Extended FP III resistance issue [resolved]

[Paul Birkeland]

Well, your center board appears to be OK.  I would imagine you still have issues on the outside boards such as backwards LEDs or possibly 2N2222/2N2907s swapped.

[Neuronal]

you were right - transistors were flipped. It is a miracle I can put my shoes on by myself. The only upside of my stupidity is that I get to use my Hakko one more time, thereby proving its value to my wife. Fixing now, more to follow....

[Paul Birkeland]

There are reasons that we no longer have the MJE340 nor the 2N2907 in our inventory anymore

[Neuronal]

OK, I flipped the transitors. They looked kinda crappy coming out - that could either be aesthetics or I smooshed them. In any case, I reinstalled and got this:

left
IA - 159
0A - -.1
IB - 0
OB - 10.3

right
1A - 48.2
OA- 28.3
IB - 0
OB - 23.3

On the left board the top two LEDs now light up; in the right both top two light up, but they are really dim.

Getting closer?

[Paul Birkeland]

Is it possible that you have some backwards LEDs on those outer boards?  Did any of the transistors measure shorted when you pulled them out?

[Neuronal]

I ordered new transistors from Mouser and have a bunch of new LEDs; updates when the parts arrive....

[Neuronal]

Progress - thankfully! - but not quite there yet. I put in new LEDs (I've left the old transistors in), and now everything lights up nicely. But...

Left
IA - 150
OA - 69
IB - 0
OB - 71.5

Right
IA - 150
OA - 0
IB - 0
OB - 6.4

Of note the moment I turn on the unit the top lights on the right board (both of them) go on; then after a few seconds the whole center board and then the whole left board and the bottom (B side) of the right board lights up. The delay for the center and left boards feels like what I am used to with my paramounts. I feel like the fact that there is no delay at all for the top right lights must mean something (current leaking somehow?). Of note, those same upper right lights stay lit up for many more seconds after the other lights have all gone off when I turn the power off.

Anyways, getting closer. I didn't replace any of the transistors, but I have replacements for all of them and can swap them out if you think that is what is going on. I am quite certain at this point everything is oriented correctly and in the correct position, etc....

[Paul Birkeland]

You can swap the outside pair of boards from side to side to see if the 0V at OA follows a board or stays on one side of the preamp.

[Neuronal]

OK, will do now. I will say, when I bought that Hakko I thought it was an indulgence, and now it is my best friend!

[Neuronal]

OK, did the swap - the voltages and the behavior follow the board, which (I think) is all and all good news. Is the right next move to swap out all the A side transistors?

[Paul Birkeland]

That's certainly an option, as well as double checking the LEDs.

[Neuronal]

One step forward....two steps?

So I rebuilt that board completely and reinstalled it. When I powered up, the LEDs were perfect (they all lit up across all three boards at about the same time after a delay). As I was taking voltages, I noticed smoke and immediately shut it down.

Turns out, in my excitement, I forgot to add the black wire that interconnects the A and B side of the board, and the two 270 ohm resistors (that interconnect 7-9 and 9-10) on the power supply on just that side burned out.

My question (for now) is: should I just replace those two resistors, or are there other parts I likely damaged? Should I rebuild the entire power supply on that side? At least it looks like I fixed the board....

Sorry for all the low-level screwing up and then asking for help. I apparently can record electrical transients from living neurons, but am incapable of following instructions carefully

[Paul Birkeland]

I have absolutely no idea.  You'd kind of need to start over on that side.  At least replace those resistors and run the preamp without the outer 12AU7 on that side, then check Kreg and O voltages on the small PC board to be sure it's regulating to 150V, then go back through the small outer board and rebuild it.  You shouldn't have to pull out the black jumper wire to rebuild the board.

[Neuronal]

OK, I think I'm good here, but just want to check to make sure I haven't missed anything. THANK YOU so much for your generosity and patience in walking me through this, Paul! You are the best!

I replaced the blown resistors on the left power supply, all of the transistors on the left side board, and both the transistors on the shunt reg board on the A side. I deduced that I had taken out the A side of the shunt reg board in addition to the left board during my little smoke event because there were some asymmetrical resistances when I compared the A and B sides of the shunt reg board (and hence asymmetrical resistances when I compared the left and right C4S boards relative to the bus ground). These became more normal when I replaced the Shunt reg A side transistors (but not identical - if I ground at the ground bus, on the shunt reg OA reads 80 kOhms, while OB reads 147kOhms; all other readings were identical across the boards).

I figured this was close enough to start measuring voltages - the shunt reg board read exactly what it read when it was working before (Krega 1.9, IA 213, OA 154, KRegB 1.8, IB 212, OB 150); there is some minor variance in the left and right boards that I (hope) is just variance:

left -
IA - 150
OA - 75
IB - 0
OB 75

right-
IA - 150
OA - 71.5
IB - 0
OB - 71.5

I took all the other voltages in the manual, and they are dead on. I am TERRIFIED of smoking this again, but I think I am ready to go? Thanks again for your help with this Paul - and I wish you and yours a very happy Thanksgiving!

[Paul Birkeland]

Your left and right C4S boards have the voltages you want. 

1.8V or 1.9V at the K terminals on the middle board still indicates an issue there, most likely something like a solder bridge on the bottom side of that PC board or you have the input and output red wires swapped possibly.