CON3 disconnected but that is expected. Neither channel volume effects the hum, but it is eliminated when the MASTER VOLUME is all the way down. Tested IC3 op amp and there is negative DC voltage on every pin.

    IC3 LM348N - all voltages DC

    pin 1 (-4.7) ... pin 8 (-16.5)
    pin 2 (-5.0) ... pin 9 (-15.4)
    pin 3 (-5.6) ... pin 10 (-14.6)
    pin 4 (-5.9) ... pin 11 (-14.7)
    pin 5 (-7.4) ... pin 12 (-12.4)
    pin 6 (-8.5) ... pin 13 (-11.5)
    pin 7 (-9.5) ... pin 14 (-10.3)

    Problem 2:
    The power tubes are redplating and bias idle current is excessive. The bias circuit was previously modded - the zener string (ZD101, ZD102, and ZD103) was removed in favor of a 56K R - 50K pot - 56K R string. I'm not able to adjust the bias with the bias pot. I have confirmed that both 56K resistors are good and the pot itself works fine in isolation. The stock JTM 30 has a zener diode regulated bias supply that is "non adjustable", so it runs hot because Marshall calls it a "class A amplifier" with two 5881s producing ~30 watts. This amp still has what appears to be the stock power tubes.

    I've done this mod myself to a couple of these in the past and have had no problems, but I had taken working amps and performed the mods entirely myself from start to finish.

    Can these two problems be related?
    Is that LM348N op amp toast?
    Attached Files Attached Files

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    Senior Member Enzo's Avatar
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    The 348 could be defective, but not from anything you have shown us. Clearly the +15v rail is missing. Are the channel switching LEDs working? And the relay? They also run on +15. Is the 78L15 getting input voltage?

    I see no involvement in bias from the +15v. The bias supply is just s simple supply.

    So what bias supply voltage is on the grids? The zeners add up to 39v, do you have anything like that?

    I am not sold on the adjust mod. After all, what it amounts to is a 150k resistance in parallel with R132. The pot will dial it down to 100k. So I think the range of adjustment is roughly 50k-60k.
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    Please take a look at the schematic. On pin #4 there should be +15V. Most probably, the REG2 (78L15) has failed, or R35 is open. Do you have voltage on the input of REG2?

    Mark
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    Supporting Member Tone Meister's Avatar
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    Will check those things with the +15v rail today.

    I guess I'll try to figure out why the bias has run away and isn't adjustable with the R string first before trying to restore the bias circuit to stock. Otherwise I'd have to order the correct value zeners. We've modded these for bias adjust before and the amps work well.

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    Supporting Member Tone Meister's Avatar
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    UPDATE

    Problem 1 - This was easily solved. The two metal standoffs for the tube PCB is ground, and without them screwed tight there is no ground. Both screws were missing, so when I replaced the screws the +15v rail was restored. Voltages at the LM348 are what they should be. Hum is gone.

    Problem 2
    - There is -17.8 volts at the grid of each power tube, measured at R127 and R129. The bias is still not adjustable with the Bourns 50k multi turn pot. I removed and tested the pot and both 56k resistors and checked each solder pad for integrity. Reinstalled the bias mod resistor string and still unable to adjust the bias. There are 0.0 volts at one end of the bias R string and those 17.8 volts at the other. Checked and verified C127, R133, R132, C126, D108, D109, F1, D2, and D3.

    No doubt this will be as simple as the +15v rail but right now I don't see it. Any insight?

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    g1
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    Did you check R107?
    There are checks you can make with power tubes removed. AC voltage on either side of R107 and C127. DC voltage at anode of D107.
    "there's another kind of party lights that I can't stand to see,
    when there's a man in that patrol car and he don't wanna party with me"

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    Supporting Member Tone Meister's Avatar
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    Quote Originally Posted by g1 View Post
    Did you check R107?
    There are checks you can make with power tubes removed. AC voltage on either side of R107 and C127. DC voltage at anode of D107.
    AC voltage on either side of R107 196 & 190 VAC
    AC voltage on either side of C127 190 & 92 VAC
    DC voltage at cathode of D107 52.5 VDC
    DC voltage at anode of D107 -22.6 VDC
    Last edited by Tone Meister; 09-29-2015 at 01:00 AM.

