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SVT-3Pro crackling

tchristian

tchristian

Nov 18, 2011
277
300
Missoula, MT
#1
Been trying to track down a minor issue with a new-to-me SVT-3Pro (May 2003). It crackles very lightly once in awhile while idling. Probably while playing too but can't be heard. Not a bias issue. (Been over that and am in good shape there.)

Tapping lightly on components and tubes with a small plastic handled screwdriver didn't reveal anything - to me at least. So I pulled the preamp board and used a magnifier to scrutinize the solder joints. I found a problem with the frequency selector:
SVT-3Pro preamp board 2.JPG
The selector had a "bubbled" solder blob at position 5. When I wiggled the switch gently it was obvious that solder joint was broken. I heated the joint gently with the smallest tip I have but it didn't flow, so I used a solder-sucker and drew the blob off. At that point it looked like the trace was damaged. I tried anyway to flow a little solder into it, but that didn't work.

My question is this: Can I leave that leg of the selector disconnected? There is no continuity between that leg of the switch and the neighboring contact to which it is supposed to be connected. (C34 I think but that whole part of the board is covered in hot melt.)

Is it a dumb idea to consider using a patch of some kind?

I need a dependable amp for gigging. I was hoping this would be it.

many thanks,
theo
 
#5
their is various methods to repair the trace

and yes a bridge needs to be made

the board is covered in a solder mask.so it has to be carefully scraped off the trace to expose the copper so you can add a wire bridge. unfortunately the traces are extremely thin, so removing the mask can damage them and heat applied for too long can lift them from the board. which is your case.

without going into great detail an internet search on trace repair could help. or of course have someone else do it. it's really not that difficult. it just requires a careful and patient approach.
and using a clean a very hot iron so the solder flows quickly and not a lot of heat is held for too long to the board.
you can take an old kids toy apart or a old vcr and practice removing the mask and soldering to thin traces usually those are loaded with thin cheap boards and traces.
 
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#6
JimmyM said:
I would have it reflowed by someone who knows how to do it, then let them check it out for any other potential issues. Even if I didn't use position 5, it would bug me to no end if it didn't work.
Click to expand...

I hear you, especially the "bug me no end" part! That's what made me open this amp up in the first place. (no no teddy :rollno:) One of my other problems is the terrible luck my friends and I have had with local electronics repair people. Big money for inferior work, recurring issues after expensive repairs . . . So I take stuff apart and try to become something I'm not.

Reminds of my grandma's classic Hoover vacuum I disassembled in my late teens. That was the coolest thing I'd seen. Just wanted to see what made it tick. All it took was a few simple tools to get it lying in a million pieces on my living room floor . . . . . . . .
 
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#8
BogeyBass said:
their is various methods to repair the trace

and yes a bridge needs to be made

the board is covered in a solder mask.so it has to be carefully scraped off the trace to expose the copper so you can add a wire bridge. unfortunately the traces are extremely thin, so removing the mask can damage them and heat applied for too long can lift them from the board. which is your case.

without going into great detail an internet search on trace repair could help. or of course have someone else do it. it's really not that difficult. it just requires a careful and patient approach.
and using a clean a very hot iron so the solder flows quickly and not a lot of heat is held for too long to the board.
you can take an old kids toy apart or a old vcr and practice removing the mask and soldering to thin traces usually those are loaded with thin cheap boards and traces.
Click to expand...

Many thanks for this line of thought. I have a microscope (thanks Charlie!), so I can see clearly what has happened. I just tried making a solder bridge between the stud and the broken trace, but the gap is a little too wide. So I'll find a piece of fine, single strand tinned wire and make a jumper.

Two of the other traces on this switch are lifted off the board but still electrically intact. It comes, I'm sure, from the switch having been physically rocked or pushed. I don't think it was me; I haven't had this board out until now. Hopefully I can patch it up and get it back in without breaking them. I'm learning to respect the fragile nature of a board with hardware that is mounted to both a PCB and the chassis. At least I'm learning something.

I'll let y'all know how it goes.
 
#9
well that's good and bad at the same time if other traces have lifted from normal use. the twisting and torque of operating the switch lifting traces. so good= probably found the crackling problem. bad most likely the repair might fail again over time.

a panel mounted switch could be added and wire leads attached to connect to the board.. unfortunately the board mounting method might make that impossible.

possibly add extra washer and locking star washer to the back and front of switch so it's firmly mounted to the panel and torque doesn't get applied to
the switch n bboard. or run small bead of super glue so the switch is attached more firmly to the board. it's possible a previous repair was done and some mounting hardware was left out.
 
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#10
I agree there was some improper force applied somewhere down the line. It's a 6-legged switch that feels strong against rotational force. But it's fairly weak and wobbly in the "rocking up/down" plane. Also, I notice that all the pots on this and my son's 3Pro lie in a straight line except the frequency switch, which needs a washer/spacer to make it line up. When installing the board, if the spacer is left out, after the pots are all snugged onto the chassis, the frequency switch sits back a sixteenth or so. I has to rock forward as the nut is tightened down. That's the motion that I believe caused this damage in the first place.

