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Author Topic: Marshall Lead 12 testing and repair questions.  (Read 4286 times)

axepilot

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Re: Marshall Lead 12 testing and repair questions.
« Reply #15 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.

Loudthud

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Re: Marshall Lead 12 testing and repair questions.
« Reply #16 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.

axepilot

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Re: Marshall Lead 12 testing and repair questions.
« Reply #17 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?

Loudthud

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Re: Marshall Lead 12 testing and repair questions.
« Reply #18 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.

axepilot

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Re: Marshall Lead 12 testing and repair questions.
« Reply #19 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.

axepilot

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Re: Marshall Lead 12 testing and repair questions.
« Reply #20 on: February 06, 2020, 11:29:12 PM »
The bc184s didn't help

phatt

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Re: Marshall Lead 12 testing and repair questions.
« Reply #21 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.

axepilot

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Re: Marshall Lead 12 testing and repair questions.
« Reply #22 on: February 08, 2020, 01:42:25 PM »
Thanks, Phil..I'll check it out.

axepilot

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Re: Marshall Lead 12 testing and repair questions.
« Reply #23 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?

Loudthud

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Re: Marshall Lead 12 testing and repair questions.
« Reply #24 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.

axepilot

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Re: Marshall Lead 12 testing and repair questions.
« Reply #25 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.

axepilot

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Re: Marshall Lead 12 testing and repair questions.
« Reply #26 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.
« Last Edit: February 13, 2020, 09:52:37 PM by axepilot »

phatt

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Re: Marshall Lead 12 testing and repair questions.
« Reply #27 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.


phatt

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Re: Marshall Lead 12 testing and repair questions.
« Reply #28 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.


axepilot

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Re: Marshall Lead 12 testing and repair questions.
« Reply #29 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?