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    g1
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    That would be minus 52.5VDC ?
    So you have -52VDC across your string of 56K resistor, 50K pot, and 56K resistor?
    What are your voltages at each those 3 components, and how is the pot wired?
    "there's another kind of party lights that I can't stand to see,
    when there's a man in that patrol car and he don't wanna party with me"

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    Supporting Member Tone Meister's Avatar
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    +52.5 at the cathode
    -22.6 at the anode

    at the top of the 56K where the 9V ZD101 once was = 0.0 vdc
    Between the top 56K and the pot = -7.9 vdc
    Other side of pot where it connects to bottom 56K = -14.8 vdc
    Bottom of 56& where ZD103 once was = -22.6 vdc

    bias-supply-jtm30.jpgervin-s-bias-supply.jpgbias-jtm30.jpg
    Last edited by Tone Meister; 09-29-2015 at 06:38 PM.

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    g1
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    You have 92VAC going to D107, you should get a lot more negative at the anode than what you are getting (-22.6).
    I'm guessing D107 anode is no longer connected to C126.
    You said you have done this mod before, I'm sure you will find this has not been done the same way.
    You need to draw out what you have for the bias circuit there, and compare it to the drawing for the way you have done it before.
    No drawings, no go.
    "there's another kind of party lights that I can't stand to see,
    when there's a man in that patrol car and he don't wanna party with me"

  11. #11
    Supporting Member Tone Meister's Avatar
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    That's what's funny, this one is wired the same way as the ones I've done. It's really as simple as removing 3 zeners and replacing them with 3 resistors, one being variable. There is continuity from the anode of D107 to C126.

    Here is a crude drawing that shows the same thing as the annotated snapshot from the schematic in post #9.


    Click image for larger version. 

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    Last edited by Tone Meister; 09-29-2015 at 06:37 PM.

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    Senior Member Enzo's Avatar
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    I don't see where you replaced or tested D107.
    Education is what you're left with after you have forgotten what you have learned.

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    g1
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    And aside from the bias circuit voltage problem, with the pot wired as a 50K resistor, it can't do anything as far as adjusting goes. At least that's how you've drawn it.
    "there's another kind of party lights that I can't stand to see,
    when there's a man in that patrol car and he don't wanna party with me"

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    Supporting Member Tone Meister's Avatar
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    Quote Originally Posted by Enzo View Post
    I don't see where you replaced or tested D107.
    D107 tested good as did R107, R105 and R106. There is something simple here that I am missing


    Quote Originally Posted by g1 View Post
    And aside from the bias circuit voltage problem, with the pot wired as a 50K resistor, it can't do anything as far as adjusting goes. At least that's how you've drawn it.
    G1, I've done this mod myself twice before and each time was able to adjust bias with the pot. Certainly my circuit prowess has a lot of room for growth, and perhaps my drawing is flawed. This one is wired the same way I recall doing it in the past. I am missing something very simple here.

    The series zener string is replaced with the series resistor string, which is in parallel with R132

    56K resistor replaces ZD101
    50K pot replaces ZD102
    56K resistor replaces ZD103
    series string in parallel with R132

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    Supporting Member Tone Meister's Avatar
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    Quote Originally Posted by g1 View Post
    And aside from the bias circuit voltage problem, with the pot wired as a 50K resistor, it can't do anything as far as adjusting goes. At least that's how you've drawn it.

    Will you explain why?

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    Quote Originally Posted by Tone Meister View Post
    Will you explain why?
    The wiper is "floating" therefore the resistance does not change. Connect the wiper to one "end" of the pot.

  17. #17
    Old Timer J M Fahey's Avatar
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    This one is wired the same way I recall doing it in the past.
    That means nothing to us, just redraw the actual circuit you wired, looking at it.
    And a closeup picture showing the bias pot/trimmer would be fine.
    Juan Manuel Fahey

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    Supporting Member Jazz P Bass's Avatar
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    Goofy way to draw a potentiometer.

    No wonder the confusion.

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    Quote Originally Posted by Jazz P Bass View Post
    Goofy way to draw a potentiometer.

    No wonder the confusion.
    Almost like the old Fender schematics where the wiper was drawn diagonally across the resistor.

    Check the values of the two "56K" they look like 5K6 to me.

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    Supporting Member Tone Meister's Avatar
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    Quote Originally Posted by J M Fahey View Post
    That means nothing to us, just redraw the actual circuit you wired, looking at it.
    And a closeup picture showing the bias pot/trimmer would be fine.
    See post #11 Juan. The drawing of this circuit is identical to the way I have wired it in the past, with the exception of the pot.


    Click image for larger version. 

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    Quote Originally Posted by Jazz P Bass View Post
    Goofy way to draw a potentiometer.

    No wonder the confusion.

    This is a 3 terminal potentiometer and I thought sure I had represented the correct schematic symbol. You do know I'm a hack, don't you? Symbols fixed in all drawings and schematics


    potentiometer.png


    The pot is a three terminal Bourns 3299Y-1-503LF (datasheet attached) and one end of the pot was left unused. I have since connected the wiper to the unused end and and still unable to adjust bias with the pot. Here is how I did it.





