Broken large ceramic Rs in combos

Mount the resistor in a custom build anodised aluminium cradle, perched on small springs with tiny hydraulic dampers to absorb any shock. Note that the springs will need to be thermally insulated from the resistor cradle to avoid any possibility of temper change through heat transference.

Hard wire the resistor to the board with mil spec extra flexible cable.

The connections should be both spot welded and soldered. The soldered connections being made in accordance with NASA specifications - available here

Re designating the guitar combo as military or aviation equipment will allow you to use tin/lead solder as per NASA specs

Hope this Helps ;)

Ron(UK)

Oh Ron, you naughty little tinker you ... :~)

Arfa

Well personally, I just resolder them.

Ron(UK)

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Perhaps it was you who "repaired" this Fender previously. I don't like bouncing repairs in the complaint sense. Whoever, resoldered an already weakened ceramic W/W lead so it broke in the pcb hole this time. Perhaps multi-stranded copper wire bulked up with swathing in solder is the material of choice for these component leads. Obviously coppery metal work hardens and fractures, brass probably does too if it moves. What metal of small cross section that is solderable, not corrodable , and survives vibration/oscillation/work hardening ?

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

I like to use gold-plated square posts on the pcb and salvaged Berg female connector pins on the fragile components leads; shock, flexing and vibration will produce other failure modes before ruining a part so mounted.

Regards,

Michael

LOL!

I guess they just don't make them like they used to? I've worked on many many combo amps and can only recall one failure as you describe and it was in my own Peavey Artist 120 circa 1975.

I bet that happened because the white gluey stuff they put on to stabilise the part had dried out and cracked off, same goes for the smoothing caps. I usually stabilise them with a squirt of high temp hot melt glue, the woodworking stuff. When it gets hot it goes softer, but doesn't run off like the clear type does.

I service amps that travel the length and breadth of the country virtually non stop being professionally used. These failures are just a part of life, beefing up one area probably means that something else will fail elsewhere. Plus of course, modern stuff is junk compared to that which was made 20/30/40 years ago

All IMHO of course

Ron(UK)

Seems to have been the cause of the failure IIRC. Have you used silicone rubber? Seems like it would be a good choice.

IMHO too.

hollow

Snap: I use what I think is the UK equivalent , 2 or 3 QM female pins slid over the long wire of on-end large Rs to make more rigid when soldered in. Can anyone identify the square section pins C,D,E in

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

graph paper is 1mm squares

Yes, silicone bath sealer works also . It just takes longer to set and they might be a slight risk of corrosion from the acetic acid, tho having said that, I`ve seen whole pcb`s potted in the stuff.

Ron(UK)

The SMEC out of my 88 Dodge Dakota was potted in silicone. I've seen it used in ham radio gear and some amps.

I too mend a lot of this gear, and for sure, you are right that the modern stuff, whilst being relatively just as expensive as that which was made in the 60s and 70s, is nowhere near as well built. That said, I too agree that failures of joints and leads on wirewound resitors is just par for the course with the stuff - as was, as is, and as ever will be. I really don't look on it as a big problem, and I'm sure that the musos just treat it all as part of the rich tapestry of life on the road. If they haven't got a spare amp with them, they just jack into the bass player's or whatever. Whilst I would never just 'repair' a leg that had broken, as I'm sure that Ron probably doesn't either, I'm sure that if the job is done properly, you are not going to get a bounce, that is going to reflect badly on you. I certainly don't see returns from doing such jobs.

I always enjoy Cooky's posts, and some of the fixes he comes up with, but really, I do sometimes think that he is seeing problems that really don't exist, and then finding fixes for them. Sorry mate, and no offense intended - just the way I see it. :~)

Arfa

Arfa

I used to pot a whole board that I used to make for a company, in a 'flowing' silicon rubber. It got in and around and under every component on the board, and then cured just like bath sealer, but without the acetic acid smell. It was designed for electronic potting, so I guess it was corrosion proof.

Arfa

Isn't 100% pure silicon just that? Or is silicon naturally acetic? Would "corrosion proof" silicon have an additive?

I think that the acetic acid thing is something to do with the curing process. Presumably, this stuff used a different chemistry to arrive at the result of it setting. I seem to recall it was a Dow Corning product, that I bought from one of the big electronic component suppliers. It came in a cartridge that you put in a standard builder's 'gun'. Came out of the nozzle with about the consistency of treacle, and just flowed around the board and self levelled. After about a half hour, during which time you had to keep an eye on it to make sure that it wasn't running off the edge of the board, it had gone to a sort of soft jelly state, and had skinned over. You could then just leave it overnight, and by the next morning, it had gone off to that really tough, but still flexible state, that you find where they've used such stuff as the insulating infill on FBTx's and triplers and the like. Unlike bath sealer, it stuck like shit to a blanket, and had a really strong peel resistance. There was no chance of repairing a board that had been covered with it - ever ...

The board was a counter that I was 'ringing' for a friend's company, and this protection method was what was used originally, so I just copied it. The counter was on food processing machines which operated in a refrigerated and generally hostile environment that was subject to washing down by hose, so the board had to be totally proof against condensation and 'wet' water.

Arfa

Is it possible that the 'vinegar smell' is some kind of mould or fungus inhibitor? I don't remember the original stuff we used to get from RadioSpares having an acetic acid smell - but I could be mistaken, it was a long time ago.

Ron(UK)

I understand that this may be PVA adhesive or something very similar.

It is actually acetic acid AIUI. In the cheap 'bathroom' silicones it's what 'makes it go off'. It's inadvisable to use this for electronics because of the acid fumes.

Electronic grades use a different and more expensive activator.

I suspect the stuff Arfa's talking about is quite different. Does it set quite hard ?

Graham