Ampeg svt 4 pro clipping too early

[diydidi]

Hi All.
I recently received this SVT4 Pro for repair. The complaint is that the amp clips too early. The bass player has confirmed with a loan unit that this is not so with the loan unit. From what I can tell its got to be the preamp. Injecting a signal to the FX return gives more than enough volume without clipping.
Anyone know if this is a common fault with these amps? Any ideas?

[Enzo]

Common? Don't know. I fix what is wrong, whether it is common or not.

Verify the problem. DOES the preamp clip? And at less than full output? Was the complaint that it clips? Or was it that the clip light comes on a lot? In other words was it audible clipping?


Inside the amp, the preamp runs probably on +/-15vDC, so are both those rails up to voltage and free of ripple? Go down the row of ICs, is there any DC offset on the outputs of any? That would reduce headroom for sure.

[MarkusBass]

This is not a common fault but I had this problem in a very similar amp: Ampeg 4B about a month ago. I don't remember what was the cause but I think that I found a cold solders both on the preamp board and on the power amp board. The amp was previosly very badly treated and fixed by a guy who had no idea about proper soldering and most probably he caused most of the problems.
Make sure that this is not a problem with limiter. Then use a signal generator and an oscilloscope to find where the problem occurs. You can also remove the preamp and the power amp board and check them with a good light and a magnifying glass.

Mark

[km6xz]

This is not a 'common' problem but as described it is too ambiguous to give any advice except to reproduce the problem and the observed conditions should point to the problem very quickly. Don't rule out operator error or dead battery in an active pickup.
It is important to do nothing to it until you reproduce and isolate the problem or you might never find the real cause. For example, retouching solder joints will make the proper diagnosis impossible if done before tracking sown the problem to those solder joints.
Cold solder joints are the most over rated problem cause in repair, it is actually rare. Broken leads or broken joints are much more common and are usually expressed as intermittent normal<>fault operation.

[MarkusBass]

Cold solder joints are the most over rated problem cause in repair, it is actually rare.

I can confirm this. I fixed many Ampeg amps (SVT3Pro and SVT4Pro) and only in two of them the problems were caused by cold solder joints. Actually, I didn't have to track down the problem because in both cases it was clearly visible (with large magnfying glass). I consider "retouching solder joints " (without previous diagnosis) as a very bad habit - something what many users here do on a daily basis. In this case you never know whether the amp was fixed, or no.

Mark

[g1]

Cold solder joints are the most over rated problem cause in repair, it is actually rare. Broken leads or broken joints are much more common and are usually expressed as intermittent normal<>fault operation.

I think you are getting into a semantic issue here. You know the precise definition of "cold solder", but in my experience, most people who use the term are referring to what you called "broken joints" (or what I call "bad solder"). As you mentioned, broken joints are quite common.
Perhaps working primarily on studio gear you did not see bad solder to the extent that is seen in the guitar amp world.
In my estimation I would say broken hardware (pots, jacks, etc.) and bad solder connections are the most common faults in amp repair.
Consider a "helper" or apprentice in an amp repair shop. If they knew nothing of electronics but could solder well, what percentage of the repairs could they successfully complete? If they were my helper, and I could say "resolder this area" or have them replace the broken pots/jacks etc., there wouldn't be much left for me to do, maybe 20%?
So maybe a more precise definition of "cold solder" would help qualify your statement a bit.

[km6xz]

