Help with Tek 2445 repair - Page 1
Forum section: electronics / repair

author: giosif
author_rank: Frequent Contributor
timestamp: December 21, 2024, 10:18:48 pm
content: Hi,I am trying to repair the PSU from a Tek 2445 scope which is not powering on (i.e. the scope doesn't show any signs of life when applying power).As such, I removed the PSU from the scope and I have been attempting to power it on its own.First thing to say is that I am not sure if the PSU requires a minimal load in order to run (I couldn't find anything along these lines mentioned in the service manual).However, reading the "Preregulator Control" sub-section in the "Low Voltage Power Supply" section (page 3-40), I understand IC U1030 is supposed to be powered separately (via Q1021, Q1022, and other nearby components).So, I would expect that IC to have power on when applying power to the PSU, even if only momentarily.However, when measuring the voltage between pins 12 and 7, it barely reaches the minimum level for the TL594 IC (i.e. 7V) and keeps cycling - please seesecondfirst attached picture.Reading the service manual further, it mentions C1025, which is part of the separate "power rail" for U1030, should develop about 21V across its terminals, but I observe a maximum of only 8V - please seethirdsecond attached picture.In my attempt to identify the fault, I have been checking the various components in the associated section, as well as in the nearby sections of the PSU, but was not able to identify anything wrong - please see the components marked with green in thefirstthird picture.Can anyone help me with any pointers, please?I am running out of ideas what to look for and what to do to further troubleshoot the PSU.Thanks!

author: calibrationfixture
author_rank: Regular Contributor
timestamp: December 23, 2024, 12:47:13 pm
content: Hi giosif,See:https://groups.io/g/TekScopesEasy to search on Topic. You don't to be a Member to search for information. Only if you want to Post.Success,Calibrationfixture

author: m k
author_rank: Super Contributor
timestamp: December 23, 2024, 07:56:38 pm
content: Quote from: giosif on December 21, 2024, 10:18:48 pmQuite fancy thing that PSU.I'm quite sure I've seen its operation explained somewhere around here.Some load wouldn't do any harm, 5V has 2x 5A fuses, so 1A there should do it.But seems that Q1022 is turning on too early.What is over C1023?

author: George Edmonds
author_rank: Frequent Contributor
timestamp: December 25, 2024, 09:18:58 am
content: HiI can help you but not via this blog, there are too many posters with no knowledge of what they are posting about creating confusion at best.First please download the service manual fromWWW.Tek.com, it is some 109MB in size. Your own posting indicates that you do not have this copy.Please then PM me and we can then correspond via direct email.I do have a 2445 power supply as I acquired an incomplete 2445 as part of a job lot deal.I also have some sixty five years of experience repairing Tek scopes.G Edmonds

author: giosif
author_rank: Frequent Contributor
timestamp: December 25, 2024, 09:03:57 pm
content: Thank you all for your input!And thank you G Edmonds for the kind offer to assist further!Since my initial message, I did a more careful reading of the service manual and, lo and behold, it has a very nice troubleshooting section.I need more patience and probably better glasses too.Anyway, after following the steps in the "troubleshooting tree" for the power supply, I narrowed down the problem to the separate power supply for U1030 - i.e. mainly Q1021, Q1022 and VR1020.And the culprit was... the 6.2V Zener diode VR1020 which, funnily enough, was still measuring fine in forward bias and even in reverse bias, this latter one for a testing voltage above 6.2V (i.e. using the 15V range for diode measurement on the 121GW multimeter).However, testing in reverse bias with a normal testing voltage (i.e. about 3V), the reading on the meter showed something like 1.x - 2.y voltage drop.So, m k was right in his assessment that Q1022 was turning on too early.The Zener diode was replaced and the PSU is now working fine.Ah, and to answer my other question: the service manual also mentions the PSU does require a minimum load to regulate properly, at least for the +5VDC power rail (at least 2.5A, if I understood correctly).Now, my next issue is related to CH2 which seems to have a problem with the VAR knob:- Even in its "detent" position, the readout shown on the display includes the ">" sign for the scale of the vertical divisions (that would normally be displayed only when turning the VAR knob out of its "detent" position).- With the VAR knob in it "detent" position, the amplitude of the signal displayed is 50% smaller than it should.I should probably DeoxIT all pots, as I suspect the one for CH2-VAR is probably not making good contact in its "detent" position.