VTM 120 molex to hardwire repair

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magnetbox
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VTM 120 molex to hardwire repair

Post by magnetbox » Wed Apr 19, 2017 2:03 pm

I recently picked up a cheap, non-working VTM 120 head. The previous owner tried to replace a flaky molex connection between the preamp and power amp board by de-soldering the connector pins from the board and hooking it back together with stranded copper wire (and cold solder joints). It worked for a while longer and then shorted out again.

I've removed the wire from the botched repair job but the 6 traces on both boards are a mess. Most of the pads are lifted at least a little bit (probably from the violent extraction of the molex pins), and one is burnt off. I've been researching common approaches for PCB rework of this type and it seems like it's something that I can tackle (I've built a couple small tube amp projects, pedals, etc). But before I get started, I wanted to get an opinion and see how others would approach it. I'd like this amp to be worthy of playing the occasional gig, so the repair needs to be sturdy.

Approach 1: cut the traces back, then make new "eyelets" with component leads and re-connect to what's left of the traces. Epoxy the bare leads to the board for sturdiness.

Approach 2: use super-glue to tack the traces back onto the board, then solder carefully. I've heard of this technique but it sounds like it might get messy and I've heard that the super glue can dissolve if exposed to too much heat. This would, however, if it worked, allow me to keep what's left of the original traces intact. Some of them are just barely lifted... almost seems like a pity to cut anything.

Other questions:

1. Solid copper wire to replace the molex ribbon-- 18 AWG?
2. If I need to epoxy a component lead to the board in place of a trace, what type / brand of epoxy should I use?
3. How the hell do I remove the power tube board? With the chassis pulled, looking at the bottom of the board, there are no visible screws. Only thing on the top of the chassis are some pop rivets on the "fins" around the 6L6 sockets.

Thanks!
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Enzo
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Re: VTM 120 molex to hardwire repair

Post by Enzo » Wed Apr 19, 2017 4:03 pm

Do you actually need to remove the power tube board? Aren't the solder sides out for you already?

In any case, you drill out 8 rivets with a 1/8" bit. And to reinstall, use 8 1/8" pop rivets from the harware store.

Rebuild the traces? WHy bother? If a pad and/or trace lifts, cut it off at the first solid point. Now bare the copper on the remianing trace somewhere convenient by scraping off the coating for about maybe half an inch. BAre a similar amount of a wire and lay it down on the bare trace and solder. Then run the other end of the wire wherever it needs to go.

For example in the top photo at the bottom, it looks like a pad and trace burnt off on their way down to the horizontal trace along teh bottom edge. Just solder to the edge trace, forget the little branch that burnt away.

SOlid or stranded is up to you, it really doesn't matter. SOlid is easier to contain sweat soldering to the traces.

magnetbox
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Re: VTM 120 molex to hardwire repair

Post by magnetbox » Wed Apr 19, 2017 4:27 pm

Enzo wrote:Do you actually need to remove the power tube board? Aren't the solder sides out for you already?
Won't remove it if I don't have to! I was thinking I'd be able to use the existing through-holes for the new wires, but insert from the component side of the board (unlike the previous repair). That's probably making things more complicated than they need to be, though, given the available space in the chassis and the lifted pads.
Rebuild the traces? WHy bother? If a pad and/or trace lifts, cut it off at the first solid point. Now bare the copper on the remianing trace somewhere convenient by scraping off the coating for about maybe half an inch. BAre a similar amount of a wire and lay it down on the bare trace and solder. Then run the other end of the wire wherever it needs to go.

For example in the top photo at the bottom, it looks like a pad and trace burnt off on their way down to the horizontal trace along teh bottom edge. Just solder to the edge trace, forget the little branch that burnt away.
Cool, thanks. That sounds like a much simpler way of doing it. I've been looking at guides like this:http://www.circuitrework.com/guides/4-0.html But wasn't sure what would be practical for something like a guitar amp.

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Enzo
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Re: VTM 120 molex to hardwire repair

Post by Enzo » Wed Apr 19, 2017 8:31 pm

There are situations where rebuilding the board is required, whether by contract or by regulation. But this is just a guitar amp, not a NASA laboratory machine. Parts and technology to do that certainly exist, but really not needed here.

magnetbox
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Re: VTM 120 molex to hardwire repair

Post by magnetbox » Sun Apr 23, 2017 10:23 pm

Okay! Today I clipped the lifted traces, scratched out some new pads and soldered new solid 20 AWG leads between the connectors. I double-checked everything pretty well, check continuity to the next point on each side of the trace, etc. But of course, no luck when I plugged it in. No sound, pre and power tubes alike won't light.

What should I check next? I don't see any other damaged components on the boards. No evidence of burns where the power supply board connects.

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Enzo
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Re: VTM 120 molex to hardwire repair

Post by Enzo » Mon Apr 24, 2017 12:33 am

OK, so the obvious wiring issues are solved, but more is wrong. SO the tubes need heater current, find out where the supply lines are broken. perhaps at the other board where that ribbon came from.

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Re: VTM 120 molex to hardwire repair

Post by magnetbox » Mon Apr 24, 2017 11:47 am

Thanks, yeah heater current is what I'm investigating next. The 4-connector ribbon from the power supply board looks visually okay but I haven't put a meter across it. Also going to check that ceramic fuse next to the big caps.

Just found this old thread, great stuff:

http://music-electronics-forum.com/t13429/

magnetbox
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Re: VTM 120 molex to hardwire repair

Post by magnetbox » Tue Apr 25, 2017 10:12 pm

Okay, found that the 8 amp ceramic fuse on the power supply board seems bad... no continuity. Got some on order. Seems like an odd enough size / type that the regular places don't carry it. Let's hope that's all it needs to get back up and running!

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Enzo
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Re: VTM 120 molex to hardwire repair

Post by Enzo » Wed Apr 26, 2017 11:15 am

You can use a glass fuse.

magnetbox
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Re: VTM 120 molex to hardwire repair

Post by magnetbox » Wed Apr 26, 2017 12:39 pm

I found some slow-blow / time delay 8 amp glass fuses at the hardware store but didn't think they'd be appropriate to replace a standard ceramic fuse... that would be okay in this application? Not sure I saw any "fast acting" glass fuses locally.

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Re: VTM 120 molex to hardwire repair

Post by magnetbox » Mon May 08, 2017 6:45 pm

Finally got the correct fuses and fired it up... it works! Replaced a broken 1M spider pot too.

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Re: VTM 120 molex to hardwire repair

Post by Nibbles » Wed May 31, 2017 2:52 am

It's great to hear you got it up and running. I've had a VTM 60 since 2004 and love it.
Bun Splitter

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