Re: Tek 7603 Followed Me Home

tom jobe <tomjobe@...>

Hi Chuck,
I think he means it appears to have a dead short (or close to it) and this is why I say that:
I have been following this 7603 thread, and had printed the power supply schematic out on large size paper to study as I followed this interesting thread.
Yesterday I thought my interest in this 7603 thread might also be a good excuse to try to fix a 7603 I have that has a large note on it about blowing the fuse instantly when it gets powered on.
There did not appear to be any short on the primary side of the main transformer, so soon this 7603's power supply was out and apart so I could check the large filter capacitors for ESR and resistance in the circuit. C808 appeared to be a full short in circuit to an ESR meter and to an ordinary Fluke DMM.
A study of the circuitry around C808 shows several other paths a short circuit could come from with everything still in the circuit, and that is where I left my search yesterday.
I figured I would study the power supply schematic and see what else I could figure out before removing any of the possible problem components.
This morning I was quite surprised to see Robert's report of finding a shorted C808 in his 7603!
Then when I saw Chuck's question this morning... I realized that my thinking there could even be a dead short in an aluminum electrolytic cap might need to be re-thought.
Other details:
This capacitor is part of the filtering of the 50 VDC supply immediately after the full wave rectifier. This transformer winding is center tapped to ground and two capacitors are used on the plus and minus outputs of the rectifier to ground. This C808 is on the negative side to ground, and a C909 on the positive side to ground. These two capacitors are identical (1800 uF/75 v) and C909 does not appear to be shorted when tested in circuit (several thousand ohms) which left me 'scratching my head' at the end of yesterday's investigation.
tom jobe...

On 9/11/2018 5:15 AM, Chuck Harris wrote:
I am not sure what you are saying when you say that
C808 tests at 0.0 ohms. Do you perhaps mean 0.0 uf?

Typically, when ripple gets to be that excessive, the
capacitor's internal connection has been etched away
and is open circuit, or the capacitor's electrolyte
has dried up. The capacitance goes to near zero uf.

As a temporary fix, you can get back in business by
paralleling just about anything across the original
capacitor at its terminals. Even as little as 1/10th
the correct value will sometimes be close for the circuitry
to limp along and work. Tektronix was very generous
with its margins.

-Chuck Harris

Robert Hay wrote:

C808 on the -50 v unregulated is the only cap that tests at 0.0 Ohms and is the only
one that doesn't charge up to some value using the cap test position on my Tek DMM916.

I ordered some replacement caps although I am not sure whether to ultimately replace
all the cans in the back section. I may use two 1000ufd 100v caps to replace the
1800 ufd 75 volt C808 on the -50 unreg. In this case the can is connected to the -50
unregulated rather than ground so I need to tie all those loose ends together.

I'll hit up the local electronic surplus tomorrow to see if I can find a replacement
cap for testing while waiting for the ones I ordered.


On 9/9/2018 9:57 AM, Chuck Harris wrote:
My guess would be the -50V unregulated supply's
filter capacitor is no longer any good.

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