Tektronix 2245; need help with dead PSU

Greetings,

My 2245 'scope recently died, with no outputs from the psu. The input
cct seems ok with ~150vdc across the filter cap, but I'm having a hard
time tracing the voltages any further without a schematic.
Does anyone have such a thing, and/or tips on what to be looking for?
In case there are different versions, the psu board has a 1986 date
code and uses a MC34060 pwm chip.

All comments welcome,

Alf in Scottsdale, AZ

 

(Alfred Green) wrote in

ISTR that the 2245 service manual has schematics for the PS.
Ebay or big-list.com may turn up a manual.
And you can download a datasheet for the Motorola MC34060 IC probably at
the Motorola website.
You can ohm the switcher FET (open?)and any rectifier diodes(short?),check
the electrolytics with an ESR meter.

 

I might be able to help.
i have a manual for something like this.
might just be the same one.
will let you know later.

 

I spoke to someone at Tektronix and he confirmed that the service manual
does include the schematics. They no longer have anything available however.
A net search has failed to uncover one either so far. I have found one for
the 2246; I wonder if it used the same psu as the 2245.

I have that info. Checking the vcc wrt the ground pin shows the voltage
varying from 0 to +5v; lower than I was expecting and I'm puzzled why it
would be changing.

The input switcher is an IRF830 which I think is enhancement mode, so I cant
really check that without supplying gate voltage. The diodes and caps seem
ok, but testing them in-circuit could be misleading.

I'll keep on exploring, and I appreciate the comments.

Rgds, Alf
Scottsdale AZ

 

I found my Service manual and its for a 2445A

 

Yes,it does.(I coulda told you TEK would nothave the manual;the 224X series
has been 'obsolete',or past the 'Long Term Support Period' for several
years now.)

The 'housekeeping' supply for the IC is started by a cap charged by a
resistor from the rectified line V supply.The cap charges,the IC tries to
start,but the xfmr winding that maintains the 'houskeeping' supply cannot
supply current,perhaps because the switcher FET is open.

That ESR meter (Dick Smith meter!!) is invaluable for testing in-circuit
electrolytics,especially for switcher supplies.

 

Not the same family of scopes. No similarity at all.

 

Thanks. I've ordered a CD for the 2246.

That makes sense. I'll dig out one of my old scopes to look at it, just
using a DVM may be giving me a false picture.
I may just replace the FET anyway and see what happens.

Sounds like a good investment!

Tnx, Alf

 

It looks like there are some similarities, but not quite enough. I
appreciate your efforts in sending me the diagram; I'll keep it for future
reference.

Tnx, Alf

 

The 2245 and the 2445 and A versions are nothing alike.
Two entirely different families. Different architectures.
Now,2445 and 2465 (and A,B versions)are very similar;and 2245 and 2246,2247
are very similar.