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Thread: Marshall JCM 800 - "splatty" Lead channel

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    Marshall JCM 800 - "splatty" Lead channel

    Hello All

    I am looking at a Marshall JCM800 Model 2205 ("old style", no chip) which suffers some problems. Mainly, the Boost channel sounds very gated and "splatty" with very low output.

    It has been worked on by someone previously, who replaced the dual-gang gain pot and the volume channel pot on the boost channel. They also replaced some of the box capacitors with combinations of radial caps in series...looks like they didn't have the right value or couldn't wait to order the right parts.

    I humbly request help with two things;

    Would someone have a schematic or service manual for this amp that has the capacitor values listed by reference designator? The replaced caps are at
    C8 C9 C36 C38 and C39, and the schematic (attached) doesn't list them that way.

    Additionally, where's a good place to start with the issue on the Boost channel?

    https://drtube.com/schematics/marshall/4210.gif

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    Old Timer J M Fahey's Avatar
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    Problem with "messed up" amps is that we donīt really know whatīs there.
    It will probably match no known (or unknown) schematic.

    I suggest a longer but safer way to solve both problms: part designations may change but they basically still must be there for the amp to work.

    Print a large copy of the schematic, even better if you print it twice the size, in two separate sheets which then you tape together, and go along the board checking parts one by one and annotating the schematic with new part designation and whatīs actually there.

    Then take a picture of the corrected schematic and upload it here.

    So, for example, you will find that "the cap which goes from V1a cathode to ground" which MUST be there, is labelled, say, "C4" on the printed schematic but "C2" on the PCB and instead of ".1" is actually ".22" or whatever.

    Do it at least for the entire Clean channel, from input jack to input of V3a

    Does the Dirty channel work fine?
    Can you set its gain pot (the dual one) down and get reasonably clean sound there?
    Or itīs also splatty gated?

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    Juan Manuel Fahey

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    Gated and splatty with low output could be down to one of the preamp grids not having a resistive path (grid leak) to ground. Check the resistance from V2a grid to ground with a cable plugged into the unpowered amp. In the original schematic the grid leak was formed by the 1M Ohm pot track and it should still read the same.

    Capacitance values to me don't account for the symptoms. I would leave them alone for the time being and pinpoint the specific fault before replacing parts.

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    Mick Bailey

    The problem with this schematic isn't only that the reference designators aren't listed, the values themselves are very hard to read in some cases.

    With a lead plugged into the unpowered amp, the resistance from pin 2 of V2 is 88K...until I turn the"Gain" knob, and the value follows the pot turn. 88K is with the pot at it's minimum.

    Resistance from pin 7 of V2 to ground is like 4.8M

    P2 of V1 resistance reading, taken as above: 7.4K P7 of V2 reads 75K

    The Boost channel is the channel that is "splatty". The Normal channel works ok, mostly...the amp may need a speaker as well.

    I am suspicious of the dual pot that was replaced. Also worthy of suspicion are the replacement radial caps.

    Click image for larger version. 

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    Looking at that job, I'd suspect that first...
    Maybe that single Mallory 150. That has no business in a Marshall. Unless you're trying to Fenderize it.

    Justin

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    Bent Member Chuck H's Avatar
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    Just trace the circuit and rewire it as per the preferred stock circuit. You really shouldn't need R# value designations for that.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

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    That's pretty bodged circuit board. Have you looked at the copper side to see if any tracks have been cut? I think it's down to what's already been suggested - trace out the connections and identify the components on the schematic. At least get to that stage and then post back here any questions on values when we know the positions. In cases where there is an unknown value that can't be read of the schematic I either go back to first principles or see what other Marshall amps use in the same location.

    If the clean channel is working you at least know that the supply voltages are close. The boost channel only has a handful of components, but if the circuit has been rewired then you need to establish what's been done.

    It would also be worthwhile measuring the plate and cathode voltages on the drive channel and posting back the results.

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    Senior Member Pedro Vecino's Avatar
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    These sets of electrolytic capacitors (five) seem to compose 320uF per pair. They seem to replace the original capacitors associated with the operation of the mutes in the switching circuit. Itīs easy to identify their functions following the scheme.
    In addition to the Mallory capacitor, there is a resistor (R12) that may not be original and the same with the other on the second socket. There is also a reddish brown plastic capacitor next to the transistor on the left that may not be original. It would be convenient to check their values in the schematic.

