Re: Tek 7603 Followed Me Home
tom jobe <tomjobe@...>
Hi Chuck,toggle quoted messageShow quoted text
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.
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.
On 9/11/2018 5:15 AM, Chuck Harris wrote:
I am not sure what you are saying when you say that