Tektronix 475 - Page 1
Forum section: electronics / testgear

author: djmwj
author_rank: Contributor
timestamp: January 20, 2014, 03:33:44 am
content: Hi,I am interested in purchasing an oscilloscope and have found one in my local area a Tektronix 475. The problem is that it looks like the beam is cut off halfway through. I was thinking maybe this would be something i could repair as a good project. I don't have any experience with this however, and was hoping to get some inputs mainly that the beam as shown in the picture below would be repairable. Please any thoughts are appreciated.

author: Bored@Work
author_rank: Super Contributor
timestamp: January 20, 2014, 06:34:34 am
content: It might (or might no ...) a simple front-panel adjust thing.There should be a knob labeled similar to "Position <---->" on the front panel. Probably below the CRT (I don't have a 475 at hand, this is from memory). Try to turn the knob to see if you can get the whole beam onto the CRT.

author: dfmischler
author_rank: Frequent Contributor
timestamp: January 20, 2014, 06:45:47 am
content: Also make sure the "HORIZ DISPLAY" buttons on the right side of the display is set to "A LOCK KNOBS".

author: djmwj
author_rank: Contributor
timestamp: January 25, 2014, 04:35:16 am
content: Ok thanks for the tips. I am going to go check it out this weekend. Any tips on things I should look out for or basic tests to run?

author: djmwj
author_rank: Contributor
timestamp: January 28, 2014, 11:58:50 pm
content: So i picked up the scope, The beam is definitely stuck, and the on board calibrator waveform does not produce the intended square wave (or the scope is not displaying it right or both) I am in the process of looking for a waveform generator that i can get access to to test the scope.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 29, 2014, 01:12:57 am
content: Guessing offhand that half the horizontal deflection circuit is toast. It's differential all the way through, so it's quite possible one side has a failed transistor or something and it's literally not reproducing the other half of the waveform. On the upside, the horizontal amp is built with discrete transistors, socketed, most of them relatively generic (ranging from 2Nxxxx to MRFxxx I think?? -- check the service manual). Could also be failed resistors, caps (check for toasted tantalums?) or broken power supplies upstream (current limiting resistors, or fuses -- I forget which supplies are fused if any). Always check and replace old electrolytics, too. All this stuff is on the main board on the bottom, though getting access to the backside is tricky at best (hopefully any soldering required can be done from the top).Tim

author: TerraHertz
author_rank: Super Contributor
timestamp: January 29, 2014, 03:30:50 am
content: As you adjust the HORIZONTAL POSITION knob, how does the visible trace move? Does the right-most end move, or is that stuck in center screen? Can you ever see the left-most end, or is that always off-screen?You checked that both the wires from the final H-amp board are actually connected to the pins on the side of the CRT, for the deflection plates? These can fall off.When you press the BEAM FIND button, what happens?

author: djmwj
author_rank: Contributor
timestamp: January 30, 2014, 02:00:44 am
content: Thanks for the replies. I have started looking at the service manual and trying to understand what is going on in the horizontal amp circuit. so i have found that the Positive right deflector plate has no voltage. My understanding is that they should be equal.TerraHertz-the visible trace does move but never past the left half of the screen. I cannot see the left end of the trace as it remains off screen.The beam find button does not produce any output on the screen. I am not sure if that is because the output is just shifted too far left off the screen. I am going to post the circuit diagram and what voltages i find on them shortly.Thanks for the help.

author: djmwj
author_rank: Contributor
timestamp: January 31, 2014, 02:51:29 am
content: Ok so I found the schematic and started probing around the right deflector plate to see what was up. I must admit i am a little confused. Looking at the schematic it would appear to me that the Q1202 transistor would never be able to switch because it is waiting for a smaller voltage on the base. I must be wrong on this, could someone clarify? I appreciate any help. I think the results here indicate to me that the Q1202 circuit is bad. I attempted to check with the voltmeter and it does seem that i get an ohm reading regardless of the polarity of applied to the transistors emitter and base.See Schematic attached.

author: djmwj
author_rank: Contributor
timestamp: February 08, 2014, 03:40:32 pm
content: Does anyone have any insight as to whether or not this is correct?

author: Nuno_pt
author_rank: Frequent Contributor
timestamp: February 08, 2014, 03:45:08 pm
content: You could try W2AEW here on the forum, he work for Tectronix, he might have a better understanding of the problem

author: Andy Watson
author_rank: Super Contributor
timestamp: February 08, 2014, 04:40:11 pm
content: My reading of the circuit is that Q1202 is providing a constant current load for the collector of Q1204 - therefore, Q1202 never turns off - the voltages look about right.However, the voltages around Q1204 do not look right. Q1204 is operating in common-base mode. R1203 and R1204 are specified to three digits suggesting an intention that the base voltage is well defined. I calculate it should be -4.98V - something is dragging it down. Need to measure the voltages around Q1174 and Q1172 to be sure, but, how accurately did you measure the -6.3V ? The collector voltage of Q1204 looks very close to its emitter voltage, even for a device that is in saturation. Without further information my inclination would be to pull Q1204 and check it for C-E short.

author: oldway
author_rank: Super Contributor
timestamp: February 08, 2014, 07:36:07 pm
content: Try to switch Q1204 and Q1214 of positions to see what happen.