Bearcat BC-300 Repair

[boochk]

I purchased an older BC-300 knowing it needed repairs. The unit had no display and after replacing the typical capacitors known to fail, I discovered the divider chip (4024) was dead in the main board. I was able to confirm it by coupling the 27Khz signal from the display board over to the main board and when I feed it to the amplifier (LM386) the display returns. I have ordered a replacement for the dead IC.

Here is my question. When the display works the first 5 digits starting on the left are noticeably dimmer than the rest of the digits. Digit 6 two of the segments within the digit are dim and the rest are brighter. The two segments that represent the number 1 in digit 6 are the dim ones. At first I thought that the 100K resistors had changed value but that would cause all the digits to be the same brightness. So I am puzzled by what I see. Do the Fluorescent displays suffer burn in kinda like older TV's? Does anyone know what the AC voltage is for pins 1 and 23 in the fluorescent display? All of the DC voltages are correct. -30v -25v etc.

Once the display works, the rest of the radio performs well. The receiver is able to listen to the local sheriff and aircraft in the area. Any advice in the display dimming in some of the digits would be greatly appreciated.

John

 

[pro92b]

The text below was lifted from Wikipedia about five years ago. Essentially vacuum fluorescent displays degrade with use much like vacuum tubes do. The suggestion about raising filament voltage might be hard to do in the BC300. Since all the supporting circuits seem to be working and the voltages are correct, the display itself is causing the dimming.



Fading is sometimes a problem with VFD displays. Light output drops over time due to falling emission and reduction of phosphor efficiency. How quickly and how far this falls depends on the construction and operation of the VFD. In some equipment, loss of VFD output can render the equipment inoperable.
Emission may usually be restored by raising filament voltage. Thirty-three percent voltage boost can rectify moderate fade, and 66% boost severe fade. This can make the filaments visible in use, though the usual green-blue VFD filter helps reduce any such red or orange light from the filament.

 

[boochk]

Pro92b,

Thank you for your post as it was very helpful! After thinking about your post, I do recall voltage boosters for old picture tubes to boost the filament voltage and restore some brightness. As the cathodes aged their emissions dropped. With this knowledge, I reviewed the schematic and noticed Bearcat used a 6v zener diode to drop the voltage to the center tap of the filament portion of the transformer. I jumped the diode with a test clip and indeed, the display brightened. However, all of the digits brightened the same so the end result is some digits are still brighter than others. I measured the AC voltage across pins 1 and 23 of the display unit and get an AC voltage of roughly 3.9 V measured with my Fluke DVM. I did not measure the voltage with the zener diode jumped. The schematic does not indicate what the correct voltage is so I am assuming this is correct.

Years ago I worked for an electronic firm in Miami and do miss working and troubleshooting electronics. My career path has changed directions, for the better, and I don't use these skills anymore. The BC-300 was a nice way to refresh some of those skills. I built a couple of Baluns to match my 12 ohm HF antenna to 50 ohms through the help of another RR member and was great fun as well.

Regards,
John

 

[boochk]

One last question. Does anyone know where I can get the alignment instructions for the BC-300? I have only been able to locate a schematic for the scanner.

Thanks,
John

 

[plaws]

C98?

C98 is a common problem with BC300s. I repaired mine ~25 years ago and surely it's dried out again. This might have some hints for you though the OP's issue sounded more complicated. Cadillac of scanners in its day.

https://www.repairfaq.org/REPAIR/F_BC_Repair.html

 

[boochk]

Plaws,

Thank you for the link, I am familiar with that link and C98 was replaced before I started. I received the 4024 divider and the display has returned with many dim digits. After a few more tests I have concluded the fluorescent display has a burn in effect. The radio itself performs remarkably well considering it's age. I will keep my eyes open for a donor radio so I can replace the display.

John