"** But it is not a class D amp in any case, just a basic class B design with a a pair of "helpers" to eliminate most of the heat. "

Class B or AB ? Things can be class B, they, according to their publications speak of "musicality" or some such IIRC. Class B is not very warming to that, though my Phase Linear does run the actual outputs in class B. It is just that there is enough feedback and low ohm resistors from the drivers that make it sound good at low levels. If it is class B there will be no bias regulator transistors or anything to do with idle bias. Is that so ? The Phase Linear you set the bias for the drivers and they are actually the outputs up to a certain level. Is it like that or what ?

"** About 200kHz, the pos and neg versions are not very symmetrical. "

And you said something about self oscillating ? That sounds like a real bitch to engineer but I suppose it can be done. But from what I know about that topology is the collector voltage just stays a few volts higher than the output voltage.

Actually in the past there were transistors that did not like that. An example would be the NEC N-965 VHS, which you probably have not seen. they had transistors for the reset the had 5 volts on the collector and then wanted 5 volts on the emitter so the put 5 volts on the base. They were a common failure. I determined that the problem was that the transistor could not maintain hfe with that low of a Vce. I replaced them with 2SD612Ks which were bias transistors and therefore made to work at low voltages. Never failed. Others had modifications running wires up to a heat sink, drilling hoes and putting the collectors to the 12 volt source. **** all that, but that is why I got the big bucks.

So in a way I wonder if these class B outputs might not fail the same way. With the ones in the reset of the N-965 they seemed to fail with an open base. They read infinity every which way on the meter. Could have been an open emitter, no way to tell.

"** It was ALL the devices on the heatsink - including a pair of 1amp, 15V regulators"

Well that pretty much clinches it that it was heat. Remember when STKs had the loops on the pins ? Obviously they were to eliminate that mechanical stress. I guess **** like that is too much trouble now. But hey, they paid the bill and you mad some money I hope. If nothing ever broke, we would be broke.


"** The power amp module was easy enough to get out and then you access the underside of the PCB by unbolting all the power devices. A common enough assembly method these days and one Yamaha uses in a few models."

So, after all that do you clean it and put new heatsink compound on ? (also known as bird ****) And then if there are insulators it is even worse. In fact I had a problem with that with a Yamaha, the insulators were big, one for the whole bank of transistors and it was stuck, no way to get it apart without damage. I was looking for a solution to that but quit that job and left them to their own device. It might be different in your country but here people like us are like gold. I know in Europe people know things and can fix things, but not in the US. They can't even change a flat tire.

"I was just trying to avoid doing that until I was sure there was a fault to be found there. It was amazing that with so many cracked joints, the PCB did NOT respond to hard impacts. "

I am pretty goddamn good but I am not sure what to tell you about this. It must be that the mechanical was so strong that even a hard smack would not affect it. Maybe if you had pressed down on the PC board right near the heatsink really hard, not hitting but applying pressure, while in fault condition of course, you may have been able to tell without taking the whole damn thing apart.

that reminds me I got a bunch of amps that have a channel cutting out and it is as if it is a bad connection, and AFTER the volume. I did prelims on them and thought it was the speaker relay(s) but found a couple which is wasn't. I see no bad joints but it has to be one, and I got it on either channel, we got 53 of these things. Not great amps, hundred a channel but can handle four ohms so two hundred a channel. HA, I AM factory service for these things and I have to operate like a boneyard.

They all got a defect in the mechanical design. The bias transistor has no thing forcing it to make contact with the heat sink. Well we know what happens then. And the factory put heat sink compound on it so that means the design wants it. It was a SNAFU between the electronics end ans the mechanics end. But if we sell them like that they are all coming back and they will have to be refunded when, not if, they fry out. They do not want that. I had an idea with a wire clip, under a screw of the output to hold it down. I prefer that because it is plastic and you got no worries about the UL or anything but the boss was asking questions, like why are all these sitting here so I am considering using a paper clip. That is a FACTORY modification !

I have seen worse.