Tek 475 No Display

[bob_e1947]

Hi All,

Been looking at a Tek 475 on eBay with no display.

I know this can be a hundred a one things, but does anyone know if this
is a common fault with the 475 and if so, what it may be. I vaguely
remember seeing something on Gooogle a few months back, but I can't
find it again.

Cheers...Bob

 

[Jim Yanik]

Hi All,

Been looking at a Tek 475 on eBay with no display.

I know this can be a hundred a one things, but does anyone know if this
is a common fault with the 475 and if so, what it may be. I vaguely
remember seeing something on Gooogle a few months back, but I can't
find it again.

Cheers...Bob

power supply failures are very common.Bad caps and/or bridge rectifiers.
Just because the front panel lamps light does not mean the PS is working
right.

Then there's HV/Z-axis failures,sweep failures,bad deflection circuits...

Possible worst;a dead CRT. or a motherboard that leaking electrolytics
ruined.

 

[Ancient_Hacker]

Hi All,

Been looking at a Tek 475 on eBay with no display.

Like Jim said, most common is power supply capacitors, then PS diodes,
then shorted tantalum capacitors, then bad HV power supply, then maybe
bad CRT.

Or a combination of the above. One 475 I worked on had all of the
above. No ait, the CRT was only weak, not totally dead.

 

[Andreas Tekman]

A service manual with the detailed trouble shooting diagram is useful
for a repair . Check bama.sbc.edu or it's faster mirror for a free
download.


hth,
Andreas

 

[Andrew VK3BFA]

--
Jim Yanik
jyanik
at
kua.net

Hi JIm - seeing your name reminded me of a discussion about a Tek 475
we had yonks ago - it had a blown eht generator transistor. It was
suggested that the closest equiv. to the TEK original was a 2n3055., I
think SPHERE list this. Experimented a bit - found a common TV
horizontal output transistor worked - cant remember what it was, but
its a BUX something - 70's vintage Philips - TO3 case- out of a junked
chassis sitting under the bencg.... Worked this out by using external
current limited power supply to feed cct, and looked at waveform on
base of transistor, as per diagram in manual - the TV one gave the
proper waveform, the 3055 had big spikes (why, I dont know - I am not a
engineer) The other problem was that huge film type resistor
(300MegOhm?) in the EHT feed back cct had gone LOW in value - this one
stumped me for a while, never seen a resistor go low, usually high.
physics is (apparently) copper migration from terminals, only happens
in HV circuits.

Anyway, now goes OK - last fault is, after an hour or two, the trace
intensity starts varying......one day, when everything else is done,
will tackle it....

Is it a nice scope - YES!!!!!
Would I do up another one NO!!!!!!!! (Chewed up heaps of time and
money ) - a previous fault (it was given to me as a junker) had me
replacing the EHT transformer, the EHT multiplier, the CRT, and the IC
that drives the vert plates........too open ended.......

Andrew VK3BFA.

 

[Jim Yanik]

Hi JIm - seeing your name reminded me of a discussion about a Tek 475
we had yonks ago - it had a blown eht generator transistor. It was
suggested that the closest equiv. to the TEK original was a 2n3055., I
think SPHERE list this. Experimented a bit - found a common TV
horizontal output transistor worked - cant remember what it was, but
its a BUX something - 70's vintage Philips - TO3 case- out of a junked
chassis sitting under the bencg.... Worked this out by using external
current limited power supply to feed cct, and looked at waveform on
base of transistor, as per diagram in manual - the TV one gave the
proper waveform, the 3055 had big spikes (why, I dont know - I am not a
engineer)

The original 151-0140-00 was a wierd,ancient manufacturing-process
type,long discontinued now.
Nobody at TEK ever figured out -why- other types would not work.
(probably T&M division's drift away from service support.)
And that xstr was used in lots of different TEK products,7000 scopes,400
series,528 and 520/A NTSC TV instruments(and PAL versions,too)

It's nice to know about that BUX-type HO-xstr,I'll try to remember that.
Thanx!

The other problem was that huge film type resistor
(300MegOhm?)

IIRC,30 megohms or in that range.The "thick-film ceramic".
It was HV divider on one side,and focus divider on the other.

in the EHT feed back cct had gone LOW in value - this one
stumped me for a while, never seen a resistor go low, usually high.
physics is (apparently) copper migration from terminals, only happens
in HV circuits.

Yeah,but generally,they were pretty durable.
I don't think TEK ever expected those scopes to last so long.

Anyway, now goes OK - last fault is, after an hour or two, the trace
intensity starts varying......one day, when everything else is done,
will tackle it....

IIRC,there's a 1uf/150V al.electrolytic cap on the wiper of the grid bias
pot that would go bad,and the DC restorer diodes were always a common
failure item in the grid bias circuit.But you need a curve tracer to test
them.(it could also just be a dirty or worn-out intensity pot,or grid bias
pot.)