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

[laatsch55]

are you getting zero volts across R34L on the control board as well?

.006 Joe

 

[laatsch55]

Hi Lee
Unless you hid it real good on me, I do not see the necessary 0.33 ohm R41 on the negative side of the transistor stack. Without this, you will get NO drive to the lower half of the output bridge.

R41 is there.... tied to R35 via a trace and the out put rail via it's lead...

 

[Gepetto]

.006 Joe

As suspected. You indicated earlier that your reading at the bias transistor E-L lead was -0.9V which should have the base emitter of Q7 turned on and drawing current through R34L. In order to just start turning on R37, you need at least 0.088V across R34L. You are not getting that because I think you have some form of connectivity problem into pad 4L from the back transistor wall.

Is R41 blown open by any chance? Were you cranking this amp earlier on and could have damaged it?

 

[laatsch55]

D13 left channel---.416 forward, 1.865 reverse. D13 right channel--.079 both directions.

 

[Gepetto]

I should have said, in order to just begin turning on Q12 B-E junction you need 0.088V across R34L...sorry for the mis-speak

 

[Gepetto]

D13 left channel---.416 forward, 1.865 reverse. D13 right channel--.079 both directions.

Yup, all tied into the same connectivity problem. You think you have an R41 in there but it is not really in there.

The ideal reading should be .022 (R34L)+.056(R35L)+.00033(R41) = 0.07833 so you meter is pretty accurate Lee.

What your left channel reading is saying is that R41 is kaput.

 

[Gepetto]

When you are measuring across D13 while in circuit you are actually reading R34 in series with R35 in series with R41. Are you seeing where that is coming from?

R41 bridges from pad 6 to pad 4 on the control board. If you sketch that in on your schematic it will be easier to see this relationship.

 

[Gepetto]

Your reverse reading is telling you that R41 is measuring approximately 1.8K ohms instead of 0.33 ohms.

 

[laatsch55]

Q5 heats up in a hurry...

 

[Gepetto]

I think all this chatter is eating up all the bandwidth to the website

 

[laatsch55]

When you are measuring across D13 while in circuit you are actually reading R34 in series with R35 in series with R41. Are you seeing where that is coming from?

R41 bridges from pad 6 to pad 4 on the control board. If you sketch that in on your schematic it will be easier to see this relationship.

Ok, during one shorting episode I toasted ALL the negative drive emitter resistors, never thought to check R41...

 

[laatsch55]

1.687 K ohms accross R41. I'll be damned!!!~

 

[Gepetto]

because it is not getting any help from the minus half of the amp output bridge. Q5 is establishing the bias through D13 and then through R35 and then through the Emitter-Base of Q7 and then into the collector of Q5. It is not used to seeing this type of load.

 

[Gepetto]

Educated guess...I was off by 113 ohms.

 

[Gepetto]

R41 is absolutely a key resistor.

Those should be 2W resistors.

 

[Gepetto]

Put a fork in it Lee.

Glad you found your problem before calling it a night.

 

[laatsch55]

Out of circuit 75K.....

 

[laatsch55]

Educated guess...I was off by 113 ohms.

My parts changer status bit me in the ass again.....Thanks Joe.

 

[laatsch55]

Had one MO .33 2watter left....

 

[laatsch55]

Educated guess...I was off by 113 ohms.

Jeezzz./.... Can ya get a little closer next time??