I stand by my assertion that cold solder joints being the problem is rare so the advice to resolder whole sections of circuits as a first resort as suggested in post after post is a bad habit and indicates a poor workflow and lack of understanding of the important, essential role of diagnostics. Diagnosis is the product being sold by a shop.
A broken solder connection is a specific defect that might or might not be resolved by shotgunning the circuit board but it prevents a definitive diagnosis.
Regarding not having guitar amp experience, I guess you are right, 60 unit repairs a day being the average throughput over years with 54 brands as warranty station, a full time staff of 23 does not quality with 10s of thousands of documented and searchable defects database by model helps create an overview, in my possibly misguided opinion, has some merit. Besides my own bench work, my time was spent walking around the shop assisting in diagnosing defects so probably determined the fault cause in 10,000 units a year. That, in some circles would be called "experience".
Broken pots, jacks and connectors in all models and all categories of units outnumbered primary cause of complaint over broken solder joints. My rule was that only techs diagnosed and repaired customer units, however the girls at the counter and an incoming triage person in the warehouse unpacking pallets from manufacturers did check operation to verify complaint. If not a qualified tech, they did not do any soldering on customer owned items. Road damage, mechanical damage was a not small part of gigging amp repair, and the majority of keyboard repair.
If beginners get into the habit of resoldering everything as a first step they will never learn to be a tech which implies a skill in diagnostics. They might even fix a few but will surely create more problems that solve. Solder bridges come from techs, not customer abuse or manufacturing defect(if it worked at all from new).
Defective tech work is the hardest to correct since it is not a logical mechanism of fault following organically developing defects from circuit operation. With my new little shop, more of a hobby, recovery from bad tech work is my primary work. Either they took it to a shade tree tech who was way over his head or they tried to repair it themselves. I ask the owner right from the start if it has been worked on by another person before bringing it to me and that helps determine the price estimate. The two types of work I get, botched tech "repairs" and eBay "I just got this shipped from eBay and it must have been damaged in shipping" repairs. This latter point convinces me that a lot of con artists are selling junk to foreign buyers knowing they will not send it back. One look inside the unit usually is enough to determine it did not work before sending it to the unsuspecting buyer.

Employers can spot a tech who will cause more problems than they solve: if they consume more freeze mist than the rest of the shop combined and if they spend most of their time soldering. Those are strong indicators of a tech who can't isolate a problem using circuit understanding or good diagnostic skills.

[Enzo]

I have to agree that "needed to be resoldered" is way over-diagnoses, at least compared to my experience. I do find cracked solder in some places more than I would like. Fender recent vintage amps like the Hot Rod series, like to crack their 6L6 socket pins solder, and lot of their solid state amps with that chassis that slopes from a tall rear panel to a small control panel like to crack solder under the large filter caps. But for the most part, resolder is not what I find.

The problem is when someone has an issue, then pulls the board and resolders everything on it, many things happened. First, the amp got disassembled, so any cable connections that were oxidized got refreshed. The boards got flexed, so little cracks get repositioned. Parts get moved and stressed, which can change the internal lead connection. And the soldering action applies high heat into each and every part soldered. That can affect an iffy part as well.


If you diagnose a fault, and correct it, you KNOW it is fixed. If you have a symptom, you resolder 100 connections, and now it works, all you can do is ASSUME it is fixed.

[MarkusBass]

... so the advice to resolder whole sections of circuits as a first resort as suggested in post after post is a bad habit and indicates a poor workflow and lack of understanding of the important, essential role of diagnostics. Diagnosis is the product being sold by a shop.

Those are strong indicators of a tech who can't isolate a problem using circuit understanding or good diagnostic skills.

I fully agree with Stan - this is what I'd tell you if my English were better .
We are selling diagnosis and not a transistor that costs $1. Lack od diagnosis and "resolder-everything-approach" cause several problems. It is hard to say whether the amp was actually fixed. It is difficult to say what is the cost. The tech is not getting any experience and with the next amp he is in the same situation as with the first. When the amp comes back with the same or maybe different problem you cannot say what was fixed previously. The list is very long - I could make it much longer by I have to go to work.

But g-one may be right regarding the terminology. Let me show you an example from one of latest Ampegs I was fixing. Or maybe not - it seems that Manage Attachments functionality does not work today. Also my previously uploaded pictures disappeared. If you could see it, you could argue that this is a broken soldering - most probably not the same as cold solder joint. But still this was a problem in less than 1% of amps I was fixing. And I fix mostly bass and guitar amps. It's interesting that other techs have completely different experience. Maybe this is caused by the way the amps are treated but I also saw amps treated badly. And in each case the "resolder-everything" approach was not used. I wouldn't know whether the amp is fixed, or not.

