Ampeg Micro VR issue

[KeroseneTrewthe]

Hey all,

It's been a very long time since I've visited here, but I've gotten back into bass a few years ago. I purchased a Micro VR because it sounded nice for it's size and weight. Out of the blue last month, it just quit on me. The amp powers on, but the limiter and pad lights don't light up and there's no output to the cab via the 1/4". I can get output via the XLR. I can use it as a practice amp with headphones and the audio in jack. That all works fine.

 

Really strange and there was no warning. It's out of warranty and I contacted Ampeg and their customer service wasn't very helpful as they want me to take it in and I think it'll just end up costing me the price of a new amp to get it fixed. Just curious if anyone has had a similar issue and what it was or what you think it is so I can possibly get it fixed myself. Thanks in advance,

[WRGKMC]

Plug your effects send into another amp and this should help you isolate the problem. If you get sound from the effects send it tells you the preamp is good.

 

Next you can plug your bass into the effects return. If you get sound, you know the power amp is working.

 

As far as fixing it goes, There may be a fuse on a circuit board some place. I haven't seen the inside of one of those so I can only guess.

Its a Mosfet amp which should be pretty durable. Hopefully you weren't running it below 4 ohms. Anything lower can blow the power amp.

 

Beyond that you'd need to signal trace the amp to find the fault. This consists of injecting a test tone into the amp, then tracing it stage by stage. If you get a signal going into a transistor, and nothing coming out, then you found your issue. The component is either bad or isn't getting voltage to operate.

 

Given the fact your limiter light doesn't come on, it may be a power supply issue. Newer amps often have over current protection and shut the power supply down when components overheat or short. I suggest you take it to an Ampeg dealer and either let them fix it or send it back to the factory for repair. There's nothing you can do besides isolate the issue to the preamp or power amp like I mentioned above. At least if you know that its less likely you'd be ripped off and told its something else.

[KeroseneTrewthe]

Hey thanks for the reply. I did as you said, and it looks like my preamp is working but my power amp is not. I replaced the main fuse just to see if that would help, but I assume if that was blown, I wouldn't get the amp to turn on at all. I looked inside the amp and don't see another fuse. Would it look similar to 250v fuse that attaches to the plug on the back of the amp?

 

Actually, the limiter and pad lights do come on when I first fire up the amp. They flicker as the amp starts up. It happens so fast, I didn't notice it before.

 

 

 

Plug your effects send into another amp and this should help you isolate the problem. If you get sound from the effects send it tells you the preamp is good.

 

Next you can plug your bass into the effects return. If you get sound, you know the power amp is working.

 

As far as fixing it goes, There may be a fuse on a circuit board some place. I haven't seen the inside of one of those so I can only guess.

Its a Mosfet amp which should be pretty durable. Hopefully you weren't running it below 4 ohms. Anything lower can blow the power amp.

 

Beyond that you'd need to signal trace the amp to find the fault. This consists of injecting a test tone into the amp, then tracing it stage by stage. If you get a signal going into a transistor, and nothing coming out, then you found your issue. The component is either bad or isn't getting voltage to operate.

 

Given the fact your limiter light doesn't come on, it may be a power supply issue. Newer amps often have over current protection and shut the power supply down when components overheat or short. I suggest you take it to an Ampeg dealer and either let them fix it or send it back to the factory for repair. There's nothing you can do besides isolate the issue to the preamp or power amp like I mentioned above. At least if you know that its less likely you'd be ripped off and told its something else.

 

 

[KeroseneTrewthe]

Oh and btw, I never had the amp more than half volume turned up and used an 8ohm cab.

[WRGKMC]

You have to realize I'm working blind here so I cant tell you, go here and do this and it will be your solution.

 

Half an amp working is good news when it comes to repair costs. The preamp is more complex and has many more components so troubleshooting costs are cheaper. Its the labor cost is what kills your wallet, not the components.

 

Your limiter comes after the preamp section, but before the power amp stage. This basic block diagram shows you there is a peak detection in the preamp section which triggers the limiter which is before the power amplifier. This doesn't have enough detail to find the exact cause of the problem however.

