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Solid State Amplifiers => Amplifier Discussion => Topic started by: axepilot on January 28, 2020, 01:20:57 AM

Title: Marshall Lead 12 testing and repair questions.
Post by: axepilot 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!
Title: Re: Marshall Lead 12 testing and repair questions.
Post by: Loudthud 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.
Title: Re: Marshall Lead 12 testing and repair questions.
Post by: axepilot 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.
Title: Re: Marshall Lead 12 testing and repair questions.
Post by: Loudthud 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 ?
Title: Re: Marshall Lead 12 testing and repair questions.
Post by: axepilot 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.
Title: Re: Marshall Lead 12 testing and repair questions.
Post by: phatt 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.
Title: Re: Marshall Lead 12 testing and repair questions.
Post by: Jazz P Bass on January 30, 2020, 11:58:37 AM
Schematic:
Title: Re: Marshall Lead 12 testing and repair questions.
Post by: g1 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.
Title: Re: Marshall Lead 12 testing and repair questions.
Post by: Loudthud 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.
Title: Re: Marshall Lead 12 testing and repair questions.
Post by: axepilot 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?
Title: Re: Marshall Lead 12 testing and repair questions.
Post by: Loudthud 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.
Title: Re: Marshall Lead 12 testing and repair questions.
Post by: axepilot 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.
Title: Re: Marshall Lead 12 testing and repair questions.
Post by: phatt 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.
Title: Re: Marshall Lead 12 testing and repair questions.
Post by: axepilot 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
Title: Re: Marshall Lead 12 testing and repair questions.
Post by: Loudthud 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.
Title: Re: Marshall Lead 12 testing and repair questions.
Post by: axepilot on January 31, 2020, 03:33:56 PM
I'm about to just start pulling each component, one by one, until I land on the right one.
I've got BC184s in.bound, then its gonna be zeners and a new IC unless anyones got better suggestions.
Title: Re: Marshall Lead 12 testing and repair questions.
Post by: Loudthud on February 01, 2020, 11:55:19 PM
Before you start that process, carefully inspect the PCB for any bad or fractured solder joints. Pay special attention to the pots and input jack. The chip is probably OK but just in case, install a socket.
Title: Re: Marshall Lead 12 testing and repair questions.
Post by: axepilot on February 02, 2020, 06:38:54 PM
Yeah, I've been thinking about putting in a socket so..ok will do.

How often do resistors fail to a short?
Title: Re: Marshall Lead 12 testing and repair questions.
Post by: Loudthud on February 03, 2020, 01:21:48 AM
How often do resistors fail to a short?

Extremely rare. Most often resistors will drift higher in value, sometimes because of too much stress on the leads during assembly. Most intermittent malfunctions are traced to faulty connections, either on the PCB or wires between the PCB and external components. One possibility is the small transistors. The lead can break right where it enters the plastic. Give each transistor a little nudge to see if there is any change in operation of the amp.
Title: Re: Marshall Lead 12 testing and repair questions.
Post by: axepilot on February 05, 2020, 10:17:23 AM
The second BC184 did impart a loud crackle when nudged.  I ordered some replacements, have yet to swap them out.  Will post back with results when I can get to it.

thanks for the word on resistor shorts.
Title: Re: Marshall Lead 12 testing and repair questions.
Post by: axepilot on February 06, 2020, 11:29:12 PM
The bc184s didn't help
Title: Re: Marshall Lead 12 testing and repair questions.
Post by: phatt on February 07, 2020, 03:13:08 AM

If the bases of Tr1 & Tr2 are as stated then it's a fair bet the most likely problem is the Output Tr's.

but maybe check this first before you swap out parts willie nilly;

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.
Phil.
Title: Re: Marshall Lead 12 testing and repair questions.
Post by: axepilot on February 08, 2020, 01:42:25 PM
Thanks, Phil..I'll check it out.
Title: Re: Marshall Lead 12 testing and repair questions.
Post by: axepilot on February 08, 2020, 02:53:41 PM
Oh...wanted to add:  the output transistors, one gets warm while the other stays room temp.  By warm, I don't mean hot.  It is just enough to be noticeable during the 2 or 3 minutes I had it powered up to test voltages and such but I was able to tell a difference in temp between the 2 output transistors.   That indicate anything?
Title: Re: Marshall Lead 12 testing and repair questions.
Post by: Loudthud on February 09, 2020, 04:14:23 AM
Oh...wanted to add:  the output transistors, one gets warm while the other stays room temp.  By warm, I don't mean hot.  It is just enough to be noticeable during the 2 or 3 minutes I had it powered up to test voltages and such but I was able to tell a difference in temp between the 2 output transistors.   That indicate anything?
That will probably change if you disconnect the speaker. With the speaker disconnected, measure the DC Voltage across R26 and R27, the two 0.33 Ohm resistors. The Voltages should be close to the same.
Title: Re: Marshall Lead 12 testing and repair questions.
Post by: axepilot on February 10, 2020, 10:34:47 AM
Hmmm... there is no C8.

