Modding a Fender Hot rod Deluxe R78 & R79 offboard

[waldo041]

The Problem:
http://www.fender.com/support/amp_sc..._Schematic.pdf

the Fender Hot Rod Deluxe Has 2 - 5 watt 470ohm resistors for the 16volt and -16 volt supply's to the opamps and relays in it. the problem with this is that they burn the PCB pads & traces quite often on these amps. they get the board so hot that D13(cr13) can become dislodged or fallout whacking the opamps and of course the 2 resistors themselves develop cold solder joints.


The Question's:

Can these two resistors and possibly the diode be rewired off of the main PCB, onto another "replaceable" PCB or Board? will it hinder any performance by extending the tracks with wire away from the Main PCB? will the wire get hot running inbetween the 2 boards(main and New Offboard)?

7 watt version of the resistors will not fit on the PCB, but will they run as hot as the 5 watts in the HRDx? if offboard wired could the 7 watt or 10 watt version be used?

thanks you all for the time you take to help so many others out. i sure hope someone can help me with this, as i gig quite often(maybe too much) and hope i don't have to start jumper wiring this PCB. i just replaced the opamps(with sockets). i know some don't like this amp, but i am not one of those.

any help is greatly appreciated.

peace,
waldo

[Enzo]

If you want to remote the resistors, go ahead. They aer not in the signal path, so you can put them anywhere. They get hot because they are dropping voltage across their resistance. Wires have negligible resistance, and unless they are very tiny wires, they will not get hot. The higher wattage they are, the cooler they will run. Same wattage dissipating over a larger area of resistor.

If you want to mount these few parts on a pc board, go ahead. They will make IT hot instead of the main board. If you are going to this much trouble, why not mount them on a couple terminal strips? Or for only $3-4 each get a couple of those nice machined aluminum ones like the Dale RH series. There are other brands too. These have mounting screw holes at each end. So what if they are 15 or 25 watters, they look great. I am holding a 50 watt Dale and it is only 2" long plus leads.

Yes, you can always use higher wattage resistors than are called for, just never smaller.

If you decide to remote the zeners as well, resist the urge to make a shortcut to their ground ends. RUn real wires back the the circuit board. In fact, ANYTHIN in that power supply that you remote needs to be wired back to where it came from. Otherwise the ground currents have the potential to make hum you won't like.

[TonyD]

R78 & R79 offboard modification

All you have to do is to glue a heat sink to the resistors. They have flat sides and are close enough together. Use JB Weld 2 part epoxy and glue a heat sink or heat sinks of the proper size to the resistor bodys. This will improve their heat dissipation and requires no soldering / desoldering or board removal. There is enough room for heat sink(s).

Another option to prevent solder reflow is to use a high strength solder like a silver solder (hobbyists) solder. This link talks about other solders that melt at a higher temperature. Using them on the joints for the resistors and the adjacent diode(s) (requires board removal) will prevent spontaneous melting due to heat.

http://en.wikipedia.org/wiki/Solder

[R.G.]

The use of a resistor and zener to regulate +/- 16V for opamps is a major design flaw IMHO in all of the similar Fender amps. I've finally figured out why they did this instead of using three terminal regulators. The bulk power is sometimes more than the 40Vdc that will kill a three terminal. But instead of doing the right thing, they went cheap.

Enzo is correct in general and specific on this one. You can in general always use a higher power-rated resistor, and in specific that it would work on this one. Tony is correct that heat sinking will remove heat better and keep the surface and lead temperature down. However, epoxy softens with heat, depending on the exact kind of epoxy. I would worry a lot about just gluing a heat sink on them in place. And I'd try it outside the box first and run the resistors up to full max power for a long time to be sure the epoxy didn't soften.

I am also very timid about using high temperature solder on PCBs. Traces lift when heat softens the epoxy (!) that holds the copper to the underlying glass epoxy card stock. The tin-lead "silver-bearing" solders with 2% or so silver are not enough higher melting to make a difference. Real silver solders melt at temperatures which would char the PCB stock. I would do some experiments first to be sure I wasn't killing the PCB by trying to cure a resistor problem.

I trust mechanical fastenings for heated situations more. I think that a clamped heat sink may work better. I personally like these: http://www.mouser.com/catalog/catalogUSD/643/701.pdf With a proper heat sink attached, they are good for 25-35W. There is a lot hidden in that word "proper", but it is possible. They're expensive, but not terribly. The trick is finding a stable mounting arrangement where the heat sink provides the mechanical support.

A shunt regulator (zeners are one of these) dissipate the maximum power all the time. A series regulator doesn't need to. I guess I really ought to design a drop-in replacement for these Fender amps.

[1ampman]

After a lot of thought, I mounted them off the board over under the speaker jack on standoffs. I changed them to 500 ohm 5w and gave them plenty of air and figured the standoff soldering tabs will also help cool them. I felt the thin cheap circuit board wouldn't hold heavier resistors considering the amount of vibration they get. Sooner or later they will work loose. There's no easy fix. The board traces are not strong. There's no room to bolt the resistors down. So for a permenant fix this was the only choice I felt good about. Time consuming, pain in %$#......

[TonyD]

Reply on suggestions

Valid concerns.

The epoxy is a high temp epoxy. I have performed the modification and there are no issues with temperature.

Solder? Yes. Select the solder alloy appropriate to the board material. I believe it is similar to FR-4. You can use a high temp solder such as Sb5 which raises the melting point. I use hobby solder which is mechanically, and electrically more robust.

Valid points. The mod works reliably. More points.....I'll get more specific.....
JB Weld is good to 500F.
Sb5 raises the melting point from about 325F of SN63 typically used, to about 450 to 500F for Sb5.
Typical soldering iron temperatures are set for 650F. They are quite high and can lift a trace on ANY board using any solder. The key is dwell time. I do not have an issue using Sb5, and it is my OPINION that 500F is sufficient - since it adds a margin of 150F over as designed parameters...and many of these amplifiers are in use without failure. That is as scientific as I get without thermocouple measurements.
Sb5 is quite useable on FR-4 PCB but you need to watch your dwell times and to assure all parts are tinned prior to soldering. This allows you to make the solder connection quickly enough to avoid lifting. I speak from experience.
The design (resistor heat) is marginally "underdesigned". I said heat - THERMALLY. The repairs I suggest are easy to make and improve design margin - THERMALLY. I make an assumption that the design is marginal and can be improved within safe limits with these mods. Until specific measurements are made on the resistor body or lead temps, any other discussion about adequacy of my suggestions is difficult. But I do appreciate the comments. There are many ways to improve a design - some are more robust than others...I just need adequate.
Recently I repaired an FR-4 board burned extensively around a cathode bias resistor in a tube amplifier. I replaced the resistor, clinched the leads to the far side of the board to improve attachment and thermal conduction over a larger area, and reinforced the traces on the component side with buss wire. However, here, I do not want to remove the PCB so I just added heat sinks. They keep the temp of the part low enough. I haven't seen softening of the epoxy so I believe the 500F limit is not breached.
If you want to redesign the circuit to reduce excess power dissipation or improve power management - by all means.