Tektronix 7603 Focus Issue - Page 1
Forum section: electronics / repair

author: wlzed279
author_rank: Contributor
timestamp: April 19, 2024, 05:13:19 pm
content: I am having trouble with getting the trace to focus on a Tektronix 7603 Mainframe. I recapped the power supply a few months on this scope and it was working perfectly. I recently fired it back up only to find that the trace had lost focus. I went through the focus calibration procedure and had to adjust the Focus Preset control fully clockwise to its limit to bring it back into better focus but it is still a little soft.Adjusting the front panel focus adjustment, Auto Focus bias and the Auto Focus Gain have no effect on the focus. The astigmatism is properly adjusted.I am not sure where to start looking for this fault. What does fully CW Focus Preset and no response from the other controls tell me?

author: BlownUpCapacitor
author_rank: Frequent Contributor
timestamp: April 19, 2024, 08:11:06 pm
content: Check voltage across R1193. Should be 180v. Ground referenced, -50v, +130vCheck voltage across R1045 too. Should be 100v DC. From ground reference, ±50v.If they check out fine, check voltage at pin 2 and pin 3 of P1194 ground referenced. Should be around +25v at pin 2. Pin 3 will vary between 0-130v.If those still check fine, measure voltage at anode of CR1255.High voltage will be present here!!!!!The voltage should vary depending on the focus setting. Report back.

author: richnormand
author_rank: Unknown
timestamp: April 22, 2024, 09:27:20 pm
content: On my 7603 I had a fuzzy trace that would not want to focus appropriately although the scope was usable (but always a bit more fuzzy than expected from Tek).It turned out to be due to a bad resistor in the "find trace" circuit.When you press the "find trace" button it will reduce the x-y scan amplitude but also go out of focus a bit.I noticed when slowly pressing the button that right at the transition I was getting a good focus for a fraction of a second.That was a long time ago but I could dig up the scope from the garage if you get stuck. I think the schematics were on the boat-anchors site at the time.Best of luck with it.Edit: added two photos Before and after the repair traces.

author: wlzed279
author_rank: Contributor
timestamp: May 06, 2024, 12:11:05 pm
content: I am finally getting back to this project. After opening up the HV cage, It looks like C1222 is having some problems. The black goo tells me it needs to be replaced. It is measure 15uF in circuit when it should be 47uF. I will replace and report back.

author: wlzed279
author_rank: Contributor
timestamp: May 09, 2024, 01:12:47 am
content: Hello Blown Up Capacitor, I finally got around to measuring the voltages you suggested. They were all normal as expected. After replacing the cap i mentioned above, no change in the focus issue. I measured the anode of CR122 as you suggested. With the focus control fully clockwise, I am getting 9.2 volts. After just a 1/4 turn of the focus control it adjusts down to zero and stays at zero for the rest of the range of that control; about 3/4 of a turn.

author: wlzed279
author_rank: Contributor
timestamp: May 09, 2024, 01:14:40 am
content: Quote from: richnormand on April 22, 2024, 09:27:20 pmThanks for the suggestion. The Find Trace function seems to be operating as expected.

author: wlzed279
author_rank: Contributor
timestamp: May 11, 2024, 03:38:39 pm
content: Quote from: BlownUpCapacitor on April 19, 2024, 08:11:06 pmHello Blown Up Capacitor, I finally got around to measuring the voltages you suggested. They were all normal as expected. After replacing the cap I mentioned above, no change in the focus issue. I measured the voltage at the anode of CR1255 as you suggested. With the focus control fully clockwise, I am getting 9.2 volts. After just a 1/4 turn CCW of the focus control it adjusts down to zero and stays at zero for the rest of the range of that control; about 3/4 of a turn.

author: BlownUpCapacitor
author_rank: Frequent Contributor
timestamp: May 12, 2024, 02:34:16 am
content: the voltage there should not be 9.2 volts as far as I can tell. Check R1258.The problem is somewhere in the image provided I believe. The cathode of CR1254 should have a significant voltage change with the intensity control. If that checks out though, might just be a bad tube?