Ampeg BA-210 SP

[MarkusBass]

Hi mark thanks for chiming in. I fixed the preamp voltages by replacing D11 and C39.

It's theoretically posiible that both D11 and C39 have failed at the same time but I think that practically it's rather not possible - rather only one of them has failed. Additionally the rail could be just shorted by one of the ICs (maybe by U2). You could check it by measuring voltage across R68 and R66 (and calculating the current from Ohm's law). Are both rails 16V now?

I then replaced U2 and the amp seemed great, until 5 minutes after reassembling. When i checked, U2A had DC on on all pins.

It can be again caused by a failure of e.g. D11. Do you still have +16V on this rail? What do you mean by DC on all pins? 1V or 16V - this is a big difference .

I guess I should check to see if the dsp is putting out dc. DC on the input of an opamp isn't destructive is it?

I already suggested to disconnect the DSP module and turning the EFFECT BLEND pot down. DC on the input of an opamp is destructive (of course, depending on DC value an opamp configuration).
You can also have DC from the power amp. There is C11 capacitor between the preamp and the power amp but just to be sure that the power amp is not causing any problems, I would desolder R25 (input of the power amp).
So my suggestion is:
1. Diconnect DSP module and turn down the EFFECT BLEND pot
2. Desolder R25
3. Check +/-16V rails (fix them if there is a problem)
4. If there is DC on the output of U2, replace it.
5. Tell us the results

Mark

[lowell]

Ok I diconnected the DSP at J5.
The noise is only there if the LEVEL knob is turned up, so I don't see any point in disconnecting R25... I'll do it though if you insist.
The rails are good, however about 200mv apart.
There was not DC on U2A's output after replacing it but now there IS some... when I put my meter on it it fluctuates from .7v-1.2v.
I signal probed and the noise is on all three pins of U2A, and there is no noise on U2B on any pins.
Also of note: If I touch the LEVEL casing w/ my finger it induces a hum. This doesn't happen when I touch ANY other pots casings. I replaced the pot and it didn't help.

[MarkusBass]

No, in that case I don't insist to disconnect R25 :-).
I would look at two other problems:
- I would check whether LEVEL pot casisng is connected to the ground. Most probably not, so I would check how other pots' casings are grounded. I would check whether the lower pin of the LEVEL pot is soldered correctly to the ground. And what happens if you take a wire a connect the pot's casisng to the ground? Does it remove the noise?

- I would check all components around U2A. Actaually it works in a very simple configuration - voltage follower. The (-) pin is connected to the output and the (+) pin is connceted with R3 and C53 to the ground and this is also a signal input. So you have just 2 components to be checked. Is R3 soldered correctly?

Mark

[lowell]

Mark,
The level pot is in the feedback loop so it's not the traditional grounded volume control. None of the pots' casings are grounded. No the noise does not go away when I ground the casing w/ a wire. All those components check out fine w/ the meter. R3 is connected and good. Maybe C53 is leaking?

[lowell]

wait a minute, yes the noise from touching the pot DOES go away when I ground it w/ a wire.

[MarkusBass]

Lowell,

In sorry, I suggested "Diconnect DSP module and turn down the EFFECT BLEND pot" and you somehow started talking about the LEVEL pot. But I was still refering to the EFFECT BLEND pot :-). My mistake. And what happens if you turn down the EFFECT BLEND pot?
You can even disconnect everything related to the DSP module by desoldering the JW50 jumper or the R14 resistor. If you still get hum in such a case, it may be coming from U4A or from the CD INPUT. Have you looked at the CD input? I think that you can even short it to the ground just to be sure that this is not the source of the hum.
I would also look at the fact that the DSP module has a separate ground which is connected the ground of the amp only in one point (at the EFFECT SELECT switch). Is this ground OK?

Mark

[lowell]

Ok R28 smoked again while I was touching the LEVEL pot casing causing the noise to happen. I also realized just before it fried that when I pushed on the RCA jacks the noise happened too. It doesn't seem that the noise happens when I touch ANY other pots. It should be noted that at this point U2A was not in the circuit. I had lifted U2A's leads, so that just U2B was connected. This problem must be elsewhere, not just related to U2. Either that or there's more than one contributing factor to this problem.

[MarkusBass]

It looks like you have high frequency oscillations. And we are not sure whether they are caused by a problem in the preamp or by some problem in the power amp (or both). So the idea of disconnecting R25 was not that bad . On the other hand, the idea to lift U2A pins is not good. It means that the pins are floating but in this case you cannot be sure that the IC is stable. I would solder the pins back. You can eliminate U2A by turning EFFECT BLEND pot down or desoldering R14, or JW50 jumper. Did you do it ?
Then, just to be sure that the DSP module is not causing the problem, you can disconnect the DSP module's supply (J21 and J26). Then I would look at all parts that prevents oscillations in the preamp: C42, C41, C51. But please don't replace them; just check whether they are soldered correctly. Another test would be to solder a small capacitor between pins 6 and 7 of U2B (this is the same and parallel to the LEVEL pot). Something like e.g.100pF.
If this will not help you, or it will not give you any other hints, I would test the preamp (disconnected from the power amp) with a generator and an oscilloscope. You could test the gain at high frequencies. BTW, I hope that you don't have the HIGH pot all the way up.

Mark

[lowell]

Ok resoldered U2A back in. EFFECT BLEND has been down all the while. Disconnected DSP supply via J21 and J26. C42, C41, C51 are all soldered and there is continuity to related components.

Most importantly I didn't realize till just now that the GAIN control affects the noise. With the GAIN off the noise goes away. So quite possibly I should try replacing U5? Also, what are D10/13 for? Are they there to minimize DC offset on U5A's input and to make sure that the input is exactly have of the supply rails?

[MarkusBass]

Of course the GAIN pot affects the noise. I think that all amps work in this way. This is the price we pay for amplifying signals . Also HIGH, MID, LOW pots and especially the STYLE switch influence the noise (because this is a set of filters). D10/D13 are to protect U5A from input signal spikes.
Can you tell me how the pots and the switch are set? If they are all the way up, it is possible that there is nothing wrong with the preamp. Maybe the power amp needs some protection against high frequence oscillations.

Mark

[lowell]

Mark,
Tone controls have been all the way up, I also turned the HIGH pot down and the noise was minimized but still there. Regarding the GAIN pot: If turning that off rids of the noise then the noise must be BEFORE it right? So that's my reasoning here. I see what you're saying that more gain always will introduce more noise, but if this problem were after the GAIN pot it'd still be there if the GAIN was off... however it'd be lessened.

[lowell]

I just replaced U5 and the amp's been on for 15 minutes w/ no issues whatsoever. I'm now suspecting that there was more than one issue w/ this amp... namely that U5A must've been damaged via of D10/13 when the preamp supply rail was bad. I'm thinking if the supply was bad then D10/13 were allowing for a reasonable DC offset to be present on the input of U5. I'll keep it on for an hour or so before reassembling and report back.