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

George S. · Oct 27, 2021

I use a even older Hakko 936. For your new iron, make sure you also get the proper tips. They've changed tip design between the models over the years. I try to use genuine Hakko tips, the tin plating is very durable. Worth it I think.

dtafil · Oct 27, 2021

George S. said:

Yep, I do the same. Only the genuine Hakko tips over here as well.

dtafil · Oct 27, 2021

Update for the day/night.

Pulled everything, control board, backplanes, the works. Re-addressed every wire ensuring proper solder flow to both sides of the boards, trimmed up any wires. Put it all back together. Appreciate all the pointers. Ordered the new solder station too, lol.

Put it all back together with all the previous wiring & routing tips.

Passed dim bulb with bottom row of transistors installed, floated ground (no control board). Discharged filter caps.

Passed dim bulb next with star ground connected. Discharged filter caps.

Added control board. Failed dim bulb the exact same way. Watching DC mV climb across left-hand F2 as bulb got brighter. Powered back down for the night.

I could try swapping in my other (working!) control board from my other WOPL'ed 700b tomorrow. Also note that I have a transistor tester, can pull and test anything on the current/non-working board as well.

Thanks guys... have a good evening!

Gepetto · Oct 27, 2021

dtafil said:

I wouldn't go there quite yet Dave. We should continue to try and isolate where a fault may lie.

You indicated that you lifted one end of R8L and had the same results, correct? If so, put R8L end back in.

Next measure the -15VL test point (the white one) when you are bringing up the variac on the DBT and see what that voltage does. Compare it to what the +15VL test point (the red one) does under the same circumstances. Report what you find.

dtafil · Oct 28, 2021

Morning Joe - Results of the above testing. Black lead of DMM clipped to left ground on control board, dim bulb + variac. Brought up slow, 3 stops at 20V, 40V & 60 V AC.

LEFT

20V AC
-15VL: -4.2V DC
+15VL: +4.4 V DC

40V AC (dim bulb starting to light)
-15VL: -4.7V DC
+15VL: +5.0 V DC

60V AC (bulb lit)
-15VL: -4.1V DC
+15VL: +4.9 V DC

RIGHT (For additional reference in case it helps)

20V AC
-15VR: -3.4 V DC
+15VR: +2.7 V DC

40V AC (dim bulb starting to light)
-15VR: -4.0 V DC
+15VR: +3.3 V DC

60V AC (bulb lit)
-15VR: -3.6 V DC
+15VR: +3.1 V DC

Gepetto · Oct 28, 2021

dtafil said:

Morning Joe - Results of the above testing. Black lead of DMM clipped to left ground on control board, dim bulb + variac. Brought up slow, 3 stops at 20V, 40V & 60 V AC.

LEFT

20V AC
-15VL: -4.2V DC
+15VL: +4.4 V DC

40V AC (dim bulb starting to light)
-15VL: -4.7V DC
+15VL: +5.0 V DC

60V AC (bulb lit)
-15VL: -4.1V DC
+15VL: +4.9 V DC

RIGHT (For additional reference in case it helps)

20V AC
-15VR: -3.4 V DC
+15VR: +2.7 V DC

40V AC (dim bulb starting to light)
-15VR: -4.0 V DC
+15VR: +3.3 V DC

60V AC (bulb lit)
-15VR: -3.6 V DC
+15VR: +3.1 V DC

Good data, thanks Dave. What is output offset doing at each of these points, both left and right channel readings.

dtafil · Oct 28, 2021

Hi Joe - Here's what I see... let me know if my understanding/process is correct for output offset:

Bias trimmers set full CCW. Nothing connected at inputs or speaker jacks. Using Variac w/ dim bulb. Bottom row of transistors, star ground connected, control board in place.

DMM - Positive lead to left + speaker post on top of the DC protect board, negative lead to - speaker post on top of DC protect board:

0.0 mV DC all the way up to 60 vAC on the Variac with the dim bulb (glowing)

Swapped over to right channel, same process:

0.0 mV DC all the way up to 60 vAC on the Variac with the dim bulb (glowing)

Gepetto · Oct 28, 2021

dtafil said:

The speaker posts measurements are only valid if the relays have clicked in which is unlikely at only 60VAC. Did the relays click? If the relays did not click, you need to measure the offset at the Phoenix connector on the DCP input. The speaker posts are the output. You can also measure the output offset at Pad6 on the Control board which will be right in front of you.

dtafil · Oct 28, 2021

Ah, good catch. The relays click ~80 V AC w/ the variac (and no control board), so the fuse will blow by then. Let me remeasure right after dinner. Thanks Joe

dtafil · Oct 28, 2021

On thce DC protect board, measured across Pin1 (L+) and Pin2 (ground) for the left channel; measured across Pin3 (R+) and Pin4 (ground) for the left channel. Also tested Pin6 ground on the left backplane to Pin1 of the DC protect board, and then Pin6 on right backplane to Pin 3 of the DC protect board to confirm readings on DMM