Mark

[km6xz]

OK, there appears to be confusion regarding terms in soldering. I was not aware that many do not distinguish between different types of solder applications. A "cold" joint is very specific in meaning. It is when soldering the workpiece is not elevated to past the liquidus temperature, or plastic phase of the solder. It is very common with beginners or those who have the habit of applying solder to the iron and then the iron to the joint. There are several causes of joint failure from this. The joint can corrode quickly or over a long period because the de-oxidizing flux has flowed to the iron and not to the joint/wire junction. The solder tends to clump on the joint but does not fully wet the joint metals. Look for convex solder surfaces. Concave surfaces indicate good wetting. Cold joints might even look shiny like a proper joint which makes the harder to spot than a "dry" joint. The type of solder and flux has a bearing on likelihood of cold joints, besides iron heat and use. 60/40 solder melts over a range of temperatures and stays in a plastic phase until it drops fully below the lower melting point. That can give the impression that the right temperature has been achieved because the solder melts onto the iron tip, but what temperature is the joint? The cure for that is not to melt the solder on the iron(except to wet it to prevent oxidation). Heat the joint so the deepest part of the joint interface between the wire and contact melts the solder directly. This allows solder to fill by flowing into the voids, and and have first contact with the cleaning flux.

A more common cause of joint weakness and breakage is easy to spot, the "dry" joint. This one is very common with joints that appear to be soldered well but the joint or wire moves, ever a little while in the plastic phase. It crystallizes the solder which becomes ganular and brittle. It is also likely to not even conduct. To reduce that, make a good mechanical connection of wire to terminal so the joint works even without solder, then heat it higher than the melting point so removing the iron will not be during the transition from plastic to old phase where the crystallization occurs. The joints that is "dry" is not wetting the metals and the solder surface looks dull grey or granular. Any flexing of the joint will cause the solder to crack and since there is no or little wetting of the joint metals, nothing supplies mechanical strength.

Don't resolder old joints that are defective. Remove the solder and apply new solder since the flux is needed to clean and deoxidize the new joint. Retouching joints can make good joints into either dry or cold joint and the beginner not realize it so that is why I stressed that telling people to randomly touch up all the solder joints is a bad idea just from the health of the joint standpoint, let alone what it does to the diagnostic process. Diagnostics involves reducing variables not adding to them.

Choice of solder and flux is important. If you are doing a lot of rework of joints that are not mechanically stable, do not use 60/40 since it has a wide plastic(called the " liquidus") range and any movement will be a problem. For that sort of work, 63/37 has a special property in that it melts at only one temperature, not over a range of melting temperatures, 183C and is called an eutectic alloy. Eutectic alloys have a very brief change period from solid to liquid so crystallization is very rare with that type of solder. The wetting temperature of lead/tin solder is higher than the melting temperature so you still have to be sure the flow is continued to the wetting temperature or else the bond and void filling will suffer.
If you are using lead free solder.....good luck, it is hard to solder well by hand. The temperatures involved are the same range as lead solder but the flow and wetting is not as good as leaded solder.

Too much heat ruins boards, not enough heat ruins boards(because heat has to be applied a lot longer than a hot iron) and connections.

I would think that basic knowledge of soldering and fluxes is the foundation of any electronics repair work and needs to be known before attempting any home repairs.

