Fender Hotrod Series Amps
Updated: Nov 10, 2021
I've had a few of these Fender "hotrod" reissue series amplifiers in for service and repair recently, and thought it was worth sharing some of what I learned about the inner workings and what goes wrong.
The range covers the "Deluxe", "Deville" and "Blues Junior" models. They are all PCB layout designs, loosely based on historic amps from Fender's golden age.
I'll spare you loads of detail about the history and different models, as this is extensively covered in sales articles all over the web, but having worked on a few of these amps recently, and read a lot of info on them - here are my thoughts from a repair / service man perspective:
On removing the rear panel on this Hotrod deluxe, the first thing that I noticed (and nobody could miss it!), was the discoloration / burning around these two ceramic resistors. It looks awful, and usually when you see things like this in an amplifier, it's the symptom of some catastrophic failure or short-circuit. In this case, surprisingly, it's a very common fault, and is caused by poor design.
The componets responible for the mess are the large ceramic resistors R78 and R79 on this drawing. Fender needed plus and minus 16volt supplies to power the reverb driver chips, and the control circuit for the channel switching. They had +- 48Volts from the transformer, for the bias circuit, so they used these five watt resistors in conjunction with zener diodes CR13 and CR14 as a way to drop the voltage down to the +- 16Volts required, effectively dumping the excess voltage (and thus power) as heat. There are elegant ways to do this without creating excessive heat, but they cost a little more in components and board space to accomplish. So having decided on this method, the designers then compounded the error by mounting the devices tight to the PCB, where they could cause maximum damage to the pads and tracks in the area - not clever.
Here's the back of the board, which I've already started to repair. The round solder pads came away from the PCB when the resistors were removed, so there's nothing there strong enough to solder new resistors to. The best option available was to mount replacement resistors well away from the PCB on their own little prototype board, and connect to the circuit with the black and red wires you see above, preparing the PCB track far enough back to get a decent solder joint. At this point, the circuit was still actually working, but it would not have been long before the tracks failed, or there was so much carbon within the PCB that it became conductive - I've not experienced this, but have heard that it can happen - I wouldn't like to predict the consequences of this, but they would certainly be exciting and expensive.
If the damage is not yet too extensive, as on the amp I was servicing last week, and the pads on the back of the circuit board are still well attached to the PCB, then it's possible to replace the resistors, and remount the new ones above the board as pictured. This allows airflow between the resistor and the board, and should prevent the overheating problem. As the rear of the board needs to be accessed, this is best carried out when a service and filter capacitor replacement job is taking place, as the removal of all the knobs, etc. to allow the board to come forward is time consuming.
If the overheating of the board is extensive enough to necessitate the repair of tracks and pads the the board, then it's prudent to replace, or at least check the caps and diodes in the immediate vicinity too.
The filter capacitors are the four large, originally blue or grey cylinders on the main PCB. They carry out two very important functions in the amp.
Firstly to filter out the 100Hz (120Hz in the USA) AC ripple voltage which is left on the DC power supply after it has been "rectified" from the AC voltage coming off the mains transformer. If this ripple voltage gets to the valves you will hear it in the speaker as a hum.
Secondly, these capacitors act as a reservoir of power, which is drawn during transient peaks in the signal, and enables the use of a smaller mains transformer.
Filter capacitors in valve amps should be considered to be a consumable. As a rule of thumb, if a capacitor is over 15 years old (regardless of the amount of use) it's wise to replace them. In reality, most amp owners are unaware of the limited lifespan of these components, and will only consider having an amp serviced when it is displaying problems. Capacitor-related fault symptoms include increased mains hum, intermittent buzzing, instability of gain stages, loss of dynamics ("flat" sounding?)
Failing capacitors can cause damage too, if they start to leak DC, they can pull too much power through the power transformer, which can burn out, if the fuse doesn't blow first. This could be quite expensive to replace when compared with just a few pounds each for new capacitors.
You can spend a lot of money on valve amp capacitors, but I've never been convinced that there's any point. I use F&T capacitors unless a customer specifies otherwise, which have proved reliable and cost-effective. There's a hell of a lot of snake-oil in the audio trade, but that discussion (rant) is best left for a separate blog.
If you remove the back panel of your amp (after unplugging and waiting a few minutes for the caps to discharge) and your capacitors have some beigey-brown gunk around the terminal lead, like the ones in the pictures, or you suspect they are more than 15 years old then it's definitely time to have them replaced before they cause problems and potential damage.
Like a lot of amps with board-mounted control potentiometers (pots) and input / output jack sockets, the hotrod series suffers from the longevity of the solder joints. The example pictured is from a recent Hotrod deluxe and is the hot pin of one of the input jack sockets. There was crackling when the lead / connector was moved, which is often attributed to dirty contacts within the socket, but in this case was down to this spectacularly failed solder joint. The failure is caused by the difference in expansion of the socket pin, and the solder itself. Normally you can just see a tiny circular crack, but in this case it was a bit more obvious! Generally, the larger the component leg, and the closer to the source of heat, the more likely it is that the joint will suffer, so the valve sockets are prone too.
Repair is a simple and and fairly quick procedure. involving heating the joint and introducing a little fresh solder. I check and re-flow all joints to pots, jack sockets and valve bases as part of a standard service on these amps, and always use a special solder flux designed specifically for re-flowing old solder joints, which contributes to a nice clean,shiny, solid connection which will be good for many years.