[Repeater-Builder] Spectrum UHF Repeater SCR Crowbar circuit activating during TX
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
Re: [Repeater-Builder] Spectrum UHF Repeater SCR Crowbar circuit activating during TX
Could it be RF triggering the SCR? Jerry W9FS - Original Message - From: Ed To: Repeater-Builder@yahoogroups.com Sent: Thursday, October 15, 2009 9:17 AM Subject: [Repeater-Builder] Spectrum UHF Repeater SCR Crowbar circuit activating during TX 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
Re: [Repeater-Builder] Spectrum UHF Repeater SCR Crowbar circuit activating during TX
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
