WOPL 400 4/ powerups went well, one PEM not so much

Gepetto said:
Yes but did you replace the original 2N3403 that was in the P strap with the 2N5088?

If this, then the leads are in the correct holes.


Or did you leave the 2N3403 installed in the P strap.

If the latter, then the leads are in the wrong holes.
Click to expand...

I used the new 2N5088 transistors that came with the Backplane board kit, along the old thermal bands. I think Ed ended up
with the old 2N3403 transistors along with the old Control board.
 
Gepetto said:
Is your output reading 0.000V or very close to it on both channels?
Click to expand...

Output readings on left channel is 0.000V. Output reading on right channel is now a full 82.7V., was not that way before.
Bias reading left channel still 0.000, Bias reading right channel is a steady 0.142, no matter what.
 
You seem to have lost your B- feed to the control board. That comes in on pin 12 on the right channel. Figure out what went wrong there and remedy. Without B- and -15V on the regulator, you will not go very far.
 
Gepetto said:
A possible guess Jer but needs to be confirmed. And if the fuse did go, the reason why must be determined.
Click to expand...

Pico fuse is blown, my guess is me and my DMM poking around caused that. I got some Pico fuses ordered.
I took the Control board, Backplane's and output Transistors back out of the chassis.
Visually everything looks good.
 
ok, I replaced the B- Pico fuse on the right Backplane board, all the other Pico fuses are good. Powered it up like the first
times and fuses are good. Reinstalled the Control board and powered it up one more time. The right B- Pico fuse blows
again at about 60V on the Variac through the DBT. Not sure what's going on now.
I'm thinking on sending my Control board to be checked out.
Joe, anyone have any thoughts?
 
You photo is blurry but seems to be showing a missing R41R resistor. That is a 10 ohm resistor.

Can you post a crystal clear, high rez photo?
 
Check R14R location as well. That is supposed to be a DNP location (do not populate). Looks OK on the left channel but the right channel seems to have a rogue resistor in there.

Double check your assembly against the Do Not Populate list at the end of the Bill of Material.

Joe
 
R41R is a Full Comp only resistor, right Joe?
Funny, I populated R14L by mistake as well. Once you get going, it is hard to slow down.
A question: Why were C10 and R14 deleted for both versions? What was their purpose in the circuit? Also, I saw that 1/2 of the long tail pair (Q2, R7, R8, and R10) are not included in either Quasi or Full Comp versions. I'm late to the Rev E party, but just want to get caught up.
 
Hi Mark
R41R is installed for all versions Mark. It is the +15V feed resistor to the right channel. Without that, not much works on the right side.

The other questions are answered in the change history document that was included in the large doc packet that I sent over to you.

C10 and R14 are a carryover from the earlier RevD and once I determined not necessary by the SPICE model, I removed them.
 
I typo'd, I meant R14R instead of R41R, darn that dyslexia.
In Change History is says "3. Changed the MPSA18 long tailed pair that formed the initial gain stage to a single 8 pin OPA134 Burr Brown audio OP Amp to increase the gain of the initial gain stage allowing a wider, flat high frequency response." I did not connect the dots that it was Q2.
All good.
 
Actually it is this doc Mark

From Revision E to E1

Reason: Reduce load on regulator Q1 (+15V regulator) and the associated heat dissipation in that transistor due to customer observations regarding the positive 15V regulator running hotter than the negative 15V regulator.

Change R38L/R from bus wire jumper to 0 ohm, 1/4W, 1%

Change D4L/R from 1N4148 to 10 ohms, 1.4W, 1% (quasi comp only)

Change R8L/R from 56 ohms, 1/4W, 1% to open

Change R7L/R from 3.01K ohms, 1/4W, 1% to open

Change R10L/R from 390 ohms, 1/4W, 1% to open

Change Q2L/R from MPSA92 to open

Change R5L/R from 3.01K ohms, 1/4W, 1% to 5.76K ohms, 1/4W, 1%

Change R6L/R from 1.50K ohms, 1/4W, 1% to 182 ohms, 1/4W, 1%

Change C14L/R from 180pF to 120pF

From Revision E1 to E2

No change, E2 was never released

From Revision E2 to E3

Reason: Decrease sensitivity of the SOA protection circuit to eliminate false tripping with low impedance loads.

Change R25L/R, R26L/R from 56K ohms, 1/4W, 1% to 13.0K ohms, 1/2W, 1% small form factor.

From Revision E3 to E4

Reason: Reduce phase lag of amplifier at 20 KHz.

Change C6L/R from 240pF to 220pF

From Revision E4 to E5 (March 23, 2019)

Reason: Change bypass capacitor value to a more commonly available size

Change C1L/R. C2L/R, C3L/R from 0.047uF to 0.01uF
 
I missed copying that file from the email group of documents. Thanks Joe, I would have never caught that.
This is R14L, the 10 Ohm resistor that I populated by mistake, it should have gone in R41L.R14L.jpg
 
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Gepetto said:
You photo is blurry but seems to be showing a missing R41R resistor. That is a 10 ohm resistor.

Can you post a crystal clear, high rez photo?
Click to expand...

I did not populate R41R, and did populate R14R. I missed it.
 

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Gepetto said:
Check R14R location as well. That is supposed to be a DNP location (do not populate). Looks OK on the left channel but the right channel seems to have a rogue resistor in there.

Double check your assembly against the Do Not Populate list at the end of the Bill of Material.

Joe
Click to expand...

Yep, I did missed it.
 

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