Laney L30B Hums, Blows Fuses

[earache]

Fixing a Laney L30B solid state bass amp for a friend. It was humming when it showed up. I checked voltages after the bridge rectifier and got +19 and -26V where I shoulda got plus and minus 28V - see the schem

L30B.pdf

I replaced the 2200uf x 35V filter caps (C29 and C30 on the schematic). When removing one of the original caps, one of them shed a leg, making me think that I must have figured out the problem. After I replaced the caps, the amp now blows the two fuses (FS2 and FS3) every time it gets turned on. I subsequently replaced the bridge rectifier and have the same fuse blowing problem. With the two fuses removed, I get voltage readings at the transformer side of the fuse holders of + and - 28V instead of +/- 17.5.

What should I look at next to get this amp running again?

[52 Bill]

What should I look at next to get this amp running again?

Check the output transistors TS5 and TS6 and the diodes connected to them. For that matter check all of the transistors in the power amp for shorts. If you have a light bulb limiter then use it and save your fuses. And be sure not to connect the speaker until you get it up and running safely.

[g1]

It wasn't blowing fuses before, but now it is. Most likely something you did or a defective new part. Check for any solder bridges or shorts related to the work you did, check the new parts for low resistance.
It could have coincidentally developed a second problem at this time, but re-check your work first.
The 17.5V at the transformer is AC, so no + or -, measure on AC range. With the fuses out it is unloaded so you will read a higher voltage, 28 is probably fine unloaded.
All that being said, 52Bill has mentioned other likely causes of fuse blowing.

[Enzo]

My money is on you installing one of your new filter caps backwards. You have positive and negative supplies. In the positive supply, your filter cap has its negatiove grounded and its positive to V+. In the negative supply, the caps POSITIVE is grounded and the negative terminal goes to V-.

Did you happen to install both caps with their negative lead to ground?

[earache]

Bingo Enzo!

My money is on you installing one of your new filter caps backwards. You have positive and negative supplies. In the positive supply, your filter cap has its negatiove grounded and its positive to V+. In the negative supply, the caps POSITIVE is grounded and the negative terminal goes to V-.

Did you happen to install both caps with their negative lead to ground?

Absolutely correct Enzo. I saw the wide trace on the circuit board and immediately assumed the negative legs of both capacitors went there... Another glance at the photos I always take during a repair confirmed what you said. Swapped the offending capacitor around and voila!..a working amp with scarcely any hum.

Thanks Enzo (and everyone who offered advice) for sharing your time and help... Much appreciated!

[Jazz P Bass]

And how much of a 'hit' did reverse polarity commit to that cap?
I'd replace the one that was installed backwards.

[Enzo]

Yes, I would not assume that cap was OK now.

[earache]

Yes, I would not assume that cap was OK now.

OK I'll replace it. Maybe a new cap will get rid of the last little bit of hum that exists at the extremes of the volume pot's travel.

[Enzo]

Be aware that hum is not generic. It can come from a whole bunch of sources, and each one has its own cures. If there is a grounding issue, all the filter caps in the world won't affect it. And likewise, all the shielding in the world will do nothing to improve lack of filtration.

And sometimes you have two sources of hum that happen to be opposite in phase. Togethre they cancel out if at the same level. So it can result in hum that is present at zero on the volume control and actuall goes down until you get to say 3 and then starts back up. What happens there is one hum is occntinuous, then the other starts at zero, and whgen you get to 3 it is at the same level and exactly cancels, then continue on and now you hear more and more of the second one.

I don't know that that is you situation, but be aware it is common.