Tektronix 2445B input shows 50Ω OVERLOAD and a Vertical error - Page 1
Forum section: electronics / repair

author: BriansBrain
author_rank: Newbie
timestamp: September 07, 2022, 01:50:30 pm
content: HelloFirstly, I did search but could not find an equivalent problem.I have a Tektronix Oscilloscope 2445B Serial # B063339(the baby brother of the 2465B)History:-I replaced four leaking capacitors on A5 Control board over 2 years ago,all tracks around the area were cleaned and found to be intact andconnecting fine with everything working perfect since then.Now:-Powers up OK, everything looks normal.Run self tests = ALL 00 PASSAll Power Supply Voltages and ripples at J119 check out OK.Errors:-Control Board A5...The -1.25V reads -1.01VThe +1.36V reads +1.103Using the ''Vertical Position'' controls for any channel it's impossible to set the trace in the centre of the screen.(however with an input signal applied the Vertical amplifiers seem to be working as normal)Horizontal system will not trigger to any input signal.CH1 or 2 with no input signal, if I select50Ω DC(50 Ohm DC) the50Ω OVERLOADerror appears at t he top of the crt,relays click and the input resets to GND, if I up select either to DC GND or AC the50Ω OVERLOADerror MSG disappears.I've looked through the manual but I can't seem to work out what circuit sets the -1.25V and +1.36V references.I would appreciate any help or advice on the problem or what to check.Thanks in advance for looking

author: BriansBrain
author_rank: Newbie
timestamp: September 08, 2022, 12:09:19 pm
content: On thefacebookOld Tek Scopespage a member replied with this...QuoteThis area

author: BriansBrain
author_rank: Newbie
timestamp: November 03, 2022, 02:49:37 pm
content: OK I found some bad componentsOn the A5 board I replaced 2x10K R2013 and R2012 with 0.1% Low Temperature Coefficient SMDs.I also replaced C2010 the trimmer R2010 a new vref compensation cap on pin 16 100PF C2110.All other resistors in the DAC circuit were measured before the new ones where put in and they all were well within tolerance.I checked all the joints with a meter and with a digital microscope and all seem to check out OK.I have also given a good cleaning of the area with with Propanol Alcohol.I can't get a DAC range of 2.500V.The maximum DAC range I can obtain is (SUM of CCW and CW) = 2.375V.Any suggestions on what to look for next?

author: Testtech
author_rank: Regular Contributor
timestamp: November 04, 2022, 10:24:41 pm
content: You still have some corrosion damaged parts, and/or, open traces. Did you remove and clean under the DAC? You will likely need to. When I restore one of these damaged boards, I remove and replace all of the SMD parts, and carefully continuity test all the traces before installing the new parts.

author: BriansBrain
author_rank: Newbie
timestamp: November 05, 2022, 11:21:46 am
content: Quote from: Testtech on November 04, 2022, 10:24:41 pmThanks for the replyNo... I have not removed the DAC... yetI was trying to avoid that as I don't really have the proper equipment to do so.I have done a continuity test on all traces and they are OK, all other resistors measure correct.

author: fant
author_rank: Contributor
timestamp: November 05, 2022, 01:09:48 pm
content: Good morning;looks very strange that on the right picture the voltage on pin 14 and 15 are both 0 V.Should be somewhere more than 0 Vdc.Mandi

author: BriansBrain
author_rank: Newbie
timestamp: November 05, 2022, 01:25:10 pm
content: Quote from: fant on November 05, 2022, 01:09:48 pmPlease note the position of the trimmer R2010 to obtain those voltages

author: BriansBrain
author_rank: Newbie
timestamp: January 27, 2023, 01:25:35 pm
content: Firstly I replaced all 9 resistors in the DAC circuit including the 2 x 681Ω in the output.Same error.I waited two months for delivery of a new DAC IC from China.Fitted it today and all is good and I have a DAC range of 2.500V.