I broke my Neve 1272

I was cleaning up some wiring on my Neve 1272 project inside the rack chassis. I forgot to hook-up the gnd wire to the chassis.

When I plug-in a mic, turn on +48V phantom power, I heard AM radio. If I touch the chassis, the AM radio goes louder and softer. So I left the room to get some alligator clips to temporary ground the chassis to the GND wire.

When I return to the room, I smelt something burning. I quickly turned the unit off, and now the channel won't pass audio. Totally dead. I traced the burnt smell to the 2N3055 power transistor.

The other channel works fine.

Question: Why would the 2N3055 burn out in the other channel? In the past, the chassis was ungrounded, but my preamp didn't burn up, nor did i hear AM radio. There was only slight hum.

The only thing I can think of, is one time in the past this same transistor smoked during my initial power testing. But after the white smoke cleared, I tested the unit and it was still working fine. Maybe the AM high RF oscillation finally burnt out the power transistor to its final death?

Hi Owel,

Picking up AM radio is a common symptom of an enclosure not having an electronic-ground reference to the internal circuit.

Did you knock or shift anything whilst cleaning out you unit? Have a look around the edge connectors and check to see that there are no shorts or loose wires. Did you knock the bias pot?

When you say that the 2N3055 smelt funny, was it a "hot-transistor" smell? It would take a lot to actually rupture the can of a TO-3 package. Plastic-package TO-92 devices can actually pop right open. Check on your board too for any discoloured resistors and other damage before just replacing the 2N3055.

Hope you fix it.

Mark

I found a resistor (R18) burned out ... see schematic here
http://www.seventhcirclestudios.com/SCA/N72/n72_schematics.htm

Can someone help me analyze this?

If this resistor is burned out, i.e. open, that would not supply power to the last stage of the Neve 1272, and that could be the reason for a dead channel?

Also, the resistor is soldered on a plated through PCB. What's your tip to getting this unsoldered? From my experience, desoldering components from plated through PCBs are a pain.

on the 283-2 is R35, C21 and R40 ok?

[quote author="Scenaria"]on the 283-2 is R35, C21 and R40 ok?[/quote]

Yes, those parts look fine.

I *think* the 2N3055 may be fine, because when I touched it during the time I smelled something burning, it was *NOT* hot.

Now that I see R18 is burned out, I think that's what I've been smelling, it is located near the 2N3055 transistor. (so I thought it was the transistor).

Okay, let me try to replace this resistor.

Update: It's fixed now... I'm happy again.

I had to replace the 12 ohm resistor R18.

Don't know why it burned out in the first place... maybe this should be a 1/2W rating.

hmm

keep an eye on it.... dont leave it unattended... until your sure everything is ok..

R18 should only dissipate about 70mW during normal operation, it doesn't need to be 1/2W. If there's more than 0.8V or so across it, you've got a problem somewhere, probably around Q5 or Q6. What's the no-signal collector voltage of Q6? Occasionally people fail to follow the biasing instructions and smoke R40, it's possible to damage R18 in the same way.

Hi Tim :thumb: ,

Tim is too polite to give himself a plug so I'll do it for him.
Seventh Circle Studios and DIY professional audio equipment
http://www.seventhcirclestudios.com/SCA/SCA.htm
there is even a forum to have fun in.

Tim,

I reread the instructions again on your biasing... argghh, I misunderstood it the first reading and I did some steps wrong. Anyways, it's taken care of now.

I'm back in business again. Thanks!