MF350 Mode 4 Trouble

[Maverick]

Hey guys total newbie here, been having some issues with my MF350 Mode4 and wanted to see if those who were knowledgeable could help me out. I've had my MF350 for about 5 years at this point, I loved every minute of playing out of it until the day it stopped working. About a year ago it just ceased to function. While plugged in, flipping the power switch now results in the "On" light turning on, and that's it. The usual hum of the tubes and fans is gone(seems like they don't receive any power or signal). A guitar plugged in no longer produces any sound, nor does the "tuner mute" button or any of the other knobs/buttons seem responsive.

Is this a common occurrence with this type of unit? A local freelance repair guy(I live in the New York City area) stated that "those types of amps have inherent problems, and are usually not worth fixing". He wanted to charge me $75 just to take a look at the amp, and then based on what I said told me that it would be at least $200+ to repair, most likely more. This is without him even seeing the unit. My immediate reaction was to call BS, but giving the benefit of the doubt I figured I'd try to get in touch with other people who have experience with the amp.

I took some pictures of the insides and recorded a video that is currently being uploaded to youtube for you guys to take a look at. Any help or advice that can be offered would be greatly appreciated. Thanks in advance guys, rock on

 

[Maverick]

[ame="http://www.youtube.com/watch?v=JRW1q-bbK8M"]http://www.youtube.com/watch?v=JRW1q-bbK8M[/ame]

 

[Maverick]

Apologies in advance for the blurry camera shots, had trouble focusing in such tight quarters.

On/Off switch(inside view)

Circuits between Power Unit and Heat Sink #1

Circuits between heat sinks

Behind heat sink #1

Tubes

 

[MajorNut1967]

I've heard of problems of these things over-heating because of fan failure and that it snow balls form there. lets see what we can find.

 

[Maverick]

Thanks for the support

 

[MajorNut1967]

If any power module IC is bad, REPLACE ALL FOUR.

DO not connect or disconnect any power module with the amp turned on.

Always make sure the power supplies are discharged before connecting power modules. If the connector sparks, you may have just blown a new module.

ALWAYS read the load/jack instructions on the rear panel and obey them.

NEVER connect or disconnect a load while the amp is running.

If the power modules are new and it no longer blows fuses, but so sound comes out, then check the mute circuit for shorted component. That is on the power supply and is a transistor, a zener, and a cap in parallel, making the STBY signal.

 

[Maverick]

If any power module IC is bad, REPLACE ALL FOUR.

DO not connect or disconnect any power module with the amp turned on.

Always make sure the power supplies are discharged before connecting power modules. If the connector sparks, you may have just blown a new module.

ALWAYS read the load/jack instructions on the rear panel and obey them.

NEVER connect or disconnect a load while the amp is running.

If the power modules are new and it no longer blows fuses, but so sound comes out, then check the mute circuit for shorted component. That is on the power supply and is a transistor, a zener, and a cap in parallel, making the STBY signal.

Going to check the IC components with a voltimeter, but there is no evidence of shorting(burn marks and the like). Same on the mute circuit. I suspect the problem lies in the power supply or the transformer(?)

Since there are no burn marks indicating shortage anywhere on the board, around the IC banks, or the tubes, I'm going to venture that the power is not making it from the supply past the transformer. Is there any way to indicate if the transformer is the faulty unit? Thanks again.

And dually noted with removing the power cable while the unit is left in the on position. At the time I wasn't paying attention, was more worried about demonstrating the issues I was having on video.

 

[Procter2812]

Going to check the IC components with a voltimeter, but there is no evidence of shorting(burn marks and the like). Same on the mute circuit. I suspect the problem lies in the power supply or the transformer(?)

Since there are no burn marks indicating shortage anywhere on the board, around the IC banks, or the tubes, I'm going to venture that the power is not making it from the supply past the transformer. Is there any way to indicate if the transformer is the faulty unit? Thanks again.

And dually noted with removing the power cable while the unit is left in the on position. At the time I wasn't paying attention, was more worried about demonstrating the issues I was having on video.

I doubt its the power transformer thats wrong, they rarely go bad.

Like Major said the most common problem in solid state power amps is the power section itself.

Id check out the IC's Try plugging into the FX loop return and see if you get any sound.

Also check the fuses that none inside the amp are blown.

If you get no sound from the fx loop return, its most likely the power amp thats bad.

One of the power transistors may have blown open.

 

[Jesstaa]

If the tubes aren't lighting up that's odd, have you checked all the fuses?
If so try what the others have suggested and replace the tubes (Couple of 12AX7s will cost $30 at the most including shipping)

Also you shouldn't really turn it on without any kind of load attached, but as it's not operational I imagine you're not damaging it too much, and it's solid state.

