Extended FP III resistance issue [resolved]

[Neuronal]

Hi - after burning in my (newly built after all these years) FPIII for a few months I decided to install the extended upgrade. When I took resistances at the terminals, everything was normal. To be extra careful, I also read across all of the accessible resistors to make sure they were still reading correctly. Everything looks fine except for the B side 127 kOhm resistor on the shunt regulator board (light blue, on the lower left when looking at the flipped chassis with the volume pot towards me), which reads 82.3 kOhms, as opposed to the A side equivalent resistor, which reads correctly at 148 kOhms. When I read across the + and - terminals on the A side, I get 148 kOms, and when I do the same on the B side I again get 82.3 kOhms.

Is this a problem, and if so is it limited to that one resistor (or instead maybe I've messed up something that lives in parallel to that resistor)?; I haven't yet fired it up to take voltages, as I'm afraid of smoke given the strangeness.

thanks in advance for any advice or help- it is really appreciated!

[Paul Birkeland]

The voltage divider that biases up the heaters sits across one of the regulated B+ rails and will throw off one of those resistances.  I would bet if you heat up and lift out one end of the resistor in question, it will measure perfectly.

Having said all that, if you have had this kit sitting around for 10 years, the probability that the included LEDs have absorbed too much moisture to be soldered is somewhat high. 

[Neuronal]

Thanks Paul - I remembered your advice from before about old LEDs, and before the build I went ahead and bought 20 from the Queen.

I fired it up, and the tubes lit up. But now a new set of problems:

1. On the left, the A side C4S board LEDs light up; the B side does not light, but the LED soldered into the tube holder does light. On the right, no LEDS on either the A or B side, and the LED in the tube holder does not light either

2. Voltages (VDC) are really off. Power supply voltages (strips 1-10) are normal. But:

H1 - 4.5 V
H2 - 11.4 V
12 - 25 V
15 - 15 V
21 - 25 V
22 - 4.5 V
21 - 208 V
21 - 156 V

Any advice? I'm not sure where to start with this....

[Neuronal]

Oh, and since I forgot to mention it, all the shunt reg LEDs light up fine

[Paul Birkeland]

It would be far, far easier if I could have:

IA, OA, and OB for the two outside boards.

IA, IB, OA, OB, and Kreg for the middle board.

[Neuronal]

on it, will send ASAP

[Neuronal]

Total newbie ? - just to verify, KReg is the K lead on the regulator chip, not the pad marked K that sits near it. Thanks - b

[Paul Birkeland]

The pad marked K.

[Neuronal]

quick update - I went back and realized that made some JV errors here - on the A side I flipped the MJE350 and MJE340 transitors, and on the B side I made some simple wiring errors. I cut the leads really short on those transistors on the A side when I first installed them, so unless you have some advice as to how to get a three lead transistor out of the board without clipping the leads, I'm going to clip them out and write the Queen for replacements, then reinstall on that side. More to follow....

[Paul Birkeland]

If you have an electrically powered solder vacuum pump, this can all be pulled apart in a few minutes.

Beyond that, trying to use solder braid or a manual pump will destroy the boards most likely, so I would clip them and just replace the parts.

The MJE340 is no longer in our inventory, but Mouser and Digikey should still have them.

[Neuronal]

now you've got me trying to justify spending $250 on one of those nice Hakko jobs, as opposed to $2.00 on new transistors. I'll wrestle with my conscience and we will see how this goes. I mean, I'm definitely going to need to desolder more in the future, right?

[Paul Birkeland]

Yeah, I bought one of the $200 Hakko pumps not that long ago, I don't know why I waited so long to do that.

[Neuronal]

OK, I bought one of those Hakko desolder guns (totally awesome!) and replaced the regulators on the A board, and went through and verified the wiring is correct throughout. Voltages seem off:

LAI - 44
LAO - 41
LBI - 0
LBO - 34.5

RAI 34
RAO - 21
RBI - 0
RBO - 16.8

center
AKreg - 4.1
AI - 217
AO - 44
BKReg - 4.4
BI - 216
BO - 40.1

All the LEDs on the center light up fine. Only on the left board, only A left lights. On the right board there is a flash from the top two leds and then they both go out.
Also -
H1 - 8
H2 14
H4 7.8
H5 14

Thanks for any help you can provide - I will not be defeated!

[Paul Birkeland]

What did you replace with what? New photos?

The lettering is "IA" for input A, "IB" for input b, "OA" for output A, "OB" for output B, etc.

Your raw power supply is working, your K voltages show that the regulators have enough voltage to operate, but you only have 40V on each rail.  Whatever issue is present must be present on both sides.

If you remove the two outer 12AU7s and leave the center one in the socket, what voltages do you see on the center PC board?

[Neuronal]

In the initial build on the left C4S board I had put the MJE350 where the 340 was supposed to go. Those are corrected now. I had also made a silly wiring error connecting the right board, which I've now fixed.

With the right and left tubes out, but center 12AU7 in:

KRegA - 1.9
IA - 219
OA - 152.8

KRegB - 1.93
IA - 219
OA- 150

Pics of boards attached; I solder on the bottom per the instructions so only solder on the top of the board is for the wiring. Oh, and the H1-2 voltages are also lower than in the manual if that means anything. - thanks again!