Tektronix 577 curve tracer not working correctly

[lazza96]

Hi all,
i bought a month ago a tek 577 with a few fixtures, since then i’ve tested it a few times and it was working but was a little out of spec, so i’ve dediced to do a little calibration and found out that 2 trimpots weren’t working (the R1245 and R528 ).
Ordered some linear ones on digikey (the manufacturer is piher) and replaced the R1245 one. In the mA range everything is working fine but when i switch the vertical/div knob from mA to lower ranges the collector protection light turns on (and the beam on the crt gets greatly out of range), i don’t believe it was the R1245 that i’ve replaced, maybe the preamp in the 177 or the amp inside the 577? I don’t have much time testing all the components, as anyone had a similar experience?
In addition, i see only now, that the 200 volt line on the power board is at around 218v…
Thanks in advance

[lazza96]

I have a major update, today when i switch throught all the minor vertical/div settings the yellow light of overrange doesn’t light up, when switching by the 50 to 20 and 10 mA i’ve noticed a lot of noise on the trace, on the 5 (50 mA , 5mA and so on) scale seems better while the noise gets higher reaching the 1 (10mA,1 mA) scale, it gets so bad that the trace becomes a concave curve.Reaching the uA and nA side you can see a part of a vertical trace appear on the very top of the screen.
Changing the Max peak or the series resistors doesn’t affect it.
Thanks

[hitecelec]

Hi

If memory serves me right , There is a matched Dual FET in the 177 test fixture . Probably faulty.
Try replacing, Try replacing , I think it has a Tek Part no 151-xxxx-xx

[lazza96]

Yeah you’re right, i’ve just discovered it yesterda, the part is the Q604a/b is a dual n-type jfet, the Q604A side was faulty, the component installed in my 177 is the 2n5556 from siliconix, i’ll order one and release an update…
Thanks for the help