Welcome to Solid State Guitar Amp Forum | DIY Guitar Amplifiers. Please login or sign up.

June 13, 2024, 07:20:04 PM

Login with username, password and session length

Recent Posts

 

Marshall Lead 12 combo (5005) - Help diagnosing weird drive 'fizzle'

Started by Ubik_Fresh, February 02, 2021, 09:58:19 AM

Previous topic - Next topic

Ubik_Fresh

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.  8)

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.

g1

Sounds like the schematic you are using does not match your circuit. (see attached)
Are you able to trace through and find parts according to their circuit function rather than going by what they are labelled as on the circuit board?
Otherwise, using component designations that are incorrect will only cause errors.

Example, R11 and R6 are voltage droppers for the low voltage supplies.  As we know the amp is mostly functional, those voltages have to be there.  Yet you are not measuring those voltages.  So it is most likely that those voltage droppers are labelled as some other name on your board, and you are measuring some other resistors.

The voltages you measured at R26 and R27, they were definitely DC mV and not volts?  And they were .33ohm power resistors?
If so, it is likely that crossover distortion is your problem as was mentioned somewhere.  But to correct that will be difficult if component designations are incorrect.

Ubik_Fresh

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.

Dino Boreanaz

I don't have a solution, but I can tell you that my Lead 12 (a 3005 Micro Stack) sounds very similar on the fading trail of a sustained chord.  I've always assumed this is simply the solid state devices' way of (not very elegantly) transitioning out of clipping as the signal diminishes below the clipping threshold.  I could be wrong and there may be something amiss, but I'm far from an expert so I can't really offer any input on where to look to attempt to change this behaviour.

In any case, good luck and welcome to the forum!

phatt

Plus x10 what Dino said. :dbtu:

I just had a listen to the audio and like Dino I'm thinking that's about what a lot of those budget amps sound like. As those amps are very VERY basic in design you can't expect a million dollar tone. 8|
Yes there maybe something not quite right but they are rather harsh.
Maybe google some vids ,, here's one,, sounds much like yours, really fuzzed out , spitty and clapped out.
Some folks love that but if you want the classic sweet  singing OD effect,, you need another Amplifier. :-X
Phil.

https://www.youtube.com/watch?v=vaAq-kckc0I

Dino Boreanaz

I totally agree that you have to know and accept the limitations.  I'm very happy with my Lead 12 within its inherent limitations, but in my opinion the settings used in that video do not produce a very flattering tone.  At maximum gain it does get harsh and having the treble wide open just makes it sound fizzy especially with the stock Celestion G10D-25 speaker.

Not that the speakers could change the amp's clipping behaviour, but I've replaced the stock Celestion G10D-25 speakers with Celestion G10 Greenbacks and I love what they've done for the tone of this amp ... strong, controlled low end, nice pronounced mids, and clear, but smoothly tamed highs.

For what it's worth, my settings are generally Gain between 4 and 8, Volume 2 (playing in a small room at home), Treble between 4 and 6, Middle 10, Bass 10 and I'm usually aiming for something ranging from AC/DC to Ramones in terms of tone.

Ubik_Fresh

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. 


Dino Boreanaz

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.

Ubik_Fresh

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.

Dino Boreanaz

If you get a reply, I'd love to hear what Johan Segeborn says about this as he's quite the champion of the Lead 12 (and probably the reason I bought mine).

That trailing fizzle is definitely present on mine as well.  I've attached a quick recording of what mine sounds like at two different volumes.  The first time is with Gain 7.5, Volume 2, Treble 6, Middle 10, Bass 10 and the second half is identical except volume on 8.  I might be imagining things, but the fade out at the higher volume seems a little smoother, but I didn't repeat it multiple times to find out how consistent it is.

Edit:  I realized that the playback volume on the file was quite low, so I bumped it up a little.  Same recording as before.

edvard

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. 

phatt

Ubik, Your last clip has insane treble so yes that will certainly enhance the hard clip effect at the end of a fade out.
Have another listen to Dino's clip *Lead 12 fizzle*
Because he has far less top end that edge is greatly reduced to the point that it's hardly audible BUT it's still there. ;)  Your guitar might have very bright pu's adding to the problem.

As a side note the main reason SS amps got a bad name is partly due to the fact that by design Valve circuits (as used in guitar amps) wipe off a lot of that fizzle due to a combination of Miller effect and the bandwidth limits of the output transformers used. (Other stuff as well but they are the main ones)
You can make a Valve amp sound just as bad as a ss unit just by building a really hi spec hifi Valve circuit.
Guitar amps don't need hifi 20/20 bandwidth to sound good.
What SS Guitar Amp designers forgot is just how important it is to wipe off all the excess bandwidth distortion. It makes a MASSIVE difference. full bore distortion over 10khZ sounds grossly awful and very non musical. xP

If you want you could try lifting one end of R5 or C1 (just under IC1A)
that will remove a lot of excess top end.
another option is add a 470pF cap across R7,, or do both.

Great OD/distortion guitar sound is totally dependent on EQ and I'm not talking about the tone controls or one fancy bit of mod kit or fancy pants pedal circuit
It's called SYSTEM TONE..
In other words the whole signal path and each stage of a guitar circuit dictates the final outcome.
Your Guitar and amp  setup has way too much bandwidth which is why you hear all the fizzle.


You can get far better treble by wiping off a whole lot of hi end crud in a circuit THEN you boost the freq that ENHANCES the sound and gives it that edge with out all the excess harmonics which just clutter the sound/Tone 
Think of it as TONE FOCUS.

i.e if you have a dark amp a bright speaker might help,, if a bright amp try a darker speaker.
Humbuckers might sound good on one amp design but turn to mud on another.

Trust me it's taken me 30 plus years to understand what MAKES GUITAR SOUND WORK. 8|
Hope it helps, Phil.

Ubik_Fresh

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.

Ubik_Fresh

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.

Dino Boreanaz

For what it's worth, I really like the G10 Greenbacks in my Micro Stack.  I found their slightly reduced high end makes the amp's treble control more usable.

I should mention that my clip was recorded with an SM57 microphone and I think it sounded quite representative of the sound in the room.  As you said, different recording methods will tend to emphasize different parts of the frequency spectrum, and some may not portray the "real" sound as well as others.

If you do get around to trying out different op amps, I would love to know what you thought ... maybe even some clips if you can.