Tektronix 7904 Power supply issues - Page 1
Forum section: electronics / repair

author: Zenwizard
author_rank: Regular Contributor
timestamp: June 19, 2023, 10:53:40 pm
content: I have a mid late serial number power supply for a tektronix 7904 that is trying to start but is not quite getting in to burst mode. I have some scope shot taken from the input of the switching transformer. The first is a working supply in burst mode the other one is the broken supply that is not starting correctly. If I put the working supply in to the scope frame everything starts up and works fine. So the frame is good but the power supply needs some help.Been spinning my wheels a little hoping to get some traction soon. seeing if there was any thoughts?

author: David Hess
author_rank: Super Contributor
timestamp: June 20, 2023, 01:22:39 am
content: The primary side uses a DIAC for self starting and then powers the switching controller IC through the primary side current sense transformer. Take a look at the supply voltage at the IC produced by the current transformer to see if the switching controller is even being powered.

author: Zenwizard
author_rank: Regular Contributor
timestamp: June 20, 2023, 02:17:09 am
content: I am assuming the switching controller is the TEK branded IC that is on the semi regulated board?

author: Zenwizard
author_rank: Regular Contributor
timestamp: June 20, 2023, 02:29:40 am
content: U1275Pin 6 +1VdcPin 7 -1 VdcIf this is correct than no the burst pulse is not waking up the controller IC.Zen

author: David Hess
author_rank: Super Contributor
timestamp: June 20, 2023, 12:29:09 pm
content: Quote from: Zenwizard on June 20, 2023, 02:17:09 amYes, the 7704 has a discrete implementation of their switching regulator controller and then they made an IC for later models starting with the 7704A and 7904.Quote from: Zenwizard on June 20, 2023, 02:29:40 amThat is right. The switching controller IC is not being powered for whatever reason. Pin 6 should be +7.5 volts and pin 7 should be -2 volts.No voltage there could be caused by no primary current, or the switching regulator controller could be shorted. I think I would remove the IC and then test pin 6 and pin 7 again. VR1279 should limit the voltage to 15 volts maximum.Tektronix published an article about diagnosing the power supply which is available here:https://w140.com/tekwiki/wiki/7904

author: Zenwizard
author_rank: Regular Contributor
timestamp: June 20, 2023, 09:49:54 pm
content: With the switching controller out of the power supply it looks like I getPin 6 + 11 VdcPin 7 -1.6 VdcSo I think my switching IC is toast.Zen

author: David Hess
author_rank: Super Contributor
timestamp: June 20, 2023, 11:42:07 pm
content: Quote from: Zenwizard on June 20, 2023, 09:49:54 pmThat would seem to be the case. My recollection is that some fail by shorting, but I do not remember anything about why.

author: Zenwizard
author_rank: Regular Contributor
timestamp: June 21, 2023, 01:24:13 am
content: A bit of HUGE progress. I am back in to TICK mode with the power supply. Turns out my values for C1271 as well as C1278 were not correct here. Good news is the switching controller is still good. I had 10 uF caps here as that is what some one else recommended. The R/C time consent is way to long it was not letting the voltage rise fast enough. Switching them back to 1uf tantulums has worked.Also in a late serial power supply they are 50Vdc rated and should be find for the long run.Weirdly enough I happened to have another switch controller chip here in the lab.Zen