    Attached Files Attached Files
    Last edited by Tone Meister; 09-29-2015 at 06:35 PM.

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    Quote Originally Posted by 52 Bill View Post
    Almost like the old Fender schematics where the wiper was drawn diagonally across the resistor.

    Check the values of the two "56K" they look like 5K6 to me.
    Alright den. Point taken on the erroneous schematic symbol.

    Bill, the resistors measure 56K. Here's a closeup.


    56k.jpg

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    Senior Member Enzo's Avatar
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    Hey, resistor with an arrow drawn across has always meant variable resistor to me.
    Education is what you're left with after you have forgotten what you have learned.

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by Enzo View Post
    Hey, resistor with an arrow drawn across has always meant variable resistor to me.
    Same here, but on this schematic the way the arrow was drawn means "wiper contact".

    As of the pot wiring, as the British say : "the proof of the pudding lies in the eating" : measure resistance across those white wires and turn the setting screw end to end : the resistance either varies from 0 to 50k or not. (I suspect the latter) .
    If not, maybe the pot was damaged with rough handling, replace it with a good one.

    And next time, avoid using this screwdriver to adjust it:

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    Supporting Member Tone Meister's Avatar
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    Quote Originally Posted by J M Fahey View Post
    Same here, but on this schematic the way the arrow was drawn means "wiper contact".

    As of the pot wiring, as the British say : "the proof of the pudding lies in the eating" : measure resistance across those white wires and turn the setting screw end to end : the resistance either varies from 0 to 50k or not. (I suspect the latter). "Don't suspect without investigating." - JMF Marshall VS65R - amp powers on but no sound

    If not, maybe the pot was damaged with rough handling, replace it with a good one.
    Schematic symbols fixed on all documents in this thread. Because this pot has three terminals, I mistakenly thought I was using the correct symbol, based on this information.

    Schematic Symbol

    With the pot out of the circuit, the resistance DOES vary from 0 to 50K when I measure resistance across those white wires and turn the setting screw end to end. I could replace the pot since it could be damaged, but the only thing I knew to check was the resistance the way you described and it checks out in isolation.

    Side Note: There is not one of you I can't strike out with a 90+ mph fastball

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    g1
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    Quote Originally Posted by Tone Meister View Post
    Side Note: There is not one of you I can't strike out with a 90+ mph fastball
    I remember a kid in school who used to say he could kick my ass on skates .

    Now that it's back wired up, measure resistance across R132 with pot set to either end.
    Last edited by g1; 09-29-2015 at 10:25 PM.
    "there's another kind of party lights that I can't stand to see,
    when there's a man in that patrol car and he don't wanna party with me"

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    Supporting Member Tone Meister's Avatar
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    Quote Originally Posted by g1 View Post
    I remember a kid in school who used to say he could kick my ass on skates .
    Now that it's back wired up, measure resistance across R132 with pot set to either end.
    I thought everyone here knew I'm a novice when it comes to circuits. A simple notation mistake turned into cauliflower ear.

    R132 = 61.5K

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    As drawn, the pot just presents a variable load to the bias supply. The pickoff point for the junction of R105 and R106 needs to be inside the resistor/pot string.

  28. #28
    g1
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    Quote Originally Posted by Tone Meister View Post
    R132 = 61.5K
    Measure with pot set to min. and again with pot set to max.
    "there's another kind of party lights that I can't stand to see,
    when there's a man in that patrol car and he don't wanna party with me"

  29. #29
    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by TimmyP1955 View Post
    As drawn, the pot just presents a variable load to the bias supply. The pickoff point for the junction of R105 and R106 needs to be inside the resistor/pot string.
    Weird as it seems, for bias supplies the tried and true method is using the pot as a variable load.
    Probably because it's fail safe: if pote opens or wiper contact is dirty, bias becomes maximum voltage and tube cools (in every meaning of the word ) while the conventional way it leaves tubes unbiased , so red hot or worse.

    That said, *some* amps wire bias the conventional way as you suggest.
    But then they often add a fixed resistor in parallel.
    Juan Manuel Fahey

  30. #30
    Supporting Member Tone Meister's Avatar
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    Quote Originally Posted by TimmyP1955 View Post
    As drawn, the pot just presents a variable load to the bias supply. The pickoff point for the junction of R105 and R106 needs to be inside the resistor/pot string.
    I've tried to go back and find the exact pot I used before, but I am sure that when I did this mod myself that all I did was remove those 3 zeners and replace them with the 56K>50Kp>50K resistor string using the holes left by the zener diodes, nothing more. I need a solution. Is this Bourns pot the wrong part for the job at hand, or is it just wired into the circuit incorrectly in your estimation?

