Marshall AVT150 Overdrive channels passed away - help

[Zzzaaappp!]

My OD1 channel started making a hissing noise - like a class A amp turned 'way up - , while still sounding OK otherwise. After a while both it and the OD2 channels faded away - you can still hear both of them at very low volume if both the Master and OD volume controls are maxed. Clean channel works fine.

Since the OD channels still make noise, I suspect that I am hearing the preamp (??) . The OD channels do respond to the gain and tone controls.

I've read up on the OD output chip problems, TDA7293, and took a look at them. Both chips measure the same DC voltage on thier legs. From Memory: Starting (counting from left to right) on pin 5 (?) around 42 v, pin 6 = 5.72 v, last two pins are -42 v, and 5.72 v. First 4 pins have zero/near zero voltage.

No sign of smoke, fire or obnoxious fumes. Wiggled everything and checked rest of boards - nothing unusual. Swapped 12AX7 tube out with know good - no change (and anyway, the clean channel works OK).

I've got the schematics for an AVT50, and the OD boards are both labeled AVT50, but they didn't shed light on the problem. Anyone have the AVT150 schematics?

Any tips are appreciated. By way of experience, one of my hobbies is making/repairing tube amps, and I am handy around electronics (OK, hollow-state electronics) but not so much with solid state stuff, so feel free to get technical.

Thanks

Glen

[Enzo]

If the output chips - 7293 - were bad, then nothing would come out. The clean is still OK, right?

Scope the outputs of the op amps in the channels affected. Any of them get hot? Oh these are the SIP packages aren't they? Not real accessible, but you can stil get on there. I'd be looking for DC on op amp output pins, adn I would trace a signal through it. OD1 and OD2 are the same channel, just a little gain switching.

[Zzzaaappp!]

Yah, clean is good.

Since I only have the ACVT50 schematic, I have been tracing the op amps back to what they call the "1C2B" side of a NMJ072BD chip (whatever that is) that is shared by the Clean and OD channels. Both channels use this chip, and since all of the tone and gain work for both channels, this is my current suspect.

I can't find the chip on the main board, and think maybe it is buried behind the control knob panel. My bench is occupied with another amp at this time so will tear into it this weekend.

Since I am an op amp novice, what am I looking for, other than signal strength? I do have a scope and logic probe, + signal generator.

Oh for discrete diodes, resistor, caps.....

Thanks

Glen

[Enzo]

Oh, get a grip. Look at the internal schematic of an op amp and tell me you really want all those millions of transistors discrete.

Send me an email with Marshall AVT150 schematic as the subject line
to:

tmenzo at msn dot com

[Zzzaaappp!]

<< tell me you really want all those millions of transistors discrete >>

Er, actually I was hoping for tubes rather than silicon - and if you could make them Mullard 6L6's that would be nice!

Thanks

[Enzo]

They used to make tube op amps. Bob Pease of National Semi often talks about the old Philbrick models.

[tim]

There is a TSB for those amps that exhibit that problem. I believe it is U8 on the preamp board goes south... I think it's a TL072 SIP...it also involves installing some back to back diodes on the input board to minimize this problem in the future. It's caused by static discharge when guitar is plugged in.....Enzo, can you confirm that? Not sure if you're a Korg ASC....I have that TSB somewhere but can't seem to locate it.....I know what chip it is when I see it on the board when it's in front of me.....

[Enzo]

I'll look. I have repaired quite a few Marshall preamp sections needing a op amp replaced - the SIP, yes - but never looked for a bulletin. I could troubleshoot to it in a couple minutes already.

Pop the board free and lay a towel in there or something. Set the board on that to insulate from other stuff. Power up. It is hard to get at things when they are mounted, but not hard to get at once free.

When you look at the schemo and consider the symptom, there are only a couple chips it can be.

The bulletin I have is for OD channel chips blowing due to static discharge into the input jack. I sent him that bulletin along with the schematics. There are no other bulletins on the Korg site for this.

Here is a tip: The dual op amps in SIP come in both 8 and 9 leg versions. They are the same except the extra leg. The 8 pins do the same jobs number for number as the DIP version. In the 9, they added a pin before pin 1 and made it another V+ pin. Check it with a meter, pins 1 and 9 are short together. Pin 1 is now the extra + power pin, and all the rest of the pins move down one, so pin 1 becomes 2, 2 becomes 3, etc.

The tip is this, if you have a 9 pin and need an 8, snip off pin 1 and stick the thing in the 8 holes on the board. If you have an 8 pin and need a 9, stick the 8 in holes 2-9. and remember to add a small jumper wire from pin 9 to the empty pin 1 hole, in case the circuit was relying on the chip internal connection to pass along the power rail.

[Zzzaaappp!]

Yep, got front panel free without extra chunks falling off. It is IC8 according to bulletin. Lucky for bulletin, 'cause my first guess would have been IC10B, the segregated OD side of the chip.

Am presently wiping heat sinc paste off of my keyboard, seems like schematics dragged on power amp modules.

You guys are great? Wanna come over for a beer (bring your tools!) - hah!

Now if I can just find the heater filament wires for these here op-amps.....

By the way, I couldn't find the in-line SIM 072BL chips on-line, and we live in a small town (we're so small that the electric company sends power one electron at a time), but DID find 50 of 'em (TL072), 8 legged paired, in my parts drawer! Now what the heck were these silly little creatures doing alongside my 12AX7 tubes??

If I don't have any zeners, I can replace 'em with 5y3 tubes, right?

If the chip is bad, I think I'll cut the legs off of the present one (ignoring those tiny little screams), and wire in one of those I have, using four jumpers for the off-side legs, plus the 1-9 pin jumper if needed.

I'll post the results when I'm done for those who may follow.

Thanks again!

Glen

[Zzzaaappp!]

Thar be noise, laddie !!!

You guys are great!

I removed the #8 Op Amp, soldered in the replacement (or as I call it, "Slash and Burn!"), did the zenier diode mod, and was rewarded by the God of Deafness! As in, 'so loud now that I may go deaf'.

Works like a charm.

Getting the old chip out was entertaining, and using a Dip replacement with jumper wires was admittidly more fun than a root canal, but she be workee!

However, methinks I like tubes bestest....

Thanks again for the schematics and advice!

Glen

[Enzo]

Mounting a DIP in there? Oh my. A SIP anything works fine. The TL072 is just a general purpose part. 4558, 4560, 2068, whatever. ANything in a SIP8 or SIP9. I stock several varieties.

[Zzzaaappp!]

Any electrical issues with the DIP? The former SIP had 8 legs, and the new DIP has four legs in holes, four using jumpers. Seems to work OK so far.

Did I lose points for tube mentality creativity? If I need the SIPs (8 legs) how much for them?

Glen

[tim]

You can get those SIPs at Mouser, Digikey, MCM or any other purveyor of fine electronic components. I think the last ones I purchased were about a buck a piece and I think I got them from MCM. I usually keep a few in my parts bins although I've only used them on this fix....everything else I work on (that quickly comes to mind) uses DIPs. Either Mouser or Digikey has no minimum order, I can't remember which one but they are both good companies to deal with...I've purchased many $$$ from them with zero issues.

[Enzo]

You get plenty of points for resourceful, no doubt, and if it works, that is all that matters. Electrically the chips are the same, only the packaging differs.

Mouser has no minimum, and neither does Allied. DOn't recall on Digikey, I only wind up ordering from them once or twice a year. Jameco doesn't have quite the selection, but they do have interesting stuff, and no minimum for online orders.