Ampeg BA115 Bias

[decades]

Hi, I was looking for a bass amp and saw a BA115, which was advertised in the local classifieds as "project". After reading the common problems with these amps, I took a chance and bought it.

Inspected it when I got home and found that capacitor (c20) had blown and had been removed from the board. Resistor (r31) had been replaced with a 100k ohm resistor. The outputs were shorted so I've replaced those. I've checked the drivers, D-6 to D9 diodes, and other resistors and they've all tested ok. Checked voltage at the speaker pins and got .2V, which I'm assuming is acceptable?

So far so good with the fix. I built a light bulb limiter and so far, there's no sign of a short when powering the amp.

I need help adjusting the bias or figuring out why I'm getting 7 volts across R61. I've tested the trimpot (ap1) and I'm getting open when fully counter-clockwise and about 480 ohms when fully clockwise. Any adjustment to the trimpot will only vary the voltage at r61 from about 7.5v down to 6.6v.

Why is the voltage at r61 not able to go down to the ampeg-specified 5mV?

[Woodstockz]

Originally Posted by decades

Hi, I was looking for a bass amp and saw a BA115, which was advertised in the local classifieds as "project". After reading the common problems with these amps, I took a chance and bought it.

Inspected it when I got home and found that capacitor (c20) had blown and had been removed from the board. Resistor (r31) had been replaced with a 100k ohm resistor. The outputs were shorted so I've replaced those. I've checked the drivers, D-6 to D9 diodes, and other resistors and they've all tested ok. Checked voltage at the speaker pins and got .2V, which I'm assuming is acceptable?

So far so good with the fix. I built a light bulb limiter and so far, there's no sign of a short when powering the amp.

I need help adjusting the bias or figuring out why I'm getting 7 volts across R61. I've tested the trimpot (ap1) and I'm getting open when fully counter-clockwise and about 480 ohms when fully clockwise. Any adjustment to the trimpot will only vary the voltage at r61 from about 7.5v down to 6.6v.

Why is the voltage at r61 not able to go down to the ampeg-specified 5mV?

Usually when fixing stuff, it's the power supply, or power amplifier.

Capacitors usually only blow when they are very old and dried out.

Usually active components blow before passive components.
The passive components bias the transistors, so I would make sure that you have the right values in place for all of your bias components. And also look at Q3, Q1 and Q4 - and maybe replace Q3. Whatever happened to C20 may have damaged Q1.

D6, 7, 8, and 9 are just clamps.

Analyze the circuit in DC for bias, then AC. You can calculate the total resistance, then calculate the DC Voltages at each point, then see how they match up with your measurements.The resistors form a voltage divider to bias the Q3 transistor to operate in the correct range. The voltage divider goes from +16 Volts to -16 Volts. R50, AP1, R44, R43, R42.

R61 is a current sense to see if everything is biased right and the FETs are off when there is no signal. Since you are getting 7 Volts, something is turning them on. Maybe, if you get a bigger pot, you can adjust the voltages further.

[B-string]

If you PM Jerrold Tiers he may have some more ideas.

[Jerrold Tiers]

Ach, ja......... lots.....

But back to discussing this amp.....

First of all R61 is almost certainly open..... it's 0.1 ohms, and 6.6V would be 60 amps.... which I would venture to suggest you do not have flowing......

Unless, of course, the mental giant who replaced the 220 ohm resistor with a 100K has been there also. It might be some wrong value.

let's see..... the trim pot should NOT go open at any point in the rotation...... that right there is an excellent reason for the outputs and anything else around them in the amp to be blown. If it opens, it raises the bias on the outputs to a high level, turning them "on" pretty hard.

Do you have a schematic? If not, PM me and I can email you one. You'll need it if "Mr 100k" has been elsewhere in the amp as well, sowing discord and rebellion..... along with wrong values.

My suggestion is to start by checking all values, replacing that bias pot that goes open, and checking every semiconductor in the thing..... In fact, most are cheap enough that if you are replacing outputs, you can just replace everything made of silicon in the power amp.... then you know you are starting with good parts. Whether they stay that way is up to you.....

[decades]

Thanks guys for the replies! I've been waiting and thought I was gonna be left hanging. I guess day jobs get in the way.

Originally Posted by Jerrold Tiers

...First of all R61 is almost certainly open..... it's 0.1 ohms, and 6.6V would be 60 amps.... which I would venture to suggest you do not have flowing......

Bingo! the R61 was the problem. But instead of being open, Its resistance actually went up to 10,000 ohms (fried). I'm a novice, but per Ohm's law, an increase in resistance is an increase in voltage, which I'm guessing is the reason why I'm getting such a high voltage (7V) at R61. That proved to be the case. I got a new .1 ohm wirewound resistor to replace R61 and that brought the bias numbers closer to what it should be...in millivolts....

ok, The lowest I can get the bias is 9.1 mv across R61. What do you think? acceptable?

Jerrold, thanks for the offer to email the schematics. I have copies.

lol about "Mr 100K"...Yes, thowing parts, without diagnosing the actual problem, can cause more grief than good. ...good thing my brother had spare resistors, caps, etc.

When I bought the amp, I was also handed a zip lock bag of loose parts. In it were the knobs and nuts...but also a bunch of blown up 2.5 and 4 amp fuses.

Iv'e actually tried playing my bass through the amp today and it sounded lovely!

[decades]

oh, and I'm so happy that I got the amp for $40...(sorry, I'm showing off)

[Jerrold Tiers]

9 mV, if you are sure about it, and measured cold with no speaker load...... is a bit "hot" for the bias. I don't know exactly what would happen, other than a little more heating of the outputs.

it's 90 mA instead of 60mA bias current.

[decades]

Originally Posted by Jerrold Tiers

9 mV, if you are sure about it, and measured cold with no speaker load...... is a bit "hot" for the bias. I don't know exactly what would happen, other than a little more heating of the outputs.

it's 90 mA instead of 60mA bias current.

oops, I was measuring with speaker load.

Finally got it to 5.2 mV across r61, no speaker connected, and the trimpot is about halfway. Will play with the amp some and order a new trimpot.

Thanks again for all the help!