Re: 1502 TDR project - using bench supply


On 12/12/2019 3:59 PM, Mark Pilant wrote:
After using the bench supply, and giving the unit a couple of minutes to warm up,
I started rotating the intensity control. As I swept to the upper end, I could see
evidence of a trace, although off the CRT. By adjusting the vertical position, I
was able to finally see the trace on the CRT.
I decided to check a couple of RG-58 test cables I had laying around, and the TDR
got them right.
So, here is a list of what I tried and whether or not it "worked":
1) Used the TekWiki cap/resistor replacement for the battery. Did not work.
Two things come to mind on that.
First, do not attempt to power on the 1502 immediately after plugging
in the AC cord or wall wart supply. Give it a few seconds to charge
the cap, then turn the 1502 on.

Second, the cap may be too small. I used 4700uF which works
fine for me - I think the wiki recommends 2200.

Finally, if using an external supply (wall wart or otherwise),
put a diode in series between the supply and the cap/resistor
combo. The diode is to protect the 1502 against plugging in a
wall wart or supply with the opposite polarity, and protect "backfeeding" the wall wart or supply. The TDR tries to put
about 17 volts across the bananna jacks to charge the battery.



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