 

[Ken]

Also you shouldn't really turn it on without any kind of load attached, but as it's not operational I imagine you're not damaging it too much, and it's solid state.

Wouldn't that be okay with a SS power amp?

Ken

 

[Maverick]

Got the brains out of the case and made a new video demonstrating me trying all of your suggestions, and still getting nothing guys

Uploading to youtube is taking forever for some reason, I'll post it here as soon as it's ready. Thanks again for all of your help, very grateful to all of you.


*Update*

Video is finally online.

[ame="http://www.youtube.com/watch?v=g8ndc3BN9JU"]http://www.youtube.com/watch?v=g8ndc3BN9JU[/ame]

 

[Maverick]

Updated.

 

[MajorNut1967]

Question? Is the heater filaments heating up when the power switch is on? Another thing just looking at the fuses doesn't necessarily mean they are good, you need to check them with an DMM or a continuity tester! Now we might not be seeing any parts that look fried or blown, but since this is an SS amp there may be a blown component that looks fine!

I don't know for sure but it appears that the filaments are not heating, can you verify that?

 

[Maverick]

Sorry for the lapse in information guys, work has taken up a bit more of my time that I'd have liked it to over the last few days and I had to delay my further testing of the head.

I took the unit to a local electronics expert(not to be confused with the amplifier expert previously mentioned) and had him perform continuity tests on various areas of the board. We discovered that none of the circuits post-transformer are receiving power as a result of the yellow wire group coming from the transformer(you can use the previous pictures/video for reference) not transferring any current. Being that this is the "power out" wiring(the dark blue is "power in"), this confirmed my suspicions that the transformer is the culprit for the problems I am having.

How would I be able to properly discharge the transformer to prepare the board for a replacement, and how would I be able to acquire said replacement so that I can get everything back in working order and play again?

Thanks again guys, those who have contributed their expertise have been a huge help so far.

Rock on

 

[saranden]

I've gone through all of this, changed all four TDA7293's measured HT volt at 99V and I am getting 99V at the connections to all four IC's.
I have replaced the zeners, caps transistor and diode in the standby circuit, and after all of that I am still not getting an output.
The preamp works perfect as at the moment I am sending that signal to a 400w external amp, but that's having to use 2 amps for one job!

Anyone have any further ideas, BTW I have also put 2 new valves in.
Seems to me that I need to know where to probe to find out if I am in fact getting the voltage sent to the TDA to switch the standby I suppose and making the TDA operational instead of staying in standby.

I will try anything that is suggested and I have a schematic. More help with reading voltages in the standby circuit will be really great.

 

[saranden]

I like all the other members, agree that it probably wasn't the transformer, but more than likely the fusible resistors R23, R44. I did read maybe 2-3 times check the fuses!
I discovered one of these to be open circuit instead of 2R2 so this would definitely give his amp the symptoms he described, wouldn't you think? I've got 4 new KITS-0004 modules, and I've now replaced all of the components in the standby circuit. Gonna check all the headers to make sure not short voltages to other pins. I have no burnt traces near the headers, pre amp works great.

 

[Kiajee]

Hey Everyone,

I hope to not get too far off topic from the forum, but I am also having some difficulty with my Mode4 and this seems to be the closest forum to my problem. I bought mine about 6 years ago. I had plugged the head into my cab, but from what Marshall told me I had a bad connection from the head to the cab and it blew the head. This resulted in the same exact issues im seeing in this forum. I sent this to Marshall a few years back to get repaired, and they sent it back and everything seemed fine. Recently now I am getting a very loud feedback noise coming from the amp. This noise occurs whether I have a guitar plugged in or not. I really cannot afford to get another repair done. Can anyone suggest some help! Thanks guys!!

 

[Marshall Mann]

Hey Everyone,

I hope to not get too far off topic from the forum, but I am also having some difficulty with my Mode4 and this seems to be the closest forum to my problem. I bought mine about 6 years ago. I had plugged the head into my cab, but from what Marshall told me I had a bad connection from the head to the cab and it blew the head. This resulted in the same exact issues im seeing in this forum. I sent this to Marshall a few years back to get repaired, and they sent it back and everything seemed fine. Recently now I am getting a very loud feedback noise coming from the amp. This noise occurs whether I have a guitar plugged in or not. I really cannot afford to get another repair done. Can anyone suggest some help! Thanks guys!!

My guess is that you have a capacitor that's fading. This is not an expensive repair, but one that should be done by a tech if you are not confident working on an amp.