Tektronix 485 Channel Reset

[Electro Fan]

If the red reset light remains on, even after the 50/10M Ohm button has been depressed twice (in an effort to reset), does that mean that something (blown resistor?) is defective in the circuit?  What's the typical fix?  Thx

[med6753]

I would say yes. If it doesn't reset something is blown in the 50 ohm circuit. Here's the section of the service manual that describes it's use.

[David Hess]

The circuits which control the overload protection are part of a custom Tektronix IC which also controls the probe readout, but there are several connectors and discrete parts which could be causing the problem.  The ICs are generally reliable unless abused.

[Electro Fan]

Thx

I saw the same excerpt.  It’s not very understandable what the manual is trying to convey.  “Pushed twice” means pushed twice in fairly rapid succession?  Or something else?  And “485-2 is non-illuminated momentary switch for RESET only”?? If anyone can explain, thanks.

Does 485-1 and 485-2 refer to two versions of the 485?  Or maybe the wording indicates there is also a 485-3 version?  If so, any easy way to ID the particular version?

On the good channel the 50/1M buttons work and the 1, 10, 100X probe LEDs work; on the 2nd channel only the 1M Ohm setting works, and the 1X LED works, but with a 10X probe the 100X LED goes on rather than the 10X LED - so maybe the IC David described is the problem, or maybe it’s a discreet (resistor) component?  How hard is it to access this area inside?

Thanks again

[med6753]

On my 485 the reset is simply a light and not a push button switch.

Assuming you have only one channel with the reset issue your troubleshooting can be made easier. Set the scope to "ALT" so both channels are active and do a comparative analysis between them. Start at IC U80/M94 and work your way back. I would suspect the defect would reveal itself rather quickly.

I've only been in that area of a 485 once to fix a channel 1 roll off issue in the Low-Z circuit and as I recall access was not too bad.   

[Electro Fan]

fwiw, looks like there were at least 3 versions:  485, 485-1, and 485-2

http://docmesure.free.fr/manuels/Tektronix/Serie%20400/TEK%20485/TEKTRONIX.%20485.%20SCOPE.%20INS.%2011528.pdf

[mr.fabe]

The differences were lower paid options with less features from the standard 485 prices. The 485-1 was less $100 and the 485-2 was less $350.  The standard 485 has the external A trigger.

[Electro Fan]

I think I’m dealing with a 485 (not -1 or -2).  So the reset process should be to press the 50/1M switch twice?  Just want to confirm the scope needs a repair vs operator training

[mr.fabe]

I have not experienced any overloads with my 485.  The push twice is what's detailed in the service guide along with removing the source of the overload.

There are also steps you can perform to verify that the attenuator is working properly in the service guide.