    The area is delimited and turning the board you can check which welds are original and which are not.

    In any case, itīs the first Split Channel model, manufactured for about a year and a half and its boost channel (pre-drive equalization included) sounds very bad.

    This can be confusing to determine if it sounds like it should or not.

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    Old Timer J M Fahey's Avatar
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    In general "splatty" which Iīd also call "chopped" means a stage which canīt pass signal for some reason until it exceeds a certain value, like a very poor noise gate.

    Typical is stage being horribly biased, so itīs basically cutoff for normal signals; strong ones can get through by sheer volume, and very distorted at that.

    In principle measure cathode and plate voltage at each triode involved as a quick mode to confirm/reject that.

    But this amp also includes new actors: the switching transistors present everywhere; a poorly biased or shorted one can also act as "terrible noise gate" .

    Since said transistors ground selected parts of the circuit through electrolytic capacitors, just for testing I would lift one of their legs so Clean channel is forced "enabled" and check whether we now have proper sound.

    Or we have a horrible soldering somewhere along the path, we have no real soldered continuity but "metal poorly touching metal" and only strong sound can jump that barrier.

    OF COURSE, a scope can show that quickly, thatīs why Techs have them.

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    Juan Manuel Fahey

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    The later version with a bad switching chip can cause this same problem, that's how I got a 2210 cheap.

    So it may be something in the switching circuit.

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    Transistors may be separate ones or an array inside a DIP package, electrically are the same: a transistor which shorts signal to ground when base is biased positive.

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    Juan Manuel Fahey

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    Quote Originally Posted by earache View Post
    Hello All

    I am looking at a Marshall JCM800 Model 2205 ("old style", no chip) which suffers some problems. Mainly, the Boost channel sounds very gated and "splatty" with very low output.

    It has been worked on by someone previously, who replaced the dual-gang gain pot and the volume channel pot on the boost channel. They also replaced some of the box capacitors with combinations of radial caps in series...looks like they didn't have the right value or couldn't wait to order the right parts.

    I humbly request help with two things;

    Would someone have a schematic or service manual for this amp that has the capacitor values listed by reference designator? The replaced caps are at
    C8 C9 C36 C38 and C39, and the schematic (attached) doesn't list them that way.

    Additionally, where's a good place to start with the issue on the Boost channel?

    https://drtube.com/schematics/marshall/4210.gif
    Given the fact that a lot of people don't like the sound of that amp even when 100% working, I suggest you pull the PCB out and make a nice turret board 2203 or 2204 with some bells and whistles since there's a lot of space inside that chassis.

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    Or just tweak it and it sounds great like I did.

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    m1989jmp

    Not my amp to mod

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    Pedro

    You sound like you are extremely versed in the topology of this amp. Do you have a gut shot of the circuit board?

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    All

    Could someone take a look at the schematic and offer some interpretation of how some parts are being referenced?

    Specifically: look at V2a and just below the grid there is what I believe is a polarized capacitor connected to a transistor labeled BC184.

    Is it a polarized cap, yes or no?

    Can you help me interpret it's value? It also says next to it "now .022" (I think...)

    In similar fashion, there is what appears to be a polarized cap just below V1a on the schematic, coming from the middle leg of the Volume pot.

    Please help me decipher what it's value should be, as it also says something like "now 1 pf" or something...

    This questions arise because, as you all have suggested, I'm trying to trace out this schematic. I'm having some good results but these legends have me stumped.

    Thanks in Advance

    Eric

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    Senior Member Pedro Vecino's Avatar
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    I'm sorry, but I don't save images from that model.

    Regarding these capacitors: they serve to filter the signal through them and make the mute depending on the channel that is activated. They have no other function.
    The original indications must be 330uF (an unnecessarily high value). The new in V2 seems to be 22n (0.022).
    But: in the most modern version they are originally 220n (0.22) and do not allow full filtering. There is a trace of unfiltered bass that with very high gain / volume settings in the inactive channel appear in the active channel. In the modern version I always install 2.2uF plastic capacitors and with them a perfect filtering is achieved by making a completely independent channel from the other.

