Music Man clean channel dead

[Garydean]

I have a MM 2275-130 chassis, 210-HD that just came in. The clean channel is completely dead both inputs. Has anyone had this problem? I do have a scope and all that fun stuff but I won't be able to get to fixing it till Monday. Just thought I would tap into the forum ahead of time and get some background on these amps as they have a solid state preamp section. Thanks in advance.

GaryDean

[Enzo]

There is nothing special here, a circuit is a circuit. Feed it an input and trace the signal through the channel. I'd bet on a bad op amp before a lot of other things, but certainly bad controls and cracked solder should come to mind.

[Garydean]

Thank you for your input Enzo. It does seem like I'm loosing the signal at IC-1. I say "seem" because the circuit board is very tight and probing was tedious. This will be my first attempt in replacing one of these little things. Any advice on what I should be concerned with. The 8 pins obviously need to go back the same way, how do I know what pins go where when I put the new one in? Iron temp I'm sure is an important factor as well. What temp is suggested so I don't burn the bugger up? I'm sure I need to replace it with the exact part Lm307H..... couldn't find one in the mouser catalog.

Thanks again,

Gary

[52 Bill]

I agree with Enzo about the bad opamp. I thought that the ICs on Music Man amps were socketed. If they are, try swapping one from the working channel and see if it fixes your problem.

If I remember correctly, the "H" case should have a small tab that references pin 1, so just be sure to note the location of the index pin. It may be a problem to find another LM307H. Worst case, solder leads to a plastic dip version and form to fit the original circle pattern.

Hope this helps.

[Garydean]

Thanks Bill!!
I have located the part.... I'm waiting to hear back from the company on price. I'm wondering too if i should replace IC-2 while I'm in there. That would be the 2nd half of the 1st channel right? IC-1 represents the first half of a 12ax7 in theory right? or am i overkilling?

thanks again,
Gary

[Don Moose]

Before you solder in the replacement, I'd STRONGLY suggest you put in a good-quality socket. One with at least some gold flash on the contact area.

This one, for instance.

It lets you concentrate on getting a good solder joint without worrying about cooking the chip, and lets you play with different types if you want.

Tie the chip into the socket with a small ty-wrap and you should be (mechanically) bulletproof. If it ever dies again, re-replacement is supersimple, too.

Hope this helps!

[Enzo]

Are the holes on the circuit board in a circle or two rows of four? Two rows of four fits the common 8 leg DIP. I have seen many older products where the round op amp has the legs arranged into two rows of four on the board.

The LM309 is an old obsolete single op amp. You could replace it with a TL071 or TL081, or even a NE5534. There are a few LF35x that would work, but those I mention are readily available. You won't find those newer parts in a round metal case though. But pin for pin they are wired the same. It would not be hard to wire a DOP socket into the round grouping of holes anyway.

[52 Bill]

I looked up the part last night and the pin references pin 8 not pin 1. Well at least I remembered that there was a pin!

[Enzo]

Oh. I thought the little tab sat between 1 and 8. I so seldom run into it I always check a data sheet anyway.

[Garydean]

Thanks guys, the LM307H is round. This application does not use a socket, it's pins are long as it sits about 1/2 inch above the board. I was digging through some old parts in my shop and found one. I'm gonna pop it in today and see what's up. Thanks for the help.

Gary