Nuts and Volts - February 2011

>>>YOUR ELECTRONICS QUESTIONS ANSWERED HERE BY N&V READERS

general-purpose pushbutton switch to the exposed ends. Take the remote apart and check the switch first. Also, I once found a bad battery connector. Make sure you measure nine volts at the board.

The receiver has stopped working a few times. Each time, I found bad electrolytic capacitors that have dried out and lost their capacitance. Replacing the bad ones usually fixed the problem. If you don't have a capacitor tester, you could just replace all of the electrolytics; there are six in the Sears receiver.

As an aid in troubleshooting, the demodulated 52 Hz tone is coupled back to the antenna for viewing on a scope. You should see a 52 Hz waveform when you press the remote button. You will need to find the circuit return on the receiver board for the scope ground. It should be the negative terminal of the main filter capacitor — usually the largest capacitor in physical size and capacitance. I don't think you'll have to get involved with troubleshooting the RF section.

Marvin Smith Harbor City, CA

#2 I would not try to repair the original transmitter as most have proprietary chips with no ID numbers.

I would instead check out a transmitter/receiver such as what is listed at www.seeedstudio.com/depot. They have a tab for communications, then subtab for wireless on their site.

The transmitter may fit inside the old transmitter case; the receivers have relays which can be connected to the manual button leads on the garage door opener. Many other sites have the transmitters/receivers which are similar, but I have ordered from Seeedstudio in the past successfully.

J.P. McGinley Stafford, VA

[#12103 - December 2010]

CONAR 251 Scope Info

Where can I find out the value of a fuse for the CONAR Model 251 oscilloscope and/or where can I get a manual or schematic for this scope? (CONAR was bought out by the National Radio Institute - McGraw Hill Continued Education which is no longer in business.) I want to power up this scope and test its output. I need the fuse type and amps before I test it, as money is in short supply. Or, is there a website to find this information?