    The new V1 mute capacitor seems to indicate 100n (0.1uF), of the same value as the cathode (.1uF).

    Understanding that the switching circuit is the same but done with transistors, using the value proposed (2.2uF) in all three sites you should have no problem with it.

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    Pedro V

    Thanks very much for the quick reply!

    They don't need to be polarized caps then?

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    Senior Member Pedro Vecino's Avatar
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    I use them without polarizing, but you can use it polarized keeping the proper orientation.

    Note: I see that the second mute string (V2a) is double. A part of it takes the signal from the plate through a 100n (0.1uF) high voltage capacitor. You can leave that capacitor there since its value is adequate.

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    Don't forget the joker g1's Avatar
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    Here's a pic from the web if it's any use.


    Click image for larger version. 

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    G1

    That’s the beast! Thanks!

    I wonder why they used polarized caps in those locations if it was not strictly necessary?

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    I wonder why they used polarized caps in those locations if it was not strictly necessary?
    Circuits don't "require" polarized (electrolytic) caps, they just need a certain capacitance. The point is that non-polarized (film/foil) caps are available only up to a few ĩFs. So if the circuit requires a higher capacitance than say 10ĩF, only polarized caps are available. In the range from 1...10ĩF both types are available, but polarized E-caps are typically smaller and cheaper.

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    What voltage rating would be needed here on these capacitors?

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    Quote Originally Posted by earache View Post
    What voltage rating would be needed here on these capacitors?
    As far as I can see from the schematic, the caps in series with the switching transistors see no DC, only AC. Means that a low voltage rating of 16V should suffice. But a higher voltage rating may increase lifetime. Generally electrolytics would live longer with some DC bias.
    If a cap value of 2.2ĩF is sufficient, as Pedro suggested, film/foil caps rated at 50V (or more) are actually the most reliable/durable solution.

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    Last edited by Helmholtz; 08-31-2019 at 03:51 PM.
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    Hey there you fine peoples

    If I were to have to replace the BC 184 NPN transistors on this amp, could I use just about any switching transistors like 2N2222 or 2N3904?

    Thanks

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    I’ve been testing the transistors in-circuit with a small multifunction test unit.

    I’ve not used the test device before in this manner.
    It’s one of those small rechargeable units from China.
    Is it a legitimate way to test transistors in circuit?

    The first three transistors returned different results. See attached pix

    Click image for larger version. 

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  27. #27
    Old Timer J M Fahey's Avatar
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    Did you test them with amp powered or unpowered/unplugged?

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    Juan Manuel Fahey

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    JM Fahey

    Tested with the amp unpowered/unplugged

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    I have a 4211 on the bench right now, basically it's the 100w version,if you need help I can give you measurements and values.
    The boost channel is not the best I've heard on Marshalls,that was one of the first with diodes.

    your switching system looks like this one
    https://el34world.com/charts/Schemat...v_50w_4210.pdf
    you can see the 330u caps used to pull to ground the stages,but there is a correction,in the new one they are all .22u poly type

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    Last edited by alexradium; 09-04-2019 at 02:43 PM.

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by earache View Post
    I’ve been testing the transistors in-circuit with a small multifunction test unit.

    I’ve not used the test device before in this manner.
    It’s one of those small rechargeable units from China.
    Is it a legitimate way to test transistors in circuit?

    The first three transistors returned different results. See attached pix
    Not very useful here, since transistors are not being used as "transisoirs" , to amplify, with bias, etc. bust as simple switches.

    Emitter to ground, collector to signal, if unbiased "switch" is open so signal is unaffected; when biased (some current getting into base) "switch closes" and signal is grounded.
    As crude as can be but popular because of cheapness.

    Looking at your pictures top to bottom:

    1) nice data but irrelevant here. At least it looks like "you have a transistor there"

    2) should be same as first one, but it shows only 2 legs
    You may have a dead transistor there or simply poor test leads connection, who knows?

    3) even worse. Problem is I canīt be certain we have bad parts or simply poor tester connections.

    4) weird, it shows Hfe of only 6, while we expect >100 , etc.

    I suggest you leave that tester aside for now,and test amp *functionally*

    What are transistors expected to do? ... switch channels? ... ok, letīs test that.

