The 5W resistors (two white blocks) control the channel switching relays and reverb opamps.

Printer Friendly version (text only)

The 470 ohm 5W cement resistors in the +/- 16V supply are the cause of so many problems in the Hot Rod Deluxe/Deville it's unbelieveable. Are your channels randomly switching on you? Are you getting an uncontrollable squealing/feedback sound that appears after the amp's been on a while? Is the reverb dropping in and out?

The main problem is that the resistors get so hot that they form cold solder joints. Cold solder joints are caused by physically stressed leads that are subjected to vibration and repeated heating and cooling cycles. (They are very common on PCBs which use a wave soldering method like the Hot Rod Deluxe.) If your amp acts fine when you first turn it on, but after a few minutes starts showing the aformentioned symptoms, then you likely have one or two cold solder joints. These are characterized by a dry/dull appearance; a good solder joint will appear shiny and clean.

The DC voltage from the resistors power the channel switching relays, the effects loop opamps, and reverb opamps. Once the joints get hot they'll stop conducting for random periods of time. This is what causes the channels to switch and the reverb to drop out. The cold solder joints also cause your amp to "feedback" whether a guitar is plugged in or not—particularly the More Drive and Drive channels. I think this is actually some type of oscillation, but I don't know exactly why it's caused. Still, doing this procedure fixes the problem.

Look at the picture atop the page. If we look closely we can see the foil traces under the PCB, which are tan or beige in color. Each "trace" links one "soldering pad" to another in the circuit. Notice how the 5W resistor's soldering pads are darker in color than all the others—they almost look "burned," when compared to the other traces and pads in the picture. Note: this picture was taken immediately before I replaced my resistors, which were causing my channels to involuntarily switch. So the picture above is an actual picture of an amp with 5W resistors in need of repair. Note that the 5W resistors could develop cold solder joints and not turn darker, though if yours are darker consider it another symptom pointing towards a need for this repair.

Another problem is how the resistors are prepped. Look at all the other resistors in your amp. The leads come straight out of the component, bend at a 90 degree angle somewhere on the lead, and then are soldered onto the PCB. The 5W resistor's leads are bent as soon as they come out of the body. You should never bend leads like this. This puts stress on the inside of the component and causes it to crack. Remember, some resistors could be burned up, cracked, and totally disfunctional and you'd never know from looking at the outside. The only way to test it is with a multimeter.

Now that you know why your amp's acting flaky, how can we fix it?

1. For a permanent fix you'll need to order a few parts. VintageJon, an extremely reliable tech at the FDP, wrote that Fender put out a bulletin recommending that these resistors be replaced with 330 ohm resistors of the same type—the old value was 470 ohms. Apparently Fender now uses 330 ohm resistors in newer amps to prevent this problem, but I haven't seem this implemented. Someone emailed me and said it was better to use a higher resistance rather than a low resistance, but I have not been able to confirm this yet. The disagreement is over how the resistor drops voltage. All the RadioShacks in my area do not carry any 5W resistors over 1Ω. Chances are yours won't either. It'd be good to call them first to save you the time and gas. The largest values I could find there were 10 ohms! Click here to buy better quality power resistors right off of Mouser. If you have trouble with link, then do a search from the main page. Its Part # is 280-CR5-470-RC.

2. Drain the power supply's filter caps! This is for your own safety. If you don't know how click here. Your life may depend on it!

3. If you've drained the filter caps, and I hope you did, the back panel should already be removed. Now remove the chicken head knobs, black screws, washers, nuts, etc. Whatever you have to do to get under the PCB. It's also a good idea to remove the green ground wire that's on the same side as the input jacks. This will make getting under the PCB easier. Be sure to store these parts safely so none are lost.

4. Look for the two problematic resistors (R78, R79). They are in at the bottom-center of the PCB. (They look like two white blocks, and are right next to the bias pot.) Now that you've found them on top, you'll need to find the location of their leads on the bottom of the PCB. This is essential if your fix is temporary or permanent.

5. (a) For a quick fix, "touch up" the solder joints by adding a little solder to all four joints. This will keep you problem free for awhile, but not for good. For a permanent fix: unsolder the two 470 ohm 5W resistors. You may need to clip the tips of any leads under the PCB—if they're bent in a hook shape. (Probably all of them.) If not, it'll make this step more difficult as they get hung up when you try to pull the resistors out. (b) Be sure to remove as much solder as possible with a solder sucking device. If not, you could damage the solder points which are just a thin piece of foil glued to the PCB. Then you'll find yourself running a lot of jumpers—like me. Learn from my mistakes. Also, it can get awkward trying to unsolder on one side and pull the resistor on the other. It's always nice if you can keep someone nearby to assist you.

6. Take the new 330 ohm resistors and bend the leads—but not right where they come out of the component. This is the proper way to "prep" a component. This will take stress off of the inside of the resistor, which will keep it from cracking and failing prematurely. (a) NOTE: The "prep" in the picture to the right is exaggerated for clarity. You don't need to bend your leads that far out, though you can if you want to. Just don't bend them right where they come out of the body. That's the most important thing to remember. (b) A good trick is to place a miniature screwdriver right against the body of the resistor and bend the lead around the screwdriver at a 90 degree angle. (c) This prep will be far closer to the body than you could get it with your fingers, and it will look more professional than any wildly prepped leads bent further out.

7. If you have some Silicon Rubber Sealer apply some under the resistor. Use it to prop the resistor up off of the PCB a little. This will help keep the resistor from vibrating when it's trying to dissipating heat, which in turn will prevent cold joints from reforming. Solder the resistors into place. NOTE: Be sure to give the silicon 24 hours to cure before turning on your Hot Rod. Once it has cured it will be completely inert to electrical current.

8. Screw everything (PCB, jacks, knobs) back into place. If you've done everything correctly you should be problem free for a long time.

By Justin Holton