Yamaha A-960 II Integrated Amp Won't power on

[KilgoreCemetery]

I've been troubleshooting this one for a while. First, it has a special power supply similar to what made Carver amps famous. I don't fully understand what each part does, but I know that it chops up the line voltage using triacs, almost like a slow version of a switch mode power supply. I've checked all of the resistors and diodes on the two "Primary" boards. I was even able to pull the IC and triacs out of a working Yamaha A-760 to rule them out. At this point, I'm beginning to think that the problem is further down stream.

I did verify that the pilot light works by running about 5vdc through it with the hot side disconnected from the board. Based on what I've read, the pilot lamp is important for this model because it's wired to a couple of transistors on the main board (TR347, TR348). Honestly, I have no idea why or what those transistors are doing. Best guess is that it has something to do with voltage/current regulation.

Also, I found that both of the output transistors on one side are shorted. However, desoldering them from the board didn't seem to change anything. Driver transistors TR334 and TR332 test fine.

Can someone help me decipher what all the extra circuitry is for and maybe give me an idea of where to look next?

 

[kellys_eye]

Probably not the power supply itself - it's doing it's job by shutting down due to an overload (fault) condition. Usual suspects are the output transistors which, being independent devices, are easily checked and replaced.

 

[KilgoreCemetery]

Probably not the power supply itself - it's doing it's job by shutting down due to an overload (fault) condition. Usual suspects are the output transistors which, being independent devices, are easily checked and replaced.

Yeah, both of the output transistors on one side are shorted. However, desoldering them from the board doesn't change anything. Driver transistors TR334 and TR332 test fine and I have yet to find any other failed components. I even soldered in some temporary replacements for the output transistors so there would be a load. No change.

It still doesn't come on fully, but there is some voltage at the output transistors. Base and Emitter measures the same on all four output transistors at -4.00v. On the NPN's, the collector voltage is about 7v. On the PNP's, the collector voltage is about -7v.

If I disconnect TR348, the amp will actually come on after a few seconds and I even get relay click. I'm assuming that this is bypassing the protection circuit in some way. I am using a dim bulb to cushion any current draw, but it's pulling almost 600mA. However, I performed the same test with a working Yamaha A-760, and it only pulls 500mA at idle.

Hmm.. I feel like I'm probably going to have to check every transistor/diode/resistor in the power amp circuit until I find the bad one