Voltage problems with 990c installed

[jwschaff]

Successfully completed to C84s and one N74 and love them!

Then came the C99. I went through all the installation of components, then started doing the voltage checking on the last page of the instructions.

1) Checked for high resistance in J7 (as in step 32 = all fine)
2) Connected power and checked voltages on U1 (as in step 35 = all fine and now adjusted to exactly +/-20 volts.
3) Checked voltages of U3 pins 4 & 8 (as in step 36 - all fine at c. +/-12v)
4) Powered down, waited, then installed J. Hardy 990c (brand new)
5) Checked for voltage between pins 4 and 6 of U1 (as in step 38 = BIG PROBLEM! = constant +/- 13 volts with no change over time)

tested every single solder joint, checked every single component for correct placement and arrangement (not an easy task) = all appear fine.

Read through previous threads and tried anything that seemed plausible, but to no avail.

One note: U4 gets very hot, no smoke or anything, but very hot to the touch (as in you would hold your finger on it for more than 3 or 4 seconds), while U5 is only barely warmer than room temperature (I don't know if this is abnormal or has any correlation or not).

Any advice would be greatly, greatly appreciated.

John
(good soldering skills, but not strong in fundamental circuit design).

[tpryan]

Remove U2 from its socket and apply power. Does U4 still get hot? Measure the voltage between the empty sockets at U2, pin 4 (PGND) and U2, pin 1. Swap 990s at U1 and measure again. Let us know what you find.

[jwschaff]

I tried your suggestion of removing U2 and checking the heat and voltage between pin 4 (gnd) and pin 1. I got two different readings depending on which op amp was installed at U1. The readings were

13.88 v
14.64 v

I don't know if these differences are significant? Also, U4 remained comfortably cool with only one op amp installed at U1.

I did discover something more ominous, however. I tried tracing resistance between the R20/21 pair and pin 1 of U1 as you had suggested in an early post on a similar subject and kept coming up with an impedence of .843 K-ohms (834 ohms?). I finally isolated the source to the transformer winding (pins 5/6) on the input transformer, a number that seems way too high for a simple winding. The other two windings (pins 1/2 and 3/4) both registered only 59 ohms, a much more reasonable number. Could this be the source of the problem?

[jwschaff]

Any thoughts yet?

[thebearingedge]

any developments with this one?

[tpryan]

Faulty op-amps, if I remember correctly.

[jwschaff]

Faulty Op Amps is exactly correct. I got two newer 990s from John Hardy and the pre worked just fine.