Blew diode on the Ortec 490B

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Dennis P Brown
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Blew diode on the Ortec 490B

Post by Dennis P Brown »

The coaxial I installed for the input apparently had issues* because it shorted out the unit (smoke.) A diode (I believe) was fired (and broken). I have no idea what diode to replace this one with. If some one has a schematic diagram of such a unit that would be extremely helpful.

I include a picture of the diode and the circuit board. The diode connects (marked as a thick red line) between two soldered lines of -24 (volts)

This a Ortec 490B Amp unit.

I assume a diode and I would speculate that it provides -24 volts to that other soldered line but no easy way to know w/o schematics.

I had a 10x reduction connector on one end. Was using it for something else and didn't think that was an issue. Either very wrong or the unit had a defect.
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Richard Hull
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Re: Blew diode on the Ortec 490B

Post by Richard Hull »

I think you have a bad tantalum cap somewhere in the -24 volt line. That is more likely the brother of those two side by side low ohm resistors. Are you positive it is a diode?

Richard Hull
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Rich Gorski
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Re: Blew diode on the Ortec 490B

Post by Rich Gorski »

Dennis,

Found this online. Hope it helps. Hard to read but based on your photos I think that's resistor R102 rated at 10 ohms.
ortec_490B-AmpSCA.pdf
(879.68 KiB) Downloaded 129 times
In the schematic (last page) R102 location is near the bottom just above where it says "output driver stage"

Good luck.

Rich G.
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Richard Hull
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Re: Blew diode on the Ortec 490B

Post by Richard Hull »

Do not replace that resistor before you find the bad capacitor, or smoke will return. It is used as a cheap fuse for the -24 volt supply line.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Dennis P Brown
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Re: Blew diode on the Ortec 490B

Post by Dennis P Brown »

Thank you Rich and Richard. I searched online and got nothing on the manual - can't say enough how useful this forum can be and how helpful everyone is here.

The schematics that Rich was so kind as to provide, does show a R102 (so NOT a diode; so Richard was correct. It must have burned black) in that approximant location. The circuit section say's 10. So I assume that means a 10 ohm resister (1/4 watt by spec. on schematic). Wow, that is a low value but there are a few resisters that also say 10 as well. Interesting fuse arrangement, I guess. It does connect directly to the -24 volt line so that follows the circuit logic, as well.

From the schematic, this fed a lot of the transistors so would certainly cause the unit not to function.

This is the "Input Driver Stage" (bottom most section, just right of center on the schematic.)

Guess with that low a resistance value, it makes sense that it dies in such a dramatic fashion if a short occurred for the input line.
Last edited by Dennis P Brown on Thu May 18, 2023 11:22 am, edited 3 times in total.
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Dennis P Brown
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Re: Blew diode on the Ortec 490B

Post by Dennis P Brown »

Richard is correct, that resister is there mainly as a 'cheap' fuse. So, I checked all the caps that were on that line and none appeared bad (so, good advice to check for that problem - never would have guessed that could lead to such an issue. Knowing about that is a nice bit of info.) Not finding any issues with any related caps, so I went ahead and replaced the resister. In hind-sight, I am fairly certain the coaxial cable caused the short.

So, after the repair, success on the Ortec amp working and better still, no smoke this time ;) .

The 10 ohm resister was correct value.

Ran it with a pulse generator and later with the Ortec preamp. Everything on the amp side appears to work. The SCA mode does not appear to work for amplification in a manner that fully makes sense to me but that is a bridge too far to concern me currently. But the amp output works exactly as it should.

Thanks again, was lost with what component that destroyed item was and that schematic was just what I needed. :)
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Richard Hull
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Re: Blew diode on the Ortec 490B

Post by Richard Hull »

A 1 ohm short on the minus 24 volt line would put about 2 amps of current at 24 volts across the 10 ohm 1/2 watt resistor. That is 48 watts heating the 1/2 watt resistor. smoke and fire is the norm here.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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