[ReadyTeddy]

OK, there appears to be confusion regarding terms in soldering. I was not aware that many do not distinguish between different types of solder applications. A "cold" joint is very specific in meaning. It is when soldering the workpiece is not elevated to past the liquidus temperature, or plastic phase of the solder. It is very common with beginners or those who have the habit of applying solder to the iron and then the iron to the joint. There are several causes of joint failure from this. The joint can corrode quickly or over a long period because the de-oxidizing flux has flowed to the iron and not to the joint/wire junction. The solder tends to clump on the joint but does not fully wet the joint metals. Look for convex solder surfaces. Concave surfaces indicate good wetting. Cold joints might even look shiny like a proper joint which makes the harder to spot than a "dry" joint. The type of solder and flux has a bearing on likelihood of cold joints, besides iron heat and use. 60/40 solder melts over a range of temperatures and stays in a plastic phase until it drops fully below the lower melting point. That can give the impression that the right temperature has been achieved because the solder melts onto the iron tip, but what temperature is the joint? The cure for that is not to melt the solder on the iron(except to wet it to prevent oxidation). Heat the joint so the deepest part of the joint interface between the wire and contact melts the solder directly. This allows solder to fill by flowing into the voids, and and have first contact with the cleaning flux.

A more common cause of joint weakness and breakage is easy to spot, the "dry" joint. This one is very common with joints that appear to be soldered well but the joint or wire moves, ever a little while in the plastic phase. It crystallizes the solder which becomes ganular and brittle. It is also likely to not even conduct. To reduce that, make a good mechanical connection of wire to terminal so the joint works even without solder, then heat it higher than the melting point so removing the iron will not be during the transition from plastic to old phase where the crystallization occurs. The joints that is "dry" is not wetting the metals and the solder surface looks dull grey or granular. Any flexing of the joint will cause the solder to crack and since there is no or little wetting of the joint metals, nothing supplies mechanical strength.

Don't resolder old joints that are defective. Remove the solder and apply new solder since the flux is needed to clean and deoxidize the new joint. Retouching joints can make good joints into either dry or cold joint and the beginner not realize it so that is why I stressed that telling people to randomly touch up all the solder joints is a bad idea just from the health of the joint standpoint, let alone what it does to the diagnostic process. Diagnostics involves reducing variables not adding to them.

Choice of solder and flux is important. If you are doing a lot of rework of joints that are not mechanically stable, do not use 60/40 since it has a wide plastic(called the " liquidus") range and any movement will be a problem. For that sort of work, 63/37 has a special property in that it melts at only one temperature, not over a range of melting temperatures, 183C and is called an eutectic alloy. Eutectic alloys have a very brief change period from solid to liquid so crystallization is very rare with that type of solder. The wetting temperature of lead/tin solder is higher than the melting temperature so you still have to be sure the flow is continued to the wetting temperature or else the bond and void filling will suffer.
If you are using lead free solder.....good luck, it is hard to solder well by hand. The temperatures involved are the same range as lead solder but the flow and wetting is not as good as leaded solder.

Too much heat ruins boards, not enough heat ruins boards(because heat has to be applied a lot longer than a hot iron) and connections.

I would think that basic knowledge of soldering and fluxes is the foundation of any electronics repair work and needs to be known before attempting any home repairs.

Excellent post! The only thing I might add is that if a dry soldering iron is applied to a joint to be soldered, heat transfer is often poor and you have to leave the iron on for a long time. Sometimes one lead from a component that's in the joint to be soldered may get lots of the iron's heat, and by the time the rest of the joint is heated properly that one component can have been overheated. My usual soldering technique is to touch the iron to the joint and then add a small amount of solder by briefly touching the solder to the iron/joint interface. This vastly improves heat conduction, and shortly thereafter, the balance of the solder can be added in the usual fashion. This technique always works for me. I also might add that when working with point-to-point wiring in which several leads may have been brought together, I also do a careful visual inspection to make sure the solder did in fact flow to all parts of the joint.

