1. TekScopes
  2. Теми

unable to adjust 7904 focus amplifier

 

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 ± 3 VDC, and it's 128 V. Unfortunately, R1757 does not change the reading at all.

DC voltages on the supply are good, and I have +50 on R1757 as expected (and voltage on the other other side of it varies smoothly). I checked transistors and diodes in the focus amplifier (Q1765, Q1769, Q1757, Q1755, CR1768, CR1764, CR1754) with a DMM diode check, and none of them are shorted. R1762 (75K) measures 12K in-circuit, but I'm skeptical that means anything.

Anyone else seen this, or have ideas on what to look for?

thanks,
Adam
 

Cleaned trim pots ?

Visually OK or bad blanking?

I would leave it alone

Jon
 

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 ± 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
 

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

On Dec 20, 2024, at 7:55 AM, Adam R. Maxwell via groups.io <amaxwell@...> wrote:


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 ± 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
 

Jim,

Your focus control could be dirty. See if it has the tabs on the back that can be bent back enough to put in some contact cleaner. The 1meg resistors and resistor parallel to the focus preset (2meg in this case) in these strings are stressed by the voltage across them. Even the 453/4/A types have this problem even though they are a higher value of resistance. Replace with 1W types. I got 1W 1% types from China on epay. I would get at least 40, if not more, for stock. The 150V zener could be bad. Also replace the two 22meg 1/4W carbons, R1659 and R1684, with Vishay VR37 series.

Mark
 

Thanks, Mark! I’ll give that a try. Might actually have time over the holidays to do so. Jim

On Dec 20, 2024, at 10:24 AM, Mark Vincent via groups.io <orangeglowaudio@...> wrote:

Jim,

Your focus control could be dirty. See if it has the tabs on the back that can be bent back enough to put in some contact cleaner. The 1meg resistors and resistor parallel to the focus preset (2meg in this case) in these strings are stressed by the voltage across them. Even the 453/4/A types have this problem even though they are a higher value of resistance. Replace with 1W types. I got 1W 1% types from China on epay. I would get at least 40, if not more, for stock. The 150V zener could be bad. Also replace the two 22meg 1/4W carbons, R1659 and R1684, with Vishay VR37 series.

Mark
 

On Dec 20, 2024, at 07:55 , Adam R. Maxwell via groups.io <amaxwell@...> wrote:

I'm waiting on an AT5058 that will hopefully be an actual fix for this.
For the archives: the transistors finally arrived, and replacing Q1769 fixed the 7904 autofocus issue, allowing me to complete all the steps in the Z-axis service procedure. Ironically, the readout now isn't as sharp as it was before, so I may play with this further when I get bored.

Adam

1 - 7 з 7