    Quote Originally Posted by g1 View Post
    Measure with pot set to min. and again with pot set to max.
    Will do this afternoon when I can get back to that amp.

    Quote Originally Posted by J M Fahey View Post
    Weird as it seems, for bias supplies the tried and true method is using the pot as a variable load.
    Probably because it's fail safe: if pote opens or wiper contact is dirty, bias becomes maximum voltage and tube cools (in every meaning of the word ) while the conventional way it leaves tubes unbiased , so red hot or worse.

    That said, *some* amps wire bias the conventional way as you suggest.
    But then they often add a fixed resistor in parallel.
    Same question as I asked Timmy: I need a solution. Is this Bourns pot the wrong part for the job at hand, or is it just wired into the circuit incorrectly in your estimation?
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    g1
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    You need a solution, but we are going by your recollection as there is no documentation of your previous mod.
    I'm aboard with what you think you did before except for you leaving R132 alone. Your new circuit in parallel with R132 immediately drops the bias voltage and makes the tubes run hotter, near as I can tell.
    But what you are trying to accomplish is to make them run cooler than the stock "class A", correct?
    I'm going to guess that when you did the mod before you removed R132 and suggest you try it.
    "there's another kind of party lights that I can't stand to see,
    when there's a man in that patrol car and he don't wanna party with me"

  32. #32
    Supporting Member Tone Meister's Avatar
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    Yep, we definitely want them to run cooler than the stock design runs them. I'm certain all I changed was swapping the 3 zeners for 3 resistors and I thought I had picture documentation to demonstrate, but thus far I haven't found it. I am equally certain I did not remove R132 either of the previous two times, and each time the bias adjustment was within the desired range (55-65% dissipation). Hope I'm not coming across as obstinate on this, but I'm sure of what I did. However, the one difference is the type of pot used.

    I think I have another of the 50K pots here that I used, not that it would make any difference. Maybe Juan is right, maybe that Bourns pot is bad. Perhaps it tests fine under no load but craps out when voltage is applied, albeit that is a minimal load.

    I still believe that I can't see the forest for the trees. It will end up being something so simple that I am continuing to overlook it, thinking I have it figured out. That's the inexperience showing.
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    g1
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    Ok, if that's how you recall doing it, but it's not logical that adding an extra load in parallel with the R132 could increase the negative voltage. It should do the opposite, which would make the tubes run even hotter. Unless those zeners load down the circuit a lot more than I think they do.
    A simple test is to disconnect one end of your series network. Now what is the DC at C126 ?
    This should show if there is some other fault in the bias circuit.
    The only other thing I can think of, if you haven't flipped the board over, maybe the other persons bias mod included some trace cutting or jumpers. If you haven't yet, have a look.
    "there's another kind of party lights that I can't stand to see,
    when there's a man in that patrol car and he don't wanna party with me"

  34. #34
    Supporting Member Tone Meister's Avatar
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    I had to pull the board to do complete testing, and there are no traces cut, in fact the board and work was very neatly done - it just doesn't work. With all the components removed, I examined every trace and every soldering pad under magnification and tested each junction for continuity, then reinstalled the two resistors and the pot.

    Came across this picture from one of my earlier JTM30 bias mods which shows a common Piher 50K PT6KH503A2020 where the Bourns is fitted now in this amp. The bias works as I'm describing with this arrangement. You can see where the one leg is lifted and not used.

    jtm30-bias-mod.jpg

    I'll lift one end of the string as you've suggested and measure. I may go ahead and try this pot to see what difference it makes.
    Attached Files Attached Files

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    Supporting Member Tone Meister's Avatar
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    Quote Originally Posted by g1 View Post
    Ok, if that's how you recall doing it, but it's not logical that adding an extra load in parallel with the R132 could increase the negative voltage. It should do the opposite, which would make the tubes run even hotter. Unless those zeners load down the circuit a lot more than I think they do.
    Yeah, well, that's exactly what's happening too. The tubes are biased hot enough to redplate.

    Quote Originally Posted by g1 View Post
    A simple test is to disconnect one end of your series network. Now what is the DC at C126 ?
    This should show if there is some other fault in the bias circuit.
    OK. First I'm gonna measure R132 with pot set to min and again with pot set to max as you suggested before. Then I'll disconnect one end of the string and check DC at C126. I should have time to get back on this in the morning. Thanks ya'll.
    g1 likes this.

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