Ampeg SVT-400T Repair Progress

[agedhorse]

I am very supportive of folks expanding their education and understanding of electronics BEFORE attempting repairs. I also see the effects of DIY repairs gone wrong, and it's always more expensive than the original repair would have been (assuming the amp was not damaged beyond economical repair)

 

[washburnbassman]

@agedhorse I think I understand where you are coming from.

I would never charge for my work. This is a hobbyist repair on my own amp, and I do not consider myself highly skilled or an EE.

I don't think I am the target demographic here, I'm not service personnel, but i accept your point.

This is just me, fixing my $200 amp instead of chucking it in the bin. I have expensive mesa gear that I would be afraid to do the same thing to.

 

[washburnbassman]

@agedhorse I think I understand where you are coming from.

I would never charge for my work. This is a hobbyist repair on my own amp, and I do not consider myself highly skilled or an EE.

I don't think I am the target demographic here, I'm not service personnel, but i accept your point.

This is just me, fixing my $200 amp instead of chucking it in the bin. I have expensive mesa gear that I would be afraid to do the same thing to.

I am very supportive of folks expanding their education and understanding of electronics BEFORE attempting repairs. I also see the effects of DIY repairs gone wrong, and it's always more expensive than the original repair would have been (assuming the amp was not damaged beyond economical repair)

Thank you and sorry that I snapped back at you.

 

[agedhorse]

To be clear, when I make comments I am addressing the potentially hundreds of folks that are trying to learn more, or who are interested in learning about how the repair world is supposed to work (and why it often goes wrong). Most of these folks do not have an electronics background, though some would like to learn more.

Since there is some danger in servicing anything that is line powered, and that there is even more danger where there is much high voltages present (>400V), this gets factored into my replies because in effect I am obligated to do so for clarity and accuracy as well as moral principles.

 

[washburnbassman]

To be clear, when I make comments I am addressing the potentially hundreds of folks that are trying to learn more, or who are interested in learning about how the repair world is supposed to work (and why it often goes wrong). Most of these folks do not have an electronics background, though some would like to learn more.

Since there is some danger in servicing anything that is line powered, and that there is even more danger where there is much high voltages present (>400V), this gets factored into my replies because in effect I am obligated to do so for clarity and accuracy as well as moral principles.

Makes sense and thanks for clarifying, I have been fiddling with stuff for over 20 years and am pretty careful but this certainly isn't an amp to start messing around on.

 

[dbase]

In 1972 I took an electronics course at a local night school (Philadelphia Wireless) taught by RCA engineers.. I learned what the engineers call the RCH factor, built a vacuum tube voltmeter, how to use a slide rule, anti logs and college math all to figure out a capacitance reactive circuit. I learned more there in one year than four years of high school. When we hit the crossroads whether to proceed down the path to TV repair or take the road to TV transmitter repairs I picked the transmitters. Fun stuff. I watched as rookies would take a test lead and slowly stick it into a microwave transmitter assembly to test for voltages while the instructor would come up behind them and slap two 2x4's together ... Needless to say ... I enjoyed that class. We all learned to be careful working with high voltages on chassis. I ended up in IT .. much safer. (off soapbox)

 

[washburnbassman]

"slap two 2x4's together"

Have had that done to me. terrifying. Also now working in IT, but unfortunately still finding ways to electrocute myself outside of my job.

I can afford to buy these things new. I don't know what is wrong with me but I prefer buying them broken and fixing them.

 

[BogeyBass]

or in reality anybody that has repaired a ampeg has probabably replaced a zener or 2

otherwise if the amp had work done to it already or a failed repair.
in general that is a good place to start.
often the failed shot gun repair has either bad connections or damaged traces.

and once you get past the poor previous mistakes
you can actually repair the amp.

good to hear the amp is repaired and working

 

[etherealme]

This seems like as likely a place for this inquiry on this 400T as any other thread here. A) this one is more comprehensive a thread on the amp B) it's also more recent than the countless other search results I got. This thread has the only decent responses on the amp I could find on TB. So, I think it's a valid resurrection.

How is the preamp section regarded with these amps and how does it compare to other Ampeg preamps? Say, if I want to buy a cheap one to use for the preamp into another power amp....

Thanks.

 

[etherealme]

Bump seeking a response.

