How do i check this bridge rectifier

[movek1]

Hi, how do i check this bridge rectifier.
Also , in the second picture, are those diodes or bridge rectifiers, if they are diodes, on my dmm shouldnt it check open in one direction and closed in the other.(they are part # 1n4007.
by the way this is all out of a marshall 8100 thats blowing fuses and its not beacause of the output transistors

[R.G.]

Hi, how do i check this bridge rectifier.
Also , in the second picture, are those diodes or bridge rectifiers, if they are diodes, on my dmm shouldnt it check open in one direction and closed in the other.

First, use your DMM to check a diode you already know is good. Most DMMs have a diode check range on the low end of the ohms scale. Set it there and see what it does on a known good diode. Generally, but not always, the red lead is positive and the black negative when checking diodes and resistors. Doing this will let you recognize what your meter does with good diodes. That way you can tell if what you read on the bridge and diodes is correct, or if your meter just can't read them. Diodes do read open when they're back biased, but they will generally read from 1 to 100 ohms on an ohmmeter scale.

After calibrating yourself and your meter test the diodes. The meter should show open when the positive lead is connected to the bar side of the diode and the negative lead is connected to the non-bar side. It should conduct somewhat when the leads are reversed.

Once you can reliably read diodes, try the bridge. That bridge is four diodes inside. The bar ends of two of them connect to the lead marked "+". The non-bar end of two of them connect to the lead marked "-". The leads marked "squiggle" are the other ends. So you test the bridge as if it was four diodes - which it is - connected as I've described. Draw it on the bridge if that's confusing.

Note that a DMM does not test whether the diodes have enough reverse voltage rating for the actual use, because the DMM only has a battery to put voltage on the diodes in reverse.

[movek1]

First off thank you for the help.
So what your saying is power would have to be on, to see if the diodes or bridge rectifier, are working correctly.

[R.G.]

First off thank you for the help.
So what your saying is power would have to be on, to see if the diodes or bridge rectifier, are working correctly.

You're welcome. Since you have the diodes out of the box, test them for what the TV-repairman guys call front to back ratio. That's just the resistance forward and reverse on the meter. If they pass that test of conducting one way but not the other, put them back in circuit.

Then use your lightbulb current limiter (check the archives here, or look at geofex.com) to keep the amp from blowing fuses while you make measurements to find out what is really wrong. A variac and current meter will help as well, but while a variac with current meter tells you "It's dying right NOW", the lightbulb limiter says "It's trying to die, but I won't let it."

[movek1]

yes the diodes checked out good and the bridge.
So what are you saying is the major difference between variac with ammeter and a light bulb current limiter

[Gtr_tech]

A transistor tester is better for checking diode junctions. It can show leakage current as well as a go/no go test. A leaky diode will check ok on a DMM but fail in circuit. Saw this a few days ago working on a Dean Markley rack tube amp I recently bought. Hum in the output was caused by a leakey diode in the bias section.

[gbono]

The rectifier diodes in the second picture our inexpensive at $.34 ea. I would replace them and the bridge since they are out of the board.