Re: TEK465 faulty power supply


Yesterday afternoon my 465 started smoking and smelling after exhibiting symptoms similar to yours. When I opened it I traced the problem to a shorted C1220, a 100 ufd/10v electrolytic. The associated resistor, R1220 was darkened, but measured OK. These components are associated with the -8V supply. It might be worth checking C1220.


--- In TekScopes@..., "Roelof Bakker" <roelof@...> wrote:

Hello all,

I have been given a TEK465 in pristine condition.
However it has developed a fault in the power supply.

The +55, +15 and +5 volt are fine.
The +110 volt line gives +120 Volt, so there might be a problem.

The major problem is the -8 volt line.
It has a ripple of about 7 V-pp and seems to be shortening twice a second.
This can also be seen on the +5 volt line as it dips in the same sequence.
The two neon lamps glow and are also dimming in sync with the -8 volt line.
This can be heard as well.

The previous owner has already put a lot of work in finding the fault.
The reservoir capacitor after the rectifier has been replaced and the -8
volt line has been isolated as per the manual.

I have checked U1554, a LM358, and it is fine.
I have also tested Q1564 and Q1568, 2N2222A, and these appear to be fine
as well.
Before I replace Q1566, I think it is better to ask for help.

The resistance between the -8 volt test point and ground is about 500 ohm.
C1569, a 33 uF Tantalium capacitor has also been disconnected and is not
at fault.

Any help is much appreciated.

Roelof Bakker, pa0rdt

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