 

[Garry7263]

For the record, I have had an Ampeg SVT 400T since it came out, running it with a Trace Elliot 4x10 and a Trace Elliot 1x15. Have AWESOME tone and power. Have gigged thousands of times with it. Would not part with it at any cost. It is a tank. I have watched roadies drop it a few times along the way, still worked perfect. I have tried other amps, really wanting to upgrade, but nothing comes close the sound I have now, or the versatility this amp gives me.

Just so happens that last night at rehearsal, it started cutting out. At the beginning of the night I noticed the EQ light was on even though I didn't have it engaged. Was searching to see if there were any known problems before opening it up. Pretty sure it needs a full recap after 30 years. Only problem I have ever had with it was the EQ sliders needed to be cleaned about 12 years ago. I have not torn into it yet, was just searching on google for possible causes before I pull it from my rack and dig into it. Thank you for your thread and the links to the schematics and to the caps you used.

I am not a service tech. I repair TVs, amps, and cell phones all the time and can read a schematic and am fairly proficient at troubleshooting things. I always do a visual check to begin with, followed by checking caps and diodes first.


*** So not only should someone that has not worked on electronics before, either leave it to a professional, or understand the risks, and that touching the wrong thing can KILL you. Even if it is NOT plugged in. That includes those capacitors. They can hold a very large charge even when it is not plugged in. Just a warning, get a 10 watt 100 ohm resistor and short the legs on all those caps before you do anything. Can use a screwdriver, but you can weld it to the cap. The resistor is better.

 

[agedhorse]

Just a warning, get a 10 watt 100 ohm resistor and short the legs on all those caps before you do anything. Can use a screwdriver, but you can weld it to the cap. The resistor is better.

Using a screwdriver can ruin the cap as well.

 

[byacey]

IMO, it's important to present the information both conceptually as well as factually.

Since I am a design engineer IN THIS INDUSTRY, with an EE, and have written numerous service and technical training manuals addressing many of the common myths that I see with service personnel that do not really understand as much at they think, I am sharing this experience with those who want to learn more about how amp repair is done in a qualified service shop.

A qualified, knowledgeable service tech will approach a repair by applying diagnostics, verifying the fault(s), identifying the cause(s) and then correcting the faults that have been identified. This would include replacing parts that test bad. They will also understand the necessary safety precautions that go along with servicing line voltage equipment.

As a follow up, they will then test after the repair to verify the performance and at the same tone check for parts that do not meet the performance spec.

Just going in and replacing parts is not something I would consider as being skilled, and if charging a customer for such work might be considered dishonest, deceitful, or even fraudulent.

Unfortunately, this is all too common, and along with the additional labor from such an inefficient (and generally less effective) way of attempting repairs, is one factor in driving g up the repair cost to the customer.

This IS helpful and informative information to those who are interested in learning and understanding. For those who already know it all, perhaps this information is not as interesting.

This post sums everything up in a nutshell. In fact, it should be made a sticky at the top of this forum entitled:
For those considering DIY repair of Electronic Equipment

 

[washburnbassman]

Hey Garry,

Hope you fixed your amp! Mine is still going strong, love these machines.

For the record, I have had an Ampeg SVT 400T since it came out, running it with a Trace Elliot 4x10 and a Trace Elliot 1x15. Have AWESOME tone and power. Have gigged thousands of times with it. Would not part with it at any cost. It is a tank. I have watched roadies drop it a few times along the way, still worked perfect. I have tried other amps, really wanting to upgrade, but nothing comes close the sound I have now, or the versatility this amp gives me.

Just so happens that last night at rehearsal, it started cutting out. At the beginning of the night I noticed the EQ light was on even though I didn't have it engaged. Was searching to see if there were any known problems before opening it up. Pretty sure it needs a full recap after 30 years. Only problem I have ever had with it was the EQ sliders needed to be cleaned about 12 years ago. I have not torn into it yet, was just searching on google for possible causes before I pull it from my rack and dig into it. Thank you for your thread and the links to the schematics and to the caps you used.

I am not a service tech. I repair TVs, amps, and cell phones all the time and can read a schematic and am fairly proficient at troubleshooting things. I always do a visual check to begin with, followed by checking caps and diodes first.


*** So not only should someone that has not worked on electronics before, either leave it to a professional, or understand the risks, and that touching the wrong thing can KILL you. Even if it is NOT plugged in. That includes those capacitors. They can hold a very large charge even when it is not plugged in. Just a warning, get a 10 watt 100 ohm resistor and short the legs on all those caps before you do anything. Can use a screwdriver, but you can weld it to the cap. The resistor is better.