Marshall AVT 150 no effects

[rockola1971]

Picked up a Marshall AVT 150 combo cheap but "broken". Evrything works except for the effects? Any other techs out there seen this fault yet?

[J M Fahey]

Yes.

[rockola1971]

Bad module? What fault did you find?

[Enzo]

Hi, welcome to the forum. It may sound like it, but I am not picking on you here.

I think what Mr. Fahey is trying to communicate is that this question is like "did you ever see a car broke down beside the road?" I don't think there is a technician who has not seen an amp with dead FX, and it can be caused by million things.

The FX control could be bad or have broken solder
the connections to the FX board could be bad
The power supplies to the FX board could be bad
Wire connections could be off
Signal routing circuits in the amp could have failed.
Switching circuit problems

There is a long list, we have to systematically isolate the problem. There is no "it's going to be THIS" answer.

[rockola1971]

Hi, welcome to the forum. It may sound like it, but I am not picking on you here.

I think what Mr. Fahey is trying to communicate is that this question is like "did you ever see a car broke down beside the road?" I don't think there is a technician who has not seen an amp with dead FX, and it can be caused by million things.

The FX control could be bad or have broken solder
the connections to the FX board could be bad
The power supplies to the FX board could be bad
Wire connections could be off
Signal routing circuits in the amp could have failed.
Switching circuit problems

There is a long list, we have to systematically isolate the problem. There is no "it's going to be THIS" answer.

Well it takes alot more to get me "stirred up".
I wont comment on "MR. Fahey's Short and Sweet answer of Yes".

But you know as well as I do that it is not uncommon at all for there to be typical manufacture/engineering induced common problems that are seen often in certain amplifiers. Marshall Cooling fans living a short life and/or not up to snuff, B52's circuit boards dropped into a obvious contaminated solder bath, JCM800 horizontal input models with broken pot solder joints, and many many more common ones.

So I figured I would try my luck and see if this model had a common problem that was seen often in respect to the DFX failing but the rest of the amp working great.

[Jazz P Bass]

The only Service Bulletin that I have pertains to the early production run.
They added protection circuitry to the input.
As to the problem.
See if you have signal going in to the FX board.(pin 11 & 13 /J1)
If you do, now see if it comes back out. (pin 15 & 16/ J1)
That will narrow it down to the FX board or not.
I had a Marshall FX board that would not output from the Fx board.
I found a very small sliver of solder jumpering two pins on the microcontroller.
So it comes down to where is your amp failing.

[Enzo]

And i am an old jukebox guy too.

I understand your intent. But this is a generic problem. After over 25 years of riding an amp bench, I can say that ther is no "usually this" answer for you.

The items I listed would be common failures. And we cannot rule out plain old bad FX card.

[rockola1971]

Thanks for info gentleman. I'll jump into it this weekend and have a look at where my signal is stopping. Guess I need to break out the O-scope and blow the dust off. I shut down my amp shop in 2007. (I took a Federal job that kept me away from home alot for the first 3 years) Ironically I am now working on RF transmitters that broadcast to aircraft.

[Gtr_tech]

Does the module have its 5v supply voltage?

[rockola1971]

Ill be jumping in this tomorrow now that I have a full set of schematics. Another thing I noticed is that when I rotate the encoder for all the different effects settings I do hear an obvious change that you would normally hear as you change from one effect to the next. I just am not getting any actual effects change in the guitar signal. It also has alot of hum/static when the mix is maxed toward DFX. It goes awat when I rotate mix to zero. Ill be checking the obvious firts...power supply voltages going to the DFX board. Much thanks to Jazz P Bass for the DFX schematic. That one seems to be the missing one when I found all the others (front and back pcb schematic)

[rockola1971]

The only Service Bulletin that I have pertains to the early production run.
They added protection circuitry to the input.
As to the problem.
See if you have signal going in to the FX board.(pin 11 & 13 /J1)
If you do, now see if it comes back out. (pin 15 & 16/ J1)
That will narrow it down to the FX board or not.
I had a Marshall FX board that would not output from the Fx board.
I found a very small sliver of solder jumpering two pins on the microcontroller.
So it comes down to where is your amp failing.

Im not sure if that is the right schematic or not. Its marked DREV--Which i assume is Digital reverb. My board is marked 012-05005 rev1 and it sits atop the back PCB. The board does have an Alesis M4C (ASIC) and the DRAM (TMS44C256DJ) is marked the same as what is on that schematic. Plus that schematic is dated 11/1998 and this is a JCM2000 amp. The little board has pins that go down into a header on the back PCB. 16pin. (2 rows of 8) The header is marked CN101 on the back PCB that the DFX board plugs in to. I did find +5vdc going to the board but I could not tell you what pin it was since no pins are marked with a number. Is this the correct schematic for the DFX of a AVT150? How much is a replacement of this board and are they available still? Im going to check the signal in and out just as you suggested. While Im probing pin to pin Ill hopefully figure out then what the numbering is for this plug/jack.

[junkcollector]

I know this is an old thread, but did you have any luck with this issue, I just picked up an AVT150H with the same issue. I am currently looking online to get info and just downloaded schematics (thanks to all that have posted them) just curious what your solution was.


Thanks

[Jazz P Bass]

I had a Marshall FX board that would not output from the Fx board.
I found a very small sliver of solder jumpering two pins on the microcontroller.
JPB.

[Lee Steel]

This thread starts off talking about an AVT150 and then someone jumps in arguing about a JCM2000??

Confusing but here is my take on this issue.

The DFX board has a tendency to fall out if the amp is banged around. One amp that I have repaired, had the DFX card rattling around inside the head and just needed to be plugged back in again. The other one, the board fell out but was still damaged (electronically). The DFX board is still available as of today (5/12/16) from Full Compass. The part number is: M-REVB-00007 and cost is $25.95 plus shipping.

When the card is inserted into the header it is still quite wobbly. I added a thin piece of foam, under the DFX card (between it and the main board) and also a thin slice on top of the DFX board, between it and the heat sink, creating a sandwich that holds the card in place. I suppose you could also use a little dab of hot glue.

[oc disorder]

"it's going to be THIS" apologies Enzo couldn't resist , been a long morning so I won't be posting fix by numbers from my coloring in layout book!

Had one recently and in my case was quite obvious the capacitor for the 5v supply near the regulator had come unstuck -think the owner had been hiding something in there or given it a bump ! So the whole board had to come out to resolder it back in. The 5v regulator had cracked solder joins too , so touched those up and a couple of others .. well while I was in there .. more effort to dismantle than actually do the repair!

So without knowing the level of expertise here I would first check the 5v rail and secondly remove the board and check over all the soldering.

Those efx boards are about $70 Au here with freight from Melb.