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10th June
FREE NOTES 2010
ON ELECTRONICS
b y d o k t o r p y t a [ a t ] g m a i l . c o m
Tektronix 7603 mainframe repair
In today's episode I'm going to describe briefly the repair
process of the TEK 7603.
Fig.1 Tektronix 7603 in good physical shape but "dead"
10th June
FREE NOTES 2010
ON ELECTRONICS
b y d o k t o r p y t a [ a t ] g m a i l . c o m
Symptoms:
Completely NO reactions after powering the unit: no fan hum,
no scale illumination, blanked CRT, "POWER ON" lamp on the
front panel was off.
Repair:
After opening the housing it became obvious that I've bought
option 01- the cheapest version without CRT readout and
without the fan. So two problems off :).
At the beginning I downloaded the service manual from the
best website for test equipment enthusiasts -
" http://bama.edebris.com/manuals/ ".
It was found that the +5V and +15V voltages were
missing, they even went 0,6V negative. I started tracking,
which voltages in the power supply section are +15V
dependent. I have disconnected supply cables from every
board in the scope EXCLUDING the main board. Why?
Because the main board provides all "SENSE" feedback
connections and without these feedbacks most of the PSU
circuits can't work properly. Please refer Fig.2 for details.
Fig.2 +5V PSU schematic diagram
10th June
FREE NOTES 2010
ON ELECTRONICS
b y d o k t o r p y t a [ a t ] g m a i l . c o m
As You can see, the +5V can't work until the +15V supply is
provided. To ensure my thesis I connected external bench
power supply set for 15V and 1A of current limit. The CRT
turned on and the scope was brought back to life.
It was a time to find the cause of +15V PSU failure.
After few minutes of measuring few potentials the diagnosis
was : Q931 fault (Fig.3). I found it's parameters on the web
and the closest equivalent I had was BSX61 (fT doesn't matter
in this circuit, voltage and current ratings are important).
Fig.3 +15V PSU schematic diagram
I checked everything once again and it worked stable and
properly. All voltages coming out from the PSU unit had less
than 2% tolerances, so I could go further with checking.
I connected all boards' supply connectors back on their
places.
10th June
FREE NOTES 2010
ON ELECTRONICS
b y d o k t o r p y t a [ a t ] g m a i l . c o m
Than I used two TEK plug-ins I was sure to work fine:
vertical amplifier 7A26 and time base 7T80. I also applied
1kHz 1V test signal to the vertical plug-in input. I tried to
obtain the correct view, but I couldn't. What I've seen was
only bright, VERTICAL trace. I've presumed that the horizontal
part was faulty. I checked the timebase and all signals
produced by 7T80 plug-in were correct (Fig.5). Using the
second oscilloscope I found, that signals coming into U510
were correct, but there was no signal at the output of this IC
switch. Rotating the "LIMIT CENTERING" potentiometer
resulted in moving of the X position of displayed trace, so I
assumed that following horizontal amplifier was OK. I needed
to ensure, that the U510 was faulty so I put out the same type
from working 7613 scope. The scope showed a proper
sinewave on the screen.
Fortunately the custom TEK IC's were available on eBay for
about 10 Euro so I've bought one.
Fig.4 Horizontal amplifier and switch PCB
10th June
FREE NOTES 2010
ON ELECTRONICS
b y d o k t o r p y t a [ a t ] g m a i l . c o m
Fig.5 Horizontal switch
The next thing - let's say less important - was no reaction on
rotating the "GRAT ILLUM" knob. Soon it was clear that ALL
three 6V bulbs were burned out. I checked few shops to find
bulbs with the same bulb holder but it was impossible to buy
them. I decided to crash the glass of the TEK bulbs and I
prepared my own bulbs using the POXIPOL and bulbs rated at
6V/ 40mA which I've bought earlier. The results are shown on
the Fig.6.
10th June
FREE NOTES 2010
ON ELECTRONICS
b y d o k t o r p y t a [ a t ] g m a i l . c o m
Fig.6 Scale illumination bulb
reconstruction
Everything was fine until one day I've seen something as on
the photo below (the trace was shaking ).
Fig.7 Strange traces on the display (1kHz 1V test sinewave
applied)
10th June
FREE NOTES 2010
ON ELECTRONICS
b y d o k t o r p y t a [ a t ] g m a i l . c o m
Once again I've checked the PSU for the AC ripple
using an external oscilloscope. The transformer/ rectifier/
filter module needed to be pulled out to do further inspection.
After removing 6 screws I pulled the module out (Fig.8 and 9 )
Fig.8 Removing the PSU assembly
Fig.9 PSU assembly ready for the repair
10th June
FREE NOTES 2010
ON ELECTRONICS
b y d o k t o r p y t a [ a t ] g m a i l . c o m
I turned out that the capacitor C811 in the -18V PSU was dry.
Fig.10 PSU failure location
The C811 was 14000uF but I had only 10000uF/ 25V so I used
one :)
10th June
FREE NOTES 2010
ON ELECTRONICS
b y d o k t o r p y t a [ a t ] g m a i l . c o m
Now It's the time to see the TEK 7603 happy face. Priceless :)
ALL. COMMENTS.
AND. SUGGESTIONS.
ARE. WELCOMED.