need ampeg B2R schematic

[melvin]

...have a B2R with a fan that doesn't want to turn on. The tip-31c in the fan circuit was bad, I replaced it, but what I gather from datasheets and testing it, the fan won't even turn on until the lm-35 sees 70 degrees celsius. I wonder if this is normal. I have a schematic for the ampeg B2, but the circuitry for the fan control is slightly different.

[Jazz P Bass]

I have a scheme for the B2-SVT. The amp that I worked on did not match the schematic. I called a gentleman at St. Louis music & he indicated a design change.
The amp that I had came on when the amp got pretty warm. The schematic I have, the fan turns on at power up & then rotates faster when hot.
There are (2) 270 ohm 5 watt cement resitors between pin 2 of the fan & ground.
This is the always on circuit.
LoudTechnologies has always been helpful as far as schemes are concerned.
You have to supply the serial # of the amp.

[melvin]

ampeg b2r

thanks for the reply. the amp I have has a single 1.5k 5W resistor from fan to ground. I tried the 'always on' config in the b2 schematic but the current when powering the amp up was too much for the power switch (arcing). I know the dc fans draw alot of current when you start them up... I thought those two 270 ohm resistors from fan to ground might be just for sharing the current once the fan did start... maybe I could amplify the dc from the lm-35 just a bit?

[Enzo]

Fan draws a lot of current? The power switch handles the whole operating current of that amp. I would have thought the fan, even its starting surge woulo be a very small percentage of the whole amp draw.

[melvin]

I see your point. And I don't understand why it arced when the power switch was turned on. Maybe I'm missing something. Here's what I did to get such a result.

Referring to the fan circuitry in the B2 schematic, I changed the circuit in my B2R to match everything to the left of R58. This caused it to arc at the power switch at power-on. Originally my B2R had these values:

R54A & R54B was one resistor, 1.5k
R55 was 100 ohms

That's it. The only difference besides the arc was that the fan came on at power-on, whereas before the fan would only start when the lm-35 gave an output of 700 mv, which I simulated at R58.

Could it be that the power switch can't handle very much during switching? The mosfets are on kind of a delayed start too, right? I've seen standby switches with 450 on one side arc when thrown and I've seen old, dirty power switches arc, but I'm not sure what's going on here. (But what's great about this forum is that what's eluding one person can be obvious to another.)

Anyway, I reverted the circuit back to it's original state. There is no arcing and it has been reported to me that the fan did indeed turn on before the relay part of the protection circuit shut the amp down. So I guess it is normal.

[Enzo]

Well, for one thing, correllation is not necessarily causality - just because two things happen at the same time doesn't make them related. They may or may not be. The same switch is probably used in the amps with both fan versions.

Also, whenever we work in an amp, and it then has a new symptom - such as burning switches - we can usually blame it on the work we just did. In other words, you didn't just change a few resistors, you also took the board out, moved things, disconnected and connected things. You could have caused two things to touch that shouldn;t, or you could have damaged another part while working in there.

And does the amp work OK? Are we having an actual problem? Or did we just notice a spark when the switch was flipped? Sparking is common, the amp may have done it all along, but you never saw it before.

I suspect your hand meter has current functions. Disconnect one end of the fan and insert the meter set for DC amps in series with the fan. How much current does the fan draw? Not much, I bet. Now set your meter up for AC amps, and wire it in series with the mains power. Easy way to do that is to remove the fuse and clip the meter across the fuse holder. Now you can monitor the current draw of the entire amp from the wall.

Mains current has a brief spike when turned on, because all the filter caps have to charge. Try it with the fan and again with the fan disconnected. See any difference?

A fan may have a starting current surge, but if it runs at 100ma, a 200ma starting surge will not likely stress the power switch.