Tek 2465B Channel 2 VOLT/DIV indicates 10x channel 1 - Page 1
Forum section: electronics / repair

author: W3KW
author_rank: Contributor
timestamp: August 26, 2025, 06:25:44 pm
content: My #2 2465B is gorgeous. It runs and looks like a champ save one issue I need to resolve (besides replacing the NRAM and PS caps). Channel 2 indicates 10x the volts per division that channel 1 indicates using both the internal calibrator or my Tektronix AWG. See the photos. I ran test 2 and got “PASS.”Curious the situation is exactly 10x. It’s not the probe or input from the AWG. It’s definitely an internal fault. Or, user fault?? Am I doing something wrong? I can’t duplicate the issue on my other 2465B. That one, aside from weak readout display, is running great.

author: Kwakerman
author_rank: Regular Contributor
timestamp: August 26, 2025, 09:03:29 pm
content: Is the automatic X10 detection ring around the BNC clean. Cant remember the resistor value but the automatic switching probes have a pin that contacts with the ring and effectively puts that resistor between the ring and ground. When the scope detects it it automatically sets the channel X gain up by a factor of 10. Seems the the scope thinks one is fitted.Edit: Is your first picture the same input on both channels? If it is I think it is a readout (A5?) issue being as both traces are 2 graticule squares high (matched) but the readout differs.

author: W3KW
author_rank: Contributor
timestamp: August 27, 2025, 12:54:35 am
content: In the example shown, I’m not using a probe. But the rings are clean and the issue is not intermittent.Yes, the cursors read the correct voltage but the Volt/Div is set 10X higher.The two attached photos show 1M ohm on the good machine, and 64M on the bad one. Weird that it shows 64M on all four channels. But channel 1 measures a perfect 1V PP signal using either a probe (1M setting) or BNC (50 ohms).I’m sure the answer will present itself. Somehow. Fun puzzle while it lasts.

author: Kwakerman
author_rank: Regular Contributor
timestamp: August 27, 2025, 07:56:16 am
content: What are you using as the signal source? If it is a sig gen with 50 Ohm output impedance then there should be a halving / doubling of the reading when switching from 1M to 50 Ohm and back again.You can also select 50 Ohm input and measure the resistance between the BNC inner socket and outer. That may show in the input attenuator has been damaged by overload.Another thing to try is setting both channel attenuators to there highest setting then slowly reduce each one a step at a time and listen for the attenuator relays clicking and see if both Ch1 and Ch2 match.

author: Kwakerman
author_rank: Regular Contributor
timestamp: August 27, 2025, 08:15:27 am
content: Just measured a 2445B I have on the bench.Ch1 & Ch2 set to DC = 1 M Ohm BNC inner to outer.Both set to 50 Ohm = 50 Ohm BNC inner to outer.Also measured from X10 detection ring to BNC outer and both channels measured 2.5 M OhmDon't know if that is any help to you.

author: W3KW
author_rank: Contributor
timestamp: August 27, 2025, 03:33:49 pm
content: Yes. I get 50 ohms and 1m ohms on my good 2465B. I get 64M ohms on the bad one in any input selection. (50 or 1M) Weird that with an input attenuator so screwed up, I actually get perfect measurements on CH 1 and an odd 10x on channel 2. Channel 2 is 10X regardless of input method. (BNC or probe, 50ohm or 1M ohm)Complete head scratcher. I’ll need to operate I’m afraid. The CRT and cosmetics are so good on the messed up one, I really want it returned to health.

author: W3KW
author_rank: Contributor
timestamp: August 27, 2025, 03:36:06 pm
content: The relays sound like they are doing their job. I’ll need to research the schematics and manual. Odd that all the internal tests pass on it.

author: W3KW
author_rank: Contributor
timestamp: August 27, 2025, 04:52:42 pm
content: Nuts! Some user error here. I was measuring the BNC's from the center conductor to the probe-coding ring and not ground. DOH! That is supposed to be about 64M ohms! You must measure from the center conductor to the BNC barrel. ta-daa. All four channels measure perfect impedance.So, another light bulb went off in this dim brain of mine. What is the probe-coding reaction to attaching my probes? DOH, again! Channel 2 is actually working correctly and adjusting to my 10X probe. Channel 1 is not!This is why I am not an EE. However, it was fun doing math in public and hunting down the problem. Now that I know the problem, I can start hunting the solution.Ok. Where in the 536 page manual is probe-coding? LOL

author: factory
author_rank: Super Contributor
timestamp: August 27, 2025, 06:18:00 pm
content: The probe code goes from the attenuator module to the A5 control board, the CH1 to CH4 "PRB" can be found on the analog control diagram, going to a 4051 mux IC.You could check continuity from the BNC to the A5 board, failing that investigate the resistors & the 4051.If it's a surface mount A5 board, then corrosion & conductive capacitor pee is always a possibility, if it has those leaking caps.David