Trouble with Marshall AVT50 Head

[outrider13]

I was adjusting the volume on my Marshall AVT50 when it started crackling then suddenly stopped making sound altogether. I switched the channel to clean, and it still wasn't making any sound. The power still turns on, and the tube lights up still. What could the problem be?

[52 Bill]

Welcome!

Here's a list of things to test/try first:
1-Are your guitar cables all ok?
2-Is the speaker and speaker cable ok?
3-Did you try putting a cable bridging the FX out to the FX in jacks?
4-Did you try giving the amp a good thump with your fist?

Check it out and let us know what the results are.

[outrider13]

Welcome!

Here's a list of things to test/try first:
1-Are your guitar cables all ok?
2-Is the speaker and speaker cable ok?
3-Did you try putting a cable bridging the FX out to the FX in jacks?
4-Did you try giving the amp a good thump with your fist?

Check it out and let us know what the results are.

My cables were all fine, but just in case, i'll try using different ones. I wasn't using any effects, so I shouldn't see any problem with that. And I haven't "thumped" it with my fist, so i'll see if that does anything

thanks for the advice.

[52 Bill]

I wasn't using any effects, so I shouldn't see any problem with that.
This is exactly why there may be a problem here. There is a switching contact on the return jack that will sometimes become oxidized from non-use, causing the signal to drop-out. Bridging the loop jacks will bypass this switch, showing up any problems with the contact, so please try this as well.

[outrider13]

silly me.
i isolated the source of the problem, being that something is wrong with my cabinet, and not my amp. any suggestions on how to find out what exactly is wrong with the cabinet?

[52 Bill]

What sort of cabinet?
Open it up and check the jacks, the wiring and the speakers.

The whole thing can be checked quickly with a 9v battery. Plug in a good speaker cable and touch the terminals of the battery to the tip and sleeve of the other cable end. The speakers should make a thumping sound.

No sound?, check each speaker the same way by touching the battery across the terminals of the speaker itself.

[Enzo]

Just had a guy bring in a Crate 412 last night. SHowed him the battery trick on the dead cab, then opened it up and determined all four speakers were blown. I get my money's worth from those dollar store batteries.

[outrider13]

I used the AVT50 on a Marshall MG412. Now I was talking with a friend, and because I don't know too much about stacks, he said that was a bad combination because for the AVT50, the minimum is 4ohms, and the MG412 cab is 8 ohms. I don't know what that really means but apparently it can mess with the electronics. I'm going to try that battery trick, but if worse comes to worse, i'm going to have to get my cabinet replaced.

[outrider13]

What sort of cabinet?
Open it up and check the jacks, the wiring and the speakers.

The whole thing can be checked quickly with a 9v battery. Plug in a good speaker cable and touch the terminals of the battery to the tip and sleeve of the other cable end. The speakers should make a thumping sound.

No sound?, check each speaker the same way by touching the battery across the terminals of the speaker itself.

Ok, so I did the battery trick and it did make the thumping noise, however the head just isn't working on it, I'm going to open it up and see what's going on

[outrider13]

Alright, so I tried the battery test, and the cabinet worked fine. I plugged my ipod in it just to be certain and every speaker was working just fine. It's just that it won't work when I use my head on the cabinet. I suppose the only thing i can do is sell the cab for another one.

[Enzo]

If the cab works, it is NOT your problem.

The mute circuits sometimes short to ground WHEN THE 7293 IC burns up.

Any solid state amp - which this is - that has a 4 ohm minimum load will be twice as happy with an 8 ohm load. There is no reason an 8 ohm cab would stress an amp with a 4 ohm rating.

SOmething is wrong in the amp.

Do the headphones jack work?

[outrider13]

If the cab works, it is NOT your problem.

The mute circuits sometimes short to ground WHEN THE 7293 IC burns up.

Any solid state amp - which this is - that has a 4 ohm minimum load will be twice as happy with an 8 ohm load. There is no reason an 8 ohm cab would stress an amp with a 4 ohm rating.

SOmething is wrong in the amp.

Do the headphones jack work?

Yea, true. but the amp works fine when i plug it into other speakers, and the headphone jack works perfectly. What's 7283 IC?

[Enzo]

The TDA7293 is the power amp IC on the little output board.

If the headphones work, then the amp itself works.

If the speaker cab works and the amp makes other speakers work, then I would have to think there was a problem with what connects them together.

[outrider13]

alright, weirdest thing. I plugged in my headphones out of curiosity, and when i unplugged them, the sound started coming out of the cab. strange

[teemuk]

No, it’s not weird at all: The headphone jack has an internal switch that breaks the signal path of the loudspeaker output. It is quite common that these switches get corroded or dirty and therefore develop intermittency. In fact, in case of any intermittency in an amp one of the first practices one should do is to plug and unplug a cord into all jacks including headphones, effects loops etc. This removes most corrosion and dirt – at least for a while. By the way, if the problem returns then clean the jack with some cleaning substance designed for the job. If it doesn’t help you usually have to change the jack but before this inspect that the solder joints are fine and not causing the problem. If you have to replace, make sure to replace the jack with an identical one.

[outrider13]

thanks for the info, you have all been very helpful.