 

I should have also mentioned, try using headphones. If you get sound from the headphones, then the limiter is good. If you don't get sound through the headphones then your problem has been narrowed down to the limiter circuit. The effect send is just before the limiter and you have sound there. The headphones come just after the limiter. No sound = bad limiter circuit.

 

Because the limiter switch bypasses the limiter circuit, I suspect you may get sound through the headphones and the problem may be with the power amp. The switch may not be true bypass however and the buffer after the master volume (before the limiter) may be blown.

 

In any case, knowing is the headphone jack works greatly narrows the fault possibilities down.

 

 

 

 

The power transformer fuse is obviously good because the preamp is working. The power amp section may have pico fuses but they can be difficult to spot and I wouldn't want you to go guessing. They can look like resistors or diodes. They can only be replaced with exact replacements and you'd need a schematic to determine their values.

 

I'd would do is inspect the power amp section and see if any of the components in the power section look like they are blackened of cooked. I'd also check for bad solder joints that may have cracked and could be causing a signal loss.

 

Next, Hopefully you have ruled out the speaker cable and speakers. you never want to use a guitar cord for a speaker cord. They are designed for low current signals coming from an instrument. When used as a speaker cable, the high current levels can melt the insulation inside the cable and short taking out the power amplifier. If you were using a guitar cable, this can be a likely cause for the amp failure.

 

I own a newer Portaflex head. I know the head has thermal breakers which will kill the power supply and shut the head down till the power transistors are cool before it can be powered back up again. Manufacturers commonly incorporate components and circuit designs from other heads they make. It saves them time and money because they don't have to reinvent the wheel with every new product they sell.

 

Unfortunately your amp isn't a Class D design like the Portaflex that uses resettable thermal breakers. Its likely a push pull A/B design.

Mosfets in that amp likely look like large square three legged transistors mounted on the aluminum heat sinks.

 

I suspect with the small VR head size use thermal fuses mounded on these heat sinks near the Mosfets. When the power transistors over heat, the fuses open to protect them from shorting out and removing them from the power supply. This is why your preamp section still works. The Power transistors have been taken off line.

 

Apparently these are thermal fuses and not resettable breakers. Unless you failed to check your speaker cable, its shorted and it pops the breaker every time you power up, I suspect they are fuses that open and require a tech to replace them.

 

In very rare cases, you have amps that have thermal breakers with a red button on them. you push the button in to rest them much like you'd flip a AC circuit breaker for a house.

 

Again, I'm working blind here throwing out some last ditch possibilities. I'm not expecting you to crack out a soldering iron and start replacing components. The headphone test is about the end of the line. You can inspect the power amp section for obvious problems but to repair it, you need the skill, tools, and access to the proper parts.

 

If you do take it to a tech however and tell him you've isolated it to the Limiter or the power amp, its like taking a car to an auto mechanic. When you tell them you have bad brakes, instead of saying "the car doesn't go" he isn't likely to suggest you need a new transmission and confine his work to the brakes. If he does attempt go beyond them, it send up red flags.

 

Sorry I cant be more helpful here. I haven't worked on these new micro amps. The layout is similar to my Portaflex but its a different amp.

[onelife]

If you plug an mp3 player into the audio in jack on the front of the amp, do you hear the player in the headphone jack?

[KeroseneTrewthe]

Thank you for all that information!

 

Yes, I get sound from the headphones and also from the audio input when I use my ipod. I pushed the gain and my bass and the limiter is indeed working.

 

I'll have to check the cable that was at the church I was using the amp. For all I knew, it was a speaker cable, but it may have been an instrument cable.

 

I don't see anything discolored at all inside the amp. It looks brand new and all the solder points are shiny silver. I don't see any other fuses inside other than the one I found that's connected to the power cord connector on the back of the amp.

 

So after this info, you're thinking it's probably the power amp?

 

Thanks again, man. Really appreciate your time with me.

 

 

 

 

You have to realize I'm working blind here so I cant tell you, go here and do this and it will be your solution.

 

Half an amp working is good news when it comes to repair costs. The preamp is more complex and has many more components so troubleshooting costs are cheaper. Its the labor cost is what kills your wallet, not the components.

