Tektronix 7B85 Repair (No trigger) - Page 1
Forum section: electronics / repair

author: BlownUpCapacitor
author_rank: Frequent Contributor
timestamp: February 21, 2024, 07:24:57 am
content: Hi everyone, I have a Tektronix 7B85 delaying time base in need of repair.Everything else works fine except the trigger. The problem is 100% A2 trigger board assembly as I have a working 7B80 that I switched boards with, and the trigger worked fine on the 7B85 with the known good A2 board.Anyway, here are the voltages I got from the test points from the manual.The format is as follows: "reference designator"-"pin number", "actual voltage", "(expected voltage)"All readings are in volts unless specified otherwise.U65-5, -3.77, -4.93U65-4, -0.726, -0.68U65-2, -0.05, -0.05U65-8, -2.05, -2.44U65-6&7, -2.05, -2.5U65-10, 0.309, 0.02U65-3, 0.743, -0.12U65-1, 3.50, 2.98_________________U55-3, 3.42, 3.51U75-5, 3.81, 3.56U75-3, 3.80, 3.50________________U85-13, 1.072, 3.88U85-14, 1.037, 3.86________________All voltage rails are reasonably okay.Unsure if U65 is the problem. The voltages change when a known good IC is used instead, but the suspect IC works in the working 7B80...

author: David Hess
author_rank: Super Contributor
timestamp: February 21, 2024, 04:23:14 pm
content: Before going further, check whether the external trigger and line trigger sources work.

author: BlownUpCapacitor
author_rank: Frequent Contributor
timestamp: February 21, 2024, 05:52:12 pm
content: They don't. Absolutely no trigger works. Even if I inject a trigger signal in, nothing.

author: David Hess
author_rank: Super Contributor
timestamp: February 22, 2024, 01:00:00 am
content: Pins 13 and 14 are differential ECL inputs to U85 and your measured voltages are way off. They should be 3.55 and 3.95 volts.The trigger output pins from U65, 16 and 17, are the collectors of differential pairs, so one draws a current while the other is off, depending on the output state. So either U65 is not operating correctly, or the bias network built around R85 is bad. Check if the wiper of R85 is open.Data about trigger amplifier and channel switch U65 is available here:https://w140.com/tekwiki/wiki/155-0126-00Data about the trigger U85 is available here:https://w140.com/tekwiki/wiki/155-0109-00I think the problem is around U65 and there is a full schematic at the above link, but it is complicated enough that it is not immediately obvious to me what could be wrong. If you try swapping U65, be careful that the supply voltages are correct first so as not to damage a good IC.

author: BlownUpCapacitor
author_rank: Frequent Contributor
timestamp: February 22, 2024, 08:18:39 pm
content: R85 does indeed have an open wiper. Measuring several kilo-ohms even though it is a 10-ohm pot. I don't have any 10-ohm pots right now though, so I'll try to repair the component itself.Edit: The component repair plan has proven unsuccessful.

author: BlownUpCapacitor
author_rank: Frequent Contributor
timestamp: February 22, 2024, 09:58:01 pm
content: I actually managed to fix R85 and the wiper now makes contact, and after replacing it, the trigger works perfectly now!Thanks!

author: David Hess
author_rank: Super Contributor
timestamp: February 23, 2024, 01:04:02 am
content: Then I declare victory!I have several 7B80/7B85 sets and they have all worked fine so far. About half of my 7B52As and 7B92As have problems, but usually with the delayed sweep cam switch.