Unstacking A Terror Bass And Looking For Tin Whiskers

An Orange Terror Bass amp came in with a very strange complaint. The owner indicated that the treble control was inoperative.

At first I chalked it up to drama but when I got it back to the shop a couple of things became obvious-the treble control did, indeed not work and had no noticeable effect, and second of all the true genius of Leo Fender's designs.

For those of you who are unfamiliar with this amp it is a hybrid with a tube preamp that's (allegedly) the same as their AD200 bass amp, and a 500w Class D solid state power amplifier. I still haven't figured out how they do this Class D business or pack all that power into a lunchbox amp but nevermind. They say it, I believe it, and that settles it-more or less.

The tone stack in the tube preamp is a direct copy of the same section in the 5F6A Bassman as a gander here will show, with a few minor component value changes. Here's the Bassman.

Well. The similarities are there, and this combination of high and low pass filters does a very nice job. Particularly when you figure that this design is better than sixty years old, it speaks to the genius of the designer-one Clarence Leonidas Fender-that has not been surpassed.

It's homage in a way that Leo's designs live on in the newest amps-or relatively new amps.

So. After consulting with the Brain Trust and studying the schematic, an order was placed with Digikey for three pots of the correct size, value and termination points. No small thing when you start shopping for potentiometers I may say. Digikey's on line catalog is excellent because the graphics are clear and sharp, they let you see exactly what you're ordering, and they're only two days away, being in western Minnesota.

Also ordered at the same time were a pair of .047uf capacitors and a 470 pf ceramic disk capacitor to replace the 120pf capacitor as I figured it was not a bad time to revoice the amp for slightly better bass response. These had to be relatively small to fit in the space allotted to them, but at least the board is keyed to the schematic diagram so at least I knew what to remove and replace.

For that task I used my handy dandy ZD915 solder removing machine. It came from Marlin P. Jones Associates, and there is a good youtube review of this piece of gear on the EEVblog. It goes by the trade name Rhino in Oz, but it's the same piece of gear. It ran me about $120 US, and I ordered plenty of spare tips, filters, and a spare gun which was only $20.  I do not know how I got along without it, as the manual solder suckers I'd been using could lift pads on Fender boards quite easily with their recoil. Not a good thing.

Keep the ZD915 clean and you can use it on leaded solder or lead free as you wish. It helps to put a dab of leaded solder on the connector before you use the sucker-that or some flux is fine, but either makes the lead free solder's exit from the scene much more easy and a lot less drama to boot.



After reassembly everything worked swimmingly and it went back to its owner today.  I will likely autopsy the pots to see what went haywire. Some of my colleagues suspect that it was a case of lead free solder growing tin whiskers and shorting the treble pot out. I will do an autopsy with my 12x lamp and see what I find. Failure analysis is always fun stuff.

Tin whiskers are real, not fanciful, and they are microscopic fibers that grow out of the tin used in lead free solder. Some folks choose to believe they're not real, but I have seen microscopic images of them and have heard reports from other technicians who should know they're a real phenomenon in the age of RoHS compliant electronic products.

You can read all about them here