 

Your limiter comes after the preamp section, but before the power amp stage. This basic block diagram shows you there is a peak detection in the preamp section which triggers the limiter which is before the power amplifier. This doesn't have enough detail to find the exact cause of the problem however.

 

I should have also mentioned, try using headphones. If you get sound from the headphones, then the limiter is good. If you don't get sound through the headphones then your problem has been narrowed down to the limiter circuit. The effect send is just before the limiter and you have sound there. The headphones come just after the limiter. No sound = bad limiter circuit.

 

Because the limiter switch bypasses the limiter circuit, I suspect you may get sound through the headphones and the problem may be with the power amp. The switch may not be true bypass however and the buffer after the master volume (before the limiter) may be blown.

 

In any case, knowing is the headphone jack works greatly narrows the fault possibilities down.

 

 

 

 

The power transformer fuse is obviously good because the preamp is working. The power amp section may have pico fuses but they can be difficult to spot and I wouldn't want you to go guessing. They can look like resistors or diodes. They can only be replaced with exact replacements and you'd need a schematic to determine their values.

 

I'd would do is inspect the power amp section and see if any of the components in the power section look like they are blackened of cooked. I'd also check for bad solder joints that may have cracked and could be causing a signal loss.

 

Next, Hopefully you have ruled out the speaker cable and speakers. you never want to use a guitar cord for a speaker cord. They are designed for low current signals coming from an instrument. When used as a speaker cable, the high current levels can melt the insulation inside the cable and short taking out the power amplifier. If you were using a guitar cable, this can be a likely cause for the amp failure.

 

I own a newer Portaflex head. I know the head has thermal breakers which will kill the power supply and shut the head down till the power transistors are cool before it can be powered back up again. Manufacturers commonly incorporate components and circuit designs from other heads they make. It saves them time and money because they don't have to reinvent the wheel with every new product they sell.

 

Unfortunately your amp isn't a Class D design like the Portaflex that uses resettable thermal breakers. Its likely a push pull A/B design.

Mosfets in that amp likely look like large square three legged transistors mounted on the aluminum heat sinks.

 

I suspect with the small VR head size use thermal fuses mounded on these heat sinks near the Mosfets. When the power transistors over heat, the fuses open to protect them from shorting out and removing them from the power supply. This is why your preamp section still works. The Power transistors have been taken off line.

 

Apparently these are thermal fuses and not resettable breakers. Unless you failed to check your speaker cable, its shorted and it pops the breaker every time you power up, I suspect they are fuses that open and require a tech to replace them.

 

In very rare cases, you have amps that have thermal breakers with a red button on them. you push the button in to rest them much like you'd flip a AC circuit breaker for a house.

 

Again, I'm working blind here throwing out some last ditch possibilities. I'm not expecting you to crack out a soldering iron and start replacing components. The headphone test is about the end of the line. You can inspect the power amp section for obvious problems but to repair it, you need the skill, tools, and access to the proper parts.

 

If you do take it to a tech however and tell him you've isolated it to the Limiter or the power amp, its like taking a car to an auto mechanic. When you tell them you have bad brakes, instead of saying "the car doesn't go" he isn't likely to suggest you need a new transmission and confine his work to the brakes. If he does attempt go beyond them, it send up red flags.

 

Sorry I cant be more helpful here. I haven't worked on these new micro amps. The layout is similar to my Portaflex but its a different amp.

 

 

[KeroseneTrewthe]

If you plug an mp3 player into the audio in jack on the front of the amp' date=' do you hear the player in the headphone jack?[/quote']

 

Yes, both of these work.

[WRGKMC]

I'm hoping its a cord/speaker issue and there's nothing wrong with the head.

You always want to use a high quality unshielded cord for the speakers.

Having a bad cord going into the amp does nothing if it shorts. The speaker output can go as high as 70V and pump allot of current. Bad things can happen with a bad speaker cable.

 

We've narrowed it down to being after the limiter.

Given the fact Ampeg makes some very durable products and because power transistors usually shut the amps completely when they go out, I'll throw the dice and say the amp is working and you just used a bum speaker cord.

