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Messages - axepilot

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Amplifier Discussion / Re: Marshall Lead 12 testing and repair questions.
« 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?

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Amplifier Discussion / Re: Marshall Lead 12 testing and repair questions.
« 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.

3
Amplifier Discussion / Re: Marshall Lead 12 testing and repair questions.
« 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.

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Amplifier Discussion / Re: Marshall Lead 12 testing and repair questions.
« 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?

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

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

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Amplifier Discussion / Re: Marshall Lead 12 testing and repair questions.
« 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.

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Amplifier Discussion / Re: Marshall Lead 12 testing and repair questions.
« 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?

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Amplifier Discussion / Re: Marshall Lead 12 testing and repair questions.
« 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.

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Amplifier Discussion / Re: Marshall Lead 12 testing and repair questions.
« 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

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Amplifier Discussion / Re: Marshall Lead 12 testing and repair questions.
« 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.

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Amplifier Discussion / Re: Marshall Lead 12 testing and repair questions.
« 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?

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Amplifier Discussion / Re: Marshall Lead 12 testing and repair questions.
« 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.

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Amplifier Discussion / Re: Marshall Lead 12 testing and repair questions.
« 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.

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Amplifier Discussion / 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!

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