I've just fitted a new pair of output transistors to a NAD amplifier...SGS-Thomson MJ2955 and ISC 2N3055. Also fitted new drivers and bias transistor in heatsink.
Everything seemed fine at first switch-on but when I cranked up the input (1K sine wave from generator) the side I just repaired went up in smoke. It was under no load, just the scope probe.
One of the output tyransistors MJ2955 and all the drivers had blown.
The MJ2955 had failed s/c B-C
I couldn't find anything else wrong so I fitted a different new pair of outputs (mospec) and all new drivers again and tried it..and it works perfectly.
Any chance the MJ2955 was a fake or was I just unlucky?
Everything seemed fine at first switch-on but when I cranked up the input (1K sine wave from generator) the side I just repaired went up in smoke. It was under no load, just the scope probe.
One of the output tyransistors MJ2955 and all the drivers had blown.
The MJ2955 had failed s/c B-C
I couldn't find anything else wrong so I fitted a different new pair of outputs (mospec) and all new drivers again and tried it..and it works perfectly.
Any chance the MJ2955 was a fake or was I just unlucky?
Attachments
If the pic is representative of the real thing, it does seem to look correct, according to 2 recent purchases of similar STmicro parts that I made. However, I paid a lot more than some Chinese stores selling what appears to be the same items through Aliexpress, Ebay and similar websites. Some are as cheap as $0.45 US ea, even in small quantities and post free in lots of 10 or more! Buyer beware if your retailer or kit maker is buying these parts to resell to you.
The question of fakes won't be settled by guessing about authentic external details though - it's the seller's purchasing reputation that's in question as much as ridiculously cheap selling prices for alleged new products.
The question of fakes won't be settled by guessing about authentic external details though - it's the seller's purchasing reputation that's in question as much as ridiculously cheap selling prices for alleged new products.
The number isn't recognised on the STMicroelectronics web site but is listed by Farnell with a data sheet dated November 2013!
You could cut the top off the TO3 package and see how big the chip is.
If it was significantly cheaper than Farnell (£1.72) I would expect it to be fake.
You could cut the top off the TO3 package and see how big the chip is.
If it was significantly cheaper than Farnell (£1.72) I would expect it to be fake.
I bought a Maplin 225WRMS amplifier off ebay.
It had a few transistors missing so I bought new ones from RS Components.
I soldered them in and powered up the amplifier.
Lots of oscillation on the output.
I tweaked the VAS capacitor and that fixed it.
I got out my component analyser and that showed the new transistors had a much greater gain than the original specced transistors.
It looks like the extra gain caused the oscillation.
Luckily I switched it off quick enough before the output transistors fried.
It had a few transistors missing so I bought new ones from RS Components.
I soldered them in and powered up the amplifier.
Lots of oscillation on the output.
I tweaked the VAS capacitor and that fixed it.
I got out my component analyser and that showed the new transistors had a much greater gain than the original specced transistors.
It looks like the extra gain caused the oscillation.
Luckily I switched it off quick enough before the output transistors fried.
I've been fighting problems of way too much gain and Ft in new transistors for obsolete designs for two weeks. Every common emitter stage is running saturated, swallowing up or chopping the bottom off the input signal when it isn't oscillating. When not saturated at 0.4 v Vce, theres lots of high steady AC voltage out collectors instead of the beating rock and roll signal that went in. I've sprinkled 33 pf capacitors everwhere not in the design, I've put resistor dividers on all the common emitter stage bases instead of OEM resistors off to one power supply. I'm not quite ready to stick in a 68W max IC and call that good, but I'm learning a lot. I'm going for 200 w music peaks ( not steady state, the heat sinks and soa won't allow that) out of a single pair of output transistors. Now about that thermally stable operating point? Silicon is now the new germanium?
I'm using ON semi and Fairchild pre-merger parts from the US division of Farnell, so there is little chance these are fakes. High gain is an expensive commodity to fake, and I've certainly cooked these thoroughly with my trials with smelly heat sink grease and no apparent damage to the transistors, except one driver where the emitter lead came unsoldered with 72 v on the collector. these parts have serious die to heat sink heat conductivity. I'm doing this on a 3" x 5" card with leaded parts and point to point wiring, so there is no room for a honey badger solution, either. Besides the lack of transformer center tap.
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I just recently designed a 250WRMS bipolar amplifier.
It oscillated with a sine wave input to it.
I tried VAS capacitor and feedback capacitor value changes but nothing fixed it.
I then tried 10R base stopper resistors on each base pin and that fixed it.
It oscillated with a sine wave input to it.
I tried VAS capacitor and feedback capacitor value changes but nothing fixed it.
I then tried 10R base stopper resistors on each base pin and that fixed it.
Yes Mile Slavkovic has got 10 ohm base resistors to his output transistor on his AX6 design I'm part copying. i started without them, but they can also be used as fuses to protect your $3 output transistors if the driver shorts across like mine did when the wire fell off. I put one watt 10 ohm resistor on the OT bases. I've got a one ohm one watt resistor added to the + rail in , to act as another fuse in case that happens again, another driver transistor short. Nice leads to solder with on those resistors, no precision drilling for fuse clips, and resistors cost $.02 to replace instead of the $.83 real fuses are going for these days. Resistors as fuses, a trick I learned from Peavey.
On the anti-oscillation front, the first thing I tried was an 8 turn ferrite stick inductor between VAS collector and the two bias diodes over on the heat sink. It is still there after I put a 33 pf on the input transistor CE junction to kill input oscillation, and I've managed to get rock and roll through the VAS so far, but I'll try to pull it out at the end. I've never seen one of these salvage PCAT switcher power supply inductors on any schematic on diyaudio.com, except maybe as the zobel output network. It might make a colpitts oscillator or something unintended. Bob Cordell would not approve, I'm sure.
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Does this amp have output stage emitter resistors? Some of those old NADs don't. Using modern 3055's without them is playing with fire. Adding 10 ohm base stoppers is the recommended fix (base stoppers add a bit of thermal feedback, but keep the crossover characteristics of the original design).
No, there are no emitter resistors as such, only one which is normally shorted by a solder link but is unsoldered and the mV measured across it when setting the bias.
The new mospec transistors are working fine after a while on soak test and bias is bang on the centre. No evidence of oscillation.
The new mospec transistors are working fine after a while on soak test and bias is bang on the centre. No evidence of oscillation.
It's not an oscillation thing. The base stoppers are used in leiu of emitter resistors, for the same purpose. It's possible that the Mospec units have high intrinsic base and/or emitter resistance. Older 3055 variants did. The modern ON units don't, and if you just put them in there it usually runs away.
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