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Old 25th September 2013, 04:03 PM   #1
AMV8 is offline AMV8  United Kingdom
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Default Lateral mosfet Protection Characteristics

Hi
The C Audio Service notes for the XR3801 advise that the lateral mosfets date coded later than 4L3 exhibit different self-protection characteristics than previous production batches. Could anyone please tell what the change is.
I need to change a K135 mosfet in an SR606, which has the same driver circuit as the XR3801. The defective mosfet is coded 2E2 and all my replacement K135's are coded 9K1. Does anyone know what changes need to be made for output mosfet protection.

The C Audio Service Notes for the XR3801 say;

"Attention is drawn to C audio Technical Bulletin "MOSFETS". Output devices date coded 4L3 onward exhibit different self-protection characteristics from previous production batches. In order to permit the use of these devices in this unit a "re-entrant protection daughter board" has been developed (PCBA042). This PCB is fitted to the main circuit board. When an output device of date code proceding 4L3 is replaced with a more recent part, this daughter board must be fitted to the affected channel.
This is done by removing D11, D10 and R gates and fitting the mother board into the holes vacated by the above components and connecting the "flying" 0V lead from the daughter board to the 0V point on the main pcb. The unit may now be tested as usual."

D10 and D11 are 6v8/9v1 zener diode clamps fitted betweeen the Ov point and the base of the driver transistors to limit the maximum gate to source voltage applied to the output stage thereby setting a current limit for protection of the output stage. R gates is a 1K resistor fitted between the N drive and the P drive to the mosfets.
The daughter board controls the turn on and turn off of the driver transisters so as to reduce the allowed Gate to Source voltage as the drain to Source voltage increases, thereby providing closer protection of the output stage.

Does anyone know the circuit for the daughter board?

Any help would be appreciated.

Don
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Old 26th September 2013, 12:21 AM   #2
burbeck is offline burbeck  United Kingdom
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Join Date: Aug 2004
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hi Don
i have repaired a quite a few of these amps and seem to remember the service notes you speak about.
i thought this was for the plastic cased versions of the MOSFETS used in the latter type amps. something about a change in spec of the device capacitance of gate, requiring change of small caps and something about the mounting of the devices.
all this is very vauge in my mind, i do have the info but am not able to access it at this time its all paper and packed away in storage.

the SR606 was a real heavy weight that used the TO3 MOSFETS it was exellent at driving bass cabs. the only mod as i remember was the soft start upgrade.
i have replaced lots of the MOSFETS in these had no problems with the mixing of date codes, however if you have to replace them with alternatives they should be all of the same brand at least, the original Hitachi devices being the most reliable.
for a definitive answer to you question i suggest you contact one of the members here Mr Lee Basham, he is the expert on these amps as he was service manager at C-Audio, very knoleagable guy
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bob
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Old 26th September 2013, 04:19 PM   #3
AMV8 is offline AMV8  United Kingdom
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Join Date: Jan 2005
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Burbeck
Thanks for your reply.
I agree with everything you have said.
You are correct the C Audio SR606 is a heavyweight amp (25KG) using 8 paralled K135/J50 T03 version metal case mosfets. ( Whereas later C Audio designs generally used plastic versions of the lateral mosfets. )
I posted the thread as I do not know whether the internal change that Hitachi seem to have made in the mosfets applied to plastic case or to plastic and metal case mosfets as I can not find any information about the change. However C Audio did seem to think it worth making a modification to amplifiers because of the change in the mosfet protection.
I have replaced a couple of mosfets and I have only used original Hitachi mosfets. I also matched the Gvs of the replacement mosfets to 3% of the original mosfets. ( Gvs - Gate to Source voltage at which conduction between Source and Drain starts - on original parts 0.37v at Id of 2.5ma and the replacement parts with a Gvs of 0.40v at 2.5ma )
The problem I had was that the C Audio amp has so much protection built in as standard that I am finding it hard to know what else to add.
The amp already contains; flying catch diodes, natural current limiting, short circuit protection, folded drivers to help turn the mosfets on and off, zener diodes to avoid over voltage and boosted driver current supplies.
I have made a change and I will test the amp and post the result.
Don
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