Ampeg BA115-HP Noise Issue

[misterc57]

My Ampeg BA115HP is making a constant noise with nothing connected to the inputs. Sounds like wind and occasional thumping. The noise can be made louder by using the gain and master volume pots.

I think the problem is at U1 (TLO-72). I can hear the noise at both pins 1 and 7.

I have the following VDC readings at U1

1 : 8.2 VDC
2 : 8.2 VDC
3 : 8.2 VDC
4 : -16.7 VDC
5 : 1 mvdc bounces around
6 : 5 mvdc bounces around
7 : 5 mvdc bounces around
8 : 16.6 VDC

Looks like pins 1,2,3 are way off? I have checked resistors in circuit which seem okay. D10 seems ok.

Would you all agree that U1 needs replacement? Anything else to check? Not sure what ICs I have sitting around, are there substitutes that can be used?

Thank you. Mark

[Jazz P Bass]

I would wonder why pin #1 is at 8.2Vdc.
It should be zero volts as it is the input in from the input jack.

Either the ic is toast or D27 is.

You could try removing the IC.
If the voltages then stabilize, the IC is bad.


ampeg_ba115hpt.zip

[misterc57]

I do not have a D27. I forgot to attach the schematic that matches my amp.. Here it is.

I see the following at the two diodes.

D10 cathode = 16.9 VDC, anode = 7.8 VDC
D13 cathode = 7.9 VDC, anode = -17 VDC

[Jazz P Bass]

Thanks for the correct schematic.

The question remains: why is there voltage at pin #1?
Remove the diode and or the IC.

Any dual opamp will work there for a test once the voltages are normal.

[misterc57]

No progress. I pulled the pre-amp board and lifted a leg on both D10 and D13. Diodes tested good. I found some new spares and replaced the TLO-72 (U1). Put everything back together.

I still have noise and now 7.7 VDC on pins 1,2, and 3.

Pin 3 is the input in from the input jack.

I did not try to remove either the IC or the diode as suggested as I had the pre-amp board out and was not sure where/what I would be testing with one of those out.

Hmmm, what next? Thank you

[g1]

I'm not sure if you are checking the diodes with a meter or listening with them disconnected? Noisy diodes will test fine on the meter.

[misterc57]

Checking diodes with a meter. When everything is plugged in I am listening at IC pins 1 and 7 (noise on both).

[g1]

Suggest you try testing with the diodes disconnected. If DC and noise is gone, replace diodes.

[misterc57]

Removed D10 and D13. DC and noise are gone.

Those diodes are 1N914, can I replace with any general purpose diode like a 1N4003 - 1N4007?

I do not understand the purpose of D10 and D13 and why the amp can operate without them?

Thank you! Mark

[g1]

The diodes clamp excessively large signals.
Use signal diodes like 1N914 or 1N4148, not rectifiers like 1N4007.

[Enzo]

If you plan to do much of any electronics in the future, you need to stock the 1N4007 for high voltage use, and the VERY common 1N4148 for little diode needs like this.

You can get 100 of them for less than $2 from Mouser on your next order.

[misterc57]

I am looking to order the 1N4148, there are numerous specs to filter on (see image) and I am not sure which if any are important to select. Thank you

[Enzo]

Forget them.

1N4148, discrete diodes. (86)
"In Stock", (58)
Through hole, (18)

With 18 on the page, I look down the offerings. First three for example are Fairchild, all $1.90 a hundred. Note the difference? First is bulk, meaning loose pieces in a bag. next is reel, where the diodes are lined up next to each other with the ends of the wire leads stuck in strips of tape. Then that is rolled into a large reel of 5-10 thousand. You only want 100? they cut off 100 from the reel. Then there is ammo box, which is the same taped parts, but zig zag stacked in a box, like machine gun bullets for the army. Same parts, just how would you prefer to get them?

At this point I am selecting the $1.90 choices, and ignoring the $6/100 options.

[misterc57]

My local Radio Shack is closing this month. I ended up getting a few things there including a box of 50 1N4148 for under $2.

I also found a couple of 1N914 and put those into the Ampeg. This amp is now working fine. Every repair is a great learning experience!

A big thank you again to all for your help! Mark