Ampeg BA115HP Loud Hum

[ca7922303]

Does the power LED come on? If so then 16V is working at the front board.

LED does come on.
with negative DMM probe on negative end of d11 instead of chassis ground.
D11(-16vdc)
D12(+0.7vdc)
R66(-42vdc)
R68(+42vdc)

[ca7922303]

Does the power LED come on? If so then 16V is working at the front board.

With DMM black probe on #1 pin of speaker jack, getting about +50/-50vdc on all four(4) FET's.
U5 pin #4(0.7vdc)
pIn#8(+16vdc).
Now that I have a good ground, I'm getting better readings.

[ca7922303]

Does the power LED come on? If so then 16V is working at the front board.

Also, getting (-59vdc) at speaker output on board, both probes on pins.

[ca7922303]

Also, getting (-59vdc) at speaker output on board, both probes on pins.

Haven't taken either boards loose yet to check caps. Any thoughts on that 0.7vdc on U5(5532) pin #4? Thanks.

[glebert]

Haven't taken either boards loose yet to check caps. Any thoughts on that 0.7vdc on U5(5532) pin #4? Thanks.

I would take the schematic and go over the whole amp and measure anywhere there should be a -16V. I would use the speaker ground as the reference for all voltage measurements. My guess is that you aren't getting -16V anywhere (since that U5 pin 4 should come directly off D12 which is hte -16V "regulator") but doing onsy-twosy measurements and making assumptions is not a recipe for success.

[g1]

There are only 5 IC's. Measure DC at pins 4 and 8 of all of them.

[ca7922303]

There are only 5 IC's. Measure DC at pins 4 and 8 of all of them.

U1 pin#4(1.3vdc) pin#8(15.6vdc)
U2. (1.3vdc). (15.6vdc)
U3. (0.7vdc). (16.4vdc)
U4. (0.7vdc). (16.5vdc)
U5. (0.7vdc). (16.6vdc)

[ca7922303]

I would take the schematic and go over the whole amp and measure anywhere there should be a -16V. I would use the speaker ground as the reference for all voltage measurements. My guess is that you aren't getting -16V anywhere (since that U5 pin 4 should come directly off D12 which is hte -16V "regulator") but doing onsy-twosy measurements and making assumptions is not a recipe for success.

R66(-16/-59vdc)
R9. (0.7/1.3vdc)
C40(0.7/7.0mv)
D12(0.7/7.3mv)

[ca7922303]

R66(-16/-59vdc)
R9. (0.7/1.3vdc)
C40(0.7/7.0mv)
D12(0.7/7.3mv)

R59(0.7/0.7vdc)
R42(0.7/5.6vdc)
AP1(6/6/6vdc)

[glebert]

R66(-16/-59vdc)
R9. (0.7/1.3vdc)
C40(0.7/7.0mv)
D12(0.7/7.3mv)

Seems like something wrong with this. D12 and R66 share a node so -16V should be at both. D12 is what drags the voltage down from -59V to -16V across R66, so it seems impossible that there isn't -16V on D12. I guess I would next do resistance measurement to see that the D12 ground is the same as the speaker ground and between R66 and D12 to see they are connected.

[ca7922303]

Seems like something wrong with this. D12 and R66 share a node so -16V should be at both. D12 is what drags the voltage down from -59V to -16V across R66, so it seems impossible that there isn't -16V on D12. I guess I would next do resistance measurement to see that the D12 ground is the same as the speaker ground and between R66 and D12 to see they are connected.

Had tried to avoid removing boards until now. Turns out R66 had a broken leg obscured by a holder that legs fit in at the board. Resoldered leg and now have normal output (no hum). Sounds suprisingly similar to the SVT-ii Pro just worked on recently. Not as warm and round of course, but definitely the Ampeg bass sound. Thanks for all the help.

[glebert]

Glad that it worked out for you, the one I just got done working on was a major PITA.