Tektronix 475a + dm44 - Page 1
Forum section: electronics / repair

author: JonteP
author_rank: Newbie
timestamp: March 09, 2015, 10:26:10 pm
content: Hi all,I'm still very much a newbie when it comes to electronics and recently got my first oscilloscope, the Tektronix 475a with dm44.First thing I noticed was that the dm44 refused to work. Turned out to be a dead 12v voltage regulator and after changing it it came alive! It was a bit of a pain to get to the lv power supply board though. Overall the oscilloscope seems to work fine now and I'm quite happy with it. It has a few quirks though.1. The .5V/div on channel 1 does not work, I just get a flat line.2. At some combinations of volts/div and time/div the calibration signal is jittering. Most notably, at 50mv/div on ch 1, it's jittering on all time/div settings except 1ms. I can improve it by increasing holdoff, but note that it happens when holdoff is at minimum. I can also improve this by pulling out the time/div knob to the b sweep position and wiggle it. Sometimes, switching to A INTEN also helps.3. The time/div knob goes out of alignment with the numbers when turning clockwise and at some positions it seems to fall between two positions, giving half sweeps or other anomalies4. According to the manual, dual delay sweeps should only be active when dm44 is in time or 1/time mode, but it is always active. I bypass this by switching to alt mode and hiding the ch2 sweep.Other than this, it works great.I guess point 3 is mostly a case of readjusting the knob but I'm not so sure what to look for in the other cases. Maybe cleaning of contacts or failed components?Any input is welcome!

author: GreyWoolfe
author_rank: Unknown
timestamp: March 10, 2015, 12:58:57 am
content: No harm is cleaning all of the pots with some contact cleaner. Before doing that, you can also rotate all the knobs several times back and forth from stop to stop and see if that helps.

author: tombi
author_rank: Regular Contributor
timestamp: March 11, 2015, 10:49:19 am
content: With the jitter, so does the trigger light come up? Not always a guarantee as the actual globe can be broken (although I can't remember if 475a has LEDs or globes).You could try pressing the trigger view button (the square bandwidth button on the left between the volts/div buttons). This should show you what the trigger circuit is seeing. You can adjust the trigger level to get it in the middle and then the scope should trigger on the signal.One common fault is the tunnel diodes in the trigger circuit fail and then the trigger stops working. If you suspect this is the case you can swap the diodes from the A and B trigger around and see if it makes a difference.The only other thing I could think of is if you have trigger set to normal and you have two unsynchronised signals on the two channels it will appear to jitter as it will alternate between triggering of each signal. Try setting the trigger to channel 1 or channel 2 and one signal should halt.With the flat signal on some volts/div ranges it is likely the switch contacts inside the attenuator. Forgive me if I get this wrong as I have a 475 which might be a bit different. On the left side board there are a pair of cans covering a PCB containing the input attenuators. If you unscrew and open the cans you can see the attenuators which are white boxes that plug into the board. You can gently pull these out and re-seat them which can sometimes help.Also, as you turn the volts/div knob you will see these tiny gold switches move up and down. For each switch, you can get bit of paper and soaked in isopropyl alcohol, turn the knob so the switch opens, stuff the paper in and close the switch. Gently move the paper around and pull it out. This will gently clean the switch contacts.There are a couple of switches underneath the rotating assembly that you can clean the same way but they are harder to reach.I don't understand what you mean about the delay sweep - this should be independent of alt/chop etc. If you put it in A Inten mode, move the B sweep up a couple of clicks (pull and turn time/div) and then turn the delay time knob (multi-turn knob top right) you should see part of the wave form appear brighter than the rest and will move with the delay time knob. Also make sure the B trigger is set to 'starts after delay'.I suggest downloading the manual too as it has complete schematics and a theory of operation.Tom