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Author Topic: Marshall Lead 12 testing and repair questions.  (Read 5156 times)

phatt

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Re: Marshall Lead 12 testing and repair questions.
« Reply #30 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.
« Last Edit: February 15, 2020, 03:54:30 AM by phatt »

Loudthud

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Re: Marshall Lead 12 testing and repair questions.
« Reply #31 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.
« Last Edit: February 15, 2020, 06:47:11 AM by Loudthud »

phatt

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Re: Marshall Lead 12 testing and repair questions.
« Reply #32 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.