Yeah, that looks horrible. I'd clean up all the sockets and remove all that gunk, especially around the gate pins. Then use some 'less runny' heatsink compound (the white stuff?) and re-mount in a clean fashion. Obviously you'd want to short the MOSFET's out with a crocodile clip while doing that so that you don't zap them with ESD. If it is aging MOSFET's then increasing the gate stoppers a bit may help. But first I'd sort out the mess. Stray R/C in abundance.
I second what Kram's said. It's probably had a hard life chugging away in a club and would benefit a strip-down / rebuild. If you don't own somethng like the Peak tester, then get one. Testing the FETs one by one has to be the first thing to do here. These O/P devices are used in many PA amps from the '80s / '90s, and failure is common (not because they aren't rugged, but simply due to abuse over time). The C-Audio SR range use the same devices and the 1st thing to do when one fails is to test O/Ps one by one.
As I wrote before, the V800 is based on the Hitachi HMA-7500, which spawned numerous versions, such as the Maplin / Dave Goodman kit. I've never seen a Lateral FET amp without a Zobel...nor can I get my head around why anyone would leave it out. I can remember making a Maplin amp and experiencing HF oscillation, even with the Zobel (check the Ebay listings for Maplin amps and see the charred Zobels...). I had a tube of spare MOSFETs and got the amps stable by swapping them around... It would seem that some are more easily agitated than others...
Around 1991, I sold a stereo Maplin / Hitachi amp to a record store and it came back with a blown MOSFET after a few days... These amps invariably have minimal protection and the O/Ps are prone to damage, especially after a hard life on the road.
Here you go: Ebay item 404219590126
(witness charred PCB under Zobel. Zobel R has been changed from the original red Maplin one).
As I wrote before, the V800 is based on the Hitachi HMA-7500, which spawned numerous versions, such as the Maplin / Dave Goodman kit. I've never seen a Lateral FET amp without a Zobel...nor can I get my head around why anyone would leave it out. I can remember making a Maplin amp and experiencing HF oscillation, even with the Zobel (check the Ebay listings for Maplin amps and see the charred Zobels...). I had a tube of spare MOSFETs and got the amps stable by swapping them around... It would seem that some are more easily agitated than others...
Around 1991, I sold a stereo Maplin / Hitachi amp to a record store and it came back with a blown MOSFET after a few days... These amps invariably have minimal protection and the O/Ps are prone to damage, especially after a hard life on the road.
Here you go: Ebay item 404219590126
(witness charred PCB under Zobel. Zobel R has been changed from the original red Maplin one).
Dear friends! (kramtweeter, BSST, thermionic, and all the others)
Thank you for all your help! What you said is the solution! The missing resistor from the Zobel circuit. I added a 4,7 Ohm resistor in series on the unaffected channel (as shown on the V800 drawing: resistors R20 and R38) the oscillation stopped immediately, I can't even detect it with an oscilloscope! I restored the experimental channel to the factory version, also the 10pF VAS Miller capacitance, still good! Only I didn't restore the C4 capacitor to the factory 22pF (left the 10pF) because I still need to put together a pulse generator to see how stable it will be under load. Perfect with sine wave 🙂 Thanks a lot my friends. One more question: how to set the quiescent current? I don't have Source resistors on the FETs to measure voltage drop (I don't even know how much quiescent current is needed for this device).
Thank you for all your help! What you said is the solution! The missing resistor from the Zobel circuit. I added a 4,7 Ohm resistor in series on the unaffected channel (as shown on the V800 drawing: resistors R20 and R38) the oscillation stopped immediately, I can't even detect it with an oscilloscope! I restored the experimental channel to the factory version, also the 10pF VAS Miller capacitance, still good! Only I didn't restore the C4 capacitor to the factory 22pF (left the 10pF) because I still need to put together a pulse generator to see how stable it will be under load. Perfect with sine wave 🙂 Thanks a lot my friends. One more question: how to set the quiescent current? I don't have Source resistors on the FETs to measure voltage drop (I don't even know how much quiescent current is needed for this device).
Congratulations! Well done! 🙂
I like your intention to test stability with a square wave, but be cautious. Use low frequency--- all the info is the edges. Let us know what you find.
Since there are no source resistors for current measurement, I suggest installing a 0.1 Ohm power resistor in each channel, inserted in the wire powering the four FET drain terminals. This resistor can be either in the N channel or P channel array--- which ever is most convenient, probably installed in the heatsink. I suggest leaving these resistors permanently in place; use them for bias current sensing, but install jumper wires across them for actual use. I can only guess at an appropriate bias current through the 0.1 Ohm---- 100mA?
Again, well done!
I like your intention to test stability with a square wave, but be cautious. Use low frequency--- all the info is the edges. Let us know what you find.
Since there are no source resistors for current measurement, I suggest installing a 0.1 Ohm power resistor in each channel, inserted in the wire powering the four FET drain terminals. This resistor can be either in the N channel or P channel array--- which ever is most convenient, probably installed in the heatsink. I suggest leaving these resistors permanently in place; use them for bias current sensing, but install jumper wires across them for actual use. I can only guess at an appropriate bias current through the 0.1 Ohm---- 100mA?
Again, well done!
Thank You BSST!
I put together a small square wave generator that gives a 400Hz signal. I used it to test the amplifier. It looks perfect, with no sign of oscillation under load or unloaded. I also made a little video of the results. With the original input transformer back on, of course the square wave signal is not as nice, but that's not the point 🙂
Thanks to everyone who commented, without You I don't think I would have found the error! I don't know why they could have left the resistance out of the Zobel tag, as it was still there in the early version. It is incomprehensible. But the point is that everything seems fine now, the amplifier seems stable.
Thank You all!
I put together a small square wave generator that gives a 400Hz signal. I used it to test the amplifier. It looks perfect, with no sign of oscillation under load or unloaded. I also made a little video of the results. With the original input transformer back on, of course the square wave signal is not as nice, but that's not the point 🙂
Thanks to everyone who commented, without You I don't think I would have found the error! I don't know why they could have left the resistance out of the Zobel tag, as it was still there in the early version. It is incomprehensible. But the point is that everything seems fine now, the amplifier seems stable.
Thank You all!
