Sunn Beta Lead Popping 6 amp fuses

[randomair]

Hello all! I recently had a Sunn Beta Lead blow a fuse on the power board. (6 amp). I tested the outputs and there were two that were shorted. Typically in the past if I had found outputs to be shot, I would juat replace them and that seemed to do the trick, but this is a different story all together. I replaced the 2 outputs (I have an excess of 2n3005/mj954's)and checked the rectifier before powering on the amp, POP went the fuse again, taking out another 4 output transistors. I immediately stopped and evaluated the situation further, thinking, well could this be the power transformer...Heres the dilemna, how do I properly test the power transformer? There are two red wires that attach to the rectifier, two black that go through the switch, and red wire that goes to the power board (these all come from the transformer). How can I determine if the transformer is faulty? Thanks in advance for the help.

[Amp Kat]

If you can disconnect the two red wires leaving the black ones connected that will take the load off the amp. Simply measure the voltage (AC ) at the two red wires and verify somewhere between 40 to 75 volts. If you get that, it's good. They should also read around .5 to 1 ohm across them if they are good. My guess is either the drivers shorted this time or some cap maybe in the voltage path shorting the power supply out. I would bet almost 80% that transformer is good and you have a shorted component.

[R.G.]

There's a good section on figuring out if a transformer is bad on the "Tube Amp Debugging Page" at GEOFEX. There's also the Secret Transformer Tester, which is the only easy way to detect an internal short.

Basically, you remove the transformer from ALL connections, then use your ohmmeter to verify that (a) all windings show connections from lead to lead, with none open; (b) no windings are shorted to other windings or the core, where they should not be. Then you use the shorted turn tester. If it passes all that and it ever worked, it's good.

The Tube Amp Debugging Page also leads you through figuring out if your AC wiring has a short or open in it, another potent source of blown fuses.

As a final filip, there is a light bulb limiter schemo at GEO, subbing a light bulb in series in the AC line into the amp. If the amp is shorted, the light bulb glows, but THE FUSE DOESN'T BLOW so you can do some debug.

[Twist]

Pull the shorted output transistors out. Check all remaining transistors and signal diodes for shorts (with the power off of course). If in doubt pull them out of circuit for testing. Also check all low resistance value resistors. Typically 2K or less. When powering up a solid state amp it's best to not hook up a speaker until your sure you have no DC on the speaker out. I prefer to use a variac with a current meter in series to power up slowly and to monitor any high current draw to avoid costly damage to new parts.

[randomair]

mystery to me....

thanks for all your help everyone! ok...so i checked the transformer, its fine.
i just happen to find another power board laying around the shop, it checked out fine, so i switched out the power board, put in new transistors, fuse still goes!!! do you think there is something in the pre-amp that could ultimately take out this fuse? this is weird....thanks!

[Amp Kat]

At this point it's a matter of finding the shorted component and it could be a capacitor and probably is. I doubt it's something in the preamp and still think something in the driver section to output is your problem. Did you check the bridge good and the regulators for the voltage rails ? RG's site has a troubleshooting guide on his site that may help in locating your problem.

[Twist]

If it's the model I'm thinking of then it has a bridge rectifier mounted on the heat sink. Disconnect the leads to it and check it for shorts. Repeatedly powering on with shorted components will eventually short the bridge rectifier. If that's not shorted then an OpAmp in the pre amp section may be shorted and sending DC to the input of the power amp. Doesn't this board have the main filter caps mounted directly on the power amp board? If you've swapped the whole board with a known good board then the problem should be in the rectifier or pre amp. If you suspect the pre amp section then insert a 1/4" plug into the "power amp in" jack. This will hopefully disconnect the power amp from any DC being fed from the pre amp and will allow you to troubleshoot the power amp. Although if you have a shorted OpAmp it may pull down one side of your +/- 15 volt supply. Look at the large resistors tied to the TIP29 and TIP30 transistors on the power amp board to see if they have severely overheated.

[Enzo]

The bridge is on the board along with the filter caps.

How did you determine the other board that was laying around was OK?

Are you sure the power transformer wires are on the right posts?

I don't think a short on the preamp board would directly cause the fuse to blow, but the 15v rails are derived from the +/-40VDC rails on the power board, and those same 15v rails are part of the power amp amplification circuits too. SO if the preamp loaded one of those down, it would seriously affect the power amp. And then THAT could blow fuses.

The power amp input has a blocking cap, so no DC should get into it that way.

So unplug the preamp connector and see what happens. If the fuse holds, then the preamp has an issue. The power amp doesn't need that connector to be terminatd for it to operate.