[g1]

OK, I guess I don't communicate well at all. From what I said to what you guys think I meant seems quite a jump to me.
About as big of a jump as if I thought you all were saying "broken solder connections are very rare in the guitar amp world"
I did not say anything about "resoldering everything" or even resoldering without first diagnosing the problem. I did say a person who did those things may have a fairly good success rate. Whether I agree with that method or not has nothing to do with my point. We all know of "tech's" who use that method, and when it does NOT solve the problem, they are probably the ones most likely to say "well, must be the transformer then".
Maybe things have changed and factory solder on boards has improved a lot. Or maybe my experience was due to the gear I worked on, which was a lot of rental equipment. Or the era (mostly 80's & 90's gear), geographical location (very cold climate=more hot/cold cycles), or some other factor. I'm not sure why, but the amount of bad solder connections on pc boards was quite high.
And I have probably been guilty of lumping in "preventative resoldering" here. After diagnosing and repairing, how many times do we resolder connections that are obviously suspect. Like pots, jacks, and tube sockets, like Enzo mentioned.
If you find all kinds of intermittent faults in a mixer due to bad solder on pots, jacks, and connectors, I would not feel confident of it being reliable without resoldering all pots, jacks and connectors. Likewise with guitar amps, after your 100th (or maybe before ) Fender H.R.or Blues whatever, you will know which solder joints are prone to go bad.

Regarding not having guitar amp experience, I guess you are right, 60 unit repairs a day being the average throughput over years with 54 brands as warranty station, a full time staff of 23 does not quality with 10s of thousands of documented and searchable defects database by model helps create an overview, in my possibly misguided opinion, has some merit. Besides my own bench work, my time was spent walking around the shop assisting in diagnosing defects so probably determined the fault cause in 10,000 units a year. That, in some circles would be called "experience".

Sorry Stan, I was not at all trying to discount your experience, which I have great respect for. For some reason I got the wrong impression somewhere that you worked primarily on studio gear, and thought that may account for the difference in the types of faults we have seen (due to less vibration and abuse).
Having said that, maybe what I said could be re-read and you will see I meant no disrespect, I just made an error with regard to the kind of gear you were dealing with:

Perhaps working primarily on studio gear you did not see bad solder to the extent that is seen in the guitar amp world.

So anyway, my main point I failed to make was that all the knee-jerk "it needs to be resoldered" advice may have some factual basis and not just be based on myth.
And apologies to diydidi, we have gotten off on a tangent here, but are hoping to hear back in regard to your svt4pro issue!

[diydidi]

No worries guys. I enjoy the discussion. I managed to get a loan working SVT for comaprison tests. Will post my findings.

[Justin Thomas]

I too have enjoyed this discussion. Very informative thread!

In regards to working on guitar amps vs. studio gear, I would imagine that more guitar amps are worked on by inexperienced owners, perhaps, than studio/hifi gear. Hey, that little amp only cost $75, so what the hell? And with the popularity of DIY guitar amps, the possibility of bad solder joints from inexperience is perhaps higher? I don't do repairs often,but most of the bad (cold, dry, broken, etc.) joints I've seen in mass-produced gear come from bad environment, improper storage, or abuse. Beer, seaspray, and gravity are not the best things for any amp...

I'm not a perfect solderer by a stretch, but I'm trying to get better at it, and I am certainly better than some, too.

Justin

[J M Fahey]

Diagnosis is the product being sold by a shop.

That's the point.

I get baffled by the common USA shop practice of charging "by the hour".

So If I'm a moron and take 3 hours to find a problem will get 6X the revenue of a bright guy who solved it in 10 minutes and charged "1/2 hour minimum"?
Don't think so.

Also, what's the point of long detailed lists like:
1 .047uF ceramic cap .... 45 cents
1 2N3391 transistor ..... 50 cents
.........................................
1 fuse ..... 20 cents
.....................
And so on.

Personally I bill; "one humming buzzing Twin Reverb amp: solved .... xxx$"
meaning I charge for "the Job", not "time+parts".

Hope this makes sense to others.

As a side note, I do not get into a useless argument on:
"Hey!!, you billed me $30 for a 6L6 !!! I can get them on EBay/GC/Tubedepot for $20 !!!!! "

As I often say customers (if they are silly enough to argue that way): "parts are FREE, I'm charging my Job"

The exception would be when there is no diagnosing made, but just a routine job, such as:
"please pull the Peavey speaker from this combo and place a Vintage 30 there".

In that case, yes, I'll sell him the speaker and charge extra for the physical act of swapping.

But that's not an actual "repair" in my book.