"Jasen Betts" bravely wrote to "MIKE ROSS" (10 Mar 04  07:00:38)
 --- on the heady topic of "{at}%^{at}#$%^ veroboard"

Thanks for replying, I was thinking the feed to this echo was dead.


 JB> 08-Mar-04 10:37:48, MIKE ROSS wrote to Jasen Betts
 MR> So if your prototype requires megohm impedances you would do well to
 MR> always clean off all flux on a pcb.

 JB> I've got a new DVD player and the TV (a monitor really, a fault has
 JB> disabled the tuner) has only one set of AV-in sockets and the AV-in
 JB> on the VCR isn't under software control,

 JB> so I decided to make an little automatic AV switch,

 JB> 4 transistors and a couple of little 12V relays.
 JB> it looks through a 1 meg resistor and a 10nf capacitor for the sync
 JB> pulses (or any high-ish frequency) from the DVD player and switchees
 JB> the AV to that source when it sees it, when it doesn't it uses the
 JB> signal from the VCR, (which I also ue as a tuner)

They don't make tuners like they used to in early VCR's. I still use
an old RCA to pull in DX channels off air. The blue screen of death on
modern VCR's is less than useless. BTW digital tv is going to make
DXing nearly impossible. At least when analog goes wrong some of it
usually still manages to get through but with digital it's all or
nothing. I'd much rather have a degraded signal than no signal at all
or a stupid blue screen!

Okay, so that's where the megohm stuff was used. More likely to see
megohm values on FET type input biasing, used to be on vacuum tubes.


 JB> 1M input impedance is probably over-kill but anyway it was fun to
 JB> build and proves useful.

Not at all reminds me of retrofitting an early solid state stereo amp
with a relay circuit to defeat the turn-on thump. Used a FET with a
cap and 1M ohm resistor on the gate to time-out energizing the added
speaker relay. Basically the cap held the gate biased off until it
charged. Then the FET turned on and the relay was energized. No
logic used at all. Only defect was one channel came on about 1/10 of a
second sooner than the other but the thump was gone.

 Mike
 ****

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