Tektronix 475A, trigger does not work?

Joerg Hau wrote in news: snipped-for-privacy@54693.user.dfncis.de:

Lamp.(grain-of-wheat,5V,IIRC;80ma) You can use a LED with the proper dropping resistor.I would.

does not light up as it should. I can only obtain a

No,you need to have 'A' sweep selected.Check to see if external trigger input works.Scope to see if the trigger signal picked off from the vertical is getting to the input FETS on the trigger board.One or both of those FETs could have failed(matched pair).You can swap the pair from B trigger. The circuit uses TEK-made IC's which aren't available from TEK anymore,just from parts scopes.(no longer made & stock depleted)

Also look to se if a wire from the trig level pot is broken or cut from putting the cabinet on.(Saw a lot of that.) Sometimes people also pulled a coax cable off the vertical assy when re- installing the cabinet.Easy to bend the center 'pin' when reinserting the Peltola connector,too.

It may only be a DC trigger level adjustment,or a trig sensitivity adjustment.

Well,getting the service manual is the first step.Read the circuit description,it's very helpful in understanding how the scope works.

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Jim Yanik
jyanik-at-kua.net

Hi Jim

No, it doesn't.

I will, but need to find the circuit diagrams first ;-)

Argh. How do you spell "hope" ?

Been there, everything fine. Pot "as such" is OK.

Verified, everything fine.

Thanks a lot for the help, and have a nice day!

- Joerg

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...

Hi,

I've send him a Service Manual, see PM.

hope this helps.

Heike

Hi Heike,

That's truly kind :-)), but I got a bounce on your mail address ("This account is not allowed"). Could you contact me via PM please?

Best regards (and sorry for the bandwidth),

- Joerg

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Hi again,

Ah ... I got a copy of the circuit diagrams :-)

For the measurements I split the input via a "T" connector to both channels & set the display to CHOP mode:

- Signal comes through the input stage, up to to TPS26 (U520 Pin 3).

- Yes, in both channels.

- Channel A, output of U520 (Pin 8+9) does not carry any AC component.

- Channel B, output of U520 (Pin 8+9) has the trigger signal.

Scope input then grounded, measured DC voltages at the output of U520 (should be 14.7 V according to circuit diagram):

Channel B, pin 8+9 = 14.58 V both. Channel A, pin 8 = 14.20 V, is influenced by R542 Channel A, pin 9 = 15.03 V, NOT influenced by R542

... is that now "just" a problem of DC voltages around U520 in Ch A, or is that IC "shot"?

Any ideas? (... pleeeease say it ain't that IC ;-)

- Joerg

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For a free 475A service manual go to

and look for Tektronix. Quality is not high, but it is for free! I used it to fix my 475.. Good luck.

Joerg Hau wrote in news: snipped-for-privacy@54693.user.dfncis.de:

I'm not sure what U520 is,are you checking signals on the -vertical- assy.?

I don't have any schematic to reference,but you may want to check the metal film resistors around that IC,one could have changed value or opened.

--
Jim Yanik
jyanik-at-kua.net

No, I'm still inside the "A" trigger generator. U520 is the TEK-made input preamp just after the FET stage.

Been there, done that, no avail ... well, as the voltages were not that much off and all the transistors were OK, I finally swapped this preamp IC with its partner from the "B" channel: Indeed, U520 is shot. One of those - as you said - "TEK-made IC's which aren't available from TEK anymore,just from parts scopes" :-((

(If anyone has a spare to give away: it's labelled with the TEK logo, then "155 0032 01" and in the second row "621 366 USA").

At least Ch A triggers now, which is the most important channel :-)

Thanks a lot to all those that provide input!

- Joerg

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I have same scope and still working. that is a all old vacum tubeS big scope. It made in '50s, so it almost 50 years old. 10 years back, I went to HAM FEST almost get a complete set of vacum tubeS of it. Normal trigger don't work was due to tube aging problem. did you open the side of the blue case yet? you should fine that trigger tube. I think it should be 12au6 or ? It is for now. let me know, I'll look into this.

The 475A is an all solid state scope from the 70s. You're thinking of a 500 series scope from earlier on. Yes, those were MUCH larger.

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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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