Electronics > Repair
Tektronix 475 in need of repair
guenthert:
A buddy of mine acquired a non-working Tektronix 475 and handed it to me to repair. I cautioned already, that such a project exceeds my level of experience and skill, but he couldn't be discouraged :-\
First it did greet me with the magic smoke. At least that was an easy fix: a blown Tantalum capacitor in the +15V PSU sektion. There was another bad one (which at first glance looked ok) and two blown Transistors in the -15V PSU section.
It works now in as much as there is a trace, albeit it is very wavy/fuzzy and I can't adjust the Y offset sufficiently to center it (the 'beamfinder' has some effect, but not exactly the desired one). At least HV and timing seems to be working.
The DC values of the various PSU subunits look Ok, there is some excess ripple, but I'm not sure how much that'll be expected from a 40+ year old instrument and how much of an impact that'll have (might explain the wavy/fuzzy trace, but hardly the offset).
Fortunately I found a service manual online (else I wouldn't have attempted the repair). There are however test points on the PCB which don't seem to be described in the manual. Is there another document out there which does? I was even tempted to buy a working unit (recently there were *two* Tek 465 on craigslist, alas on seller didn't respond at all and I was apparently too slow for the other) to compare them.
I also am a bit stumbled by some quite high frequency noise (with components as fast as ~140MHz), seemingly (I used a small loop of wire as make-shift EMF probe) emanating from the area where U1454B and Q1446 sit. I'm pretty sure, they're not meant to do that (and don't think U1454B actually can oscillate that fast). What could that be (I *think* it comes from the area and not the nearest FM transmitter ;-} ? Is that normal? If not, how is that meant to be suppressed?
m3vuv:
dont know if it helps,a few weeks ago i repaired a 453a with no trace,found a couple of bad transistors,that got a trace of sorts,it was fat and fuzzy and wouldnt focus,asked on a few forums with no joy,noticed i was testing if with the hv aluminium covers off,as soon as i put them back on,it had a clean nice trace,no one seemed to spot it here!,just a thought m3vuv,73
guenthert:
--- Quote from: m3vuv on October 22, 2019, 06:05:28 pm ---dont know if it helps,a few weeks ago i repaired a 453a with no trace,found a couple of bad transistors,that got a trace of sorts,it was fat and fuzzy and wouldnt focus,asked on a few forums with no joy,noticed i was testing if with the hv aluminium covers off,as soon as i put them back on,it had a clean nice trace,no one seemed to spot it here!,just a thought m3vuv,73
--- End quote ---
Thanks, but I didn't have a need to remove the HV covers (and am not equipped to test/repair anything in that section anyhow).
james_s:
The excess ripple is absolutely going to be an issue. The very first thing I would do is check/replace all of the electrolytic capacitors in the power supply, the 475 is old enough that some of these are likely to be bad. Get all of the power supply outputs in spec before you even start to troubleshoot other areas.
guenthert:
Earlier today I noticed that the fuzzy/wavy waveform displayed changes a bit when I tap the PCB at various places with a pen (around the middle of the PCB on the button it seems to be most sensitive). Not sure, if that's to be expected. Forgot to mention, but I checked most, if not all, transistors for seating.
The fuzzy/wavy waveform (seemingly independent of input signal) is only displayed when the vertical offset is adjusted to one of the maxima (and even then it's off-center). Earlier I had it display 'GND' signal, triggered on line. I tested with the buitl-in (in spec) square wave test signal and noticed that the trigger lamp is lit only when the vertical offset is adjusted sufficient to also display the waveform.
More interesting perhaps, the aforementioned high frequency 'ripple' (that might be a misnomer, as others might think of ripple as a line frequency component) or rather noise varies with the setting of the vertical offset! On the +50V line, there is this noise with a pk-to-pk level of up to 350mV, but with the vertical offset, it can be adjusted to as little as about 20mVpp. Unfortunately, then the wavy/fuzzy signal is displayed off screen. Baffling.
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