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Tektronix 475A B Delayed only triggering with A Trigger Slope


Timothy
 

Hi folks! Once again I am stumped and asking for some assistance.

This is another scope I have been refurbishing. The power supply has been fully gone through and has all new filter capacitors and 1 new bridge rectifier. It has been well cared for in regards to being kept indoors, loved and not abused. But the previous owner did not attempt to make repairs and it had 6 or 7 different issues. I traded a fully refurbished 2445A for it (plus front cover and a 475A service manual) since I liked it so much (I have way too many scopes!). Anyways, this is the final issue.

Tektronix 475A, when Horizontal Display - B Delayed is selected only the A Trigger Slope knob allows me to trigger and light the triggered bulb. The B Trigger slope knob was previously showing some changes to the signal. When there was a single bad tunnel diode in the circuit and you turned the B knob to the right the trace would just fold up and disappear. But in regards to actually triggering the signal it has had no effect even after I replaced the bad tunnel diode and corrected that situation.

When B'Dly'd is depressed J2-6 changes from 5v to -8v and I did check transistors in the various horizontal circuits branching from B Dly'd but I was hoping someone can help narrow down what circuit might allow this to happen. I have not checked any components on the A7 Timing board yet since its hard to get to.


robeughaas@...
 

I have had a problem with a couple of 475's and 475A's caused by Q904 in the A sweep circuit. This part is specified as a 151-0190-01. The base -0190 is a 2N3904, but the -01 version is a Teledyne part in a round white ceramic package with a triangular lead pattern, rather than the black plastic TO-92 of the regular 2N3904. I don't know what the selection criteria for this part are, but it is specified as being supplied by Teledyne. (There are 9 variants of the -0190 listed in Tek documentation). I have found that these tend to be lower beta than other 2N3904's, and I have had some success using the lowest-beta part I could find in this position.

--
Bob Haas


Timothy
 

Thanks Bob. I did not see any problems in that circuit. Now I admit that I am still learning about how these circuits work and often I am just checking voltages, looking at signals and testing components to find problems. Im not a engineer. I also may not have described the operation of the Tek 475A incorrectly.

What I found was the second tunnel diode on the B trigger, was also bad. Once replaced I was able to trigger with the B Slope knob. However it does not light the triggered bulb, only A slope seems to do this, this may be normal operation.

Hopefully I can get replacement tunnel diodes in the future. I dont think I have many used spare ones left.