Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => AmpTools/Tech Tips => Topic started by: jordan86 on August 05, 2020, 02:42:55 pm
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Wanted to consult some much wiser minds.
It seems that even the simplest of repairs because unnecessarily complicated on a PCB assembly line amp. Say I wanted to replace the first two stage bypass caps on the PR Reissue with some 5uf's. Simple right? Not on this bad boy. How do you guys tackle repairs in a PCB? I presume the "right" way to do it, is to pull the pcb and access soldier joints from underneath. Is there an acceptable alternative that's any easier?
A while back, I had a friend with two leaky filter caps in his Hot Rode DeVille. He wanted to save money and venture into DIY repair, but did not want to pull the entire PCB to desolder form underneath. So we ordered the fancy new F&T caps and just clipped the old ones out. We clipped the old ones right next to the caps and left the leads in place. We bent them upward and used them as sort of makeshift turrets. Wrapped new leads around those and soldered. Ghetto, I know. He gigs it regularly though with no problems after 18 months. Is this just totally bad form, or could it work in a pinch?
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PCB repair is tedious, frustrating, done correctly, they can be just as robust as a turret board, done 1/2ask and you get what you get.
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I think you already know the answer to this. :icon_biggrin:
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I do know the answer to this :) It is ghetto and not the best way to do it. That said, you do run the risk of breaking a lot more things or creating new problems when you completely pull the PCB.
So let me ask it another way...What if you took extra precautions on this alternative to ensure solid mechanical connections? Like using an eyelet/turret to make the connection between old and new leads, like in the attached. Would this still be mechanically/electronically/structurally inferior? Or could it actually function as a time saving alternative to PCB removal? The component could be secured to the PCB for stability and the opposing tension of the two leads would pull on the eyelet in each direction.
If the only answer is "there are no alternatives", then I can accept that. I am more so asking WHAT they are, IF there are any. I presume it also is dependent on the nature of the repair as well. A small resistor or cap vs a tube socket or some other major/heavy component, etc.
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have I done it, yes;
here's the main problem, you have to heat the "old" lead to ~ the same temp as new lead, that melts the "old" solder, starting at the lead and "radiating" out. Solder doesn't really like to be re-heated. So "it works", but MTTF came down from years to months. a 3 wrap around the "post" with good crimp is a good 3 month fix for bouncin, maybe a year on the shelf
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That said, you do run the risk of breaking a lot more things or creating new problems when you completely pull the PCB.
I totally agree. Especially with these cheap chinese boards. Even if you are a good tech that really cares about his workmanship, it's a crap shoot every time you pull a board such as that PR.
But it's also a crap shoot to do what you are wanting to do. Gotta remember, that board was "wave" soldered, so not the best solder connections to begin with. Some component leads may have even been missed by the wave. When you cut the component lead near the component lead and then solder your replacement component, its possible that the original lead may heat up enough to disconnect from the solder pad. What then?
Pull the board. Take the time to do it right. And be careful.
If you want to make that PR bullet proof and easy to work on, rip out that cheap board and replace with a Hoffman PR board. Then you can easily modify with little worries.
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Great point Shooter on the unintended heating of original solder. That’s the sort of stuff I was hoping to gain insight on.
So the best solution is start with handwired to begin with. :)
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That’s the sort of stuff I was hoping
to look up, NASA soldering program :icon_biggrin:
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If you do the ghetto method (Lord knows I never have :icon_biggrin: ) clip a little clamp or alligator clip on the old lead down tight against the board, that should help dissipate the heat away from the board.