Hey folks, tried a long list of different Op-amps today. I have to admit, they all had issues compared to the original op-amp. None of them really sounded 'good' to my ears, so there's a definite reason for the 1458 in the circuit.

For now, I am happy to stop tinkering. The fizzle, I think is just part of how the circuit behaves. I may upgrade the speaker to a greenback at some point. But, not in any rush per se.

Quote from: edvard on February 06, 2021, 03:48:57 AM
Sounds exactly to me like hard rail clipping from overdriving the op-amp itself and is a reason why I don't like circuits that do that.  I don't know of any remedy that doesn't involve a few invasive mods to the circuit, but you can try something a little less complex;
Replace the 1458 with a socket and test out a few different dual op-amps to see if any are better for avoiding the fizzle. 
Try a TL072, NE5532, RC4558, and LM358.

Will definitely give this a shot, as I think I have a few ICs lying around from pedal builds. I already put in a socket, as I suspected the IC might be at fault.

All useful, thank you!

The additional clip was recorded on my phone instead of a proper mic, as it was just for demonstration purposes. So, that explains the harsh top end sound somewhat. Setting were roughly the same as the first clip.

Overall, I don't mind the tone of this thing. I have nice point to point valve amps, but this thing is meant as a living room practice amp so I don't piss my neighbours off.

I will likely change the speaker out for a 10" greenback.

Quote from: Dino Boreanaz on February 05, 2021, 03:26:03 PM
I just re-read your thread on the Marshall Forum.  I remember having read it a while back, but didn't have any insights to offer ... and I suppose the same is still true.

I'll try to get a recording of mine later today and post the results here.

Thanks man! I appreciate it. I don't know if I'm being too picky, or if it's a genuine thing that can be fixed. I just didn't want to go down the road of randomly replacing parts hoping I'd find the issue.

Would love to hear from more Lead 12 combo owners of this configuration. I actually emailed Johan Segeborn to see if he has any insight.

At this point you guys have me second guessing myself, BUT a few folks on the Marshall forum couldn't replicate the issue. I'm not talking about tone here, I like the tone, I like the drive... it's just that weird fizzle out like something is dying. If it's inherent in solid state amps, that's cool... but I am struggling to find another example of it in any lead 12 video. The video above has no sustained chords, so you can't tell.

Here's another recording where it's a bit more prominent:

https://drive.google.com/file/d/1waGMninBkkm7RYm6TkfwfJNZcf4GR0zp/view?usp=sharing

Surely this can't be normal?!? I also tested with another speaker, a V30 loaded 1x12 and the same issue persists. 

Thanks for your input. I got the time to measure the 0.33ohm power resistors again. If you see my photo in the original post, these are the two large grey resistors that sit between the two big motorola Transistors. NOTE: My R25 equates to R26 on the schematic.

I measured DC across he two resistors with my DMM. So, black probe one side of resistor, red probe the other side. Readings  as follows:

R25: -4.8mv
R27: 8.5mv

I think this isn't too far off as the schematic shows about 12ma at this point in the circuit.

Anything else I should be measuring? Frankly, I only have a loose understanding of the schematic and the circuit as a whole. It does look like the schematic doesn't quite match.

Hey folks,

First post here, and also my first venture into fixing up an amp. Please treat me like a complete beginner as I am still learning. I have built some FX pedal kits, but that's about it. My soldering skills are solid. I bought this amp second hand knowing it needed some attention. The tools I have at my disposal are a Tenma DMM, cap discharge tool and a soldering iron.

So far I have done the following:

-Checked over board for cold/broken solder joints, re-flowed any that looked suspect
-All pots replaced with new like for like values PCB mounted pots
-All input jacks replaced with new like for like replacements
-Fitted external speaker out
-All electrolytic caps replaced including power filter caps
-Power stage resistor replaced at r21
-Socketed the IC and replaced with a new MC1458
-Sorted out ground connections to chassis that were loose

The amp now works and sounds pretty good. However, there's one niggling issue that's driving me nuts. With a driven tone, i.e. the gain cranked a bit, sustained chords seem to fizzle out (kind of like a voltage starved fuzz) as they fade out. I made a quick recording to demonstrate this (you can hear this fizzle at 0.15 when I start letting chords sustain).

At this point I am struggling to fault find the issue. I have scoured most of the Lead 12 threads here and have taken the below readings with my DMM at test points on the schematic. At this point I suspect it may be one of the transistors, or perhaps the zenner diode is on its way out. A friend suggested what I am hearing might be cross-over distortion? Or that the bias of the power stage transistors is wrong.

Below are the results of the test suggested in other threads, I tried to hit most of the voltage test points on the schematic. I THINK the combo version (5005) circuit differs ever so slightly from the head schematic 3005 which seems to be the only schematic I can find online, so that's what I'm using. I have the combo (5005) version. For clarity, my process was: amp on, speaker attached, volume down. My DMM set to measure Voltage at AC range, black lead clipped the chassis for ground, red lead / probe to the appropriate test points for reading. Left hand in pocket at all times! I haven't fried myself yet. 

DMM across the speaker and set to DC Volts to verify there is no DC across the speaker.

No DC voltage across speaker, my DMM wavers a bit, but it's all in the 0.001 DC range

Next set the meter to AC volts. Is the reading almost zero ? You are looking for an ultrasonic oscillation that may read several volts depending on your meter.

AC reading 0.034 V

The Emitters of Tr1 & Tr2 should be -.7Volts, Collector of Tr1 should read +18Volts.

TR1 emitter: -0.7v
TR2 emitter: -0.7v
TR3 emitter: 18.87v

Junction of R15 & R16 should read -9Volts.

Wasn't sure what the 'junction' is, so measured each resistor.

R15: -9v
R16: -070.5v

Zenner diode ZD1: Neg side -9.12v / pos side -000.8mv

Power section Filter caps (replaced)

C17: 18v
C18: 0v (measured at + side)

Check that pins 1 and 7 of the IC are near zero Volts. If not, that indicates a bad IC, component or broken connection problem.

IC Pin 1: 0 volts
IC Pin 7: 0.046 mv

IC Pin 4: -17.34v
IC Pin 8:  17.30v

Lift (unsolder) the input side of C8 (it's the input to power amp) now use your walkman or any audio device that has line/headphone output.You should hear a reasonable medium level CLEAN sound. If the sound is severely distorted or very low volume then it's a fair bet the power transistors are shot or TR3 is dead.

Not done

Measure the DC Voltage across R26 and R27, the two 0.33 Ohm resistors. The Voltages should be close to the same.

R26: -068.3mv
R27: 068.1mv

Check R 11 & R 6 check they have aprox 19 volts on one side - I think I should take these readings again!

R6 -000.1mv on both sides, reading jumps around a lot. Hard to get exact reading.
R11 4.5v and 3.2v

Check they have approx 19 volts on one side and 16v on the other or another way measuring across the resistor should give a reading of 3 volts the amount "dropped" by the resistor.

R21 -4.9mv (should be -10V?) - replaced with new resistor

Does any of that strand out as 'not right'? Not really sure what to do now, as far as diagnosis goes.

Gut shot as it currently stands.

Any help gratefully received. Keen to learn and work out what is the cause of this issue.