Tektronix 7603 not sweeping horizontally  - Page 1
Forum section: electronics / repair

author: turtius
author_rank: Newbie
timestamp: May 31, 2021, 06:11:44 pm
content: Hello everyone i'm new to this forum and electronics in general. I have this Tektronix 7603 oscilloscope that stopped functioning after not being in use for couple of years.Now the problem is that the horizontal sweep is no longer functional. So i opened up the service manual and started to debug. Now according manual, the horizontal sweep is generated by the time base and in my case its the 7B53A which i checked by probing the SAWTOOTH OUTPUT using a DMM. Now with the slowest time on the time base, i saw a voltage ramp going from 0v to ~5V and then back to 0V so with this I figured the sweep generator might be functional so i moved to the mainframe instead.From here i took a closer look at the horizontal amplifier and probed the P501 header since this came from the time base. This time though i saw a voltage ramp going from 0V to ~1.7v and then back to 0. So i figured that the sweep gets passed from the time base to this circuitry and the problem is the horizontal amplifier(maybe) but i'm not sure if this voltage range is correct or not.I must add that the 'beam find' button is also functional as it causes horizontal deflection.Now there was the unlabeled chip that i had suspicion as its the horizontal switch but i dont know how i'd go about checking if this chip is functional or not.So any help would greatly be appreciated.Here is a video demonstrating this problem:

author: lowimpedance
author_rank: Super Contributor
timestamp: June 01, 2021, 01:44:15 am
content: The PCB looks rather crusty (dust and/or corrosion ?), I would say a good clean of the PCB and probably the rest of the instrument. Thing to note here is most of the semiconductors including the transistors are in sockets. I would suggest giving the transistors that are socket-ed a good wiggle up/down to 'wipe' any lead oxides off that may have formed in storage , do this step after cleaning and inspection/repair of any bad PCB traces from corrosion etc

author: David Hess
author_rank: Super Contributor
timestamp: June 01, 2021, 03:44:37 am
content: A vertical amplifier can be used in the horizontal slot to test the horizontal circuits in the mainframe.

author: turtius
author_rank: Newbie
timestamp: June 01, 2021, 06:38:12 am
content: Quote from: lowimpedance on June 01, 2021, 01:44:15 amI see, Now the way i thought about 'cleaning' the board was using a brush and alcohol but saw the transistors present which i believe are not ESD tolerant as the brush will most likely cause some static electricity by the strokes. So how would this be approached with these vintage boards?Quote from: David Hess on June 01, 2021, 03:44:37 amAh!, that'll clear with the time base/amplifier problem, will give this a shot after 'cleaning' the boards.Thanks.

author: lowimpedance
author_rank: Super Contributor
timestamp: June 01, 2021, 11:20:01 am
content: I would give Davids suggestion of swapping the plug in and using the vertical to test the mainframe horizontal section first before disturbing any PCB with cleaning just to isolate the fault location for certain.Then clean and/or repair. If just somewhat dusty I would use an unused 20mm paint brush to loosen the crud and gently blow it off to start. I have not had any issues myself with this method.If your concerned use an anti static brush which I guess you will need to buy first. Plus a wrist strap and mat to work on.If more cleaning is needed then IPA with cotton tips or a small bristle brush will help.

author: turtius
author_rank: Newbie
timestamp: June 01, 2021, 03:38:27 pm
content: Yes, i reinstalled the amplifier back in without doing anything and swapped the vertical unit with the horizontal. This confirms that the time base is sweeping, thanks for that!