Ampeg B-410 DC On Output Diagnostics Help

[misterc57]

So this one is showing 47 VDC at the output and the entire amp is drawing 1A of current on my variac.

Unlike my other B-410 (with the overcurrent issue) this one does not have a polarity switch or an internal 6A fuse. However both of my B-410 share the same schematic which is actually factory taped to the chassis cover.

The schematic is attached here.

Please show me the way to diagnosing the DC on output issue.

Thank you, MC

ampeg_b115_b410_sch.pdf

[misterc57]

Using a bulb limiter I have 34 VDC on the output. I also see that on both sides of R224

[misterc57]

Strange reading at Q207? Should emitter be negative V?

Q207 B = -33.3V, C = -33.4V, E = 33.3V

Q206 B = 32V, C = 33.7V, E = 33.3V
Q205 B = -34V, C = -34V, E = -33.3V
Q204 B = 33V, C = 34V, E = 34V

And I just noticed Q207 all 3 legs are loose on the board and each via pad is lifted. Going to need repair... Will report back.

[The Dude]

Q207 emitter should be at or near 0V. It directly connects to the amps output through coil L201.

[misterc57]

So I repaired the connections to Q207. I still have +35 VDC on the emitter. Would this indicate a bad Q207? The B and C are around -34 VDC.

[g1]

You have 35V on the Q207 emitter because that is what is at the output.
But it is a PNP so the base can only be 0.7V more negative than emitter. So either Q207 is bad or something is mixed up with regard to it's pin-out.

[The Dude]

Respectfully, it would be beneficial for you to learn about transistor testing. There are a number of videos and tutorials on the web. Knowing how a transistor works and how to test one is essential if you are doing solid state repairs. Again, respectfully- there is no disrespect intended in this post.

[misterc57]

I removed Q207 (a NTE129) and it failed testing on the bench. I did both B to E and B to C ohm tests and diode tests and they both failed.

I replaced it with the Q207 (a 610263-4) from my other amp (the one with the overcurrent). It tested good on the bench.

DCV on the output is now 1.5 VDC

This is with a bulb limiter

Q207 B -50.5 V
Q207 E 1.5 V
Q207 C 0.847 V

Something seems very off.

Here is a pic of the replacement Q207. Pretty sure I have the orientation correct.

[misterc57]

Respectfully, it would be beneficial for you to learn about transistor testing. There are a number of videos and tutorials on the web. Knowing how a transistor works and how to test one is essential if you are doing solid state repairs. Again, respectfully- there is no disrespect intended in this post.

I hear you and appreciate the message. I have watched videos and read up on this, and will continue to do so. I just have not done enough of these to feel confident yet. And when testing in circuit I feel completely lost.

Removed Q207 and did both B to E and B to C ohm tests and diode tests and they both failed. In both cases reversing leads looking for opens.

[The Dude]

Are you sure you installed it correctly or that you don't have pins mixed up in your measurements? Look up the datasheet for the transistor and find the pinout. Assuming the transistor is good, those measurements make no sense at all.

[misterc57]

Are you sure you installed it correctly or that you don't have pins mixed up in your measurements? Look up the datasheet for the transistor and find the pinout. Assuming the transistor is good, those measurements make no sense at all.

I have been trying to find a Fairchild 610263-4 pinout but no luck yet. I attached a picture of the installation (you will need to refresh the page). I installed it identical to how it sat in the other ampeg amp.

I double checked me readings. What should I be seeing for readings at Q207? I am thinking all legs should be close to around -34 V.

Thank you

[The Dude]

Q207 approximate readings should be E= 0V, B= -.5, C= some higher negative voltage.

[misterc57]

Q207 approximate readings should be E= 0V, B= -.5, C= some higher negative voltage.

What about Q206 where I had

Q206 B = 32V, C = 33.7V, E = 33.3V

I thought these two kind of mirrored each other.

[The Dude]

Look at both of those transistors (Q206 and Q207). Both of the emitters tie to the output line that goes through L201 to the speaker. Therefore, both emitters must be at or near 0V.

[misterc57]

Another round of readings with a bulb limiter

Q206 CBE 1.3V, 1.39V, 1.4V
Q207 CBE -51V, 0.8V, 1.4V

D204 C,A -51.1V, -51.1V
D205 C,A -1.3V, -1.4V

Q205 CBE -51.3V,

Suddenly, BOOM. C203 (330 uF 10V) explodes