At 20 V AC:
L: 3.58V DC
R: 4.82V DC

At 40 V AC:
L: 0.004 V DC (not a typo, confirmed 2x)
R: 5.38 V DC

At 60 V AC:
L: 2.29 V DC
R: 5.11 V DC

Gepetto · Oct 28, 2021

dtafil said:

On thce DC protect board, measured across Pin1 (L+) and Pin2 (ground) for the left channel; measured across Pin3 (R+) and Pin4 (ground) for the left channel. Also tested Pin6 ground on the left backplane to Pin1 of the DC protect board, and then Pin6 on right backplane to Pin 3 of the DC protect board to confirm readings on DMM

At 20 V AC:
L: 3.58V DC
R: 4.82V DC

At 40 V AC:
L: 0.004 V DC (not a typo, confirmed 2x)
R: 5.38 V DC

At 60 V AC:
L: 2.29 V DC
R: 5.11 V DC

Are all the readings positive or are some negative? You have rail fuses installed correct?

dtafil · Oct 28, 2021

Gepetto said:

Hi Joe - Yes, all voltages were positive, sorry. All rail fuses installed.

Gepetto · Oct 28, 2021

dtafil said:

Thanks Dave

Let's lift one end of D52L, either anode or cathode and rerun your 20/40/60VAC test. Record voltages on Pad 12L and Pad 11L with respect to ground. Any change in the results?

dtafil · Oct 28, 2021

Gepetto said:

Here we go (pulled leg of D52L)

At 20 V AC:
L: +6.5 V DC
R: +0.65 V DC

At 40 V AC:
L: +8.22 V DC
R: 0.00 V DC

At 60 V AC:
L: +2.0 V DC
R: 0.0 V DC

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

At 80 V AC:
L: +0.25 V DC
R: 0.0 V DC

At 100 V AC:
L: +0.25 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 teh respective Phoenix connector per pad location (hope that's OK)

At 20 V AC:
12L: -7.37 V DC
11L: +7.36 V DC
Grabbed the right as well for reference:
12R: -7.37 V DC
11R: +7.36 V DC

At 40 V AC:
12L: -9.67 V DC
11L: +9.65 V DC
Grabbed the right as well for reference:
12R: -9.66 V DC
11R: +9.65 V DC

At 60 V AC:
12L: -11.47 V DC
11L: +11.45 V DC
Grabbed the right as well for reference:
12R: -11.45 V DC
11R: +11.44 V DC

At 80 V AC:
12L: -13.6 V DC
11L: +13.6 V DC
Grabbed the right as well for reference:
12R: -13.6 V DC
11R: +13.6 V DC

At 100 V AC:
12L: -15.5 V DC
11L: +15.5 V DC
Grabbed the right as well for reference:
12R: -15.5 V DC
11R: +15.5 V DC

Thanks Joe.

Gepetto · Oct 28, 2021

You made it to 100VAC without blowing the fuse it seems. All Pad12 and Pad11 voltages symmetrical and opposite, just what you would expect.

The right channel output came into regulation as it should (0.0V) and the left is trying hard for an op amp that has no negative power supply (you took it out of the circuit when you lifted one end of D52L).

What wattage bulb are you using? You are getting rather low raw DC voltages at 100VAC.

dtafil · Oct 28, 2021

Gepetto said:

Was supposed to be 100W, but just checked. Had a 72 watt one in there instead. That's now clearly labeled w/ a sharpie.

I can rerun any of those measurements in the AM with a 100w or 150w bulb if that's helpful.

Gepetto · Oct 28, 2021

dtafil said:

It would be worth rerunning the last group of readings with a 100W bulb in there to ensure that the fuse is not blowing on the left backplane.

Seems something is up with the -15V regulator on the left channel Most likely suspect is the TL431 shunt regulator device.

dtafil · Oct 29, 2021

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.

WOPL Sniffer · Oct 29, 2021

dtafil said:

Skip the pliers when soldering. Leaving the marks and damaging the insulation looks bad and will make your work look better without the "Tiger Stripes". I use my fingers or a paper towel strip to hold the wires while soldering.

WOPL Sniffer · Oct 29, 2021

dtafil said:

Pulling and testing transistors (i.e. heat-reheat-heat-reheat) takes YEARS off components due to excess heat. Pretty much all the transistors can be tested "In Circuit". Some of the guys here may argue that point but it's true. The shotgun technique is better than damaging components from heat. Unless you are proficient in using a heat sink between the heat source and the component, just test in circuit and replace. Use your tester, or Fluke.... Either or....

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

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