Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Amplifier Discussion => Topic started by: somguitar on October 14, 2012, 12:56:16 AM

Title: AMPEG B2R Problems-Help Please!
Post by: somguitar on October 14, 2012, 12:56:16 AM
Hi, So I purchased an AMPEG B2R bass head used and I had no problems with it for quite some time. After a while though it started occasionally dropping a lot of sound and going very distorted. Then I would turn it off and unplug a few cables and what not and it would magically come back to normal. This occurred numerous times. I tried different cables and guitars with no results. Eventually I figured it must be a bad solder joint inside. Took the cover off and removed the components. I saw nothing wrong with the joints and when I reassembled it, the amp constantly had the weak sound and distortion issue. I then proceeded to disassemble it and clean all of the ground locations. Reassembled to no avail. I'm starting to wonder if its the power supply or maybe a set of bad capacitors? These are really just shots in the dark but if anyone has some advice that might save me hours of testing and trial and error it would be much appreciated.
Title: Re: AMPEG B2R Problems-Help Please!
Post by: DrGonz78 on October 14, 2012, 06:39:34 AM
One time I was working on a Crate GX130c guitar amp and it was acting strange. Just as your amp it would turn on okay and then get a weird volume or crackle even then it would  start to drop the signal. Eventually it would shut down and turn back on by itself... All this happened within about 2 minutes. This would happen every time you turned on the amp. What I found was a 120 ohm 10 watt wire wound resistor was cooking up so hot that it was getting loose from trace or something. I don't think wire wound resistors typically get intermittent but this was the issue possibly. The wire wound cement was cracking and even though it read around 120 ohm cold it was heating up and loosing the battle, then the amp would shut off. Now these resistors on this amp would heat up anyhow, but after years of use it was toasted. I replaced all the wire wounds on the amp and it stopped that problem. The amp lives on now!

In your case this might not be what is happening exactly, but what is happening (very probable) is that some component is heating up too much. Each time it heats up something is loose inside and at that point the amp becomes unstable. When it starts to break up and act funny does tapping on the amp have any effect to bring the signal back momentarily? A cold solder joint is hard to detect but chopsticks help.
Title: Re: AMPEG B2R Problems-Help Please!
Post by: Roly on October 14, 2012, 06:58:54 AM
Amps with actual electronic problems generally don't "magically come back to normal".  This is much more likely to be what you went looking for, a bad connection somewhere.  One of the most common causes are where external sockets are mounted on a printed circuit, and physical strain (e.g. tripping over a cable) causes the solder joints to fracture.  Even if you know what you are looking for these can sometimes be quite hard to spot, even with a lens.  One giveaway is that there is movement in the middle of the joint when pressure is applied to the socket on the component side, but not always.

These can be localised by applying gentle sideways pressures to a plug while signal is being sent through the amp (generally easier from a source like an MP3 player so you can concentrate on what you are doing without having to twang a guitar).  If you are lucky/careful you may be able to identify a socket where you can switch the fault on and off at will.  Then it is simply a matter of resoldering the broken joint, BUT, if it happens once it will eventually happen again because it is generic to PCB mounted sockets.

Another common cause where the amp is fitted with self bridging Fx Send and Return or Pre Out/Main In are the bridging contacts in the sockets.  These can often be localised by plugging in an external patch lead and the problem disappears - something to try.  If they are open style sockets you can clean the contacts with a slip of paper, but the enclosed type often need replacement.

Looking at the circuit it seem this amp has both Fx loop and Pre/Main breaks, so there are two points where you can monitor the signal path to see if the fault is before/after.  There is also some sort of multi-pole board interconnect.  These are normally pretty reliable, but you might try unplugging and replugging each end to freshen the contacts, and see if that helps.

It may not seem very scientific, but when the amp is in fault mode try giving it a thump and see if the fault seems to be physically sensitive - if so it is almost certainly a dodgy contact (but the reverse is not true, just because it isn't physically sensitive doesn't mean it still isn't a poor contact)

Many thanks to;

http://music-electronics-forum.com/t7704/ (http://music-electronics-forum.com/t7704/)

for the attach.

HTH
Title: Re: AMPEG B2R Problems-Help Please!
Post by: Enzo on October 15, 2012, 06:34:53 PM
My vote is the jacks.  You have two pair that can do this.  You have the FX loop send and return, and you have the pair marked preamp out and power amp in.

Take a spare cord - a good cord - and plug it from FX send to FX return.  ANy help?  Now move the cord over to preamp out to power amp in.  ANy help?
Title: Re: AMPEG B2R Problems-Help Please!
Post by: Ric5 on August 10, 2019, 10:14:47 AM
I fixed my Ampeg B2 by plugging the short cables in the back

Title: Re: AMPEG B2R Problems-Help Please!
Post by: joecool85 on August 12, 2019, 04:07:46 PM
One time I was working on a Crate GX130c guitar amp and it was acting strange. Just as your amp it would turn on okay and then get a weird volume or crackle even then it would  start to drop the signal. Eventually it would shut down and turn back on by itself... All this happened within about 2 minutes. This would happen every time you turned on the amp. What I found was a 120 ohm 10 watt wire wound resistor was cooking up so hot that it was getting loose from trace or something. I don't think wire wound resistors typically get intermittent but this was the issue possibly. The wire wound cement was cracking and even though it read around 120 ohm cold it was heating up and loosing the battle, then the amp would shut off. Now these resistors on this amp would heat up anyhow, but after years of use it was toasted. I replaced all the wire wounds on the amp and it stopped that problem. The amp lives on now!

In your case this might not be what is happening exactly, but what is happening (very probable) is that some component is heating up too much. Each time it heats up something is loose inside and at that point the amp becomes unstable. When it starts to break up and act funny does tapping on the amp have any effect to bring the signal back momentarily? A cold solder joint is hard to detect but chopsticks help.

I had a very similar issue on a Vox I got cheap a while back.  Turned out the cement resistor had broken a leg (dropped amp?).  Sometimes it worked, sometimes it didn't.  I found the break and, being cheap, I soldered it to itself rather than replace.  It worked fine for a few years, then I sold the amp.  Still working as far as I know.

Regardless, sounds like a mechanical issue.
Title: Re: AMPEG B2R Problems-Help Please!
Post by: DrGonz78 on August 13, 2019, 03:39:16 PM
Amps with actual electronic problems generally don't "magically come back to normal".

I sure do miss Roly. Very good quote to get stuck in your head. I have many statements like this one in my memory bank from so many great techs I have met here and other places online.