Fender Blues Deluxe Reissue bias doesn't match schematic, has intermittent jump in bias voltage

[MichaelNuzum]

What baffles me is all I did was rehab a few mostly working solder joints. When they were working, so was everything else. Nothing was changed or modified, no parts removed.

I re-flowed one leg of R76, both legs of R77, two of the legs for the bias pot, and one leg of R78. All have been checked for continuity to traces and components upstream and downstream, and they seem to check out. Being a double sided board I checked to see if there were any component side traces I might have missed, there are none.

But, apparently in doing that, I somehow negatively affected a previously working low volt system. I don't understand how I might have taken out some other component downstream, but I admit, there is a lot I do not understand.

I checked continuity on ribbon cables just to make sure one didn't go funny, but they check out.

I am at a bit of a loss right now. Complete head-scratcher.

[The Dude]

The zeners are suspect. Check them out of circuit and repost. FYI, you can just unsolder one leg of each and test. Also, there are other things in parallel with the zeners, so it could be something else loading the supply, but I would check the zeners out of circuit first anyway.

[MichaelNuzum]

Hey all, had to go out of town for a week, but I am now back at it. I pulled one leg of one of the zener diodes and got .658 one way, open the other.

I don't have benchtop power supply to test for voltage drop. I was hoping to get two fresh 9v batteries and make an 18v test supply. I would then wire the batteries, a 27 ohm resistor and the diode (2v drop/75mA test current = 26.66). Then I should see 16v drop across the diode. Did I do the math for that right?

Is this a reasonable way to test, or is there a better way? Can one leg remain on the circuit board for the voltage drop test, or should it come fully out ? Thanks again.

[nevetslab]

What baffles me is all I did was rehab a few mostly working solder joints. When they were working, so was everything else. Nothing was changed or modified, no parts removed.
[ATTACH=CONFIG]55280[/ATTACH]
I re-flowed one leg of R76, both legs of R77, two of the legs for the bias pot, and one leg of R78. All have been checked for continuity to traces and components upstream and downstream, and they seem to check out. Being a double sided board I checked to see if there were any component side traces I might have missed, there are none.

But, apparently in doing that, I somehow negatively affected a previously working low volt system. I don't understand how I might have taken out some other component downstream, but I admit, there is a lot I do not understand.

I checked continuity on ribbon cables just to make sure one didn't go funny, but they check out.

I am at a bit of a loss right now. Complete head-scratcher.

Looks like the one Zener nearest the chassis mtg hole isn't soldered well to Ground. Over time, the resistor leads on those 5W bathtub resistors AND the zener diode terminals become quite oxidized. I've had to remove them, scrape away the oxidation to get to fresh metal tin them and then reinstall them. Your PCB looks to be in excellent shape....not seeing the usual long-term discoloration from the 5W dropping resistors. Look at an equivalent Hot Rod series PCB, and you'll appreciate what you have here a lot more.....PCB quality on the Hot Rod series leaves a lot to be desired.

[MichaelNuzum]

Well, it's not the zeners. I broke down and purchased a bench top power supply. I put one of the zeners on it, and it dropped 16.16V. I suspect the other will be similar.

The way the power supply is dropping symmetrically makes me suspect one of the ic's. So far, I pulled U1 and no change. There are three others, I'll see what happens there. Hopefully pulling one of them will make a difference. At least help me isolate it to one leg of the system vs another.

I really don't like doing it this way, but I am out of ideas at this point.

[The Dude]

Unfortunately, when there's a short on a supply line, not much else you can do except unsolder connections until you find it. PITA, but sometimes necessary.

[MichaelNuzum]

Yep Dude, total PITA. I unsoldered all four ic's, and wouldn't you know it, the last one finally brought the voltage back up. Four ic's x 8 legs each = 32 solder joints (really 64, it's a double sided board), and not one lifted pad!! Told you my solder skills were pretty decent!

So now I have to figure out if it is the ic or something else in that leg the circuit. I just went ahead and ordered replacement ic's so I have a good new one to throw in there. Hopefully that will fix it and let it be happy once again.

Thanks again for all the help, y'all.

[MichaelNuzum]

I am still left wondering what the heck happened? I am careful to discharge the power caps before I work, I use a solder station that claims to be esd safe, and I never arced or sparked or noticed any sort of shorting. How in the world did I take out the reverb drive circuit?

I am someone who likes full knowledge of their mistakes to learn and avoid a repeat. I am big enough to admit when I am at fault, acknowledge what I did incorrectly and grow from there. But with this one, while I have to believe I am at fault for the failure, I can not for the life of me, figure out what I did incorrectly. Hard to learn from your mistakes when you are not sure what the failure action was. Kinda frustrating.

Anyway, thanks again for all the help and logic. I do this by myself out in the middle of nowhere and have no local resources/peers here to bounce ideas off of. You all provide invaluable feedback and ideas and I am truly grateful for folks like you that feel inspired to share your knowledge. Much appreciated.

[Leo_Gnardo]

Yep Dude, total PITA. I unsoldered all four ic's, and wouldn't you know it, the last one finally brought the voltage back up. Four ic's x 8 legs each = 32 solder joints (really 64, it's a double sided board), and not one lifted pad!! Told you my solder skills were pretty decent!

So now I have to figure out if it is the ic or something else in that leg the circuit. I just went ahead and ordered replacement ic's so I have a good new one to throw in there. Hopefully that will fix it and let it be happy once again.

I hope you're installing sockets for those IC's, in case you ever need to change one again.

Murphy's law, part 672 subparagraph "n," states, if you don't socket, you WILL have to unsolder that IC again sometime. OTOH, if you use a socket, that IC will never go bad again. Most of the time...

[g1]

Yep Dude, total PITA. I unsoldered all four ic's, and wouldn't you know it, the last one finally brought the voltage back up. Four ic's x 8 legs each = 32 solder joints (really 64, it's a double sided board), and not one lifted pad!! Told you my solder skills were pretty decent!

Good job, but a tip for future reference:
since you are only concerned with removing the short from the supply rails, you only need to desolder the power pins on those IC's, so 8 connections in total. Double sided is more tricky, but you can do resistance checks to make sure the pin is disconnected.
If you do them one at a time, you probably won't have to do all 4 unless it turns out the last one is the bad one (usually my luck ).
Sometimes, when the unit is much more complex, you end up cutting traces to isolate sections, then reconnect them later. It is not ideal, but if it ends up saving hours of labour, it ends up being in the customers best interest.

[MichaelNuzum]

Hey g1- thanks. You know, I was curious about that. I wasn't sure if just un-soldering the power feed leg would work or not. I was concerned that because it is a double sided board with through-plated holes, just removing the solder might not break the connection (and I was hoping to avoid cutting traces). So, I erred on the side of being certain. Maybe too much work, but with this amp's failure being so confusing for me, I wanted to be absolutely sure I found the fault.

And yeah, of course, it was the last of four ic's that was the guilty party! Oh well.

[52 Bill]

Just out of curiosity, did you read any voltages on the pins of the ICs before removing them? Did any of them seem to be getting warm?

[MichaelNuzum]

52 Bill - I just checked that the ics were seeing the low voltage. I didn't think to check the temp. I just recently got an IR laser thermometer gun, it seems fairly sensitive. I have honestly only used it as a quick way to look at power tubes to make sure they are all conducting. Would an over-consuming ic heat up enough to show up on something like that?