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Thread: Marshall Schematic

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    Member patlaw's Avatar
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    Marshall Schematic

    Does anyone have a better copy of this Marshall schematic? It appears to be the right one for this amp, which has two channels with two inputs each. I can't read all of the values on this one.

    It's an M1989 Lead Combo Mark II, US version.

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    No, that's a 70's 1987 just like my '73.

    That's the only version of that schematic I've seen but it's pretty damn close to a regular Plexi of which there are plenty of nice clean re-drawn versions.

    Print one of those and pencil in the differences?

    Mine just rips, sounds phenomenal with all Mullards tubes including EL34's I converted it to.

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    Is Plexi a model of a Marshal amp? I'm fine with penciling in the changes if I can find one of them.

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    Plexi just refers to amps made from around 66 to 69 that had a plexiglass front panel.

    The amp circuits were basically the same with small changes made to the different models over the years.

    The Plexi models fetch more money, even though my 73 model 1987 is almost identical to a 69 model 1987.

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    Okay, after pulling out my Marshall amp book-

    M1989 is a 50w 4 input 2 channel Organ amp, quite rare.

    Both channels share the same cathode resistor and cap as the normal channel of a 1987.

    Very similitude with just a few differences.

    I'd love to see a picture of the cabinet.

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    Quote Originally Posted by drewl View Post
    Plexi just refers to amps made from around 66 to 69 that had a plexiglass front panel.

    The amp circuits were basically the same with small changes made to the different models over the years.

    The Plexi models fetch more money, even though my 73 model 1987 is almost identical to a 69 model 1987.
    FWIW, also late (65/66) JTM 45s (including the Bluesbreaker combo and the 100W JTM45/100) had plexi panels. Those were different (KT66 power tubes, different transformers, different NFB).

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    Quote Originally Posted by drewl View Post
    I'd love to see a picture of the cabinet.
    I'm happy to provide one when I get home. So we're back to the original question. Which schematic should I use? The Normal channel is very bass-y. I'm having trouble following the components to know whether that's normal for this amp. There's also a low amplitude 120Hz hum.

    Thinking the power supply filter caps were not up to par, I paralleled a 40uF 600v polarized Weber cap around a 50uF can cap. The amp's fuse blew. I still don't know why. Here's what I do know.
    1. I hooked up the new cap with the correct polarity.
    2. The new cap is not defective.
    3. The plates in the new cap have been properly formed by slowly applying power.
    4. The leads connecting the new cap were not shorted to another lead or cabinet.
    5. I've been doing repairs for almost 50 years, some of them commercially.

    Your thoughts are welcome.

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    Don't forget the joker g1's Avatar
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    Was the case of the cap touching anything?

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    I paralleled a 40uF 600v polarized Weber cap around a 50uF can cap.
    Did you connect the additional cap to the powered circuit? If so, it would draw a huge charging current surge that may blow the fuse.

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    Quote Originally Posted by Helmholtz View Post
    Did you connect the additional cap to the powered circuit? If so, it would draw a huge charging current surge that may blow the fuse.
    No, and the case didn't touch anything. It has to be something stupid, but I don't know what it is. And this is not my first rodeo.

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    Try pulling the output tubes and see if it powers up okay..

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    Here are the speakers.

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    From the schematic in your first post, which cap would be the equivalent?
    Which fuse blew, mains or HT? Was it slow-blow? What value?
    I know it's not your 'first rodeo', but you also said it must be 'something stupid' so we do like to ask the simple questions in case anything was overlooked. We all have those momentary lapses (of reason ).

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    Quote Originally Posted by The Dude View Post
    But, I did learn something. There are protons, neutrons, electrons, ............ and morons.

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    Quote Originally Posted by g1 View Post
    From the schematic in your first post, which cap would be the equivalent?
    It was paralleled across the two 50uF caps in the same can at the cathode of the power supply diodes.
    Which fuse blew, mains or HT? Was it slow-blow? What value?
    Mains, 3A slo blo
    I know it's not your 'first rodeo', but you also said it must be 'something stupid' so we do like to ask the simple questions in case anything was overlooked. We all have those momentary lapses (of reason ).
    I understand and am guilty as charged. When I get back on the bench, I'm going to experiment more - this time with the light bulb current limiter, which I had no reason to use before since the amp is working almost correctly.

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    Is the bias supply still stock, ie standby mode deenergises HT AND bias?
    Marshall had a decade long ‘lapse of reason’ in sending their 50 watters out like that.

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    Quote Originally Posted by pdf64 View Post
    Is the bias supply still stock, ie standby mode deenergises HT AND bias?
    Marshall had a decade long ‘lapse of reason’ in sending their 50 watters out like that.
    It appears so. Since I don't have an accurate schematic, and since this is the first Marshall I've serviced, I assume it is stock. It does not appear to have been modified. The standby switch definitely removes the HT.

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    Quote Originally Posted by pdf64 View Post
    Is the bias supply still stock, ie standby mode deenergises HT AND bias?
    Marshall had a decade long ‘lapse of reason’ in sending their 50 watters out like that.
    I had a 1986 wired like that. There was a huge current surge when switching from standby to play (as you'd expect) so I moved the bias supply input to the other side of the standby switch where it should be.

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    I think I did with my '73 50watter like in the 1st schem up top as I had it open recently to check something and bias comes up with the power switch.

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