Diodes and resistors from hell

NORAA · 8th May 2006, 03:44 PM

Hi, guys,

I am trying to fix a fender m80 solid state amp. It works, but there is one section of the board that is literally burning hot.

If you would please look at the partial schematic, my problems are resistors #94 and #95, and zener diodes CR 25 and CR 26. All the other resistors, diodes, and caps test OK (I think I know what I'm doing ).

All the voltages are OK. No other components on the board are even warm. I've replace the four components (although I had to put in 1N5352 instead of the original 1N5353 zener diodes), and the problem is still there.

I'm assuming that there aren't supposed to be burning hot components on the board!!

OK, I've thrown down the gauntlet...please help!!

Netlist · 8th May 2006, 04:04 PM

NORAA,
Looking at the power ratings of the resistors and zeners (5W) it could be normal behaviour.
If possible, I try to mount them elevated from the PCB or at least resolder them. What current is running trough the R's?

/Hugo

anatech · 8th May 2006, 05:17 PM

Hi NORAA,
Yup, they will be hot as heck. 50 V (ish) dropping to 16 V through a 330R resistor means the diode will dissipate 1.65W with no other load connected. The resistor will always dissipate 3.5 W. That is very hot.

How much is the current draw from the op amps? I'll assume that is all that the circuit supplies power to. A voltage regulator stage might be more intelligent.

-Chris

Edit: Hugo, just over 100 mA. C48 and C49 should be rated at 63V. They may be opening up by now.

djk · 8th May 2006, 06:46 PM

Most 5W resistors de-rate to zero at 250*C, some are 275*C.

This would indicate an operating temperature at 3.7W of about 185*C, enough to cook the board and melt ordinary solder.

anatech · 8th May 2006, 06:52 PM

Hi djk,
Exactly. I wonder who designed this one.

Oh, ° (ALT-0176)

-Chris

NORAA · 9th May 2006, 04:21 PM

you guys are amazing...since this is a Fender amplifier, for a company in business for around sixty years, I thought for they knew what they're doing, and I was sure there was a problem with the amp....I guess a problem in the design stage is a problem with the amp....

Well, if I was an electrical engineer, I don't think I would design a amp circuit that could double as a frying pan!!

thanks a zillion..........

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8th May 2006, 04:04 PM	#2
Netlist diyAudio Moderator Join Date: Jan 2003	NORAA, Looking at the power ratings of the resistors and zeners (5W) it could be normal behaviour. If possible, I try to mount them elevated from the PCB or at least resolder them. What current is running trough the R's? /Hugo

8th May 2006, 05:17 PM	#3
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi NORAA, Yup, they will be hot as heck. 50 V (ish) dropping to 16 V through a 330R resistor means the diode will dissipate 1.65W with no other load connected. The resistor will always dissipate 3.5 W. That is very hot. How much is the current draw from the op amps? I'll assume that is all that the circuit supplies power to. A voltage regulator stage might be more intelligent. -Chris Edit: Hugo, just over 100 mA. C48 and C49 should be rated at 63V. They may be opening up by now.

8th May 2006, 06:46 PM	#4
djk diyAudio Member Join Date: Feb 2001 Location: USA	Most 5W resistors de-rate to zero at 250C, some are 275C. This would indicate an operating temperature at 3.7W of about 185*C, enough to cook the board and melt ordinary solder.

8th May 2006, 06:52 PM	#5
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi djk, Exactly. I wonder who designed this one. Oh, ° (ALT-0176) -Chris

9th May 2006, 04:21 PM	#6
NORAA diyAudio Member Join Date: May 2006	you guys are amazing...since this is a Fender amplifier, for a company in business for around sixty years, I thought for they knew what they're doing, and I was sure there was a problem with the amp....I guess a problem in the design stage is a problem with the amp.... Well, if I was an electrical engineer, I don't think I would design a amp circuit that could double as a frying pan!! thanks a zillion..........

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