Fender BDRI +16/-16 Supply

[bobloblaws]

2. One thing you can do to determine if the connected circuitry is loading down the voltage is to lift the junction of R78 & D13 and the junction of R79 & D14 up off the board and then measure the voltages at those junctions with respect to ground. If the voltage rises to ±16V then you would have your proof that the down-line circuitry is causing a problem. If the voltage is still low, then you have have only a few parts to investigate.

I did this and the zeners are giving me +16V/-16V. I have opamps in stock so I reckon I'll either remove one at a time and look for a bad one or replace all 4 and try again.

[g1]

I reckon I'll either remove one at a time and look for a bad one or replace all 4 and try again.

Instead of removing, you can just de-solder the supply pins, til you find the bad one.

[bobloblaws]

Instead of removing, you can just de-solder the supply pins, til you find the bad one.

Geez, never thought of that. Thanks!

[Tom Phillips]

Geez, never thought of that. Thanks!

You can also carefully touch the case of the ICs with you finger and determine which one is getting warm or Hot. Only if you feel safe and comfortable with doing that.

Edit: To be clear for people who may want to try this technique in the future, you don't need to have the amp powered on when feeling the IC cases for excess heat. Just leave the amp on for a while (as long as nothing is smoking, redplating etc. ) Then turn the amp off, unplug the power cord, discharge the filter caps and you will be safe from electrical shock while feeling for excess heat. In a case like the one that bobloblaws just solved, you should be able to tell that one IC was significantly hotter than the others. Use common sense just as you would around other potentially hot items.

[bobloblaws]

Instead of removing, you can just de-solder the supply pins, til you find the bad one.

Yes! U3 was the culprit. The opamp supply is restored. Thanks everybody for the suggestions!

[bobloblaws]

Hey guys, I'm hoping you can help me with a another small problem without having to start another thread. I have the amp all back together and thought I was done, but it is making a crackling sound when adjusting the guitar volume. I've never run into this before and trying to google it just results in suggestions for pot cleaning related to the guitar itself or the amp itself. In this case the amp is reacting to the guitar volume pot. This is the case regardless of the guitar used. Any ideas?

[nosaj]

DC on the pot or cracked or cold solder joint?
nosaj

[Helmholtz]

Sounds like DCV at the amp's input.
With guitar connected measure DCV between V1-A pin 2 and ground.

[g1]

DC on the pot

This is possible as the first tube stage (V1A) has no cap between the input and it's grid. Sometimes an iffy preamp tube will put DC on it's grid. Try a couple different ones for V1 and see if it gets rid of the guitar pot issue.

[bobloblaws]

Sounds like DCV at the amp's input.
With guitar connected measure DCV between V1-A pin 2 and ground.

OK, that reads -0.197 VDC. I don't know what would cause that off the top of my head.

[bobloblaws]

OK, that reads -0.197 VDC. I don't know what would cause that off the top of my head.

I just thought of something. When I was working on the bottom of the PCB there was a lot of damage around the first couple of pots, looked like heat damage as well, strangely. I re-soldered as well as I could and checked continuity, but I'm wondering if there is conductivity on the board as xtian was talking about above.

[Helmholtz]

That's a voltage caused by excessive grid leak current of V1.
Try a new tube.
Board conductivity would cause a postive voltage.

[bobloblaws]

This is possible as the first tube stage (V1A) has no cap between the input and it's grid. Sometimes an iffy preamp tube will put DC on it's grid. Try a couple different ones for V1 and see if it gets rid of the guitar pot issue.

Hi, I tried 3 different tubes in place of V1, same issue.

[bobloblaws]

That's a voltage caused by excessive grid leak current of V1.
Try a new tube.
Board conductivity would cause a postive voltage.

Hmmm, I tried a few different tubes and still had the same problem.

[bobloblaws]

This is possible as the first tube stage (V1A) has no cap between the input and it's grid. Sometimes an iffy preamp tube will put DC on it's grid. Try a couple different ones for V1 and see if it gets rid of the guitar pot issue.

Do you think adding a cap might fix it?