[Solved] Tek 465B : Channel 2 always grounded

[Snowolf842]

Hello,

I have recently bought a Tek 465B for 20€, sold as broken. I had the same power supply problem as here https://www.eevblog.com/forum/testgear/tektronix-465b-help-needed!/  So I've just unsoldered one lead of the C4331 capacitor until I get a replacement.

Everything seems to work fine now (triggering, channel B, B delayed, ...) except channel 1. I googled for the same problem, but I didn't find anything close to it.

I get a horizontal line in all conditions as if the coupling switch was in GND position. I ended up to open the 465B and checked with another oscilloscope I borrowed how far the signal goes. Signal stops right after a resistor behind the BNC connector ! I measured the resistance between the center lead of the BNC and ground : 76 for channel 1 and for comparison, more than 1M for channel 2.
The circuit seems shorted right after R13010 and before C13012 (see attachment). The 2 leads of C13012 are not shorted. From the PCB side facing the case, I can't see anything that could explain the short.

How can I find the short ? Should I remove the vertical preamplifier board to check the other side of the PCB ?
Am I looking at the right part of the circuit ?

Thanks for reading !


PS: If I put a wire from a function generator close enough to the attenuators of channel 1 (shielding removed), I can get a noisy sine wave to show up. I think it's good news ?

[bd139]

Firstly I would avoid removing the vertical preamp and attenuator. This is mega-difficult on the 465B. Actually most work on them is a pain.

When diagnosing problems with these the first thing to do is to pull the individual attenuator modules out one at a time. They get damaged from overload easy and can go to short to ground. Then re-check everything from the front end back towards the resistor that connects the attenuator assembly to the vertical amp for continuity/resistance. The leaf switch contacts are likely dirty as well which may compound any issues. They can be cleaned with some IPA on some quality paper by soaking and sliding them through the contacts if you need to do that.

You got a good deal if it works or not for €20 though!

[Snowolf842]

Okay thanks a lot ! I will check that tomorrow.

[bd139]

Before I forget, pull them dead straight out. They have very fragile legs on them!

[Snowolf842]

I have removed all the attenuators and still the same measurements as my first post.
As I was writing this post and testing, the problem just disappeared ! Nice trace shows up in all scales.
As you said earlier, the contacts were perhaps dirty and moving the buttons several times seems to fix the problem.

[bd139]

Ugh intermittent problems. This will come back at the most inopportune moment. I'd probably pull the attenuator assembly out and clean the switches on both sides. Also get your finger in there and poke it hard on bits of the board and make sure it stays away

[tggzzz]

The V/div and s/div switch contacts can sometimes be cleaned with a paper strip soaked in IPA. The contacts are relatively fragile and can be damaged by careless movements.

The attenuators can get oxide building up on their pins. Sometimes insertion/removal is sufficient to dislodge the oxide, sometimes a very small amount of Caig DeOxit can help.