PPI A600 gate resistor value

[Perry Babin]

Are the legs that read 0.2 ohms directly connected to the large emitter resistors?

[Sean Morrison]

No, they are just kind of mounted in the middle of the board. They aren't on the edge clamped to the heatsink. They are also very small and stand of vertically on three legs.

[Perry Babin]

Are the legs connected to the emitter resistors by traces on the top or the bottom of the board?

[Sean Morrison]

I'm just an amateur working on a first amp repair so I don't ultimately know what an emitter resistor is, but I know that one of the terminals on a transistor is called the emitter, so I went to the closest ones on the heatsink and found a small tan 1 ohm resistor electrically connected to a leg. From the other side of that 1 ohm resistor I was then able to get direct continuity to the small barrel shaped transistors. I actually think I spoke incorrectly of the component numbers - they are Q12 and Q30. Again, two legs on each of these are showing direct path to what I assume are the emitter resistors.

This picture shows the transistors in question and the corresponding flat heatsink transistors. One thing I found odd was that it is a BDT81 on one side and a BDT82 on the other. The 4 transistors immediately next to each are all BDT82's.




I'm guessing that the MPS A06 & MPS A56 transistors are defective?

[Sean Morrison]

FYI - it did blow my 10 amp fuse upon testing but didn't harm the newly replaced power supply fets or 2 legged rectifiers.

Also, I found continuity between pins 5 & 7 on UC2535AN and not sure if there should be.

[Perry Babin]

Have you read the basic amp repair page?

The transistors that you're referring to are used as part of a protection circuit for the voltage regulators. The large transistors circled are used as regulators.

Emitter resistors connect to the emitter leg of the output transistors. 0.68 ohm in this amp.

There's generally a 100 ohm resistor between 5 and 7. It's used to set the deadtime for the IC. On the Rev.S, I think there may have been a 10 ohm resistor.

[Sean Morrison]

I read through the basic amp repair page and am trying to learn as I go. I have a background in mechanical engineering, but some of this electrical stuff is pretty unfamiliar to me.

Based on what you said, then I believe the emitter resistors are the large grey ones toward the RCA end of the board in front of 8 transistors per side and they all check out fine.

[Sean Morrison]

I'm a little leary of soldering in the new 16 pin pwm IC for fear of applying too much heat and damaging it. What are the experts thoughts on using a 16 pin dip socket and plugging the chip into that? I figure it would make any future repairs easier as well. Drawbacks?

[Perry Babin]

Cheap sockets are a problem. If you use a socket, get the type that either has milled pins or the ones that have contacts on both sides of each of the individual sockets.

I've never heard of anyone damaging a DIP IC by using too much heat. The board would be charcoal by the time the IC got hot enough to suffer damage.

[Sean Morrison]

I just wanted to update this to let everyone know I solved the problem and to thank everyone for their guidance.

It turned out to be the 16 pin IC. After I replaced that, using a good machined socket, it powered up and didn't blow the 10 amp fuse. I then connected an old bookshelf speaker and music sounded normal and clean. Once I get my crossover set up I will test it on my 10w3 and see how it does on bass duty.