Ampeg B200r trouble

[jonradford]

Hi guys,

I have an Ampeg B200R which I'm trying to repair. The symptoms where that it was very low on power and therefore to get any volume out of it it needed to be turned up a lot which introduced hum and hiss. As I had the board out I replaced all the mosfets the power transistors the 1N914 & 1N4740 diodes in the power section. I also replaced resistors R56, 57, 70 & 71. The amp seems to be louder but when I went to bias it one of the .33 10w resistors is measures a lot higher than the other 3. R73 measures around 18mv when the other three measure around 5.8mv each. Is this ok? Or do i have a problem here? I know I need to take an average of the 4 and get that between 15-20mv but that means one is going to be quite a bit higher. Do I have a problem with Q8?
I hope someone can help!

Cheers,

Jon

[52 Bill]

Welcome to the place.

R73 measures around 18mv when the other three measure around 5.8mv each. Is this ok? Or do i have a problem here? I know I need to take an average of the 4 and get that between 15-20mv but that means one is going to be quite a bit higher. Do I have a problem with Q8?

If Q8 is the one that is drawing 3Xs the current of the others, then it is really mismatched to the others and probably should be replaced.

When you ordered the outputs, did you order any extras or just enough to fill the slots? Did you replace all of the ballast resistors as well or at least check them all?

[jonradford]

Welcome to the place.
If Q8 is the one that is drawing 3Xs the current of the others, then it is really mismatched to the others and probably should be replaced.

When you ordered the outputs, did you order any extras or just enough to fill the slots? Did you replace all of the ballast resistors as well or at least check them all?

Hi Bill,

I only ordered enough mosfets to replace Q4, Q5, Q8 & Q11. When you say ballast resistors do you mean the 10w resistors connected to the Source terminals or the smaller resistors connected to the Gate terminals (I changed these ones).

I can get some more mosfets etc

[52 Bill]

When you say ballast resistors do you mean the 10w resistors connected to the Source terminals or the smaller resistors connected to the Gate terminals (I changed these ones).

The ballast resistors are the 10 watt 0.33R resistors. If one has a different resistance than the others, it will cause the different voltage readings.

[jonradford]

The ballast resistors are the 10 watt 0.33R resistors. If one has a different resistance than the others, it will cause the different voltage readings.

Ok, no I didn't check those. I shall do that. If they are similar i will replace Q8 and report back or replace any resistor that is out.
Cheers!

[jonradford]

The ballast resistors are the 10 watt 0.33R resistors. If one has a different resistance than the others, it will cause the different voltage readings.

ok, I've checked the ballast resistors and they all read 0.4 ohms on my meter. Q8 test ok using a diode test function on my meter but I have ordered a few more and will try replacing it anyway.

[jonradford]

Ok, I've replaced Q8 and it is reading lower than before but still amore than the others! (about 9mv) The amp sounds ok. Should I just bias it with that one slightly higher than the others?

[52 Bill]

Ok, I've replaced Q8 and it is reading lower than before but still amore than the others! (about 9mv) The amp sounds ok. Should I just bias it with that one slightly higher than the others?

So the new one is reading 9mV or is it reading 9mV higher than the others? You could try swapping two of the mosfets to see if the high current moves with the transistor or stays with the circuit position.

You may be at the point of bias it a little on the cold side and give it a good test.