 

Just given the fact it was something left at a church might indicate its no good. If it was good someone would have taken ownership of it.

 

When I suspect a cord like that, I instinctively unscrew the capes on the plugs and look to see if the two conductors are good and separated from one another. Wiggle and twisting will tell you if the plug is beat and falling apart too. many of the jacks use a single rivet to hole them together. When they get beat the rivet loosens and you loose electrical contact to the ground. Wanking the cord can break the wires. Two seconds and a quick look and you can discover the problem most of the time.

 

In fact, Anything an operator of gear can touch on the outside of an amp, knobs, switches, jacks can get accidentally damaged and they are always the first things you want to check. Even if the speaker cable shows as being good, there's a possibility the speaker jack on the amp is damaged. Some of these cheap PC mount plastic jacks only need one good tug on a cord and they split wide open. In a busy area with people walking around its easy for someone to accidentally yank on a cord and never know they did any damage to the cord or the jack.

 

 

[onelife]

Thank you for all that information!

 

Yes, I get sound from the headphones and also from the audio input when I use my ipod. I pushed the gain and my bass and the limiter is indeed working.

 

I'll have to check the cable that was at the church I was using the amp. For all I knew, it was a speaker cable, but it may have been an instrument cable.

 

I don't see anything discolored at all inside the amp. It looks brand new and all the solder points are shiny silver. I don't see any other fuses inside other than the one I found that's connected to the power cord connector on the back of the amp.

 

So after this info, you're thinking it's probably the power amp?

 

Thanks again, man. Really appreciate your time with me.

 

Unlike tube amps that run on high voltage (enough to obviously burn componants) solid state amplifiers run on relatively low voltage so the damage is much less obvious.

 

The most common problems I have encountered with solid state amplifiers involve the output transistors. Based on all the symptoms you have described I see that as a possibility in your case.

 

Looking at the block diagram posted above by WRGKMC, we can see that the power amplifier section is only used to drive the speakers and that seems to be the only part of your amp that is not working.

 

There are some external considerations as well such as the speaker wire (as previously mentioned) and the speakers themselves. Do you have another cable you can try? Do you have access to another amplifier that you could use to test the speaker cabinet?

[KeroseneTrewthe]

Just more information: No, this is my old amp that I was using at the church. I was given a cable to connect my cab to the amp but never thought to check if the cable was for a speaker or instrument. (I'll have to check next week when I play there again). But I play at such low volume I figured it didn't really matter in that situation which cable was used. When the amp stopped working at the church, I thought it was the connection to my cabinet since the amp has barely been used.

 

Once I brought it home, I used my heavy duty speaker cable hooked up to the cabinet and there is no output. That was the first thing I checked. I tested the cabinet with another amp and it is in good working order. So it does seem the power amp section of the cab is dead. I honestly don't know if it's really worth getting it fixed. My Ampeg center is about an hour away. I think I am stuck with a Micro Ampeg practice headphone amp.

 

 

 

 

I'm hoping its a cord/speaker issue and there's nothing wrong with the head.

You always want to use a high quality unshielded cord for the speakers.

Having a bad cord going into the amp does nothing if it shorts. The speaker output can go as high as 70V and pump allot of current. Bad things can happen with a bad speaker cable.

 

We've narrowed it down to being after the limiter.

Given the fact Ampeg makes some very durable products and because power transistors usually shut the amps completely when they go out, I'll throw the dice and say the amp is working and you just used a bum speaker cord.

 

Just given the fact it was something left at a church might indicate its no good. If it was good someone would have taken ownership of it. If it is the cord, keep that in the back of your mind next time as being something you want to rule out first.

 

In fact, Anything an operator of gear can touch on the outside of an amp, knobs, switches, jacks can get accidentally damaged and they are always the first things you want to check. Even if the speaker cable shows as being good, there's a possibility the speaker jack on the amp is damaged. Some of these cheap PC mount plastic jacks only need one good tug on a cord and they split wide open. In a busy area with people walking around its easy for someone to accidentally yank on a cord and never know they did any damage to the cord or the jack.

