Fender HR DeVille probs...

[gtrjny99]

Hey all...
My USA HR DeVille has problems...
Have the no switching or reverb issue. Footswitch lights change but not the amp itself. I have very little power on the clean channel. I touched up the solder just to see if it would make a difference and have -16 volts across R79 but 39 V across R78... Did I lose a diode (CR11 or CR13)?

Suggestions appreciated!!

John

[Jazz P Bass]

16 Volt Zener diodes

CR13 is your +16V zener diode.
CR14 is your - 16V zener diode.
Poor man's voltage regulator.
CR 11 & 12 rectify the transformer AC to the 330 ohm 5 watt cement resistors.
Check the copper trace from R78 to CR13. I'll bet it is cracked or deformed.

[Enzo]

Look at your schematic. You have two 33v supplies with resistors to 16v zeners. SO normally that means about 16v across the zener leaving about 17v across the resistor.

If you have 39v across the one resistor, that means the far end of it is at zero volts. Now either the far end is shorted to ground, or the resistor is open. Easy to check. if the resistor is amazingly hot, the far end is shorteed to ground. If the resistor is cold, then the resistor is likely open, or the solder to it has failed. It is normal for the resistors to get pretty hot, but there si hot and then there is REALLY hot.

Check for open R78 or shorted CR13. C40 could be shoprted, but a shorted zener is far more likely. If R78 is open, it could have failed on its own, or it could be it burned out because of a shorted CR13.

[gtrjny99]

OK...

First, I really want to appreciate the help...
R78 is not hot to the touch (R79 is). I have a .6 drop across CR13. How do I test the CR13 for a short?

Is there an on line source for parts?

Thanks,
John

[R.G.]

It's amazing just how many of these amps have the same problem: overheating and death of the two power resistors and two zener diodes that provide +/- 15V to the low voltage circuits.

Power zeners and shunt regulation is a reliability disaster. Theoretically it all works, but in practice this approach to regulation usually burns up.

The reliable way to fix the HR Deville and the Blues Deville is to replace the resistor/zener with a 7815 and a 7915 voltage regulator. These regulators are self-protecting for many faults, as they include both current limiting and thermal shutdown for overloads.

Even better, the total power wasted as heat inside the amp will usually go down. This is because a resistor/zener regulator has to allow the maximum peak current the circuit needs to run through the power-waster resistor all the time. The circuit takes what it needs, and the zener eats the rest, wasting it as heat. Three terminal regulators let through only what the circuit needs for power, and this is almost always less than the peak power, so there is less power wasted.

I guess if I was a true entrepreneur, I'd cobble together a how-to booklet and package that with a pair of $0.50 regulators and sell that for about $30 to people with dead/dying HR Devilles and Blues Devilles. Maybe I should. It just irks me that something so simple wasn't put in at the factory.
=======================================
Updated. I got all motivated to go publish a definitive fix for this, and in the process discovered more about why Fender didn't do it right. Sure enough, it was cheapness, but it was more complex than I thought.

One additional reason they didn't use a three-terminal for these supplies is that they cheaped out on the power transformer, using the same 39Vac winding to make the negative bias supply as they did the +/- 16V. They used the 39Vac winding and a dual half-wave rectifier setup to make + and - 50V as a raw supply for the low voltage stuff. Taps on the winding would have done that, but cost more, as would the solution I suggest later here.

The use of the bias winding for low voltage stuff is the reason they didn't use three terminals - the easy, cheap three terminals have a 40V input voltage limit, so the dead-simple thing of tossing in a three terminal won't work. Too much voltage to wipe off. But there are simple ways to avoid this, as we see later.

The current into the low voltage stuff is small. They state on the schemo that the test point voltage is 48V and the output is 16V for both + and - sides. The max current in that 470R resistor is then (48-16)/470 = 0.06805A. That's also the min current and the average - it has to burn that all the time. The power in that resistor is 2.2W.
You'd think that would be enough, right?

But what happens to those 470R resistors when you get an accidental almost-short-circuit, or if the zener shorts out? The power is then 48V*48V/470 = 4.9W. Under that condition, the 5W resistors will have a surface temperature typically about 200C, near the melting point of solder. And the charring point of PCB material under long term exposure. Notice that the current under melting-down conditions is still small, only 48/470 = 102ma.

So we have an amp which is undergoing near-death and total-death experiences to get at most 1W of +/-16V, and this because we have a +/-48Vdc raw supply. We can't use three-terminals because the input voltage is too high. What to do, what to do?

Easy - wipe off some of that voltage. A trivial pre-regulator will keep the input voltage to the regulator down to acceptable limits. A transistor, a zener and a low power resistor make a simple pre-regulator to knock down the input current.

I'm designing up a workable setup, and will post when it's done.

[Jazz P Bass]

16 Volt 5 Watt Zener

Where are you measuring a .6 volt drop.
One end of CR13 is at 0 volts (ground) Do you have continuity fron R78 to CR13? ( I Repeat: Check the copper trace from R78 to CR13. I'll bet it is cracked or deformed.)
A zener diode can be checked with a Digital Voltage Meter. On "Diode Check" your black lead will go on the end with the stripe. Red lead on the other end.
You should read .600 or close to it on your meter.
If you get a 0.000 reading, it is shorted (not good).
If you get no reading or O.L. then it is open (also not good)
16 volt 5 watt zeners can be had at Mouser, Newark Electronics, DigiKey, MCM.......
Not a hard part to find.

[Enzo]

If the resistor is cold, it is open or not connected.

[Jazz P Bass]

cold 5 watt resistor

Right on Enzo!

[gtrjny99]

I measured

.6 across CR13
When I used the diode setting on my DMM, I noted .6 the one way and 1.24 the other.

Again, thanks to all for any help. RG, really appreicate the knowledge on the design of my cheaply built amp... Nice going FMIC

[Jazz P Bass]

Zener forward V

That looks good to me.
Is the one end at ground potential? (non stripe)
You never acknowledged a connection between the 5 watt resistor & the zener diode.
You are missing a very important piece of information.
As stated previously, the resistor is cold. That implies that is is not doing any work.
Which leads to : is it attached to anything?
please advise.
John G

[R.G.]

.6 across CR13
Again, thanks to all for any help. RG, really appreicate the knowledge on the design of my cheaply built amp...

You're not alone - there's a Blues DeVille in my garage right now.

How you think I came to know about those amps?

[gtrjny99]

OK.....

So...
I am going to check the trace from R78 to CR13 this evening... I am suspecting an open R78. Should I replace both the zener and the resistor? Should I replace the resistors with higher wattage units? Any suggestions?

Thanks,
John D.

[Enzo]

We have suggested more than once over the last few days that the resistor might be open. Have you not in that time stuck your meter across the part to find out?

[gtrjny99]

Yep...

Enzo,
I am open across the resistor. What are the thoughts for replacing the resistor. Is the diode not far behind?

Thanks,
John

[gtrjny99]

Diodes and Resistors

Hi again,
Do I need to replace the diode and resistor as a pair?

John