There is a spot for a C9 but no cap was ever installed there.

R25 and R26 are both directly behind the headphone/line jack but there is no R27.
Title: Re: Marshall Lead 12 testing and repair questions.
Post by: axepilot on February 13, 2020, 09:50:57 PM
Alright, kids.  Replaced the PTs with no change.

I did inject an ipod signal and, with the neg lead of the spkr tied to the chassis, I can red probe pin 1 and 7 and get audio.  At pin 1, the audio is clear.  At pin 7, it is a bit distorted and louder.

My next suspect is TR3.  Can anyone tell me what i should be hearing at the bce of that TR?

Bu the way...yes, with the spkr disconnected, both output TRs remain cool.
Title: Re: Marshall Lead 12 testing and repair questions.
Post by: phatt on February 14, 2020, 06:53:20 AM
Hmmm... there is no C8.

There is a spot for a C9 but no cap was ever installed there.

R25 and R26 are both directly behind the headphone/line jack but there is no R27.

Maybe we are reading different schematics?
What ever cap connects to the base of TR1 will be the input to the power amp section.

As for the 2 x OR33 that *LoudThud* mentioned they are 3Watt resistors (they will be Larger than the others).
They connect the Emitters of the 2 power transistors to the speaker output.
so maybe a good time to go and measure the voltages as was asked.
Phil.

Title: Re: Marshall Lead 12 testing and repair questions.
Post by: phatt on February 14, 2020, 06:59:49 AM
Alright, kids.  Replaced the PTs with no change.

I did inject an ipod signal and, with the neg lead of the spkr tied to the chassis, I can red probe pin 1 and 7 and get audio.  At pin 1, the audio is clear.  At pin 7, it is a bit distorted and louder.

My next suspect is TR3.  Can anyone tell me what i should be hearing at the bce of that TR?

Bu the way...yes, with the spkr disconnected, both output TRs remain cool.

This is starting to get a bit confusing xP

Can i suggest you just do some measurements of the power amp and report your findings then folks here might be able to pinpoint the problem.
I may have missed it but what schematic are you working from? Maybe wise to post it so we are all on the same page.
Phil.

Title: Re: Marshall Lead 12 testing and repair questions.
Post by: axepilot on February 14, 2020, 12:11:50 PM
The difference in schematics is likely due to there being a head version and a combo version.  Then, I believe, there are some variances within those 2 forms, as well.  I have a combo.

What ever cap connects to the base of TR1 will be the input to the power amp section.

I'll look for it.

How can I tell, in the circuit, which lines are the emitters of the PTs?
Title: Re: Marshall Lead 12 testing and repair questions.
Post by: phatt on February 15, 2020, 03:40:49 AM
How can I tell, in the circuit, which lines are the emitters of the PTs?

All you want to do is measure the voltage drop across those resistors,, so set your DMM to DCV and measure the voltage *Across* those Resistors. so one probe on one side and the other probe on the other side of those resistors.
They are 3Watt (Big) and most of the others will be 1/4Watt (Small).
Phil.
Title: Re: Marshall Lead 12 testing and repair questions.
Post by: Loudthud on February 15, 2020, 06:43:59 AM
Most of the industrial suppliers of transistors will have links to pdf data sheets on their websites. Download and look at the data sheet for the MJ3001. Google is pretty good at finding data sheets, even for obsolete parts. Just google "MJ3001 data sheet". Much of it you won't understand, but usually on the first page is a drawing of the part with Base Emitter and Collector identified. It takes a little getting used to because in some drawings you are looking at the bottom of the part.

The name "Lead 12" is on a couple of different amps. What you need is the model number. I think the combo version is the "5005". It should say somewhere on the back or inside of the amp. Try to find the schematic for that exact model.
Title: Re: Marshall Lead 12 testing and repair questions.
Post by: phatt on February 15, 2020, 08:34:21 PM
This is a long read for the novice but it may help a lot of folks to understand the basics of *Fault finding* in Power Amp Circuits.
Understand this most basic principal, I quote Rod Elliott from his page:

"If the output voltage is not close to zero, all other voltages are likely to be wrong!
If the output voltage is close to zero, then the amp should be working, but only if it has power."

So check Your power supplies FIRST otherwise you can go in circles and quote these voltages in your postings will help speed up the repairs. :tu:
So many folks just start replacing parts and actually complicate the problem even further. :duh :duh :duh :duh

Rod Elliott's page on trouble shooting:
https://sound-au.com/troubleshooting.htm#volts1
Phil.