Advice on troubleshooting a Boogie DC-3

[GregN.]

So this Boogie combo I have is driving me nuts. It has an intermittent buzz or pop that happens randomly but quite frequently. I've swapped the tubes w/ known good ones. I've reheated all the solder joints on the power tube pcb. Haven't done it w/ the preamp tubes though because that pcb is a little more cluttered. FWIW, I can make it pop by tapping on the top or back of the amp. It SEEMS to me that it happens after the amp gets hot. That's why I went after the solder joints. I figured next I would try tightening up the tube socket pins. Anything else I should be looking into? Any advice appreciated!!

Thanks,
Greg

[imaradiostar]

I had a DC-5 with dirty effect send and return jacks. Cleaning them made a big difference. A friend's DC-5 had a bunch of cold solder joints. I know the PCB is a mess so all I can do is wish you luck and hope you find it!

jamie

[Enzo]

I don't know the model, but does it have 16mm controls, each with three wires running over to the board?

If so, power off, go down the row of controls, grasp each one's rear cover and wiggle it. ANy of them loose? Any loose pots, demount the pot, and use pliers to crimp down the little tabs that hold the cover on. Reinstall the pot. Usually the knob/shaft on loose-cover pots will be looser as well.

What happens is that something hits the knob out front, like bumping it in transport or maybe a guitar neck falling into it. Whacks the knob, which pushes the shaft in, and that pops the back loose. The back is what holds the wiper assembly in contact with the resitive element. Loose back = loose wiper = intermittent noises.

It may not be happening to yours, but takes only a second to find out. This is an automatic check for me in any Mesas constructed that way.

[GregN.]

Thanks for the responses. I'll definitely try these things when I get time to open her up!

Greg

[soundguruman]

Most common cause:
You will notice a number of resistors, sitting directly on top of circuit tracks, with no space between the resistors and the board.
The pop is caused by a resistor sitting on top of a circuit track, and arching down to the track. Lift the resistors one at a time until you find the one that's arching. (It will be burned on the underside) The resistors should be lifted off the circuit tracks, leaving a space between the board and the resistors.
Second cause:
Arching through the fiberglass board, between heater track and plate voltage track. OR between grid track and plate voltage track.
Once the arching burns the fiberglass, the carbon becomes an extra resistor in the circuit. The carbon must be removed completely, by drilling, cutting, etc...

Frequently mistaken by techs as a cold solder joint, even the factory techs don't get it. Read This:

http://music-electronics-forum.com/t27094/

[GregN.]

Well, I tried cleaning the effects loop jacks, tightening the tube socket pins, checked all the pots and none of that has helped so I guess I need to look into the resistors (ugh....). So Soundguruman, when you say "lift" the resistors off the circuit tracks, do you mean unsolder one end and lift or do you mean, physically pry the resistor up a bit?

Thanks for the help!
Greg

[Jazz P Bass]

I believe SGM is refering to a slight air gap between the resistor & the trace.

[imaradiostar]

Open it up, prop up the chassis on a home made amp stand made out of 2x6" lumber. Set yourself up so that you can safely turn off the lights and play while looking for arcing. If it's arcing you'll likely see it in a tube socket or between a resistor and the board. BE CAREFUL!! Do not attempt to touch anything in the amp with the lights off. Even with them on keep one hand behind your back (and off a guitar!) and probe with the other. Use a chopstick if you need to poke or move something! This may be old info but I have to say it for safety sake.

Is the problem limited to the clean or dirty channel or does it happen for either? That could help you track it down as well. The last DC5 I worked on exhibited similar symptoms after being on for a while. It turned out a new set of output tubes fixed it. You said you've swapped your outputs but are you 100% certain they're good? EL84's take a lot of abuse and are pretty cheap to replace should the need arise.

jamie

[soundguruman]

Lift one end of the resistor and look underneath, look closely at circuit tracks that are spaced very tightly, and look for black soot between the tracks, indicating the tracks have arched through the fiberglass. It really helps to use a magnifier lamp...
Well these amps are really time consuming to troubleshoot, that's true.

[Jazz P Bass]

Well these amps are really time consuming to troubleshoot, that's true.

PITA is a more apt statement.

[soundguruman]

Go ahead and take out V1 preamp tube, run a test and see if the popping stops..
Take out V2, and try it again, you may be able to narrow down the stage where the pop occurs...pulling tubes or disconnecting stages one at a time.
At one point I had to take out all the parts from one stage before I found that there was actually a carbon track in the fiberglass, between two closely spaced circuit tracks. I found it with a mega ohm meter and a magnifier. The resistance of the carbon between the tracks was 2.2 mega ohms. The carbon spot was about the size of the head of a pin.
It was occurring where a grid track was too closely spaced next to a plate voltage track. So follow the plate voltage tracks and look for resistors sitting directly on top of the track. That's the prime candidate.
After the carbon was drilled out of the fiberglass, the amp worked again correctly.
Boogie used to put ceramic beads on the resistor leads, so the resistor would not touch the board. Later they coated the board with green shellac, and assumed that the coating would prevent the burn through.
It really only happens in locations of high humidity, near the ocean, rivers, lakes, or where it rains a lot. Amps that live in the desert, in dry conditions, don't experience the problems. In Hawaii, you are going to see problems...

If you do not have test equipment such as a scope, it is going to make finding the offending stage much harder. If you do not have a temperature controlled soldering station, I would not try soldering the board at all.

One other trick is to power up the board in a dark room. You might be able to see where the arch flash is occurring, but no guarantees. It's a very tiny arch!
You might be able to SMELL the section of the board that is burning.
You may be able to freeze the resistors one at a time while the amp is running, to spot a thermal defect, a cracked resistor...
You may be able to wiggle each resistor, one at a time with a wooden chopstick. When you reach the resistor that is arching, you will hear the resistor pop when it is moved around.