Ampeg BA112 - Low volume issue

[misterc57]

I am looking for some trouble shooting advice. I picked up a BA112, it has very weak volume.

The volume and EQ pots have been cleaned as well as all the jacks. If I crank all the pots to 10, I can slightly hear the guitar, but the volume is way below what it should be, I'd say under 10% of where it should be.

If I connect a device to the CD input I get plenty of volume. So I am assuming the problem is in the pre-amp section.

I have 13.7+ dc volts at D1 and at 13.7- dc volts at D2. The schematic is stating 15 volts at those points.

I plan to run a tone through the circuit and probe around. Looking for specific suggestions and help.

I have attached two schematic pdf docs.

Thank you so much for your help and advice!

[Enzo]

Don't plan, DO it.


Parts of the system work, so i start to think of things like an input cap broken free or something. And of course a dead op amp comes to mind too.

[g1]

If the cd input is working, then you can ignore everything after pin 6 of IC3. Make sure that the style switch is ok.

[misterc57]

Here is what I have found so far. Hooked up a 440 hz signal from my pc.

All these tests are by listening...

IC1 is boosting the tone at pin 1.
IC2 is boosting the tone at pin 1.
IC2 has NO audible tone at pin 7, 6 or 5. If I touch pin 5 of IC2 with my DVM probe I get a nice strong crackling sound sent to the speaker.

I am suspicious of the style switch. I can hear a tone on the caps and resistors on each of the five signal paths before the switch. I hear no tone after the switch; at R27 or IC2.

I am thinking the next test would be to bypass the switch using one of the five signal paths, probably the one coming from IC2 pin 1 direct to R27 or the output side of the switch. It looks like I have to pull the board out to get to any contact points under that switch. Can I just run a jumper wire from R26 to R27?

Am I taking the right path or do you gents have other suggestions?

Thank you. MC

[g1]

Can I just run a jumper wire from R26 to R27?

Yes, that will bypass the switch, but have you tried giving it a shot of cleaner?

[misterc57]

The switch has 1M ohms resistance. I have not tried to clean the switch as I do not see any access point to spray into???

I bypassed the switch and the amp works fine. If cleaning it does not solve the problem, where can replacements be found? I am thinking of permanently bypassing also.

Thank you!

[Enzo]

The switch shows 1Meg from its common to all five poles when selected?

Did you jumper/bypass the switch right at its solder terminals? Or did you jumper the circuit at a more convenient spot which was electrically the same? In other words is there a possibility there is some break between the common pole of the switch and the pin 5 of the IC?

But if the switch itself is indeed bad, we are at the what is there to lose stage. Is there any wiggle room in the pins, or are they molded into the plastic? Pots are usually assembled by bending little tabs over to keep the back cover on. Unbend those and it opens. Is the switch built this way or is it riveted? Some tiny selector switchs even have screws holding them together, maybe god loves you?

One time honored technique is to dribble cleaner alongside the shaft, and it gets inside.

There used to be a thing called a "Gozinta", which you stuck on an aerosol spray. It was a thing that threaded down over the pot shaft and had a tube to your spray can. It sort of force sprayed at the shaft and some would be forced into the control. Once upon a time all the pots had 3/8 bushings, but now days you'd need a selection of them, still:

STEWMAC.COM : Pot Cleaning Cap



And if all else fails, the thing is part number 88-108-01, maybe Loud Tech even has some in stock?

[misterc57]

I was wrong when I said the switch has 1M ohms resistance, I was measuring across R26 and R27 and not the switch itself.

After pulling the board out I noticed that every single contact pin for the switch had a broken solder joint.

Amp and switch now work fine!

Thank you all for your help, it is much appreciated! MC