Dipping Toes Into Boat Anchors: Sencore FS 134 Field Strength Meter
A few months back I got a call from my Dad saying, hey, I found this thing I picked up from somewhere, do you want it? It was a half-century old Sencore FS 134 Field Strength Meter used for UHF/VHF television and FM broadcast frequencies. Fast forward to Christmas, and there it was sitting in a box he handed me.
Opening the case, I found analogue dials, knobs, and meters, suitable for a vintage sci-fi movie. This particular device would have been used by a television (or perhaps FM broadcast) field technician who would take readings in various locations and situations to assess how strong the frequency of interest was at that spot. My interest was piqued…you may remember this post.
I found the manual online and have uploaded it here for safe-keeping. If anyone stumbles on this and has a problem with this, drop me a line.
The signal strength meter measures both in microvolts and in decibels. Specific instructions for using the attenuator (10x and 100x) and using 300 ohm ladder line as the input feedline (“when using a matching transformer”) are inscribed on the device to ensure accurate interpretation.
A VFO allows tuning of 53MHz to 109MHz, 173MHz to 218MHz, and 465MHz to 895MHz. A switch, which is seen at the top of the dial, allows you to choose which range to tune in to, namely UHF, television channels 2-6 and FM broadcast (cut off in the top of the picture), and television channels 7-13.
The Sencore boasts that the only input into the system is directly through its antenna ports. Below you can see an interesting arrangement whereby you can directly attach a male f-type coax connector into a f-male connector with either x1 (i.e. NO attenuation), x10 attenuation (i.e., 1/10th the voltage), and x100 attenuation (1/100th the voltage). The Sencore boasts that the only input into the system is directly through its antenna ports, with shielding from any potential leakage through the chassis. Below you can see an interesting arrangement whereby you can directly attach a male f-type coax connector from the feedline of an antenna of choice into a f-male connector with either x1, x10, or x100 attenuation. Or you can hook up a 300Ω ladder line feedline which will go through a 4:1 impedance transformer, reducing to 75Ω, which will be available at the jumper wire attached to the device labeled “75 OHMS OUT”. This jumper wire with a male F-type connector can be plugged into any of the female f-type connector ports (x1, x10, x100). If you do use ladder line for readings in this manner, you need to double the recorded output (as shown in the photo of the meter above).
Now, I turned on the device and the meter’s needle moved a little bit, looking as though it had a little power. The unit came with a power cable with a standard 2-pronged 120Hz AC wall plug on one side, and an odd non-polarized jack on the other. There is a plug on the chassis (see arrow below) to insert the cable into. In reviewing the manual, I learned that the Sencore FS134 could be powered by AC mains or by batteries. The batteries could either be nine “C” cell 1.5V batteries (with eight of them providing a positive 12V energy source, and one of the providing a negative energy source), or an optional rechargeable battery circuit with a 12.5V sealed nickel-cadmium battery could be installed.
Well, at this point, I was a bit skeptical that if I plug this thing in that it may explode. Particularly if it had some bad rechargeable batteries in it. Or I thought I might be able to reverse polarity and electrocute myself. I figured I should open the Sencore up.
Accessing the device electronics required removing two screws from the back of the case and two from the front. It was pretty obvious that this had a non-standard battery in it.
And I could quickly see that one of the battery terminals was quite corroded, in fact possibly involving the underlying capacitor.
Removing one more screw allowed me to dislodge the power supply compartment. A standard 1.5V “C” cell battery and a 12.5V rechargeable sealed nickel-cadmium (Eveready brand labeled “Y 5383”)
Here’s another top down view with the power case pulled out. You can see that the batteries sit in one portion of this section, while another subcircuit is seen in the other.
A closer look at the second subcircuit shows an isolation transformer (single arrow) labeled “28B31-1 Sep 1966” and two 1000mF 40V electrolytic capacitors. More difficult to see are two cylindrical components with colored markings that I think are diodes. In short I believe this subcircuit isolates the mains power (power supply cable) from the unit via the isolation transformer, then proceeds to rectify the AC current into DC current through the diodes and capacitors. At this point, I believe the DC current can now both charge the rechargeable 12.5V battery and/or power up the device.
Here are some more views of the device:
And with the batteries removed.
And a close up of the batteries…
For now that is all. I am still trying to decide if I feel safe plugging this thing in to a wall outlet. Or maybe I just power it up with a 12V supply across the battery terminals? I do not know if I can find a replacement for the nickel-cadmium battery at this point. I am still thinking about it, but I will be sure to write an update when I figure it out!
KM1NDY