New 700b WOPL build... blowing fuse 2 on left backplane

[George S.]

Hemostats are your friends.

 

[Gepetto]

New numbers w/ 100W bulb:

At 20 V AC:
L: +8.3 V DC
R: +0.0 V DC

At 40 V AC:
L: +0.25 V DC
R: 0.00 V DC

At 60 V AC:
L: +0.25 V DC
R: 0.0 V DC

Kept going for a bit more... bulb got brighter, etc

At 80 V AC:
L: +0.26 V DC
R: 0.0 V DC

At 100 V AC:
L: +0.26 V DC
R: 0.0 V DC

At the same time for each test, grabbed these using the L ground on the respective backplane to ground, and positive DMM lead to top of the respective Phoenix connector per pad location:

At 20 V AC:
12L: -10.1 V DC
11L: +10.1 V DC
Grabbed the right as well for reference:
12R: -10.1 V DC
11R: +10.1 V DC

At 40 V AC:
12L: -15.0 V DC
11L: +15.0 V DC
Grabbed the right as well for reference:
12R: -15.0 V DC
11R: +15.0 V DC

At 60 V AC:
12L: -20.1 V DC
11L: +20.1 V DC
Grabbed the right as well for reference:
12R: -20.1 V DC
11R: +20.1 V DC

At 80 V AC:
12L: -24.4 V DC
11L: +24.4 V DC
Grabbed the right as well for reference:
12R: -24.4 V DC
11R: +24.4 V DC

At 100 V AC:
12L: -28.4 V DC
11L: +28.4 V DC
Grabbed the right as well for reference:
12R: -28.4 V DC
11R: +28.4 V DC

Still have continuity across both left backplane fuses.

Much better voltage readings.

 

[dtafil]

Thanks Joe. Next steps to proceed?

 

[George S.]

Hemostats are your friends.

On retrospect, I made a unintentional and flippant remark to Sniffs post and would like to apologize.
I agree, very easy to overheat components when soldering, especially if soldering twice.
But I think my technique negates overheating. The secret is using a screwdriver or chisel tip on the bottom, and a long thin tip on the top.
I place the screwdriver tip flat on the solder pad not touching the component lead but close, wait a second or two to preheat the pad and through hole. Then move the tip to touch the lead as I touch the solder to the joint once, twice, or three times. I watch the flux flow over the joint followed by the solder. I'm not trying to fill the through hole, as I don't linger, just want that nice meniscus. I get in and out, and I usually find the other side needs similar.
The long thin sharp tip has less thermal mass, but allows me to precisely hit the target on the populated board. I rest the tip on the through hole edge, preheat two to three seconds, then hit it with solder as I move the tip in to touch the lead. I love watching that flux travel like a wave, then followed by the solder. Then I'm quickly out and joint is done.
I wear a Optivisor to see and do this, both elbows on the bench and face in close, little fan blowing the fumes away. Very positive I'm not overheating anything as I'm in and out quick.
Now, the most amazing thing is building SMD surface mount boards with solder paste and a hot plate or hot air. Some people even use toaster ovens or infrared lamps. Built several devices this way and they still work to this day. It's amazing the heat those SMD components will take.

 

[Gepetto]

Thanks Joe. Next steps to proceed?

Replace TL431 and MJE15033. PM me your address and I will mail you replacements.

 

[laatsch55]

With no negative low voltage in the low voltage section, it creeps in from somewhere else?

 

[Gepetto]

With no negative low voltage in the low voltage section, it creeps in from somewhere else?

The op amp finds an escape path through the B-E junction of Q51L and the A-K of D51L to ground. That is why you can only get to +-.25 offset at the output because the op amp is trying its best with only that path to ground instead of a negative voltage rail.

Clear as mud?

 

[laatsch55]

Yepperts...

 

[George S.]

So, one mystery solved. New Hakko FX888D digital station on the way. The temp I had my old station set to is off enough that it's time to relegate it to back up duties or the scrap heap. Looks to be off ~ 50 degrees, at least according to my Fluke IR probe. Sheesh. Getting much better results with the temp turned up to compensate on the wire work. It is about 10 years old.

View attachment 55981

Dave, you might be able to "restore" your old Hakko by "exercising" the calibration potentiometer. Chances are it's never been adjusted since it left the factory. Should be a small hole usually on the front of the unit, takes a jewelers flat tip screwdriver. Try gently moving that pot back and forth a few times and then calibrate to your thermocouple. Betcha it works correctly then.

 

[dtafil]

Dave, you might be able to "restore" your old Hakko by "exercising" the calibration potentiometer. Chances are it's never been adjusted since it left the factory. Should be a small hole usually on the front of the unit, takes a jewelers flat tip screwdriver. Try gently moving that pot back and forth a few times and then calibrate to your thermocouple. Betcha it works correctly then.

