unwanted DC voltage on the speaker terminals help

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#81
indianajo said:
Read and heed post 66. Transistor testers test at 2 volts or less. Your power sections operates at +-25 v or higher. Junctions that can withstand 2 v can sometimes not withstand dozens of volts. You need to put a tungsten lightbulb series the AC input, say 60 to 75 watts. Then attach negative of DVM or scope to center of power supply with alligator clips so you do not use 2 hands probing for voltages. >25 v across your heart can stop it. Tale all jewelry off hands wrists & neck. Then probe for voltages. I would start at q445 q407 and q409 and then go on to q405 and q407.
Problem could be a bad solder joint or leaky electrolytic capacitor in stead of transistor. Look for b to e <0.6 volts npn -0.6 volts pnp. look for Vce <2 v npn or <=2 v pnp. Or Vce opposite polarity, which means something ahead of that is wrong. Check the voltages differences by subtraction of notations on paper, not by using two hands to probe the power section with two probes.
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it is true, but when they are in the circuit and I poke q421,q425, the voltage on the speaker terminals moves and behaves strangely.
wanted to remove them and check that no pin to the transistors not was damaged and took the opportunity to measure with transistor tester.
yeah i know to put the aligatorclip like some plaze as ground on the filter capacitors. safty first protection glases is also a god 🙂

the schedule is a little messed up when you see the tension a little blurry in some places.
eaky or shorted capacitors are checked... can be hard to see if there is any conductivity fault but I check around a bit from time to time.

two hands probing sounds awkward when measuring voltage
 
#82
JMFahey said:
My point is and many share it, that only removing parts to be tested outside is not enough.
Broken tracks, cracked solder, poor contacts, lack of supply voltages, no or poor bias, etc. all play a part.
That's why turning amp on, with a bulb limiter if necessary, and measuring DC voltages, with no signal applied, no load, can lead to the suspect area.
THEN you remove a couple parts as needed if suspect.

This week a Musician in The Gear Page is repairing a Marshall guitar amplifier, he replaced everything, every single component, except jacks, pots, power and output transformers.
It still does not work 🙁

EDIT:
In post #70 you tested the transistor. Cool, it works.
Then you have not found the problem ... yet.
Post #71 gives you a set of circuit, not parts tests .
Follow them and report back.
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bulb and transformer are probably preferable. just haven't had time to build one yet
 
#83
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