    We have 3 grounding switches, 3 BC184, Iīll call them Left, Center and Right (I hate unlabelled parts schematics)

    Turn amp on, set it to "Clean".

    Measure transistor Base voltages.

    Left should be 0V ; Center and Right should be about 0.6V .

    Does Clean channel work properly?
    Test it LOUD, at rehearsal/stage levels, preferrably with a humbucker equipped Guitar.
    Does it become "splatty?"

    Now swiotch to Distortion/Boost.
    Left shoukld show 0.6Vbe ; Center and right should show 0V.
    Repeat test as before.

    Comment on results.

    If on any test splatty sound appears, remove guitar and inject 100mV sinewave at input, scope output ... any weird signal?

    Move volume from 10 to 0, is there any weird thing happening to signalbesides clipping at high level?

    Splattyness could be completely unrelated to switching, simply instability/distortion which appears at certain levels.

    Ok, do the tests and post results.

    Notice I am neither shotgunning nor even measuring "parts" expecting to hit the jackpot and by sheer chance find "a bad one".

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    JM Fahey

    Here are the results of the tests you suggested above, to the extent that I can complete them.

    Base measured to ground, Clean setting

    Left. .06 Middle. .69 Right. .06

    Boost setting

    Left. .69. Middle. .07 Right. .69

    The Clean channel never exhibited any splatter or gated sounds even when turned up

    The Boost channel was never without the gated sound, output fell off drastically with attack of plucked string or chord at any volume

    I do not have a signal generator or scope so I couldn’t apply the sine wave.

    Note: “left” “right” and “middle “ transistors correspond to “TR1 “ “TR2” and “TR3” on the circuit board art and Also are the order, top to bottom, of the first three pictures I posted yesterday.

    Earache

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    Alexradium

    That is exactly the schematic that I am working from. I asked earlier in the posts for a clarification on that “now .022”
    Note on the schem. It only appears in one place; there’s another note that says “now .1”

    Haven’t seen crappier schematics anywhere. Poorly drawn, poorly reproduced, difficult to follow.

    Would the switching not work reliably or well with the 330uf caps and that’s why the change to .022?

    Earache

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    Don't forget the joker g1's Avatar
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    Pedro outlined the function and value of those caps back in post #17.
    I took it that the 330uF caused too much latency in switching time, and .022 was insufficient.

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by earache View Post
    JM Fahey

    Here are the results of the tests you suggested above, to the extent that I can complete them.

    Base measured to ground, Clean setting

    Left. .06 Middle. .69 Right. .06

    Boost setting

    Left. .69. Middle. .07 Right. .69

    The Clean channel never exhibited any splatter or gated sounds even when turned up

    The Boost channel was never without the gated sound, output fell off drastically with attack of plucked string or chord at any volume

    I do not have a signal generator or scope so I couldn’t apply the sine wave.

    Note: “left” “right” and “middle “ transistors correspond to “TR1 “ “TR2” and “TR3” on the circuit board art and Also are the order, top to bottom, of the first three pictures I posted yesterday.

    Earache
    Thanks.

    Not too sure we agree on transistor labelling and schematic says nothing (while silkscreen does) , so letīs try this:

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    I called them Left - Center -Right as shown on schematic.

    You tell me which is TR1 - 2 - 3 as on silkscreen

    Also measure (P)late and (K)athode voltage at V1b and V2a

    As a side test, Diode scale measure clipping diodes, all of them,letīs call them D1 - 2 - 3 - 4 left to right and top to bottom.

    No need to post values measured if normal (high one way, 0.6V the other) *unless* 1 or more are abnormal, just tell me "all pass" or whatever you find.

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    Last edited by J M Fahey; 09-05-2019 at 05:14 AM.
    Juan Manuel Fahey

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    Quote Originally Posted by J M Fahey View Post
    Thanks.

    Not too sure we agree on transistor labelling and schematic says nothing (while silkscreen does) , so letīs try this:
    I was also unsure whether you were both talking same Q's, but I did note that the logic levels seem reversed.
    You said clean would have two Q's turned on, gain one Q turned on.
    His measurements seem to show one Q on for clean, two on for gain ch.

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