SWR Bass 750 (original, not 750x)

[nickb]

Also, just to the right of the bottom melted cap, is a component missing? There seem to be two close pads that look as though they have had recent solder action.

I count five caps that have been damaged by careless soldering.

[chrisdski]

Update. I've just about completed the rebuilding of the power amp board. Replaced all the capacitors, and all the large and medium sized transistors. Have the smaller transistors on order, tackle them when they arrive. All the large transistors were shorted.

[The Dude]

A couple of things, just FYI.

1) I wouldn't just randomly replace parts. Troubleshoot and find the actual problem(s).
2) When you replace electrolytics, IMO, it's a good idea to leave the leads longer in some instances and keep them away from high heat sources. If need be, cut some insulation off of a piece of wire and place it over the cap leg leaving the legs longer. Heat is the worst enemy of electrolytic caps.

Just my $.02.

[Jazz P Bass]

My $.02.

Touch up the solder on all of the components.
Some look awful thin.
Some look rough & grainy.

If there is ANY carbon between component leads, it must be removed.
The TO220 transistor on the left appears to be pretty bad between the legs.

[chrisdski]

Update- all put back together and it works. I replaced the 4700/100v caps on the pre amp board. On the power amp board, I replaced most of the transistors, all the caps, and a couple of resistors that looked heat damaged. I found it hard to test individual components while in the board so i figured if I was going to pull a 15 year old component, I'd replace it. One issue still to be resolved is what I think is a ground hum/buzz in the speaker. The hum's volume is not affected by turning the gain or volume up or down- it stays constant. Thoughts? I do have access to an oscilloscope.

[chrisdski]

searching threads- seems I should smack the amp and see if the buzz gets louder/less, or it's a cap issue. Would the 4700u/100v caps on the pre amp board be the main suspects?

[Enzo]

Whacking the amp reveals bad solder joints to the caps rather than bad caps per se.

Determine if your hum is 60Hz or 120Hz, it points us in two different directions.

determine if your hum is preamp or power amp related

[chrisdski]

Plugging a set of headphones into preamp out, tuner out, and effects loop send out- I get no hum when I play my bass thru it. I would surmise issue is on the power amp section. I assume that also means the 4700u/100v caps I replaced on the preamp board that supply the +/- 100v to the power amp board? After comparing tones on youtube, I am going to say its the 120hz hum. I could not change the hum by whacking the amp, or stressing various components/wires with a wood stick. Again, volume/gain controls do not change the hum, they can be all the way down or up- same intensity of hum.

[chrisdski]

Seem to have isolated it as a ground loop between the power board/heat sink and the chassis. If I unscrew the power board/heat sink assembly from the chassis, it eliminated the loud buzz/hum. If a run a ground wire from the heat sink to ground, it's pretty quiet. The power board has a ground wire to chassis with apparently good ground according to my meter. I will try covering any bare metal I see with paint between the heat sink and chassis, but can't image that's the fix.

[g1]

If grounding the heatsink with wire gets rid of the hum, I would expect it is not getting a good ground, so maybe removing some paint rather than adding it should be a proper fix. Is there any kind of thermal compound between heatsink and chassis?
Do the output transistors use the heatsink for grounding? If not them, what does?