Ampeg B100R Combo Amp...Problem :(

[soma89]

Ok so I have learned so much from this site and almost feel guilty for asking this, as I know some other thread has tackled this issue. I can assure you I've thoroughly gone through all of them and still have this issue..

Problem:
Fizziness BEHIND the note. Often sounds like the amp is trying to "clear its throat" especially if I play a low note. Amp goes through phases of not having this issue but it does always come back.

What I've tried:
-Replacement speaker...issue was still there.
- plugged in to the -15db input...issue still there
-Headphones...fizziness still audible through them
-replaced input jacks..both normal and -15db...problem still there



How can I go about diagnosing this? Board looks fine. All caps look good. If I monitor the continuity through the traces in the schematic, would that give me any ideas?

I take electronics engineering at school and find this an interesting challenge. I understand i could just bring it to a trained professional, but what else do you recommend I check?

Thanks again!!

 

[anderbass]

I remember Jerrold Tiers mentioning the Bias settings can drift on solid state powered Ampegs causing this issue. You could reserch that and try sending him a PM for info: Jerrold Tiers

 

[Linnin]

Just grab a schematic and start running it down circuit by circuit and component by component.

 

[Jay Corwin]

I had a very similiar problem on mine. Mine had a buzz or a sizzle that actually rode on top of certain notes. and decayed with them. Might not be the same issue as yours.

If it is, mine was a very easy fix. One of the connectors that plugged into the speaker terminals was loose (or probably just poorly formed from the plant). Certain notes would rattle it the right way and cause the problem.

New connectors, problem gone. It's worked for years without issue now.

 

[beans-on-toast]

If it hasn't been serviced in a while, the first thing the amp needs is a general maintenance.

All electrical related contacts need to be cleaned with deoxit. This includes connectors, jacks, cleaning and lubricating pots.

There is a shunt (a switch) in the headphone jack that can be problematic, it need to be cleaned with deoxit when the contacts become oxidized. Test and see if the problem goes away when phones are plugged in.

The sort of problem that you are describing can be due to bad connection as was mentioned above. Connectors are the easiest thing to address first. It could also be a bad solder joint or component.

The amp needs a careful inspection by a tech who an take readings and compare them to reference values on the schematic. The power stage output transistor bias should be checked.

 

[AstroSonic]

Just grab a schematic and start running it down circuit by circuit and component by component.

+1

If you have replaced jacks and applied Deoxit to switches and pots, and checked the speaker connectors, and if you are in an electrical engineering program, you may be ready for a signal tracing approach to this problem. Inject a signal (using a signal generator or phone ap) at a frequency that causes the fuzziness. Using the schematic and an oscilloscope, trace the signal from input to output, stage by stage, until you pick up the fuzziness. Check that stage for specified voltages. You'll likely find at least one of them to be off. Check the associated solder joints, connectors and components. Fix connections and replace components as needed. If you have NO experience at this or feel insecure about it, have a tech do it.

 

[Crater]

Your symptoms definitely sound like mis-biased power amp transistors. A factory schematic for this amp should have the bias adjustment procedure printed on it. I fixed a Crate B 200 amp head by re-biasing the power transistors. (Crate was owned by Ampeg parent SLM at the time). Once the amp is opened up and the bias trim pot is accessible, it's a 5 minute job with a multimeter and a screwdriver. That fuzziness you're hearing is crossover distortion.

 

[soma89]

Ok guys, thank you so much for your help so far. Ive noticed another odd thing...

When the amp was on, i gently poked around with a wooden drum stick. I noticed when i tapped one of the large caps it made a loud POP! & when i tapped the face plate it also popped. Maybe some residue AC from a leaky component? Anyways, im probably going to have to bring it to a tech as i dont wanna cause anymore problems.

 

[Linnin]

Anyways, im probably going to have to bring it to a tech as i dont wanna cause anymore problems.

I thought you were learning to be an an electronics tech?! This is the perfect platform upon which to learn. It's only worth $75 CND and it's not like you can screw it up.

 

[beans-on-toast]

Ok guys, thank you so much for your help so far. Ive noticed another odd thing...

When the amp was on, i gently poked around with a wooden drum stick. I noticed when i tapped one of the large caps it made a loud POP! & when i tapped the face plate it also popped. Maybe some residue AC from a leaky component? Anyways, im probably going to have to bring it to a tech as i dont wanna cause anymore problems.

That pop can damage your speaker(s). Caps can become microphonic. A bad solder joint can also cause problems. It might not be the cap, but something near by. It requires close inspection.

The scematic has reference numbers at test points. These should be checked out. It can help you determine if the electrolytic caps need to be replaced.

 

[soma89]

Ok....just an update.

I brought it to a tech. He said there is absolutely nothing wrong with the amp.


I'm confused.

I'm going to start from scratch again. What can this be??? It distorts at lower frequencies, I have no idea what to say. I dont think the guy tried it out with a bass, as he was more of an audiophile type probably just a ran a signal generator through it.

What are other things that can cause distortion other than the electronic components?

 

[Lobster11]

Ok....just an update.

I brought it to a tech. He said there is absolutely wrong with the amp.


I'm confused.

I'm going to start from scratch again. What can this be??? It distorts at lower frequencies, I have no idea what to say. I dont think the guy tried it out with a bass, as he was more of an audiophile type probably just a ran a signal generator through it.

What are other things that can cause distortion other than the electronic components?

I suspect that everyone who responded previously assumed that you had already done the necessary tests to confirm that the problem was in the amp and not in your bass -- i.e., playing a different bass through the amp and playing the same bass through a different amp. Now it appears that the problem is indeed in the bass -- or a cable or something else (tuner pedal maybe?) along the line.

The first and simplest thing I can think of is that if your bass is active, a weak battery could cause symptoms similar to what you described. Any chance that might be it?