Tektronix 485 Hi-z Attenuator Adjust

May/23, 2016

Tektronix 485 oscilloscope

these days my old  Hitachi  V1050F oscilloscope got  some glitch. the 5mV, 10mV, 20mV band had some contact problem.the oscilloscope use some cheap band switch without electroplated gold.

the whole structure of the oscilloscope is hard to tear down, hard to clean up the band switch.  it's time consume to clean the contact point.  but it does work after polish with sandpaper.  the 5mV band still not stable, my solution is switch in and out many many times, make it self polish, and it does work, mainly reason is the copper cover with Oxide layer, switch them  will attrit the oxide layer, make it better.

there is better choice , get myself a Tektronix oscilloscope. recently it's price is get lower and lower, about $125 in online shop.I decide to choose a Tek 485.  there is full of discuss of choice of the analog oscilloscope. Tektronix 485 is one of good candidate, low failure rate and easy to fix.

the shop is not that far, i get there, choose one of them, it's quite pretty.  checking all knob working perfectly and all functional include X-Y, B-sweep, is OK.

Problem

After i reach home, play it again and carefully, found a problem. the 0.1 V, 0.2 V, 0.3 V band seems abnormal, it's amplitude is 2 times big than channel 1 while connect to an 60 Mhz signal source. 

connect the calibration signal,  fund the raise edge looks wired. and other attenuation  band is ok,  50ohm is OK, 1:1 probe is OK, only  AC 10:1 probe seems get this problem.

tear down 485, the channel amplifier box is under the box,  don't need to open the shield box,  first thing is tune C24, C26.   While tune C24, the flat top get tilt, effect equal to adjust the compensation inside the probe. this is not what i want.

C26, lucky, it's not cause by  poor contact of C26, it's seems compensation been tuned, turn around C26, the problem almost gone.

another little bit problem is  there is a little bit tail on flat top end.  i did not refer to the schematic, just randomly  pick up another adjustable thing trying to see what happens(mark the correct position before doing so, and DON"T do this). 

i trying 2 thing, first one is, C12, nothing noticeable, put it on it's original position. second thing i touched is R53, and, this time that tail disappear.  R53 adjust the HF frequency gain according to service manual.

NOTE to reading attenuation band switch

• switch status:         5 mV vertical sensitivity  , no attenuator switched in. 

           50ohm, AC input

•     Black dot markers of the switches:          black dot corresponding to the nearest switch( from the black dot) is go-through status.

•     3. probe 1:10 1:1 mode don't change anything in the scope.  the 1 M ohm input resistor always hook in. 

•      X10 white box is compensation of transit response of the amplifier for 0.1 V,0.2 V,0.5 V range.

NOTE

After simple recover the CH 2 from this failure, the 2 CH still get about 0.2 Div difference while measuring the 60 Mhz signal source. before i know what exactly thing make this difference(same signal ,same probe), i won't touch it again. it's good enough.

1M ohm attenuator include a 1:1 buffer amplifier, start from Q34 to Q70.  The buffer include bunch of compensate circuits, control and compensate different frequency response.  It also include a OP amplifier for DC to HF gain control.  Such a topology almost a common solution for oscilloscopes.

Attenuator Full Recover

Hi-z adjustment did not address all problem,  All it did, just postponed the defect few months late.  

Refer to a fully restoration Tek 485, include Full Hi-z Attenuator recover procedure:   Restoration Tek 485