Bugera 333 Blowing Fuse - F2

[PepeSilvia]

The 1A slow blow fuse F2 is blowing. Being that I'm not a licensed repair dude I don't have a schematic so I'm going in blind. I've got my light bulb limiter ready to go (first time to use it!). Could someone with the schematic inform me what that supply is and what it feeds?

[olddawg]

Have you tried pulling the power tubes and powering up the amp with a new fuse? If the tubes are months old it's the first thing I would suspect. The amp would benefit from the replacement of those cheap Sino tubes anyway.

[PepeSilvia]

Yep! Gave that a shot, it still blows without power tubes. I've got the bulb limiter hooked up now, so it's not blowing, but my bulb is glowing. It starts smelling like something is toasty, the fuse, as I said, isn't blowing, but it's getting that little layer of film on the inside like it's getting ready to.

[Jazz P Bass]

Next thing to try is disconnect the output transformer.
What is the fuse fusing?
The mains, the heaters, the B+?

[PepeSilvia]

I believe it's the B+ supply. It's coming off a rectifier that's at ~500Vdc.

Disconnected the output tranny and my bulb limiter is still lighting up.

[R.G.]

With the OT disconnected and power tubes out, that leaves wiring, rectifier, filter caps, choke and power dropping string to the preamp. The power dropping string is (usually!) high enough impedance that it won't blow fuses.

Disconnect the choke. I've seen one with an internal short. This also disconnects the choke smoothing cap and the rest of the dropping chain in most amps.Still glow on the bulb limiter?

[PepeSilvia]

Any idea where the choke is on this damn thing? Also, the fuse is after the rectifier, doesn't that rule it out?

[R.G.]

I don't know the schematic of the amp, but most amps have rectifier, then first filter cap. The first filter cap goes to the OT center tap, and also to the choke. The choke goes to the second filter cap to make the screen supply, and also from there the R-C dropping chain goes to the preamp. Some amps play fast and loose with the choke supply, but only in low powered amps. Usually.

From what you say, if you have a too-leaky first filter cap, that would pop the fuse too, as would a choke with a short to its frame, or a leaky second filter cap.

Or any of the HV wiring. Or a shorted rectifier.

The choke is the smallest "transformer" on the chassis, with only two wires coming out.

[PepeSilvia]

There is no third transformer....

[PepeSilvia]

Well, I've narrowed it down. Unplugged everything from the main board started plugging things back in one at a time (turning the amp off in between) until the limiter bulb lit up. For anyone with a schematic, it's connector X48 on the control board that goes to X36 on the main board. Guess I'll dig around and see If I can figure out what purpose those wires serve.

[Jazz P Bass]

Keep on sleuthing!
You have the amp.
We do not.

[PepeSilvia]

Turns out it's somewhere in the BDM1 board (reverb and channel switching controller). About the only component on there that I would be able to replace (besides some capacitors) is a voltage regulator. I removed it, bulb limiter still lights up. Unless it's a capacitor it looks like I'm screwed. I haven't called Behringer yet, but I assume they won't sell the board to me since I'm not a repair center. All in all, a frustrating time. The only consolation (as late as it was) was finding the schematic on some Romanian site while searching for whether or not I could buy a replacement BDM1 board. So, if anyone stumbles upon this in the future, and wants the Bugera 333 XL schematic , let me know.

[olddawg]

Have you static tested the components in that board for shorts. Simple enough. You might have a shorted diode or cap. Is there a separate rectifier circuit on that board. The number on the regulator should give you the voltage. Is it correct. Are the rails there. Is anything hot to the touch. Are there relays? What are their operating voltages, etc. Lots of specs can be infered without a schematic.

[Jazz P Bass]

Bugera 333XL Schematics

O/k.
Here are the schematics.

[Jazz P Bass]

According to the schematic, F2 (1 amp slo blo) is the Fil- & the -24V fuse.
The schematic may be labeled differently than the amp.
I see three connectors on the BDM1 board .
X1: 5 pin/ foot control.
X2: 5 pin/ programming
X3: 20 pin
Just what number they correlate to on the main board I do not know.