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Marshall 5010 Needs Help

Started by El Scorcho, March 18, 2018, 09:32:21 AM

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El Scorcho

Total noob here, with little electronics experience beyond wiring guitars. I just picked up a 1982 Marshall Master Lead 30 combo for 50 Euros, so it's not like I was expecting much. I had a Lead 20 back in my teens that I remember liking alot, but this thing sounds like a moist fart.
   I opened it to clean it out and saw that D2 is missing entirely and D1 is not a diode, but a 2n2 Cap. Someone has tagged the bottom of the chassis with a permanent marker W.A.M Workshop And Music.  Has someone actually done something clever, or really stupid? Even on low gain setting there is an unpleasant 'fizzy'coating. I gave W.A.M the benefit of the doubt and swapped out the Red McKenzie out for a Celestion G12L, which made the moist fart sound like a bit more carefully executed fart, so now I'm leaning toward stupid.
I've researched the other 5010 posts here, and bought a 500k reverse log to replace the V1, but now I'm wondering if it's worth the bother. Looking at the Schematic, D1 and D2 seem to have something to do with Voltage supply, and I wonder if this mod hasn't burned out something else. The amp powers up and seems to work  - it just sounds more like a 1-knob, particle board starter amp from an old Sears catalogue than a Marshall.
    I'm going to go ahead and replace D1 and D2 with fresh 1N914's, and hope that helps. Is there anything else I should worry about? Like maybe R23/24 and C19/20, which seem to be in the same loop? Would this mod have ragged out the op-amp? What would the point of such a 'mod' be? A poor man's variac?
    Sorry in advance if I'm wasting anyone's time, but certainly would welcome some insight.


Hi and welcome,, please post the schematic or link it,, other wise quoting part numbers means nothing. :tu:

Jazz P Bass

On the input to the first opamp/ pin 5, there are two back to back diodes.
These diodes are used to limit the input signal to the opamp. (to prevent damage to the IC)

Why D2 was removed and a cap was installed at D1 I do not know.


D1, D2 are only there as protection in case the input signal gets larger than 15v peak.  Unlikely from a guitar.  Removing them damages nothing.  A cap in place of one?  Might be there to squash unwanted oscillation.  I doubt it is the problem, but it surely can't hurt anything unless it is shorted.  Unsolder one end of the cap and lift it from the circuit if you like, that tells you if it is causing trouble.

R23,24 are just power supply parts.  The amp runs on something like maybe 25v plus and minus?  20v?  18?   In any case, too much for the op amp ICs, so those two resistors drop the voltage down to 12-15v, you tell me which.  Does the IC have similar plus and minus power on the corner pins?  C19,20 are just decoupling caps for those 15v supplies.

So it sounds bad?  First, are the two power supplies right at the IC similar voltage, other than polarity?  Pins 1 and 7 of the IC are output pins, is there DC voltage on either?

VR1:  you replaced a 22k pot with a 500k pot???

As to fizzy, have you disconnected the internal speaker and listened to the amp through some other speakers?  Possible rubbing voice coil.

Aside from it not sounding good, do the gain and master controls and the tone controls seem to work as expected?

El Scorcho

Thanks Jazz and Enzo for having a look
    I dont have any testing equiptment yet to measure the IC output voltages. I'm just getting into making my own pedals and will have one soon. Just from looking in the amp-which is similar to the Guv'nor pedal, I could see parts were missing from the scematic. I'm glad it isn't a problem, but plan to return it to stock
   The sound quality isn't good-even though the amp works. Replacing the speaker did make a noticable difference in opening up the high end, but there is still an unpleasant and unatural sounding distortion as the note decays.
     I Have the new 500k rev log pot, but haven't installed it yet. I'm waiting on a parts order for the 1N914's. But the preamp and master gain work as expected.
    The low input jack was pushed inside the chassis so it couldn't be used, but is jumpered to the high sensitivity jack.There is no oscillation from the high gain jack. The preamp gain doesn't do much until the 8-10 range, as expected. Not sure if someone tried a jumper mod on the inputs, or if it is stock-on the scematic there is only one input jack.


May I suggest not "returning it to stock" until you know that is part of any problem.  It could even be something Marshall did in later production.

Sometimes a change in one place is involved with a change elsewhere.  If we "change back" the one thing we see, we might not change back the other half of the equation that we didn't see.  Then the amp has a new problem on top of the original problem.

It is tempting to start changing parts just to get something going, but it is better to consider each action.

You really need to buy or borrow a good meter at the very least.

El Scorcho

OK. I resisted the urge to spray hot solder and got a meter to check the IC voltage.

The only two pins that gave me a reading were on opposite corners, I think the + /- Vcc pins.  I got -13.68 and 13.81. The readings fluctuated a little bit measuring them again, -13.87 and 13.78.

Jazz P Bass

Those voltages will work.
The second test is to flip your meter to read Volts AC.
That will measure the 'ripple' of the supplies.
Ideally, it should read zero.

El Scorcho

I got zero's across all but 1 of the pins. I think pin 8 is the Vcc+, which read 28.8. It gave me alot of fluctuations from 0 to 5, 16, etc, but 2x it gave a stable 28.8 reading.


Try reversing the probes where you have that funny reading.

El Scorcho

I reversed the probes, which gave me a reading of zero at the + Vcc pin, and every other pin but the -Vcc, which read 28.8. Switching the probes again gave me 0 across all pins but the +Vcc, which read 28.8 to 29.0.


It is unlikely your 13v supplies have 28v of ripple.  Your meter is confused trying to read AC voltage while DC is present.   Two options:  one is to get a more advanced meter.  Two is to put a cap in series with your probe.  Like a 0.1uf or some such.

El Scorcho

I'm certainly willing to admit I could have set the meter wrong. I just got it in yesterday. I followed the manual and set the dial to V symbols, but aside from that not sure if it's dialed in tight.

El Scorcho

Things You Can Tell Just By Looking At Her:

I found alot of electrolytic caps on the board that are not the same value as the scematic.

C2 is supposed to be 0.1p, its 2,2u, 125V

C13 is supposed to be 47n, its 100uf, 125V
c14 is supposed to be 2u2, its 100uf 25v
c17 supposed to be 100u, its 100uf 25v
c18 supposed to be 470p, its 22uf 25v

c11 is supposed to be 47n, its a strange looking disk, with 5000 printed on it
c12 is supposed to be 220p, its .047 orange drop

c20 is supposed to be 100p, its 2200uf -35V
C21 is supposed to be .22u, it's 2200uf 25v