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2465B Screen artifacts
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Hi everyone.
I recently got a 2465B scope and replaced all leaky/high ESR CAPs. The Dallas NVRAM was missing so I hacked in a FRAM and the machine did switch on; have to press A/B TRIG to proceed when the unit self tests (presumably TEST 04 FAIL). The first thing is that, I had to adjust the GRID BIAS before I could see anything. After adjusting, I noticed there're dots and weird characters on the screen. I've read somewhere else that this has something to do with the CAL data. So I tried to do the CAL procedures. But I cannot finish beyond the first step of CAL 08, the center dots dim with other dots as I adjust the GRID BIAS. The Intensity trim pot seems to do nothing. Readout Intensity is either on or off. The problem is very similar to this post on EEVBlog, and there's a video in this post: Feeding in 10MHz sine wave into all 4 channels and they looks good except the flashing dots and weird characters. Checked all voltages according to the 5-1 table, and traced down all lines related to the DI, ROI to the U650. Before R361 and R353, dramatic changes on voltage when turning the pots. But checking the 22 and 23 pin of U650, changes were, however, in 100mV range. Removing the U950 chip restore the amplitude to reasonable levels. But this also did nothing the the screen. Everything is still in place - nothing disappeared, nothing changed - weird characters, dots, signals remain the same. Test signals 22, 23, 65, 66 shown in the SM are way out on amplitude. Feeding 3Hz 5Vpp signals into the Ext, Z-axis in did absolutely nothing to the screen. I don't have another U950 on hand to test but the U950 seems to be the culprit? |
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Hey there,
Do you have a name we could call you by? The most common problem by far with the 24X5B scopes is a bad DAC reference due to electrolyte leakage from the SMD caps on the A5 board. The quick and painless way to assess the state of the DAC reference is to measure the 1.36V and the -1.25V references on the A5 board. If those are not good, then that needs fixing first. These scopes are all drive by wire, and the DAC on the A5 board generates a whole host of analog voltages that control sundry functions of the scope, including trace and OSD brightness. Siggi On Sat, Mar 15, 2025 at 9:02?AM jammythewinner via groups.io <jammythewinner@...> wrote: Hi everyone. |
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Hi, my name's Hachiroku.
1.36V and -1.25V Ref tested OK according to this post: /g/TekScopes/message/13732 Here are the voltage measurements (reference to ground) at the DAC pins. DAC Pin 14=0.151V DAC Pin 15 =0.147V More voltages: Voltage drop across R2013 (10K)= 8.82V Voltage drop across R2016(10K)= 1.02V Voltage drop across R2011 (4.75K)= 0.639V Voltage drop across R2014 (221K)= 7.97V Voltage drop across R2015 (82.5K)= 8.18V It's a shame that I've decided to give it up as l observed different Serial Numbers on different parts of the scope. l assumed parts may have been replaced/repaired before by someone else. Have a nice day Siggi and thanks for helping. |
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Hey there Hachiroku,
nice to meet you, welcome to the group :). On Mon, Mar 17, 2025 at 9:46?AM chester.jamie.jammy via groups.io <chester.jamie.jammy@...> wrote: Hi, my name's Hachiroku. I guess one other thing to check is whether J504 is in the correct position, it should be jumpered between pins 1-2. If it's jumpered between pins 2-3, it's possible that the DACs reference is bad, which would be good as that's a well known, repairable fault that can lead to the symptoms you see. Here are the voltage measurements (reference to ground) at the DAC pins.These all look reasonable. It's a shame that I've decided to give it up as l observed different I don't think parts or PCBs were serial matched on these scopes, nor generally in Tek instruments. l assumed parts may have been replaced/repaired before by someone else.Seeing how the NVRAM was missing, you know that someone was in there before you, and without knowing the story it's hard to say whether the scope might have e.g. been a parts donor. Usually it's reasonable to assume there's going to be a single fault to fix, but with an instrument that's been a parts donor, that may not be the case. That being said, this might be as simple as some other fault on the A5 board or e.g. U950. There might be some easy diagnostic steps you could try to see if you can narrow this down, like do the HORIZ/VERT POS controls work and so on. Have a nice day Siggi and thanks for helping. No worries. I find it highly rewarding to get these old beauties working, and every instrument I've repaired has been a learning experience for me. If ever you decide to circle back to this, you might enjoy reading this document: . Have a nice one, Siggi |
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