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Tektronix 475A, trigger does not work?

J

Joerg Hau

Jan 1, 1970
0
Hi,

I'm looking for a copy of the repair/maintenance manual, or at least
the circuit diagrams, for the Tektronix 475A/DM44 scope.

The trigger for channel A does not work; the corresponding LED (or is
that a lamp?) does not light up as it should. I can only obtain a
stable display if _all_ the four buttons of the HORIZ DISPLAY kerys
(to the right of the CRT) are out & I use the "B" trigger button to
adjust ... ?!

Any idea what might be wrong here (better yet: how to fix it ?)

Please feel free to contact me either via the newsgroup, or per direct
mail.

Thanks in advance for any help,
and greetings from Lausanne!

- Joerg


[ xpost to s.e.equipment & s.e.repair, f'up to s.e.e. ]
--
joerg dot hau at swissonline dot ch * Lausanne, Switzerland
http://www.mysunrise.ch/users/joerg.hau/
"All standard disclaimers apply".
remove ".nospam" from my address to reply
(this became necessary due to increasing SPAM)
 
J

Jim Yanik

Jan 1, 1970
0
Hi,

I'm looking for a copy of the repair/maintenance manual, or at least
the circuit diagrams, for the Tektronix 475A/DM44 scope.

The trigger for channel A does not work; the corresponding LED (or is
that a lamp?)

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
stable display if _all_ the four buttons of the HORIZ DISPLAY kerys
(to the right of the CRT) are out & I use the "B" trigger button to
adjust ... ?!

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.
Any idea what might be wrong here (better yet: how to fix it ?)

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

Joerg Hau

Jan 1, 1970
0
Hi Jim
Check to see if external trigger input works.

No, it doesn't.
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.

I will, but need to find the circuit diagrams first ;-)
The circuit uses TEK-made IC's which aren't available from TEK
anymore,just from parts scopes.(no longer made & stock depleted)

Argh. How do you spell "hope" ?
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.)

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

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

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

- Joerg


--
joerg dot hau at swissonline dot ch * Lausanne, Switzerland
http://www.mysunrise.ch/users/joerg.hau/
"All standard disclaimers apply".
remove "nospam." from my address to reply
(this became necessary due to increasing SPAM)
 
T

TekMan

Jan 1, 1970
0
Jim Yanik said:
....

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


Hi,

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

hope this helps.

Heike
 
J

Joerg Hau

Jan 1, 1970
0
I've send him a Service Manual, see PM.

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


--
joerg dot hau at swissonline dot ch * Lausanne, Switzerland CHANGED
URL: http://homepage.sunrise.ch/mysunrise/joerg.hau/
"All standard disclaimers apply".
remove "nospam." from my address to reply
(this became necessary due to increasing SPAM)
 
J

Joerg Hau

Jan 1, 1970
0
Hi again,
No, it doesn't.

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


--
joerg dot hau at swissonline dot ch * Lausanne, Switzerland CHANGED
URL: http://homepage.sunrise.ch/mysunrise/joerg.hau/
"All standard disclaimers apply".
remove "nospam." from my address to reply
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E

Emanuele Girlando

Jan 1, 1970
0
For a free 475A service manual go to http://bama.sbc.edu and look for
Tektronix. Quality is not high, but it is for free!
I used it to fix my 475..
Good luck.
 
J

Jim Yanik

Jan 1, 1970
0
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

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.
 
J

Joerg Hau

Jan 1, 1970
0
I'm not sure what U520 is,are you checking signals on the -vertical-
assy.?

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

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


--
joerg dot hau at swissonline dot ch * Lausanne, Switzerland CHANGED
URL: http://homepage.sunrise.ch/mysunrise/joerg.hau/
"All standard disclaimers apply".
remove "nospam." from my address to reply
(this became necessary due to increasing SPAM)
 
O

Oil4war

Jan 1, 1970
0
Subject: Tektronix 475A, trigger does not work?
From: Joerg Hau [email protected]
Date: 1/19/04 2:48 PM Pacific Standard Time
Message-id: <[email protected]>
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.


Hi,
I'm looking for a copy of the repair/maintenance manual, or at least
the circuit diagrams, for the Tektronix 475A/DM44 scope.

The trigger for channel A does not work; the corresponding LED (or is
that a lamp?) does not light up as it should. I can only obtain a
stable display if _all_ the four buttons of the HORIZ DISPLAY kerys
(to the right of the CRT) are out & I use the "B" trigger button to
adjust ... ?!

Any idea what might be wrong here (better yet: how to fix it ?)

Please feel free to contact me either via the newsgroup, or per direct
mail.

Thanks in advance for any help,
and greetings from Lausanne!

- Joerg


[ xpost to s.e.equipment & s.e.repair, f'up to s.e.e. ]
--
joerg dot hau at swissonline dot ch * Lausanne, Switzerland
http://www.mysunrise.ch/users/joerg.hau/
"All standard disclaimers apply".
remove ".nospam" from my address to reply
(this became necessary due to increasing SPAM)
 
J

Jim Adney

Jan 1, 1970
0
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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