by SteveP » Sat Oct 31, 2015 6:46 pm
Good Evening,
I powered up the ST35 two weeks ago, with no major problems. But, per the table in the manual, several measured voltages are off.
Referring to the voltage table in the manual:
Regarding V1 and V2, all 116 volt readings read around 100 volts and all 207 volt readings read around 220 volts.
Also the 780mv readings measure about 660mv.
The results are essentially the same with three different sets of 12ax7 / 12au7’s.
Line = 119VAC
320V = 313V
380V = 367V
Here are the actual readings:
V1 V2 V3 V4 V5 V6
Pin 1 100 222
Pin 2 -0.2V 99 15mv 15mv 15mv 15mv
Pin 3 664m 104 13 13 14 13
Pin 4 3.07 3.07 3.07 3.07 3.04 3.07
Pin 5 3.07 3.07 3.07 3.04 3.04 3.07
Pin 6 103 220
Pin 7 -0.1V103 358 359 358 358
Pin 8 650m107
Pin 9 3.07 3.07 363 363 363 362
Last weekend, I verified all part values and connections and found no errors. I reflowed and inspected all solder connections.
I also replaced R11 because it had a funny bulge even though the resistance measured right (33K).
I used the amp for a couple of nights, thinking maybe the voltage readings might be acceptable.
But honestly, the amp does not that good.
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I used the 4 ohm output from the z565 output transformers for negative feedback at J3/J4 pin 3. The resistance at J3/J4 pin 3 is set to 20.3K.
Did I use the correct (4 ohm) output for feedback? Is the negative feedback set to the recommended value?
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Notes on initial resistance measurements:
The secondary of the PA-774 transformer reads about 71 ohms rather than 94 ohms as indicated on page 4 of the manual, initial impedance measurements (with all transformer connections wire to the board).
The resistance readings of pins 1 and 2 of J2 - J5 read over 10M but drifted lower as I measured, as though a capacitor was charging as I made my measurements (PC board not connected to the transformers).
I am baffled. I found no errors with components values or connections, but I must be missing something. Please help.
Thanks,
Steve