Gibsonian's Cherry Pl 700 Ser I, soon to be a PLWO-1000

[Gepetto]

Lee, sorry to hear that.

I always put a small dab of silicone on the ends of those screws and at the clamp tie down points. That meter hold down scheme was the worst.

 

[laatsch55]

Why do I keep blowing R2 after applying a slight load??

 

[jbeckva]

Why do I keep blowing R2 after applying a slight load??

R2?? That the 2 ohm on the input??? (goes looking for skeezers...)

 

[jbeckva]

No Joe I haven't. Took a break. Ken nelson's 400 did not arrive in good shape. The meter holddown allen head 6-32 machine screws came loose and fell against various locations on the backwall and he plugged it in.

So Lickety Split got back here 2 days ago and I finally opened her up last night-------you would not believe the damage that resulted....

PICS PICS PICS!!! WE WANNA SEE THE BLOOD N GORE!!!

 

[jbeckva]

PICS PICS PICS!!! WE WANNA SEE THE BLOOD N GORE!!!

Really (just cuz..)



Don't you feel special that I had enough motivation to go find this smilie and add it, just for this??

 

[laatsch55]

Yes I do Jer. Let me post this in the Ken Nelson thread, that's the one 2.7 ohm R2.

 

[laatsch55]

I still have this on anything over 3K frequency....

 

[laatsch55]

Square wave..........

 

[Gepetto]

you have a positive going slew rate problem to start with. Both channels?

Things to check
C11 bootstrap cap
Damaged Q10
Damaged Q11

Carefully scope out Q6 C (top of R18) on channel 1 while displaying that square or sine wave on channel 2. I want to see if the earlier stages are attempting to compensate but cannot due to a croaked (technical term) output stage.

 

[laatsch55]

Here ya go---- Also, minimum bias obtainable is 545mv.

 

[laatsch55]

Just the right channel Joe, the left looks great.

 

[Gepetto]

Really hard to get to but the right side of R12R on channel 2 with channel 1 still showing the square wave output.

The last trace showed that the collector of Q5 has the same distortion that the output does so the output is following that signal rather faithfully.

Need to go further back, some stage should be trying to compensate for that abnormality.

 

[Gepetto]

and be careful, you don't want to blow anything up.

 

[laatsch55]

Amen...

 

[Gepetto]

OK, the signal at R12R right side looks correct.

You can see the signal at that point driving really hard to compensate for the the slow slew rate but Q5 and following does not seem to be listening. I would check D4 and D5 (doubtful anything wrong here) and Q5.

Putting it in purely technical terms, you can see Q4 driving the crap out of the base of Q5 but Q5 is not responding. My hunch is a leaky Collector-Emitter junction on Q5.

 

[laatsch55]

That's why the bias is so high......

 

[laatsch55]

That particular Jct is impossible to check in circuit...

 

[laatsch55]

Q5 checked good out of circuit, diode test. D4 however, showed open both ways. well, infinity.

 

[laatsch55]

Put Q5 back in and replaced D4, waveforms look the same...

 

[laatsch55]

If the base -emitter jct is leaking my Fluke wont have enough voltage to trigger it. Q5 checked good on the Fluke, But I'll swap it out cause we are right back where we started.