 

When I have a dead amp like that, I instinctively unscrew the capes on the plugs and look to see if the two conductors are good and separated from one another. Wiggle and twisting will tell you if the plug is beat and falling apart too. many of the jacks use a single rivet to hole them together. When they get beat the rivet loosens and you loose electrical contact to the ground. Wanking the cord can break the wires. Two seconds and a quick look and you can discover the problem most of the time.

 

 

[WRGKMC]

Google around and see if you can find a local music store amp tech. You may luck out and find someone who knows what he's doing.

 

Mosfets are field effect transistors so they are tricky to measure. When they go bad they normally short between the Drain and Gate. If you did pull the old ones out you could go here to test them. https://www.utm.edu/staff/leeb/mostest.htm Mosfet transistors are usually cheap too. If you get the numbers off of them you can probably find replacements for around $5 each then just change them all. All of this depends on your soldering skills of course. If they are PC mount you do want to use a solder sucker to get the old ones out. Makes a neater job putting the new ones in too.

[KeroseneTrewthe]

So they have to be pulled to be tested? I'll see on the back of the circuit board but I'm pretty sure they are simple soldering points.

 

Great link, that will surely help!

 

Google around and see if you can find a local music store amp tech. You may luck out and find someone who knows what he's doing.

 

Mosfets are field effect transistors so they are tricky to measure. When they go bad they normally short between the Drain and Gate. If you did pull the old ones out you could go here to test them. https://www.utm.edu/staff/leeb/mostest.htm Mosfet transistors are usually cheap too. If you get the numbers off of them you can probably find replacements for around $5 each then just change them all. All of this depends on your soldering skills of course. If they are PC mount you do want to use a solder sucker to get the old ones out. Makes a neater job putting the new ones in too.

 

 

[onelife]

Power transistors are usually mounted on a heat sink seperate from the circuit board.

[KeroseneTrewthe]

Okay, just tested the Mosfets. Looks like 4 out of the 8 are short circuited. And I didnt really notice until now, but the tops of the silver metal part is discolored to a sorta copperish hue. Now how do I find the right ones to replace them? Thanks so much everyone for your help with this!

[onelife]

It's difficult to find technical information on the new breed of Made In China brand name amplifiers.

 

you could try here:

 

technicalsupport@loudtechinc.com

 

 

[onelife]

I might be able to get some information for you tomorrow - I spoke to a friend of mine who services Ampeg amplifiers in the Montreal area and when he is in his shop tomorrow he will look up some numbers for me.

 

Do you feel confident you can replace the MOSFETs yourself?

[WRGKMC]

Yes they have to be removed. Otherwise you'll be reading other components in the circuit that will give you a false reading.

[WRGKMC]

Need the numbers off the components themselves. Should be a combination of letters and numbers. We can then google them up to find the same parts or find a cross reference chart to find other brands of the same part. There's usually a half dozen or so manufacturers that make the same part with their own part numbers. Once we can find the specs from a TTL manual data sheet its usually pretty easy to find the what they are and who sells them at the best prices.

 

One thing to keep in mind. This transistors are supported by other components that supply them voltage. There's a good chance your issue is only with the blown Mosfets but its not uncommon to have a voltage regulator go bad too. You may get the Mosfets in there and they may go bad again.

 

Again, I'm working blind here so I can only speculate. The odds are good its just the Mosfets but I don't want you to get your hopes up and be disappointed is the problem winds up being more complex.

 

As a tech I would go through the rest of the circuit and test the rest of the components. Having a schematic makes it allot easier too because they give you reference voltages. The only way to know for sure is to test all the components then roll the dice when you power it on.

 

Maybe you can take some photos and post them. Its not a schematic but It will at least give us a clue to what else you might want to check. I always like to check the voltage regulator. Sometimes there's a zenar diode there too to maintain the DC level. Again, a photo is better then nothing.

 

If we can get it all on the first attempt even if we change a few we aren't sure about you're quickly forget the few extra dollars it may cost when that baby starts percolating.

[KeroseneTrewthe]

Here's some pictures. I circled the mosfets that tested defected. I feel confident to be able to replace the mosfets. The other parts that were mentioned, I don't know until I know which ones they are and how they are connected to the board.