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Tektronix 7603 with 7A26 and 7B53A plug ins

 
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Joined: Wed May 02, 2012 3:35 pm
Location: Worksop

Re: Tektronix 7603 with 7A26 and 7B53A plug ins

Post by Refugee » Fri Jun 05, 2015 11:35 pm

It looks like it has been assembled from plugins and a frame from a pile of stuff cleared prior to refurbishing the technicians store at a college.

 
Posts: 4092
Joined: Wed May 02, 2012 3:35 pm
Location: Worksop

Re: Tektronix 7603 with 7A26 and 7B53A plug ins

Post by Refugee » Tue Jul 07, 2015 12:51 am

I have been sorting out the dodgy 7A26.
It had a very intermittent loss of all gain on all ranges on channel 2 while channel 1 was all right.
You can see all the tool marks where the twiddler has been swapping the chips around to try and clear the fault.
I did many in circuit tests with a diode tester including the input junctions on the bipolar chips. They are transistor arrays. I eventually got to the wires and switches and found that the channel 2 invert switch was totally open circuit. It is a bit disappointing for Tektronix being just a little slider switch.
With twiddler evidence and the trace moving when some switches are operated indicates that the twiddler has been at the pots too.
Time to go through the calibration procedure now.
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Posts: 4092
Joined: Wed May 02, 2012 3:35 pm
Location: Worksop

Re: Tektronix 7603 with 7A26 and 7B53A plug ins

Post by Refugee » Mon Jul 13, 2015 2:53 am

I have this evening been tackling the other 7A26 and it had also been twiddled and this time one of the DC balance pots had been moved so far off that the gain could not be set at all. The clue was that the manual warns you that they interact. The DC balance affects gain mostly.
On this plugin the twiddler had used a different tool to swap the chips around and all the legs are bent :ccb
I am now ready to finish off calibration and then it is sort out the graticule lamps and intermittent power switch and the electrics will be complete.
The only real fault was that FET in the timebase and the rest has just been twiddler related apart from dirty switches.
It was FOC with faults.
I am pretty sure it was "knocked up" from a load of faulty bits.
The search for something to replace the bottom panel and heat sink cover can now begin.
I have even made up a cable to compare two plug ins for calibrating one with another newly calibrated example. It will be a lot quicker than that a bench PSU and meter like I used on the first one.
As it is to be used as a main bench scope the cosmetically challenged side panels can stay as they are as the other equipment either side will hide it from view.
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