Hi George - Yep, actually found the small covered port to do just that. However, Amazon was kind enough to get the newer Hakko out to me, it's already on the bench and in use. I'll probably calibrate the old one and keep it as a backup at this point. Thanks though!

 

[Gepetto]

Thanks Joe. Next steps to proceed?

Hi Dave
With your 100W bulb in the DBT, you can further debug what is going on if you desire to do so. To do this, you will need to reattach the end of D52L that you had removed in prior testing.

The procedure would be to remove the OPA134 from its socket to prevent damage to it in an overvoltage situation.

Once that is done, on the left channel observe both the +15V and -15V regulated outputs on that left channel.

Bring your AC voltage up in steps and see if your -15V regulator tracks the positive one and if both stop around the 15V mark.

You may blow the fuse doing this based on the earlier problem that you reported.

I have one other debug step beyond this to perform after you do this step.

Parts mailed to you this morning.

 

[dtafil]

Afternoon Joe - Hope I grabbed these correctly for you... DMM neg lead to (left) control ground test point, DMM ps lead to +15VL and -15VL test points respectively. Also, D52L reconnected, OPA134 removed from socket, dim bulb + variac results.


LEFT

20 V AC
+15VL: +6.0 V DC
-15VL: -4.9 V DC

40 V AC
+15VL: +6.9 V DC
-15VL: -4.0 V DC

60 V AC
+15VL: +17.2 V DC
-15VL: 0.0 V DC

80 V AC
+15VL: +20.3 V DC
-15VL: 0.0 V DC

100 V AC
+15VL: +22.3 V DC
-15VL: -0.0 V DC

No fuses hamed in this attempt, lol. Bright bulb by end of test. Happy to run next debug step if the above meets your request above. Thanks!

 

[Gepetto]

Afternoon Joe - Hope I grabbed these correctly for you... DMM neg lead to (left) control ground test point, DMM ps lead to +15VL and -15VL test points respectively. Also, D52L reconnected, OPA134 removed from socket, dim bulb + variac results.


LEFT

20 V AC
+15VL: +6.0 V DC
-15VL: -4.9 V DC

40 V AC
+15VL: +6.9 V DC
-15VL: -4.0 V DC

60 V AC
+15VL: +17.2 V DC
-15VL: 0.0 V DC

80 V AC
+15VL: +20.3 V DC
-15VL: 0.0 V DC

100 V AC
+15VL: +22.3 V DC
-15VL: -0.0 V DC

No fuses hamed in this attempt, lol. Bright bulb by end of test. Happy to run next debug step if the above meets your request above. Thanks!

Did you reattach D52L for this experiment Dave? You need to.

 

[dtafil]

Did you reattach D52L for this experiment Dave? You need to.

I did indeed (noted at the top )...

Just went back, D52L still in circuit. Seeing the 0.6 V DC voltage drop anode to cathode, 1.6 V DC cathode to anode (no AC voltage applied) to amp.

 

[Gepetto]

Thanks Dave, appears that the TL431 (VR51L) may be shorted Anode to Cathode producing the zero volts you are seeing OR the Zener diode in D51L could also be the shorted component.

Next step is lifting one end of D51L out of the circuit and doing that experiment over again. This will isolate if the zener or TL431 is shorted to ground

 

[dtafil]

I'm on it; see my edited post above as well for reference. I'll wait a few minutes for you to review, then go pull that leg again.

 

[dtafil]

OK, went back in. D52L pulled one leg; opamp remained out of socket. Dim bulb + variac. Grabbed the right side as well for reference.

LEFT SIDE

20 V AC
+15VL: +9.7 V DC
-15VL: 0.0 V DC

40 V AC
+15VL: +15.1 V DC
-15VL: 0.0 V DC

60 V AC
+15VL: +20.2 V DC
-15VL: 0.0 V DC

80 V AC
+15VL: +24.5 V DC
-15VL: 0.0 V DC


RIGHT SIDE (all components in place)

20 V AC
+15VL: +7.4 V DC
-15VL: -7.5 V DC

40 V AC
+15VL: +11.0 V DC
-15VL: -11.1 V DC

60 V AC
+15VL: +13.5 V DC
-15VL: -13.6V DC

80 V AC
+15VL: +14.7 V DC
-15VL: -14.9 V DC

 

[Gepetto]

I'm on it; see my edited post above as well for reference. I'll wait a few minutes for you to review, then go pull that leg again.

Hi Dave

The current request is D51L NOT D52L . You reconnected D52L (the larger 1N4004) earlier. Leave it that way. This is about the small zener diode that operates in parallel to VR51L. Either D51L or VR51L could be shorted and this is to isolate the easier of the 2 to nail the culprit down.

My money is on the TL431 (VR51L) but this will assure that.

 

[dtafil]

LOL, my bad. Will go back out there. Sorry Joe - over-caffeinated today...

 

[laatsch55]

Sghit happens...