Marshall Mode 4

[Valvehead]

I just picked up a Mode 4 head. It powers up with no sound. I opened it up to find that someone has completely removed the output chips.

Theres the big gold heat sinks with fans--but no chips--no nothing.

So, im sure some of you have worked on these. What am i missing? is there 2 or 4 little pcb's for the chips that would mount to the heatsinks? It looks like they would have multi-wire connectors going to the main board ??

anyone have one around or perhaps a picture of the guts of the amp?

[Enzo]

I just picked up a Mode 4 head.

Bad move.

I opened it up to find that someone has completely removed the output chips.

SOmeone usually has to.

These things love to blow up, and when they do, POOF, four dead 7293s.

Yes, each TDA7293V has a small pc board to hold it, and there are a few small caps on there too. Each mounts on the heat sink, and there is a 15 pin cable to connect each to the main board. FOur little boards, four 7293s, four cables.

Can you contact the seller and ask if any of the dead parts are still there?

I replace the 7293s on the small boards, but Korg sells the little boards complete. I suspect they would special order the cables.

[Valvehead]

Thanks enzo--thats what i figured .
i emailed the guy asking if he has the old parts , still waiting . You would have to be pretty stupid to throw away the 4 boards and cables ....

[Enzo]

A long time ago - like 1975 - I was doing bench work for a company servicing arcade games. This was the era of Pong, back when Pong was a real game and not a quaint retro joke. The logic board for the game was about 100 14-leg ICs on a thing a little larger than a legal pad.

I swear to God this is true. A guy had been calling for help and he determined a particular chip was bad. He was told to replace it. next day he shows up with the bad chip and about a square inch of circuit board still soldered to the IC. He had managed to cut the IC out of the board, board and all. Somewhere is a Pong board with a square hole in it. He was asking for a replacement, fully expecting it to come with pc board. I guess he thought it was like replacing a tile in the kitchen floor.

Stupid is everywhere.

How many times have I seen someone clip the transformer wires in the middle of the wires rather than right at the end? Now I have to splice the wires instead of just stripping the ends.

The Marshall part number for the TDA7293V board for the MF350 (Mode 4) is KITS00004. List is $40, dealer cost $20. The IC itself is cheap from Mouser, but you will need the little boards. Maybe someone has some dead modules from prior repair they would part with. Usually the little parts are still OK. If a trace burns, repair it.

There are several versions of the little 7293 board depending upon amp model, so get the right one. Differences are like 15 pin cable on one, and on another 10 pin and 3 pin connectors.

There is no part number listed for the cables. SO that pretty much means special order from ENgland.

And they are not making this amp any more, so who knows how many they have there.

[Alex R]

Valvehead, are you in the uk? I have a few of these boards, probably blown but not sure.

[Steve Conner]

Best mod anyone could do to a Mode 4, IMO

[Alex R]

Bloody things. Whatever happens seems to take out all 4 power units, and sometimes some stuff on the big board below too, which is when the repair can get uneconomic & the repairer unpopular. So 'what's it worth to you?' is my first question when one comes in. The fans are quite short-lived, I've learned always to replace them with the power units even if they seem to work. I must say though that the fans are cheap, and the power units reasonable, from Marshall, thought whether its cheaper to fit a new IC depends on your hourly rate and speed of working...

Anyhow Valvehead I think I have a few burned out ones that I lazily replaced with new boards, so ask if you're in the uk and they will be yours for a small sum via Paypal.

[Alex R]

btw how would you check that the main board is ok before shelling out on new power units?

[Steve Conner]

Doesn't it have a line out or headphone jack or something? Surely if the main board is OK, those outputs should still work even with the power amps missing. Enzo is sure to know more though, maybe there's some mute or protection circuit that'll shut it down completely.

If this happened to me, I'd be tempted to chop the Mode4 head down to size and use it like a kind of ghetto Pod ;-)

[drewl]

Funny stuff Enzo.....made me laugh, thanks.

[Valvehead]

Alex--- it would help me out in a HUGE way if i could get the 4 boards from you. If you could ship them to the usa? Do you have the 4 cables for the boards? my main board has 4 slots for 15 pin cables.

I can order the chips from Mouser.

I ran the amp into a another amp today and the preamp section seems to work fine.

[Enzo]

The four power modules are wired in a sort of bridge arrangement.

There are two pairs, one pair 180 degrees out from the other and bridged. But within each pair, they are not simple parallel like transistors might be. One unit drives the output while its mate is some sort of follower.

The arrangement is not obvious on the schematic, so I traced out the circuit to see what was going on.

[Valvehead]

well, i have the boards ordered from alex r.

now i need to find the cables somewhere....anyone on here a dealer that could get me some at cost ?

[Alex R]

...I think there is some kind of mute circuit on the main board that complicates things, specially if you're dumb like me. When you fire it up after fitting the boards, do it on a light bulb current limiter or something. Those little boards just go pop pop pop if you're not careful. Pop (that's the last one gone).

[Enzo]

The mutes sometimes are damaged by the exploding 7293s, but all they do is short to ground - a cap or a zener - which leaves the IC muted. A shorted mute won't damage the 7293.

I am a dealer, and there is no part number for the cables. FInd me a part number and I can look into ordering them.