TDS 744A diplay driver board. Help please. - Page 1
Forum section: electronics / repair

author: eurgenca
author_rank: Regular Contributor
timestamp: May 04, 2020, 08:13:23 am
content: Hello.I am having trouble with TDS 744a display driver board. Picture is narrow on horizontal deflection. Horizontal position and size potentiometer are working, but still missing 1/3 picture.So far:I replace all elcos in horizontal stageCheck all power lines 25V, 21V and -12V all o.k.Deflection coils are O.K. Tested with other CRT.I made some measurement on friends scope (Green color) and only difference on my board (Red color) is on emitor of Q205 and collector Q245. I desolder and check Q245, CR156, CR208 and Q205 but it all looks o.k.Like I say C238 and C243 was replaced.So what on earth can be wrong with this board?I test other caps and resistors (with DMM) and looks fine.Any help or clue will be fine.Thanks, Sebastijan.

author: shakalnokturn
author_rank: Super Contributor
timestamp: May 04, 2020, 10:12:28 am
content: C323, C221 then C302, C305, C216 ?

author: eurgenca
author_rank: Regular Contributor
timestamp: May 04, 2020, 11:12:25 am
content: Hello.C323 and 221 was replaced. C302, 305 and 216 was tested, but with DMM. I don't know if this is o.k. could be leaked on high voltage?Seba.

author: CaptDon
author_rank: Super Contributor
timestamp: May 04, 2020, 12:14:36 pm
content: Possibly something in the size regulator section is wonky?Perhaps the I.C. is bad? Not often recommended, but perhapsrun a fly lead from the output of that I.C. to the outside of thechassis where you could just for a brief moment touch it to ground.That would turn off the size regulator transistor and allow moredeflection. If nothing changes then you may have a clue to whatis going on. Is either of the size regulator transistors (the onesafter the I.C. leaky or bad? That would hold the sweep down.What about the drive to the horizontal output transistor. Doesit compare to your friends good board? Those boards are a P.I.T.A.to troubleshoot do to their location. I had a complete externaltest jig for repairing those CRT driver boards. You must be extremelycareful to ground the external aquadag/shutter assembly or thewhole thing will be destroyed if you are going to 'bench test' it!!

author: shakalnokturn
author_rank: Super Contributor
timestamp: May 04, 2020, 12:45:33 pm
content: Any loose or missing inductor cores?

author: eurgenca
author_rank: Regular Contributor
timestamp: May 04, 2020, 02:05:19 pm
content: Hello.Further testing.Replacement C302 is not helping. If I ground pin 7 of LM358 got VERTICAL line on center CRT. If I disconnect horizontal yoke clearly got VERTICAL line but on colector Q245 then I have 23V. Is this mean that problem is betwen j305 and Q205.Aha no lose or missing core around inductor.

author: CaptDon
author_rank: Super Contributor
timestamp: May 04, 2020, 03:41:09 pm
content: On closer look at the schematic I see the 'size' section is actuallythe B+ supply to the horizontal output stage. Sorry, I missed that.When you said a 'horizontal' line on the display, I would assumeactually there was no horizontal sweep at all and the line on thedisplay was vertical up and down??? Looking at the schematic,grounding pin 7 would have turned off the B+ to the horizontalyoke driver and collapsed the horizontal sweep.

author: eurgenca
author_rank: Regular Contributor
timestamp: May 04, 2020, 09:00:19 pm
content: Sorry my mistake.I get vertical line not horizontal.

author: eurgenca
author_rank: Regular Contributor
timestamp: May 04, 2020, 09:20:43 pm
content: Yes B+ go thru Q245 and CR 156 to T150. So if horizontal yoke is disconnected and I have 20V on collector of Q245 I suppose that I have leaking thru C216 or CR307 or this can affect on feedback to pin4 of U350?I will order C305, C216 and CR307 becouse this is my first target.Just for info I found R311 with resistence of 40K but no signh of burning or discoloring.

author: CaptDon
author_rank: Super Contributor
timestamp: May 05, 2020, 12:43:55 am
content: I would suspect you would still have voltage on the collector ofthe output transistor even with the yoke disconnected. The feedis to the collector and not through the yoke look where the transformerlead goes to the collector path.

author: eurgenca
author_rank: Regular Contributor
timestamp: May 23, 2020, 08:54:17 am
content: Hello.Yesterday I have time to play with board. I replace C305, C216 and CR307 but no luck. So I remove Q245 and put DC/DC converter between E and C. Voila it works. If I adjust voltage I can set picture. So Q245 is marked with F2955 can anyone confirm that this is original transistor, becouse it was soldered. Is this transistor MJF2955 or something else?Sebastijan.