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Author Topic: Marshall Lead 12 testing and repair questions.  (Read 1442 times)

axepilot

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Marshall Lead 12 testing and repair questions.
« on: January 28, 2020, 01:20:57 AM »
There have been some threads about fixing Lead 12 problems and, yes, I have read them.  But they either don't address my problem or the thread just fizzles out.  First post here...always been a tube guy but I have this thing and I wanna fix it.

I've got the Lead 12 combo and it is making poop sound.  Hard to describe.  Sounds like I plugged my guitar into an AM radio and way overdrove it.

So I think the trans is good.  I've replaced the bridge rectifier and filter caps but no change at all.  Pulled the output transistors and tested them (ok) then replaced the thermal paste and reinstalled.  Not blowing fuses.  Just hardly any output at all unless you get the volume turned way up.  Anything less than 75% volume and the speaker just spits out this weak, fizzy resemblance of distorted guitar strumming.

Where should I be looking?  Such a simple layout it shouldn't be hard to track down but I'm not a tech.  There's some smart dudes here...sending out an SOS!

Loudthud

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Re: Marshall Lead 12 testing and repair questions.
« Reply #1 on: January 28, 2020, 02:05:06 AM »
First thing, try the amp with a known good speaker.

Next, if you don't have an oscilloscope, connect your DVM across the speaker and set to DC Volts to verify there is no DC across the speaker. 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.

axepilot

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Re: Marshall Lead 12 testing and repair questions.
« Reply #2 on: January 28, 2020, 11:26:16 AM »
For starts...thank you for jumping in!  I'm a putz at this stuff.

Okay, DC volts across 8ohm speaker:  18mV.  Across 4ohm speaker:  9mV  (I tried 2 just to be sure the results were repeatable and...they weren't.  Probably something to do with the impedance being half?)

Both of these speakers are known good.

AC voltage across the speaker leads was 0 for all tests.

Loudthud

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Re: Marshall Lead 12 testing and repair questions.
« Reply #3 on: January 29, 2020, 03:45:56 AM »
Does your amp have a Line Out on the front panel ? Does that work or have the same problem as the speaker out ?

axepilot

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Re: Marshall Lead 12 testing and repair questions.
« Reply #4 on: January 29, 2020, 06:15:04 PM »
There is a headphone out and testing shows it to have the same fizzled, garbled output heard from the speaker.

phatt

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Re: Marshall Lead 12 testing and repair questions.
« Reply #5 on: January 30, 2020, 12:10:52 AM »
Measure your DC supply voltages,,Might be a good place to start.
IF? the schematic I'm reading is the correct one,, then you should have ~20-0-20VDC for power amp and about ~15-0-15VDC for preamp.

The test point voltages are written on the schematic for the power amp section.
The Emitters of Tr1&Tr2 should be -.7Volts,, Collector of Tr1 should read +18Volts.
Junction of R15&R16 should read -9Volts.
If one of those readings is way off then that will help folks work out what'sup. :tu:
Phil.

Jazz P Bass

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Re: Marshall Lead 12 testing and repair questions.
« Reply #6 on: January 30, 2020, 11:58:37 AM »
Schematic:

g1

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Re: Marshall Lead 12 testing and repair questions.
« Reply #7 on: January 30, 2020, 01:59:52 PM »
Headphone jack comes right off the output, so doesn't isolate the fault like a line-out would.
Do you have something to use as a signal tracer?  I would check right at pin1 of the input IC. 
If it's bad there (fairly common for described symptom) it must be replaced with 1458, not 4558.

Loudthud

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Re: Marshall Lead 12 testing and repair questions.
« Reply #8 on: January 30, 2020, 08:32:59 PM »
I have two of these things. The combo version (just opened it up today) has the headphone jack on the front and the speaker jack on the bottom. The head version has a line out on the front, two speaker jacks and a headphone jack on the back panel. They have different PCBs.

axepilot

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Re: Marshall Lead 12 testing and repair questions.
« Reply #9 on: January 30, 2020, 10:46:29 PM »
Wow...okay.   So my readings...


TR1
B to E = -.59 v
B to C = +19.73 v

TR2
B to E = -.56 v
B to C = +18.49 v

*when probing TR2, B to C, it puts a hum to the speaker.  Sounds like 60hz.

Cannot determine "junction" of R15 and R16.

Signal tracer?  Are we talking about inputting a low-level audio signal and testing for it at IC pin 1?

Loudthud

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Re: Marshall Lead 12 testing and repair questions.
« Reply #10 on: January 30, 2020, 10:53:18 PM »
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.

axepilot

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Re: Marshall Lead 12 testing and repair questions.
« Reply #11 on: January 30, 2020, 11:13:02 PM »
Pins 1 and 7 both show identical readings.
Measured with the 2v range on my dmm, they swing between plus and minus .02V.

phatt

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Re: Marshall Lead 12 testing and repair questions.
« Reply #12 on: January 31, 2020, 05:17:24 AM »
Wow...okay.   So my readings...


TR1
B to E = -.59 v
B to C = +19.73 v

TR2
B to E = -.56 v
B to C = +18.49 v
Test point voltages are referenced from Ground.
So black probe on Common/Ground/Chassis, then with Red probe check the test points.
Then post your readings.

*when probing TR2, B to C, it puts a hum to the speaker.  Sounds like 60hz.

Cannot determine "junction" of R15 and R16.

not important, just check the test points you can find.

Signal tracer?  Are we talking about inputting a low-level audio signal and testing for it at IC pin 1?

Yes you can inject a low signal into certain points on the circuit to find the offending part.
but I would be checking you have close to correct voltages at those test points on the poweramp first.
Phil.

axepilot

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Re: Marshall Lead 12 testing and repair questions.
« Reply #13 on: January 31, 2020, 10:53:57 AM »
Black probe to ground I get the same voltage noted on pin 1 of the IC,  moving from + .02v to -.02v.
That's TR1.

On TR2, the voltage is ever so slightly higher, like going to +/- .028v sometimes hitting .03v

Loudthud

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Re: Marshall Lead 12 testing and repair questions.
« Reply #14 on: January 31, 2020, 03:24:06 PM »
Black probe to ground I get the same voltage noted on pin 1 of the IC,  moving from + .02v to -.02v.
That's TR1.

On TR2, the voltage is ever so slightly higher, like going to +/- .028v sometimes hitting .03v

Close enough for Rock and Roll. A Voltage of 10 to 15 Volts would indicate a problem that could cause the symptoms you describe.