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Sqwirl2001
| | Posted on Friday, July 25, 2003 - 10:29 pm: | 
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There seems to be a buzzing type sound that I think may be caused by the frequency counter.It is more pronounced on channels 1-13 and a couple of the LED segments that are
not lit up flicker.This stops when I disconnect power to the counter,and it does this either ac or dc.Any ideas? |
Adshar64
| | Posted on Saturday, July 26, 2003 - 4:43 pm: |
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When you say "disconnect power to the counter", does that mean you are switching back to clock mode on clock/counter pushbutton?
cheers Adrian |
Sqwirl2001
| | Posted on Saturday, July 26, 2003 - 7:08 pm: |
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No.I disconnected the 3 pin plug that supplies power to the module.No more buzz,but it was still present when switching back and forth from clock to counter with the power still connected.The first LED to the counter pulses and the buzzing starts after the radio has been on for about ten minutes. |
Adshar64
| | Posted on Saturday, July 26, 2003 - 9:19 pm: |
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A tricky one then seems like display playing up, like a led diode failure half wave oscillation. If you want all the schematics etc to nut it out go here;
http://groups.yahoo.com/group/Cobra_2000/,http://groups.yahoo.com/group/Cobra_2000/
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Sqwirl2001
| | Posted on Sunday, July 27, 2003 - 9:06 am: |
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Well it is a starting point.Thank you for the info. |
307
| | Posted on Sunday, July 27, 2003 - 5:51 pm: |
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Common problem , The Cobra 2000 will "buzz" IF you loose and L.E.D in the counter OR the channel display. The filtering design was based on ALL L.E.D.'s functioning and pulling a constant current. If any one L.E.D. went out , it would buzz AND you will notice the channel display is very dimly lit when the power is OFF (AC).
307 |
Adshar64
| | Posted on Monday, July 28, 2003 - 3:58 am: |
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If this was happening to me(keeping in mind that a replacement display or counter pcb is probaly near to impossible to find). Id try finding a pulsing feed near the offending led on the pcb feeding display or front conter plugs. Id try a 100uf electrolytic from this point to pcb ground(-) if I could find that scenario. |
2600
| | Posted on Sunday, August 03, 2003 - 3:31 am: |
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Replace EVERY electrolytic capacitor on the counter/clock board. Hasn't failed for me, yet. Tends to fix the "phantom" digit segments that refuse to stay dark when they should, flickering dimly and irregularly. The temperature inside the shield box for the counter gers a lot higher than in the rest of the radio. Tends to make the elctrolytics inside there fail a few years before the rest of them elsewhere in the radio. The counter circuit board is double-sided, so spending ten bucks or so for one of those "desoldering" irons from RatShack with the squeeze bulb is cheaper than damaging the "through-hole" metal sleeves inside the component holes. They also sell a small squeeze tube of rosin flux. Smear that on the pads you need to desolder. It reduces the heat needed to melt the solder, and reduces the stress (and risk of damage) to the circuit board. The flux that was used to build the circuit board was completely washed off at the assembly line. Just touching an iron to that 20-plus year-old solder won't melt it easily enough to avoid circuit board damage.
When you pull a capacitor from the board, look at the leads to see that neither one has a brown "lump" on the lead near the body of the part. If it does, this WAS a metal sleeve that connected the foil on the top side of the board to the foil on the bottom side of the same hole. This "through-hole" connection is usually important, so you'll have to "repair" this problem. Stand the capacitor up on an extra half-inch or so of lead wire, instead of pushing the wire leads all the way into the holes. Leave enough clearance to solder the lead to the foil on the TOP (component) side of the board, as well as underneath on the "solder" side foil. This will preserve the connection from the top side of that hole to the bottom side foil on that hole. If this happens, and you miss it, puzzling and strange symptoms usually follow.
Until the power supply filtering inside the counter is restored, odd noises will "leak" out of it into the audio circuits of the radio. If you just poke around to find "one" bad filter and replace only that one, another will fail soon, and you'll be at it again. Replacing all 11 or 12 (13?) of them sounds extreme, but it's a lot less trouble than doing it six or ten times, one or two parts each time.
73 |
Adshar64
| | Posted on Sunday, August 03, 2003 - 3:40 pm: |
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"The temperature inside the shield box for the counter gets a lot higher than in the rest of the radio"
Yes I leave the lid of on my counter section for that reason and dont seem to have any problems with rf shielding etc.
Good post and worth a try I guess if you cant diagnose the original problem to exact cause. |
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