Marshall Mode 4 (mf-350) IC's blowing up

[Mars Amp Repair]

Hey,
I wonder if anyone has considered one reason that these Mode 4's like to blow up so much....it might be because this amp uses a floating speaker ground output.
One set of IC's drives the tip contact of the speaker jack in the positive direction & the other set of IC's drives the ring contact in the negative direction. Hence, leaving the ring contact to float.

If the customer accidentally or other incident shorts the ring contact to ground, that could explain at least some of the failures. I'm not certain of how effective the short circuit protection is in these TA7293's..or if possibly having the 2 sets push/pulling together could further create havoc when the ring contact is grounded.

The ususally reserve this kind of set up for lower powered amps that have no extension speaker avaiable or the possiblility of shorting.

Everytime I see these 7293's used at all I have to cringe Nice savings for the manufacturer ie;Line6 & Marshall, but not a very solid way to power an amp. I imagine it has a lot to do with not being able to efficiently remove the heat from the IC's.

Any thoughts.....glen

[drewl]

TRS- tip, ring, and sleeve
So you're saying the output jack is stereo?
One set of IC's driving the tip, another driving the ring?
Using the sleeve as a common?
If this is the case a standard 2 conductor speaker cable would be shorting one "channel" to ground.

[Enzo]

No, they are TS. One pair of chips drives the tip and the other pair drives the sleeve at opposite polarity.

Make sure not to ground the sleeve with your scope.

[Mars Amp Repair]

Sorry guys,
Before there were 'stereo' jacks, the phone company just referred to the 1/4" plug tip & ring...no sleeve. As Enzo indicated (using the tip-ring-sleeve terminology), speaker jacks are typical mono speaker jacks...tip & SLEEVE.

My main job is at ATT in switch translations. I deal with PBX vendors all the time & they still refer to the 'tip' as the hot & the 'ring' as the neutral eventhough they are referring to RJ45 hardware and usually T-1 level lines not the loop pair that we usually have as our home phone lines.

Amuzing at times as there haven't been any 1/4" jacks anywhere to be found in the phone system for about 40yrs! Old habits & terminology die slowly.

glen

[Mars Amp Repair]

hope the point was not lost on this technicallity...glen

[tim]

I've noticed two failure modes on these amps (actually any of the Marshall amps that use these powere IC's).

One is if after you replace the IC and then plug the white sip plug onto the little PCB and if there is still residual voltage left in the filter caps that you will blow the IC when it is plugged in. It's best to drain all voltage out of the filter caps before plugging connector onto the little PCB...

Also, those little fans they use to cool the heatsink are barely adequate. They tend to collect dust on the leading edge of the fan which drastically reduces their ability to cool the sink...if you blow the dust off (and tell the owner to carry around a can of compressed air to do the same), the amps will not fail nearly as often... the fans also run much quieter (noisy fans being another chief complaint on the amps)...

[Mars Amp Repair]

Thanx for the info Tim on discharging the caps before installing those *cheapA$$*IC's. I guess as long as they'll going to use them, we gotta deal with 'em.
I agree as would most about the poor cooling.

I thought at one point that having these little boards it would be a great way to troubleshoot which on was bad. NOT..anyone who has tried to fire anyone of the amps up with one IC unplugged has found IC shrapnall heading their way. EEK!!

glen

[Enzo]

False economy. If one is bad, replace them all. The "good" ones have been stressed.

[Valvehead]

i will never go near a Mode 4 again.......

[drewl]

.

Amuzing at times as there haven't been any 1/4" jacks anywhere to be found in the phone system for about 40yrs! Old habits & terminology die slowly.

glen

Yeah, I grabbed a box of like 100 1/4" jacks that were headed for the dumpster as they weren't needed in telecom anymore! gotta love freebies from work!

So, if in fact the mode4 drives the sleeve with a bank of IC's, I believe you will see many fried modules from idiot musicians hooking cabinets up wrong.

[Enzo]

I don't know how to hook a cab up wrong, after all it plugs into a jack like any other amp. If somehow you ground the cab common - no easy feat - then you have trouble.

[Alex R]

If you plug one of these into your dummy load and you've hooked your scope up across the load, the scope ground will of course ground the screen and blow the ICs. Oh yes. Been there.

Might as well put my latest problem with one on this thread.

OK this Mode 4 came in with an IC blown open, so I got four TDA7293 and a new fan to replace the noisy one, and soldered them all carefully on.

I don;t know how sensible what comes next is, so do tell me, as it's what I usually do and it's been ok so far. I do know that you need all the ICs connected before firing up properly.

I switch in the light bulb limiter and put the amp on the variac with everything plugged in and bring it up slow. At about 30% supply it's lighting the bulb up and it looks a bit bright. So I unplug all the modules and try them one at a time bringing the amp just up to about 20% variac and on the light bulb again (is this really stoopid somehow btw?). One of the ICs lights the bulb up more than the rest. Maybe bad solder? I take the unit off the heatsink and clean it up and it looks ok. Reassemble, discharge caps, plug all the ICs back in again,slowly up on the variac. This time I check the temperature and the IC that lit the bulb is getting warm, the rest are cool.

OK the question is, does the reason for that one single IC getting hot (and the others not) have to be on the IC board/in the IC, or could it be on the main board?

Thanks as always.

[icefloe01]

Off topic aside for Glen

I deal with PBX vendors all the time & they still refer to the 'tip' as the hot & the 'ring' as the neutral eventhough they are referring to RJ45 hardware and usually T-1 level lines not the loop pair that we usually have as our home phone lines.

glen

Are you amused or aggravated at how the RJ-45 spec was adopted as the name of the ethernet 8P8C connector? I just shake my head and go with it as resistance is futile. lol

Eric.

[Mars Amp Repair]

OHHH NO!
You cannot run this amp unless all 4 of the ic boards are connected! One IC bad or disconnected will throw off the balance & immediately blow the others!

If you even suspect that only one IC is blown...don't just replace that one IC...if one blows, the others are blown for sure.

Also make note that you should NEVER fire this amp up (or any solid state that you have just rebuilt for that matter) with a load connected if you have replaced the power IC's. You should connect a voltmeter to the outputs & look for any offset while bringing it up with the bulb limiter & variac.

Once you're sure there is no dc offset, then shut it down & connect a load (along with the voltmeter & bring up again with the limiter in ckt. As you can see by the earlier entries in this thread, I had one that had intermittent connections on the IC board plugs that would intermittently blow all four before I ever realized what was happening.

Anyway...without all four connected or good, they all blow in 'sympathy'...but none for the tech ;-] glen

[Alex R]

Thanks glen, noted!

But with all four new ICs connected, no load, variac low and limiter operating, one of the ICs is getting hot and the others not. Could that be a main board problem? Or must it be somethingon the IC board - or the IC?