I hope that once I make the patch and install the board firmly, the switch - with it's damaged traces - will be solid.
 
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#11
Back together - I soldered a single strand of copper between the lug on the switch and the leg of C34 (presumably). I covered it with a thin layer of hot melt, I guess in case that wire ever broke - won't swing loose and short.

All 5 mid-frequency ranges work. Time will tell if it lasts.

BTW, still have the damn crackle! :banghead: Just have to roll with it for now and keep a backup amp handy. Grrr
 
#12
26 gauge jumper wires to the nearest accessible component lead connected to the same trace. Be careful though, I see more amps damaged further by failed repair attempts and in some cases they become no longer repairable because of the extent (or location) of the additional damage.

Guys, we are losing the quality repair techs in the quest for cheaper and cheaper. Support the good techs out there, and if a tech can't cut it, and can't/won't make it right, they need to be held accountable IMO. Think about where this might end up for you in 5 years when you really do need a good tech...
 
#13
I appreciate where you're coming from. Here's the abbreviated story line behind my choices:

For the last 30 years in Missoula, MT, I haven't found anyone with enough integrity to do a professional repair for a reasonable cost. I've tried, and so have some of my friends.

So, I looked online for a trace repair kit. I have enough technical skill and professional level tools and equipment to pull off a repair like that. They want $200 for the only kit I could find without wasting half my day with internet search engines. (I think I read somewhere on here that a populated preamp board for the 3PRO is available for about 200 bucks.)

So I kloodged it. It came off clean enough not to have caused further damage; the real damage was already there. When the crackling becomes unbearable, or the frequency selector fails, I will most likely take on the repair again if I can find the right materials to work with.

Many thanks Horse and BogeyBass and Jimmy and all the others who contribute here. It means a lot to me.
 
#14
Trace repair products that include conductive inks and such are inappropriate for products subject to lots of mechanical vibrations. A skilled tech will know this and maintain an inventory of suitable repair materials such as very fine Kynar insulated wire, teflon insulated wire in 26-28 gauge, bare bridging wire, etc.

I talk with techs all the time. The good ones are pretty burned out with this industry and are leaving when presented with a "better" opportunity. This leaves fewer good options, and getting fewer every year. To be a really good tech requires an education, intuitive troubleshooting skills, good mechanical aptitude, and a tough skin when handling customers who are sure they can do it better/cheaper because the Internet says they can fix it themselves! A little knowledge or false assumptions, when misused, can be dangerous, and highly misleading.
 
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#15
agedhorse said:
I talk with techs all the time. The good ones are pretty burned out with this industry and are leaving when presented with a "better" opportunity. This leaves fewer good options, and getting fewer every year. To be a really good tech requires an education, intuitive troubleshooting skills, good mechanical aptitude, and a tough skin when handling customers who are sure they can do it better/cheaper because the Internet says they can fix it themselves! A little knowledge or false assumptions, when misused, can be dangerous, and highly misleading.
Click to expand...

This has become prevalent in most trades and professions. Many seem to lack intuition and aptitude in their field and try to get by on their education alone. This is further compounded when those that possess only 90% of their trade/professional knowledge become the teachers. They then go on to impart only 90% of their knowledge. Teaching 90% of 90% is never a winning combo. I come from an aviation background where the results of this are very scary. To call me "crusty" as a sim or flight deck instructor was just the tip of the name calling. I was pretty unforgiving in my mission debriefs.

Sorry about the derail OP. I hope your repair continues to work for you. I'm very prone to repairing most of my possessions as well.
 
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#16
quickfix said:
This has become prevalent in most trades and professions. Many seem to lack intuition and aptitude in their field and try to get by on their education alone. This is further compounded when those that possess only 90% of their trade/professional knowledge become the teachers. They then go on to impart only 90% of their knowledge. Teaching 90% of 90% is never a winning combo. I come from an aviation background where the results of this are very scary. To call me "crusty" as a sim or flight deck instructor was just the tip of the name calling. I was pretty unforgiving in my mission debriefs.

Sorry about the derail OP. I hope your repair continues to work for you. I'm very prone to repairing most of my possessions as well.
Click to expand...
Agreed. A very good friend of mine has his general, power plant and airframe certifications as well as 25 years in the field working as a factory service mechanic for a major manufacturer of small business/commercial jets. He is detail oriented, meticulous, does't skip steps, doesn't jump to conclusions based on internet myths, and always welcomes double-checking of work. He is now a shift/shop supervisor because the planes he works on do not have a history of coming back needing additional work. He is also not well liked as a "buddy" because he expects the work to be done right the first time (helps him sleep at night) and will call out shoddy work without blinking an eye. So, in spite of his personality, who would you want working on the aircraft you are going to fly in?
 
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