Tektronix 335 teardown - Page 1
Forum section: electronics / testgear

author: mnemotechnika
author_rank: Newbie
timestamp: October 10, 2024, 11:34:38 am
content: Hi everyone, I'm new to this forum. Please help me figure out what might be wrong with this old device. This is what appears on the screen no matter what I adjust. The center part moves slightly when adjusting the horizontal position. Thanks.

author: frantisek
author_rank: Regular Contributor
timestamp: October 10, 2024, 01:04:09 pm
content: First of all you need to check all voltages and ripple of the power supply to make sure they are OK! Then we will see.

author: David Hess
author_rank: Super Contributor
timestamp: October 10, 2024, 11:27:59 pm
content: After checking the power supply voltages, investigate the focus resistor chain and measure the CRT cathode voltage if you have a suitable high voltage probe.

author: TurboTom
author_rank: Super Contributor
timestamp: October 11, 2024, 06:31:40 am
content: Some time ago, I had a 335 with a damaged CRT. Two of the glass rods that align the electron gun assembly were cracked so there was some distortion, though the trace was still half-way visible. But I can imagine that more severe damage inside the tube may also lead to the effect that you're reporting. I hope that's not the cause in your case.It may as well be one of the deflection amplifiers being completely out of balance. You may want to check the voltages at the deflection plates and if they change (within the pairs) in a mirror-like way when you adjust X- and Y-position.I used my broken 335 as a parts donor for another damaged scope but may have most of the components still here, so if you need an "unobtainium" spare (except for the CRT) I may be able to help.

author: David Hess
author_rank: Super Contributor
timestamp: October 11, 2024, 05:36:20 pm
content: Quote from: TurboTom on October 11, 2024, 06:31:40 amOh, that would do it also. If the beam is far enough off the screen, then the reflections make shadows like that. More than once initial inspect revealed that one of the deflection plate connections fell off.

author: mnemotechnika
author_rank: Newbie
timestamp: October 21, 2024, 09:14:26 am
content: Hello Dear Colleagues!I disassembled and cleaned the device, checked the power supplies, and everything is fine with those, the voltages are correct. I reconnected the horizontal and vertical deflection connectors, then set it to calibration mode, where everything seemed good. However, in the 10V and 5V positions, the channels appear noisy. I’ve attached images, including ones with 10x magnification. What could still be the problem?Regards, Tamas

author: TurboTom
author_rank: Super Contributor
timestamp: October 21, 2024, 09:29:26 am
content: When I refurbished my 335, I found that all the switches were a little stubborn vs. my cleaning approaches. It took quite some switch toggling and IPA to get everything smooth again (at least most of the time). Moreover, the open cam switches that Tek utilized in the attenuators and time bases may be a problem of their own, being very delicate and operating at rather low contact forces. IPA-soaked lens cleaning paper may help to clean individual points, but accessing them requires a lot of disassembly work in these areas. But those switches may very well be the cause of the noise that you reported.I also found some of the plastic end-caps of the pots that provide the detent mechanism for the "calibrated" position to have come loose. Since the material used (polyethylene or similar) virtually cannot be glued, I used short pieces of shrink-wrap to attach these caps back to the pot bodies and touched it up with a small amount of CA glue. Works okay so far...

author: mnemotechnika
author_rank: Newbie
timestamp: October 21, 2024, 09:37:19 am
content: Thank you, I'll try cleaning it.I found something else strange. In calibration mode, there is some extra light visible between the signals. Is this normal?

author: David Hess
author_rank: Super Contributor
timestamp: October 21, 2024, 11:34:25 am
content: Quote from: mnemotechnika on October 21, 2024, 09:37:19 amThat is caused by the electron beam hitting the scan expansion mesh and is completely normal. It is why CRTs which use a scan expansion mesh have lower contrast.