Ampeg B2-RE

[lowell]

I have a schematic, but not a pcb layout for the PA. Anyone have this? All the components are on the underside. Or is it necessary to pull up the board to troubleshoot? I know I've fixed these before by resoldering the whole thing... but didn't work this time.

https://supportloudtech.netx.net/lou...category/14542

[doctor]

I have a schematic, but not a pcb layout for the PA. Anyone have this? All the components are on the underside. Or is it necessary to pull up the board to troubleshoot? I know I've fixed these before by resoldering the whole thing... but didn't work this time.

https://supportloudtech.netx.net/lou...category/14542

Here is the PA layout.

[lowell]

YES, thank you! Maybe it's on their portal, but I didn't see it. Most likely because of their odd filenames.

[Enzo]

For any model file, there are subheadings: Assemblies, and Schematics. The assemblies are the board layouts, and sometimes parts lists too. The old SLM files were that way, and Loud does the same with them.

[lowell]

So the amp sounds farty. Bias pot isn't doing anything. There is 0v on the MOSFET gates, and +1.5v between the ballasts (output rail). High v rails look good at +/-72v. The speaker relay is NOT in protection, so there is +1.5v on the output.

The drivers collectors should be +/-5.5v, which feed the MOSFET gates. Here's what I have.

The drivers:
Q4
Vc: 0v
Q13
Vc: 0v

Driver transistors test good. The input transistors Q2/17 test good and DO have +/-5.5Vb.

I've traced back to the input and there is +8v there where it says INPUT. Traced back further and there is +15v on output of IC4 PIN 7. There is +600mv on PIN 5 non-inverting input.

I'm confused at this point as to what is the cause of this offset. It's equal to the low voltage + rail, so I'm thinking the opamp is bad.

Idears?

https://supportloudtech.netx.net/lou...category/14739

[Enzo]

And I would have had a new one in that spot as fast as I could solder. How much of your labor rate do you spend wondering if that 40 cent part is bad? I don't believe in just changing out a ton of parts hoping to hit the jackpot, but I also know parts are cheaper than labor. If that chip looks suspicious, just change it. Now either the problem is corrected, or at least we know it wasn;t that chip. The whole amp is a big loop, if you look at IC4 you will see the output signal is sampled back into the input of the IC.

Cure that 15v/8v offset, and I bet your 1.5v offset at the output goes away.

If you have zero volts on the gates, and collectors of Q4,13, then are Q5,6 shorted E-C? Did we already check the four zeners acrosss the gates?

[lowell]

How much of your labor rate do you spend wondering if that 40 cent part is bad?

Would you just be more direct already?

Sorry my MAIN question and reason for my post was unclear. I wasn't sure if the offset was from the power amp output being fed back to the opamp or vice versa. But yeah, putting a new one in there would be the obvious shot at seeing if that fixes it. I was non-specifically asking for more theory on why this offset was there.

If you have zero volts on the gates, and collectors of Q4,13, then are Q5,6 shorted E-C? Did we already check the four zeners acrosss the gates?

Q5/6 aren't shorted E-C. I get the combined series resistance of the bias adjust network when measuring there. Also pulled C12 to see if was leaky or shorted - didn't help. Each of the zeners had a reading across them, making me believe they're not at least a dead-short. Q4/13 also measure fine concerning semi-junctions.

It's gotta be that offset, lets see.

[The Dude]

I don't see why IC4-7 would have full +15V on it. I suspect it's bad. Unsolder that pin and make sure it's carefully isolated from the pad (or remove the IC). Is there still +15V on the pad?

[lowell]

I just replaced it - fixed the +15v offset on pin7. However, there was still 0v on those driver collectors. Ended up finding one of the bases solder pad had cracked just enough to disconnect from the trace. When I was measuring it, touching it with the meter probe, the relay was kicking on/off. Upon closer inspection I could see the disconnect. Amp is fixed now... I think. Put the bias as high as it would go at +/-4.3v. Schematic says +/-5.5v

Played the amp for a couple minutes, sounds great. The fan however, boy this fan is raging. Is it typical for the fan to be going so fast in these heads? I double checked the bias to be sure it wasn't too hot and is still 4.3v.

[MarkusBass]

Amp is fixed now... I think. Put the bias as high as it would go at +/-4.3v. Schematic says +/-5.5v
Played the amp for a couple minutes, sounds great. The fan however, boy this fan is raging. Is it typical for the fan to be going so fast in these heads? I double checked the bias to be sure it wasn't too hot and is still 4.3v.

The post is confusing. By turning the BIAS trimmer you change the DC offset between gates of the output transistors (which should be +/-5.5V). But in order to check the amp (whether the BIAS trimmer works correctly) you should measure the voltage on 0R47/5W resistors. In general, you should measure the quiescent current through the output transistors and not the DC offset on their gates. If you set the the OFFSET voltage to maximum value, you are in trouble (or you still have shorted transistors/diodes).
The fan in this amp has two speeds (slow for cold amp and fast for warm amp). If you have high speed just after you turn on the amp, you have another problem with the temperature sensor in the amp. Take a look at it before you give the amp back to the owner.

Mark

[Enzo]

The gate voltage was a spec on the schematic. As is the output idle current, so both are legit items. KNowing we needed about 11v across those gates instead of zero was a broad clue.

[MarkusBass]

The gate voltage was a spec on the schematic. As is the output idle current, so both are legit items. KNowing we needed about 11v across those gates instead of zero was a broad clue.

That's true. But now you have 8.6V instead of expected 11V. How can you tell whether the quiescent current is set up correctly, or not (without measuring it)? Incorrect quiescent current will cause malfunction of the amp, or even a failure.

[Enzo]

Not arguing for one or the other. Both as specified on the drawing. I think he needs to do both. With the low bias, I might expect low idle current. But I wonder why the bias tops out so low as well.


I always considered idle current more an indicator than a target. In other words my impression - right or wrong - was that they would adjust an amp circuit for proper crossover rejection, and then determine when it is best set, the resulting idle current was thus and so. ie, it wasn't so much the 20ma did the trick, it was the proper setting resulted in 20ma.

[lowell]

Thanks guys - so I want 20ma? That's about 9mv across those .47 Re... rounding down. Correct? Also, why 20ma? I can't find anything in the schematic stating this, nor can I find anything in the datasheet saying something VgsON = ? Or IdON =?

http://www.mouser.com/ds/2/427/91210-22396.pdf

[Enzo]

No, I just used 20ma from memory, read the schematic. It says 25milliVOLTS across each resistor. I apologize for being over casual in my writing.

That spec sounds like a lot to me, but that is what it asks.