BIG problems

[Michael F]

I invested in a pair of those mini probe type clips just for this project
Base voltages on both Q3 and Q4 are 100vdc

 

[laatsch55]

CheckD3 , pull one leg and check it.

 

[laatsch55]

And R7

 

[laatsch55]

Also, what do you have for voltage at the emitters and collectors of Q3,Q4. And then what for all 3 pins on Q1,Q2

 

[Michael F]

D3 measures good out of circuit
R7 measures 330K

Q1
E 2.6vdc
B .023vdc
C 104 vdc

Q2
E 2.6vdc
B 3 vdc
C 104 vdc

Q3
E 104vdc
B 100vdc
C .023vdc

Q4
E 104vdc
B 100vdc
C minus 106vdc

 

[Gepetto]

I invested in a pair of those mini probe type clips just for this project
Base voltages on both Q3 and Q4 are 100vdc

That puts the problem in the front end, Q1 and Q2. You should have -0.7V on the combined emitters, approximately 0V on Q1 base and approximately 2.2V on the base of Q2 (this is abnormal but the case when you have 100V on the output) and you should have approximately 10V on the collectors of Q1 and Q2 (but you told us you have 100V because something is wrong). Check R6. If it is open, you will have 100V on the output as you seem to have.

 

[Gepetto]

D3 measures good out of circuit
R7 measures 330K

Q1
E 2.6vdc
B .023vdc
C 104 vdc

Q2
E 2.6vdc
B 3 vdc
C 104 vdc

Q3
E 104vdc
B 100vdc
C .023vdc

Q4
E 104vdc
B 100vdc
C minus 106vdc

It appears you have lost your negative bias for Q1 and Q2 that comes from R6.

 

[Gepetto]

It appears you have lost your negative bias for Q1 and Q2 that comes from R6.

BTW
Q1 and Q2 are very likely shot Mike. They only have a Vce withstand voltage of 25V and you have nearly 100 across them.

 

[Michael F]

It appears you have lost your negative bias for Q1 and Q2 that comes from R6.

Damn your good :
R6 in my case is comprised of a 200K and 30K in series for some reason. An in circuit test showed up as open, turned out to be the 200K however, when I pulled them from the board, one of the pads was lifted but he 2 resistors now tested fine
I put them back in and fired up the amp, .023 on the output
Can a resistor behave that way? I wish I knew if the problem was the resistor or the pad.
I have some 220k on hand, should I go ahead and replace the series combo??

 

[Michael F]

BTW
Q1 and Q2 are very likely shot Mike. They only have a Vce withstand voltage of 25V and you have nearly 100 across them.

Oh? how would that manifest itself? Read my previous post

 

[laatsch55]

Michael, to long distance troubleshoot like Joe does, you have to understand that board backwards, forwards, sideways and upside down. I'm starting to get a handle on it, but I've got a long way to go. There will be no one you will meet in your lifetime that is better with this system than Joe. Period.

 

[Michael F]

Michael, to long distance troubleshoot like Joe does, you have to understand that board backwards, forwards, sideways and upside down. I'm starting to get a handle on it, but I've got a long way to go. There will be no one you will meet in your lifetime that is better with this system than Joe. Period.

He does seem to know a thing or two about these amps, I`m sure Bob would be impressed.
I could never have gotten this far without BOTH of you guys, I`m sincerely grateful.

What`s the next step?

 

[laatsch55]

The protect board. AFTER you replace vQ1,2,3,4 in bith channels with MPSA 18's in Q1,2, and MPSA 92's in Q3,Q4.

 

[Gepetto]

Damn your good :
R6 in my case is comprised of a 200K and 30K in series for some reason. An in circuit test showed up as open, turned out to be the 200K however, when I pulled them from the board, one of the pads was lifted but he 2 resistors now tested fine
I put them back in and fired up the amp, .023 on the output
Can a resistor behave that way? I wish I knew if the problem was the resistor or the pad.
I have some 220k on hand, should I go ahead and replace the series combo??

Absolutely Mike, get rid of that series combo.

 

[Gepetto]

Oh? how would that manifest itself? Read my previous post

As walking wounded as we know them in the industry. They are no longer to be trusted now that you have impressed them with 4x their design rating.

 

[laatsch55]

So, that's what happens when you take 4 times your rating, you're -----impressed??

 

[laatsch55]

I'm usually smokin when that happens....

 

[Gepetto]

So, that's what happens when you take 4 times your rating, you're -----impressed??

You bet Lee. I am impressed that they still work somewhat.

 

[laatsch55]

Amen. I wouldn't go Mach 8 with that combo thougfh....

 

[Michael F]

The protect board. AFTER you replace vQ1,2,3,4 in bith channels with MPSA 18's in Q1,2, and MPSA 92's in Q3,Q4.

OK, I`ll place an order at Newark for those transistors. Lee, will you still be sending that driver board out?
If your referring to the output protection board, there is no way in Hell that things going back in