Fender Rocpro 700 problem

[beerbunny]

For some time, my Rocpro has been making a hissing/rushing noise after it's been on for a while. Kind of like the sea breaking on the shore.

Eventually, I decided to investigate, expecting to find that one of the op-amps had gone noisy. What I actually found was that CR29 and CR30, both 16V 5W Zener diodes, had got hot enough to unsolder themselves from the PCB and were lying in the bottom of the chassis. I tried soldering them back in, and they unsoldered themselves again. The noise I was getting was presumably one or more of the op-amps complaining about the supply being too high (these diodes regulate the op-amp +/-16V supplies). Another result of the supplies being too high was that C53 and C54 were bulging. This is probably because they're both 16V components and would have been running at too high a voltage. If I'd been designing this circuit, I'd have used 25V caps, but I suppose the 16V ones are just that little bit cheaper.

Although I couldn't find anything electrically wrong with them, I replaced D29, D30, C53 and C54. I now find that the replacement diodes are getting too hot to touch after a while. I've now put extra 16V 5W zeners in parallel with D29 and D30, not an elegant solution, but it seems to be helping, although all four of the diodes are getting hotter than I'd like.

My questions are:- Is there a design fault with these amps that anybody knows of, and if so, is there a cure?

I've had this amp from new, btw.

Thanks in advance,

John

[Jazz P Bass]

Did you test the voltage that is feeding the zeners.
There are two dropping resistors after the stated 40V supply.
120 ohm 10 w & a 47 ohm 5 watt.
Measure the drop across both resistors on the + & - 16v rails.
There should be a similar drop across them on both rails.
Also, it would be nice to know the effective resistance that the opamps are presenting to the circuit.
You may have a loaded down rail.

[beerbunny]

Hi, and thanks for the come-back.

The only resistors between the +/-40V rails and the +/-16V rails are 2 470ohm 5W resistors in parallel (235ohms) on each side. I haven't checked the drop across these resistors recently, but I did when I first started investigating the problem, and everything looked okay (40V on one side, 16 on the other, giving a 24V drop across 235ohms =102mA). This was a couple of years ago, so I can't remember the exact figures. I'll check again.

The way I see it, the only way the zeners could get too hot is if they're passing too much current i.e. the rails are insufficiently loaded. If the load was too high (which is what I understood you to mean), the zeners would be doing less work, so they would get less hot than they should.

I've just remembered why this problem is puzzling me; a 16V 5W zener should be able to pass 312.5mA, but as I mentioned above, 24V across 235 ohms gives a current of 102mA, so even if the zener was passing all 102mA, it should only be running at a third of its wattage rating. So why did these two zeners get hot enough to melt solder? The only thing I could think of was that the zeners were faulty, so I replaced them. Unfortunately, this doesn't seem to have had the desired effect.

Cheers,

John

[Jazz P Bass]

My thoughts were more along the lines of "what is the current draw" downstream of the zener circuit.

[beerbunny]

I'll look into that when I've got some time free. But, as I wrote earlier, the only way the load (numerous opamps and the 12AX7 filament) could be causing the zeners to overheat is if it's drawing insufficient current. And, as I also wrote, the zeners should be able to cope quite happily even if the load was removed completely and the whole of the current (102mA) was flowing through them. They'd still only be dissipating just over 1.6 watts, which they should be easily able to do.

I should perhaps point out that I was an electronics technician/engineer for 25 years, right up until my retirement. Having said that, I suppose I can't rule out the possibility that my understanding of how zeners work is flawed. Obviously, I don't think it is, but then I wouldn't, would I?

To get back to the subject, I'll get back to youwhen I've done some more checking. This may get a little difficult, as the pcb has been somewhat charred by the overheating zeners, and the tracks are lifting off in that area.

Thanks again for your time.

John

[Jazz P Bass]

Here is a nice link for a zener voltage regulator circuit.
The only way that I can see a high current through the zener is by increasing the load resistance.
To be sure, the current limiting resistor will not be the same as 235 ohms.
250 ma's will put the zener at 4 watts.
Although the current resistor will change to 96 ohms at 6 watts.
Link: Zener Diode Voltage Regulator - Electric Circuit
I poked in Fenders values & came up with 1.63 watts through the zener.
That calculates to a 102ma load draw.