Another MARSHALL Valvestate 2000 AVT150 Blowing fuses......

[blearyeyes]

I'm not a component level tech but I can sorta read a schematic and I know how to solder.
Other than that I really need help. No local techs worth their salt.

I unplugged the output IC boards and the fuse still lights up like a light bulb. poof...

I pulled the secondaries wires off of the transformer and used those wires as a way to test the rectifier bridge for shorts.
I ran a continuity check on both sides of both secondary leads without any shorts showing up....
I don't know if that is the way to test the rectifier bridge as there is a resistor in the circuit.

Do I need to check the rectifier bridge pins directly?

I appreciate any help...

[blearyeyes]

Also if anyone has bothered to come up with a MOD to make this amp more reliable.

Maybe I should fix it and sell it but I would feel bad passing on the curse..

[52 Bill]

Build yourself a light bulb limiter so thet it will light up and your fuses will not go poof.

Now that you have pulled all of the transformer secondaries, when you turn on the amp does the fuse still blow?

You should use a multimeter to test the rectifier diodes. Testing an amp by fuse blowing may work, but it will cause more damage to the circuit with every power on.

What kind of meter do you have? Does it have a diode test setting?

[blearyeyes]

Build yourself a light bulb limiter so thet it will light up and your fuses will not go poof.

Now that you have pulled all of the transformer secondaries, when you turn on the amp does the fuse still blow?

You should use a multimeter to test the rectifier diodes. Testing an amp by fuse blowing may work, but it will cause more damage to the circuit with every power on.

What kind of meter do you have? Does it have a diode test setting?

I will try the power with the secondaries removed....

How would one go about building a light bulb limiter? clips, wires and a 125 volt bulb with?

Yes I have a low end digital meter with a diode test setting.

And thanks for the reply!

[52 Bill]

The simplest light bulb limiter that I have ever seen was a 60 watt incandescent bulb with two clip wires soldered to its' socket. The fuse is removed from the holder and the light bulb is clipped across the fuse holder terminals, taking the fuses' place in the circuit. I do not suggest this as there are far too many ways to screw this up and hurt yourself.

Search the forum or the web and there will be lots of instructions on how to build a simple, safe one.

In the mean time with the power removed from the circuit, use the diode test function to check the rectifier bridge. When you test a diode, you place the leads across the two ends and read the meter. Then you reverse the two leads and read the meter again.

There should be a low reading and a high reading. A typical low reading will be 0.6-0.7 volts. A typical high reading will be infinity. When testing in circuit, the low reading should still be around 0.6 volts, but the high reading may be lower due to circuit interactions. In no case should either reading be 0 (zero).

[blearyeyes]

The fuse holds with the secondaries disconnected from the transformer.

The diode test shows basically what you said, a high reading and a lower reading on both:

L100 0.9v / 0.4v and L101 0.9v / 0.5v...no zero readings.

Does that make sense?

I'll look up a light bulb limiter..only got one fuse left

[blearyeyes]

rectifier bridge vrs diode test

In the mean time with the power removed from the circuit, use the diode test function to check the rectifier bridge. When you test a diode, you place the leads across the two ends and read the meter. Then you reverse the two leads and read the meter again.

There should be a low reading and a high reading. A typical low reading will be 0.6-0.7 volts. A typical high reading will be infinity. When testing in circuit, the low reading should still be around 0.6 volts, but the high reading may be lower due to circuit interactions. In no case should either reading be 0 (zero).

When testing a rectifier bridge, there are 4 pins coming out of the bottom. Do you test the outside pins as if it were one diode?

[The Dude]

It's 4 diodes in a single package. Check each one as if it were a single diode (see attached picture). There should be no shorts across any pins.

[Enzo]

Disconnect the power transformer when testing, otherwise the low resistance of the transformer will make it seem like the bridge is shorted betwen pins 2 and 4 in the above image.


As to making the amp more reliable, consider that the vast majority of them do not blow up. People only come here to talk about them when they fail, so it seems like they blow up a lot. But out of the thousands and thousands that have been made, only a small percentage have blown up. And conversely, show me an amp model that has never blown up.

[blearyeyes]

Disconnect the power transformer when testing, otherwise the low resistance of the transformer will make it seem like the bridge is shorted betwen pins 2 and 4 in the above image.


As to making the amp more reliable, consider that the vast majority of them do not blow up. People only come here to talk about them when they fail, so it seems like they blow up a lot. But out of the thousands and thousands that have been made, only a small percentage have blown up. And conversely, show me an amp model that has never blown up.

I understand that about forums. There is defiantly a bias toward the down side. I must be lucky. I had Pat Quilter (Quilter Sound Company which became QSC) hand build me an amp in the 60s. It never blew up. I replaced it with a Gallien Kruger guitar head that worked for years of heavy road use. This is the first amp I have owned that has ever blown up. It was used in church a few times and it was just sitting "on" and blew up. Oh I forgot the bassman head I used that had smoke coming out of the input holes and an upside down waterfall of smoke coming out of the back at the Golden Bear. So ok. Still this one I don't trust for some reason. But it is what I have. So I fix it and it is a good amp. 8-))

[blearyeyes]

I want to add that I really appreciate what a great thing this forum is. What you all put out to help guys like me. It's a good thing and I really appreciate you all.
Dan.

[blearyeyes]

After I get a chance to check the rectifier bridges properly and get a bulb limiter put together, would it make sense to connect the secondaries one at a time to find which circuit has the problem?

[Enzo]

Perfectly reasonable. It is probably power amps, but being systematic is always good.

[blearyeyes]

Perfectly reasonable. It is probably power amps

First thing I did was to pull all the wires off the two output amp boards and tried another fuse which blew up. I guess it's somewhere else.

[Enzo]

OK, on any amp that blows fuses, the first suspect is the power amp stage, specifically the output device. That might be a 6L6, a set of transistors, or a power amp IC. After that we look at power supply, mostly for shorted rectifiers/bridges. SHorted caps are not common, but should be checked for. The transformer is at the bottom of the list, and you have already eliminated it. Remember you have both main power rails and the low voltage rails as separate supplies. Also there is a voltage tripler on the high voltage one to make B+ for the tube. And we need to look for loose hardware - a loose nut from some bolt wedged under the circuit board can surely cause trouble. We can check to see that none of the various power rails is shorted to ground, but since this amp has split supplies, we also want to check that the positive is not shorted to the negative on any. Ground is not the only place things can short to. If both power amps are disconnected be don;t have to worry about the power IC shorting to chassis.