EBS FAFNER REPAIR (some advice needed)

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EBS FAFNER REPAIR (some advice needed)

Postby doombox92 » 16 Jan 2018, 15:51

Hey Folks,

An EBS FAFNER rolled into my hands for repair. The power amplifier appears to be fried. Since no schematics are available, I'm trying to reverse-engineer the relevant sections to identify the broken- and suspect components. The speaker leads had -48V DC on them (that's half the supply voltage!) and one diode seems to have exploded. I'll draw up a schematic this week and would really appreciate some advice. This is just a warm-up post I guess, the real work starts when the schematic is there.

Cheers,

dB 92
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Re: EBS FAFNER REPAIR (some advice needed)

Postby doombox92 » 16 Jan 2018, 18:37

Good and well: here's what I've traced so far. I've left out the detail that there's 5 NPN MOSFETS and 5 PNP MOSFETS paralleled to supply the output 'cause none of that should matter for principal understanding of what's going on. Going back to the symptoms, (-48VDC on the speaker output) I think the following could be the case:

- One, or all J351 PNP powermosfets are toast, and fully conductive between drain and source
-One of the flyback diodes is toasted (the ones between supplies and output)
-An element in the MOSFET driver circuit is misbehaving, forcing the output towards rail voltage. This could have been caused by a failed component ofcourse.

To add to the heap: 2SK2221 and 2SJ351 MOSFETS have been discontinued so it'll be a tough one to find matching replacements...

Let me know what you think
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Re: EBS FAFNER REPAIR (some advice needed)

Postby doombox92 » 17 Jan 2018, 09:31

Rise of the living dead MOSFETS :twisted:

I took out the powermosfets yesterday evening and placed +-12V on the dual power supply pins. The DC-voltage on the speaker output is gone (leading me to believe D9 and D10) work properly. I found this hard to assess with the components in-circuit otherwise. The DC-offset on the pads that lead to the Mosfet Gates can be controlled by a trimpot on the board and is circa at a minimum of 0.3V and a maximum of 6 V or so (the minimum voltage would not be sufficient to turn on the junctions, so if I were to sub the mosfets, the DC-voltage on the speaker out would not be there I assume)

I've got two questions:
1. D5 and D6 --and D7 and D8-- are back-to-back diodes between the Mosfet gate and the amplified output. What is the purpose of these diodes? Since they're back to back, they don't seem to have any function really.

2. Which Mosfets would be a drop-in replacement for the 2SK2221 and the 2SJ351 pair? Any suggestions?

Lastly: Some pictures to keep y'all happy
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Re: EBS FAFNER REPAIR (some advice needed)

Postby doombox92 » 19 Jan 2018, 11:00

It took a good night sleep I guess. D5,6,7,8 are of course supposed to clam the gate to the source above a threshold voltage. One 15V zener, back to back with an 1n4148.
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