93 Blues Deluxe-Nothin' But Hum!

[earache]

I am looking at a 93 Blues Deluxe for a friend. He tells me he ran the output from a Hammond organ into it and since then all it does is hum. He didn't say which of the inputs he used when doing this. Anyway when you take the amp off standby it emits a
loud hum which is unaffected by any pot twiddling, button pushing or insertion of jacks. The speaker responds to having a 9 volt battery Cross the leads so I assume it is ok. Where to start with the troubleshooting?

[txstrat]

When no pot affects the hum the culprit is after the last pot. i.e. even after the reverb section. It might be a faulty power tube or PI tube. Pull the tubes one by one and see if the hum disappears.

[earache]

Just tried replacing all of the tubes one by one with known-good tubes...no difference. I have also looked over the pc board, and see no obviously bad parts, save for a little bit of browning of the circuit board at the 5w resisters R85 and R86.

[txstrat]

Those resistors are prone to failure (they desolder themselves cause of the heat they dissippate).
Check if they are OK (solder joints) and measure the voltages at the zener diodes bisides them (+16v and -16v).

[Enzo]

I could be way off, but my guts are telling me to look for a burnt open ground trace. It probably won;t be all charred or anything, just a narrow place in the copper tracework that opened like a fuse - it became a fuse.

On most boards, the copper traces are covered with a green lacquer except where soldered. When a trace heats, it first bubbles up and away that coating leaving bare copper, and then the copper can break in two. This would be on the solder side of the board, not the side with the parts. Look at the ground traces from each jack, and follow them around. Sometimes they are wide, but they often have to sneak around some other things to connect two areas together, and the narrowest places are wher they burn out.

This happenes somethimes when something ungrounded gets connected to an amp, and the ungrounded thing has chassis currents looking for a way to ground, and your amp circuit is it. Signal coming out of some hammond organs has B+ riding on it. Not what you want plugged into a jack on your amp.

The battery test tells if a speaker functions or not, but the hum coming out of it tells us that already.

[Enzo]

WHat would I do? Isolate the problem. Pull the phase inverter tube and power up. Still hum? If so, then we look to the output stage, if it kills the hum, we look further back.

Does plugging something into the power amp in jack make any difference?


Hmmm, my guts are changing their tune. If he plugged into the powr amp jack, it could have damaged the op amp IC that comes into play or even mute JFET Q1. SO check what txstrat said, the +/-15v supplies. Are those ICs getting both at their corners? And is Q1 shorted between any two legs? And even if both +15 and -15 are at the ICs, then check pins 1 and 7 of U1 and U2 for DC voltage. They should sit near zero volts. The other ICs are in the switching circuits and do have large DC voltages.

[earache]

WHat would I do? Isolate the problem. Pull the phase inverter tube and power up. Still hum? If so, then we look to the output stage, if it kills the hum, we look further back.

Does plugging something into the power amp in jack make any difference?


Hmmm, my guts are changing their tune. If he plugged into the powr amp jack, it could have damaged the op amp IC that comes into play or even mute JFET Q1. SO check what txstrat said, the +/-15v supplies. Are those ICs getting both at their corners? And is Q1 shorted between any two legs? And even if both +15 and -15 are at the ICs, then check pins 1 and 7 of U1 and U2 for DC voltage. They should sit near zero volts. The other ICs are in the switching circuits and do have large DC voltages.

OK - so here's some finding based on your suggestions:
I pulled the PI tube and the hum stopped.
Putting a plug into the "Power Amp In" changes the hum but doesn't eliminate it.
Measured voltages at both sides of R85 WRT ground: 43.5 3.23 R86: -43.7 -2.8
VDC at the opamps
U1 P1 +.63 P7 -.01 P4 -4.3 P8 +3.09
U2 P1 +4.06 P7 +12.2 P4 -3.28 P8 +3.08

Diode CR1 checks OK, CR2 and CR3 measure open
Unsure how to check Q1


While you mull these readings over, can I ask a couple questions:

First, should my friend avoid plugging this Hammond into any other amp if it has "B+ riding on it's output"?
Are the opamps U1 and U2 blown if they see as much voltage as I am reading?
If so, can I sub in TL082's for the TL072's? They are readily available at Rat Shack

[g1]

Your +/- 16V supplies have a problem. You should have +16VDC at pin 8 and -16VDC at pin 4 of the IC's. I would suspect Enzo's first gut reaction about a blown ground trace may be the issue. Measure from pin 4 to pin 8 (instead of measuring to ground). If you get around 30VDC then it should be a grounding problem. Check for continuity from the effects loop jack grounds to chassis, and from chassis to the +-16V ground (junction of CR22 and CR23).
If it's not a grounding problem and one of the IC's is shorted, the bad IC should be getting quite hot.
TL082 will work, I think the noise spec. is a bit higher than the TL072.
As far as the Hammond goes, you would have to measure if there is any DC voltage to the output jack. If so, it should not be connected to any other gear.
My suspicion is that the Hammond does not have a 3 prong plug, and connecting it to the Fender blew a ground trace, as mentioned by Enzo.

[earache]

Enzo you have proven your psychic abilities once again. I took the board out and sure enough a ground trace was cooked. Here's a shot of the back of the board:


The trace that is cooked goes between the ground connection to the chassis and the ground side of the the caps C43 and C44. Here's an "X-ray" view:

The burnt trace is between the ground wire with the spade lug, and the vertically-oriented jumper marked "W25"

I tacked a wire that bypasses the burnt trace for now to see what would happen. The amp works now but there is still a bit of background hum. I will have to go thru and check some of the surrounding components, and maybe replace the open diodes I mentioned earlier. I will be back to ask more questions as I go. You guys have been a great help so far - thanks!

[earache]

So last night I replaced the temporary repair of the burnt trace with a neater more permanent wire. I turned on the amp and, although the amp passes sound now, the controls have no effect! The guitar signal is loud enough to be heard but can't be increased or decreased by the volume control, nor do any tone knobs or channel switching buttons work!
This caused me to look closer at the board and I see a few other burnt ground traces...I wonder if this amp is now a basket case...I ran out of time but will test voltages at the places I did before when I next get a chance.

[earache]

I ask the readers of this post- in your experience, would an amp which has had it's board stressed to this degree always be suspect? I am trying to determine if it is worth the effort of replacing faulty components if the board may always be intermittent.

[Enzo]

I don't know that your board has risen yet to the level of intermittant. That implies it works 100% some of the time. Have we achieved that?

I don;t know what the Hammond organ was, not how he connected to it. If he connected to the large cable pins, there could be all manner of unwanted voltages. I was thinking those, but g-one has a good point if an ungrounded chassis is hot and connects to those grounds, PFFFT.

Errant current looking for a path to ground can often find multiple paths to do so, burning each out in turn. You have found more than one here.

[earache]

I hear what you're saying Enzo, there's no way to tell yet if the amp is a basketcase. I'll dig further in and see what I find.

[olddawg]

If the board isn't carbonized it should just be a matter of jumping a few traces and replacing defective components. Might be a little time consuming but the repair should be reliable I would think?

[earache]

Wow it has been a while for this one! The solution kept eluding me. There kept being a distorted sound especially on low notes. I finally found that the culprit was the (2) power resisters on the circuit board. I had replaced them after seeing that the PCB around where they are mounted was sort of scorched. When I remounted them, I left them up off the board to allow better heat dissipation. What happened after was that they vibrated on the low notes and caused the crackle and distortion. Once I dropped them down tight to the board and resoldered them the crackle went away.
Thanks again for all the help everybody!