The paralleled Electrolytic caps into the power supply test I mentioned is a relatively easy test before one goes in for a mod. The Spectrum Crowbar Circuit was working and something obviously changed with age.
Then I would focus my attention to the SCR Crowbar Circuit. Something not mentioned was... if the SCR circuit fired with the transmitter into a non-reactive load/termination (dummy load). s > Glenn Little WB4UIV <glennmaill...@...> wrote: > I had a similar problem with a home brew supply. > My fix was a choke (100 uHy)in series with the gate as close to the > gate a possible. > I bypassed the choke on both sides with 0.01 uF capacitors with leads > as short as possible. > This fixed my problem. > You are probably rectifying RF inside the SCR. > You might want to also bypass the anode of the SCR. > Spectrum never was real good with engineering. > > YMMV > > 73 > Glenn > WB4UIV > > > You might need to experiment with the values for the choke and capacitors. > At 10:17 AM 10/15/2009, you wrote: > >Having problems with a SPECTRUM 4000 UHF repeater that the voltage > >regulation works ok on RX, but when you kick it into TX, the SCR > >fires and blows the fuse or frys the resistor. > > > >This is the newer version power supply board that has the current > >limit control on it. > > > >We replaced the SCR and .25 ohm dump resistor, and it still triggers. > >I watched it trigger 2 times last night with a digital meter on the > >circuit and saw no overvoltage contition, at least not what the dmm > >registered. > > > >Theory: Could a bad cap be letting RF back into the crowbar circuit, > >causing the SCR to trigger from the RF rather then DC? > > > >Please Advise > > > >Ed N3SDO > > > > > > > > > >------------------------------------ > > > > > > > >Yahoo! Groups Links > > > > > > >
