Troubleshooting a Beta Bass Power amp

[kingnimrod]

I bought a Beta Bass combo the other night, and need some help troubleshooting. It was working fine until I plugged the master accessory OUT from a beta lead head into the master accessory IN on the beta bass (to try out slaving it).  I got no sound from the bass speaker, so i unhooked the beta lead.


now all i get from the beta bass combo is a muffled, distorted sound. If I strum hard, the sound is louder, but it seems like it's trying to fight its way through, like the sound you get from a distortion pedal with a dying battery.

Wondering if I did something wrong, or what the possible cause might be. Have not opened the head just yet.

Would appreciate any pointers.

[loudthud]

Do both amps have undamaged three wire power cords?

Do the channel lights operate properly when you plug into different inputs? This indicates that the logic is operating properly.

[kingnimrod]

both amps have undamaged power cords.

And yes, on the Beta Bass the input lights function correctly.

I'm just wondering what plugging something into the power amp in on the Beta Bass could have done to cause its current condition. Could it be something as simple as dirty master accessory jacks screwing with the power amp portion of the amplifier?

[loudthud]

Yes, the From Accessory Jack is one of those with the contacts that feed the signal from the preamp through to the power amp when a plug is not insterted. If you have any contact cleaner like DeOxit you might be able to spray a little in the jack without opening the amp up. Then insert and remove a plug a number of times to remove crud from the contacts. Another thing to try is install a path cord from the Channel A or B Out to the Master In jack. If that doesn't work, I'd have to say it suggests that the power amp is bad.

[kingnimrod]

I'm sitting here with it right now, with the head open, doing some testing.

Here's what it's doing:

Channel A preamp line out and Channel B preamp line out send a good signal -- I am running a line into the return on a small practice amp here. All preamp controls seem to work fine, and I get a good signal from each discrete channel, plus signal from both when plugged into the "both" input.

Master Accessory line out gives a good signal.

Patching any of the preamp outs into a power amp in on the Beta itself gives the same crappy fizzing sound, as does running an external preamp into any of the power amp inputs.

So it does sound like something is going on with the power amp section. Looking at the power amp circuit board, nothing looks fried to my eyes. Fuses are in fine shape, and at a first glance no caps are leaking or bulged.

Tracing back to when this problem originated the other night, it happened immediately when I plugged the preamp out from my Beta Lead head into the master accessory line in, so my questions are:

Could running an external signal into the power amp do something to cause this? Are there any prime candidates for malfunctioning in this situation?

Could it be something as simple as a cold solder joint on the line in input jack that could cause it? Or are the symptoms indicative of something else?

I am not an amp tech but have built some small solid state stuff, repaired effects, etc. I'm hoping it's an easy fix...

[loudthud]

Most any blown transistor on the power amp will cause the output to go to the rail or at least put some DC on the output. I'd check that first coupling cap. C1 2.2uF NP. Those caps are not very reliable.

[kingnimrod]

OK -

Upon closer inspection, one of the legs of CR4 is broken / snapped.

Could that be the culprit?

if so, is it easy to find a replacement for this? Marking says "40" stacked over "04". Would appreciate help in determining the right replacement.

[loudthud]

There is a full Beta service manual here: http://sunn.ampage.org/site/svcmans/beta/

If the diode is on the power amp pcb, it's a 1N4004. A 1N4007 will work fine. Probably not causing your problem.

[kingnimrod]

Will look at the manual and see if I can figure it out.

Wondering if there's something in the circuit that could have caused that leg to "pop" apart - when I touch the broken legs of the diode together, I get a very loud hum from the speaker and a bit of a spark... broken leg ends now blackened....

[loudthud]

The old diode is probably shorted. It's just in the circuit as a protection against voltage spikes from the speaker.

[kingnimrod]

just replaced both of the 1n4004 diodes and that did not fix it. Will be replacing caps now, starting with the coupling cap you recommended. Looking at the power circuit board, none of the caps are bulged or leaking noticeably. Could they still be failing even if they look OK on the outside?

[loudthud]

Those Non-Polar caps can do strange things. They are really like two caps in series. Sometimes one side will short out or go leaky creating lots of distortion.

[kingnimrod]

Well, I've replaced all the electrolytic caps on the power board, including the filter caps, and it's still exhibiting the same symptoms. I double-checked to make sure the preamp is sending a good signal, and it is.

But the power amp is delivering a distorted, sputtery mess to the speaker. There seems to be a threshold where I have to strum at a certain strength the get it to make a sound - kind of acting like a gate...

I guess the next items to replace are transistors. Are there any particular candidates that would be the most likely to cause this problem?

[loudthud]

Q1 and Q11 should be checked to confirm that they are undamaged and operating properly. They normally run fairly hot.

Q2 and Q3 are suspect but the best thing to do is look at the DC voltages in-circuit to confirm if they are functioning properly. There could be other problems in the circuit that would just blow them out again possibly causing additional damage. You could end up with a big smoldering mess. Power amp circuits such as this are like a tower of dominos that can come crashing down because of one little fault.