Fender BDRI +16/-16 Supply

[bobloblaws]

Hi Everybody, I haven't been on here for awhile, it's good to be back. Anyway, I have a Blues Deluxe Reissue here that someone had been messing around with and apparently gave up and brought it to me partially disassembled. The main PCB is badly damaged from heat in the area of R78, R79, D13, D14, etc. D13, D14, C40 and C41 tested OK but I decided to replace them anyway since they had obviously been exposed to a lot of heat. I replaced R78 and R79 with 470/5W ceramics with sturdier legs than the originals and mounted them 1/2" above the PCB. I had to run a few jumpers on the bottom of the board to account for damaged pads/traces. My problem is this: when I power on the amp there is only +3.8V/-3.8V where I'm expecting +16V/-16V. The other side of R78/R79 is +48V/-48V as expected. I double checked my order from Digikey to make sure I ordered the correct diodes, they are supposed to be 16V zeners, that looks fine. Then I noticed something strange, when I looked at the power resistors I removed they are actually 47ohm rather than the specified 470ohm. That likely explains why everything got so hot. What do you think is going on here? Could one or more of the opamps be damaged and pulling the voltage down? Is there a reliable way to check the opamps in circuit? I've been using the diode checker on my DMM but haven't found anything glaringly unusual. Any suggestions are appreciated, thanks!

Fender-Blues-Deluxe-Reissue-2004-Schematic-Rev-A.pdf

Bob

[xtian]

If the PCB itself is darkened too much from heat, it becomes conductive--you can actually measure this with your meter, for example, between traces that should normally be isolated from each other.

[bobloblaws]

If the PCB itself is darkened too much from heat, it becomes conductive--you can actually measure this with your meter, for example, between traces that should normally be isolated from each other.

If that was the case do you think it might explain why the opamp supply is only +3.8V/-3.8V rather than +16V/-16V?

[The Dude]

What voltages do you have with the correct value resistors at R78 and R79 (470 instead of 47)? Also, if you had the incorrect (too low) value dropping resistors, you may have damaged the zeners. It's not uncommon for the board to show signs of heat in that area. It's one of the reasons Fender changed the resistor value to 470 ohms.

[glebert]

I had a Hot Rod something or other in for repair a while back and it had a blown op amp that was bringing the 16V rails down. IIRC that one wasn't doing it symmetrically but it can happen. If you have an IR thermometer see if any of the op amps are reading hotter than the others.

[bobloblaws]

What voltages do you have with the correct value resistors at R78 and R79 (470 instead of 47)? Also, if you had the incorrect (too low) value dropping resistors, you may have damaged the zeners. It's not uncommon for the board to show signs of heat in that area. It's one of the reasons Fender changed the resistor value to 470 ohms.

I measured +3.8V/-3.8V after replacing D13, D14, C40, C41, and R78/R79 with 470 ohm resistors. I went by the schematic, I noticed the "originals" had been 47 ohm after the fact.

[bobloblaws]

I had a Hot Rod something or other in for repair a while back and it had a blown op amp that was bringing the 16V rails down. IIRC that one wasn't doing it symmetrically but it can happen. If you have an IR thermometer see if any of the op amps are reading hotter than the others.

OK, that is worth looking into.

[xtian]

If that was the case do you think it might explain why the opamp supply is only +3.8V/-3.8V rather than +16V/-16V?

Sure, it could. Depends on PCB's condition, obviously. With amp off and caps drained, measure conductivity from one 470R/5W resistor to ground. Any conductivity? Try the other one. Anything? In a healthy amp, there should be no conductivity.

[g1]

As far as the resistor values, sometimes the color bands are not what we expect as far as multiplier goes, did you measure the old resistors?

edit: never mind, I see they are the 5W ceramic type that does not use color codes

[Tom Phillips]

My thoughts:
1. First it is confusing regarding R78 & R79 values being discussed as 47 Ohms vs 470 Ohms when the posted schematic shows them as 330 Ohms. However, 330 vs. 470 does not explain the voltages you are measuring.
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.

It's suspicious to me that the low voltage is symmetrical. I'm wondering if you may have mis-marked zener diodes.

[g1]

1. First it is confusing regarding R78 & R79 values being discussed as 47 Ohms vs 470 Ohms when the posted schematic shows them as 330 Ohms.

There are 2 supplies shown on pg.2, the Deville uses 330R, but the Deluxe uses 470R.

[The Dude]

There was a tech bulletin for those resistors to change from 330 ohm to 470 ohm because they were getting too warm. That is why the confusion.

[Tom Phillips]

There are 2 supplies shown on pg.2, the Deville uses 330R, but the Deluxe uses 470R.

There was a tech bulletin for those resistors to change from 330 ohm to 470 ohm because they were getting too warm. That is why the confusion.

Thanks. That clears it up for me. The 47 Ohm resistors that bobloblaws found in his amp were a total error.
So now I'm curious what will be found if bobloblaws performs my #2 suggestion from post #10.

[bobloblaws]

It's suspicious to me that the low voltage is symmetrical. I'm wondering if you may have mis-marked zener diodes.

I was wondering the same thing, but I thought what are the odds? I still have the old ones that appear to be fuctional still. I could try swapping them in as well as your other suggestions.

[Tom Phillips]

I was wondering the same thing, but I thought what are the odds? I still have the old ones that appear to be fuctional still. I could try swapping them in as well as your other suggestions.

At least the test would determine if the problem is in the first section of the LV power supply or further down line in the load.

Edit: The test only needs to be done on one side (the + or the -) in order to gain some good troubleshooting information. So just lift two leads and tack them together up above the PCB.