Watching this topic with interest because I have a 7904 with a focus issue as well. Some resistor(s) has (have) drifted, most likely. Symptom is that I have to turn the focus knob fully CCW to get the trace just barely in focus. Even touching the focus knob at that point will blur the trace into oblivion. Someday I will get around to fixing it¡..
Jim Ford
Laguna Hills, California, USA
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On Dec 20, 2024, at 7:55?AM, Adam R. Maxwell via groups.io <amaxwell@...> wrote:
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On Dec 1, 2024, at 21:53 , Adam R. Maxwell <amaxwell@...> wrote:
HI all,
I have an older 7904 (SN < B260000) that I've been happily using, but decided to check Z-axis calibration. Step 4k is where I've hit a wall, so basically right out of the gate. Voltage on TP 1778 to chassis ground is supposed to be 123 ¡À 3 VDC, and it's 128 V. Unfortunately, R1757 does not change the reading at all.
I kept trying to figure this one out.
Checking DC voltages and annotating the schematic, I had a moment of panic when the 15V test point on the Z-axis board read 14.7, and started looking for a dying tantalum. It turns out the +15 and -15 TPs come after a 4.7 ohm resistor (which had drifted out of tolerance, so I replaced them).
I'm not sure what the autofocus signal should look like, but there's no waveform after Q1775, and replacing Q1775 and Q1777 did not change anything. It's just DC on collector of Q1775, which didn't seem right. At this point, I suspected Q1765 or Q1769, but do not have replacements, and each produced a family of curves on the 7CT1N.
I finally noticed TP1778 voltage read low (<120 VDC) for a few moments as I was starting the scope up, and for that brief time, R1757 could vary that voltage. This inspired me to hit Q1765 and Q1769 with freeze spray, and sure enough, freezing Q1769 temporarily fixes the problem. I'm waiting on an AT5058 that will hopefully be an actual fix for this.
thanks,
Adam
