Schematic Precision Power A600

[pergo]

Hi everyone!


I have a PPI A600 (not .2) with a problem in drivers-predrivers section.

I'm becoming mad because something is missing, or i'm to tired to find the problem

Anyone has a schematic or semplified schematic like Perry's?

The amplifier works without predrivers, but when a put them in, they heat and the amp draw excessive current.

Another channel works ok with new predrivers (2SC2389 + complementary) instead of original 2N5551 and 2N5401.
I don't have thats 2N's at home.

The right channel has this voltage on output stage:
- base: +-0.8V
- collector: 40 or -40 (depend of NPN or PNP)
- emitter: 0V

But broken channel, with predrivers, is:
- base: +-39V
- collector: +-40V
- emitter: +-39V
that voltage are egual with or without output stage.

predrivers & PNPs (new BD912) heat very very much
drivers are cool and ok (i replace them but nothing solved)



I have a Perry's schematic for a 2150AM but it's too different from A600


thank you!
ciao!

[1moreamp]

Pergo is this the amp your talking about < see link below >

Precision Power A600 - Amp Guts


If so you most likely has a bad SIP driver card IMO. SIP cards are not available but many times they can be repaired by carefully touching up the solder joints on them. Careful too much heat will destroy the solder pads on these ceramic cards, and most liquid solder flux's are not well accepted either. < I.E. flux does not help in most cases and may lead to pad failure...Been there, done that, Not good.

I do not have a A600 pdf. Perhaps someone else might though....

[Perry Babin]

Email me.

babin_perry@yahoo.com

[pergo]

the amplifier is similar to that:
Precision Power A600 inside - Amp Guts

but i've:

- diode fast bridge is AKA & KAK, not 4 single diode.
- filter toroid (after AKA+KAK bridge) is different, much "older" and handmade.
- drivers are different. Mine has 2SCxxxx + 2SAxxxx in plastic to220.


After the first repair (replace output stage with new BD911 & BD912) the amplifiers shows a little DC offset in the bottom channel (see image).
The DC offset was 100mV.

Then, i turn the potentiometer to the bottom right angle (see image) for set DC offset and it goes to 8mV.

I test that channel with 3ohm pure resistive dummy load, but over 100Wrms the BD912s fails (only 1).

I replace it, and re-test. Again, over about 100Wrms, same bank of BD912 fail (but another position).

Before the failure there's no distortion or other.

It takes about 3 minutes @100Wrms continuously before failure.

Then, i re-replace the BD912s...but the amplifier draws excessive current and goes on overcurrent protection.

I kept it in protection for 1 minutes and i heard a "chop!" (little high frequecy explosion) but i can find the component that fails. No smoke!
I touch everything and i've noticed that SIP card heat too much.

I desolder BF722 & BF723 but still same problem.

I desolder 1B & 3B (BC546 & BC556 in SOT23) that had 0 resistance between two leg (copying the other SIP card).

Now, the pre-drivers (or VAS?) heat. They're 2N5551+2N5401. I replace with different new components (BC546 + BC557, or 2SC2389+complementary, or MPSA06+MPSA56) but still same problem.

If i remove pre-drivers (or VAS?), the amplifer work at idle. No protection, but that channel cannot sound (obviously).

The other channel is ok.

I test every resistor, changed (with new parts) every transistor for broken channel, but the problem isn't here.

Remains only the SIP card, my fear!!

There are BF722 & BF723 and some pairs of 3B & 1B in SOT23. When i desoldered them, no fluid


and image:
http://img534.imageshack.us/img534/6791/immagineiw.jpg



ps: Perry, email sent

[Perry Babin]

Were you running it with a pure sine wave at 100w when it failed?

[pergo]

yes, for burn-in test for rock-solid stability operation.
I use 50Hz from Mp3-home-player

other channel works fine with that signal.

every amplifier i've repaired, then i test it with dummy load and max power.

is it wrong?

thank you

[Perry Babin]

A constant sine wave into a dummy load is much tougher than the same output level into a speaker with music.

Are the mica insulators perfectly intact? If they're split or separating, they may not be conducting heat as well as they should. Some of these amps have a lot of powdercoating under the transistors. If you think the failures are related to the heating of the outputs, you may want to sand the powdercoating off to promote better heat transfer.

Have you removed 100% of the old compound and replaced it with new heatsink compound?

If the amp survives playing music up to and into clipping, it's probably OK. Of course, you have to get it repaired before any of this information is of any use.

[pergo]

i check mica insulator, it's ok. i don't replace thermal compound.
The amplifier dissipates well.
Before the failure, output stage and chassis were at about same temperature.

Not untouchable, but little hot (maybe 50°C). No overtemp protection goes on.

I know that sine is more than music, but i avoid clipping when i use dummy load.

I always start with low power continuously (like 50W), then i increase.

Long time ago i used speakers, but the persistent loud music make me crazy

I test it @lab, where 500Wrms destroy the walls

I hope to finish my car-audio system in a month, to test it in the car

[Perry Babin]

Most amps can survive music up to and into clipping with a dummy load but a pure sine wave is more than some amps can take.

If the amp wasn't hot, there may be other problems. With these amps, you have to make sure that the transistors are laying flat against the sink before reinstalling the cover.

[Perry Babin]

In the attached photo, you can see that the transistor isn't laying against the sink. The corner of the bottom/base of the transistor dug into the mica and wouldn't lay down. The 'finger' applying pressure to it was perfectly inline (not sprung) when the cover was removed to lay it down properly. This sort of problem can cause the transistors to fail prematurely.