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Cobra 2000 GTL frequency counter noise

Cable Guy

Growth must be chosen again and again.
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Dec 29, 2010
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I have above radio, the counter is noisy but only noticeable during transmit. You can drop the mic gain to zero and the noise is greatly reduced, but still noticable. Unplugging power to the counter module (J409) removes all noise. Unplugging the first connector (J410 pins 1-19) also removes the noise. This module was recapped and appears to work great otherwise. It proactively had the TSB 1232 done to it also. All the solder joints look good and the caps are oriented correctly.

Carrier, mic gain max, fc on.
IMG_20250709_180837113.jpg

Carrier, mic gain min, fc on
IMG_20250709_180859783.jpg

Carrier, mic gain down, fc off (unplugged)
IMG_20250709_180908214.jpg

Switching functions, counter, auto or time, has no effect. It is there when the led segments are lit on the display basically. Someone strapped a 1k resistor to the voltage divider of the power supply causing near 16v DC to the radio/counter. I thought this may have cooked something in the counter. I repaired that to get 13+ volts. It is present when using the onboard power supply or external.
 

If the mic gain affects it, I would imagine it would have to be some strong noise to get from the counter to the mic circuit. Try floating your active probe around the counter to see if anything shows up on the scope. If it is impulse noise from a bad transistor or something, the rise time might be fast enough for the active probe to see it.

This is the sort of thing I really like the Carlson Super Probe for. You can hear it with that.
 
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If the mic gain affects it, I would imagine it would have to be some strong noise to get from the counter to the mic circuit. Try floating your active probe around the counter to see if anything shows up on the scope. If it is impulse noise from a bad transistor or something, the rise time might be fast enough for the active probe to see it.

This is the sort of thing I really like the Carlson Super Probe for. You can hear it with that.
Thanks! I just looked into the super probe, definitely going on my project list. I will float the active probe around and see if anything is radiating.
 
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Here is a timestamped link to one of his videos where he finds bad parts with it if you want to see it in action. Sharing because his main video (which this isn't part of) is an hour long, forgive me if you already saw it..

 
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Any chance that there's a solder bridge in the counter between chassis ground and circuit ground?

For that matter, how many original electrolytics are in the main radio pc board?

The clock/counter isn't powered by the 13.5-Volt regulator circuit. Won't care if it's goosed to 16. The electrolytics in the counter module tend to die first, just because that's where the highest temperatures are found. One or more of them on the main pc board is probably marginal by now if they're not all new. The caps in the counter are the canary in the coal mine.

73
 
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Okay, I should have said this, the whole radio was recapped, not just the counter. I will make sure board and chassis are not shorted.
 
Any chance that there's a solder bridge in the counter between chassis ground and circuit ground?

For that matter, how many original electrolytics are in the main radio pc board?

The clock/counter isn't powered by the 13.5-Volt regulator circuit. Won't care if it's goosed to 16. The electrolytics in the counter module tend to die first, just because that's where the highest temperatures are found. One or more of them on the main pc board is probably marginal by now if they're not all new. The caps in the counter are the canary in the coal mine.

73
With the dmm, black connected to chassis ground and red to board ground of counter it's OL (inf). Reversing the leads, I get a continual bouncing ~.5 sec rate between OL and 13-1.3k ohms using min/max. Looks to me like the meter probe on auto ohms is making chassis hot and board ground, causing something to oscillate that shouldn't? There are some caps between chassis and board I think I will look at.
 
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If the mic gain affects it, I would imagine it would have to be some strong noise to get from the counter to the mic circuit. Try floating your active probe around the counter to see if anything shows up on the scope. If it is impulse noise from a bad transistor or something, the rise time might be fast enough for the active probe to see it.

This is the sort of thing I really like the Carlson Super Probe for. You can hear it with that.
I did probe around, I can see strong timing signals near chips, strong oscillations near the xtal and other noises that I can't decipher around other components. The transistors are all steady with no spikes. I did see the most overpowering noise around the chassis grounded shielding. I think it's either a capacitor or a trans/diode junction causing it. I feel if it was just a ceramic cap then when dmm ohm probing, I would see the same result regardless of probe polarity. But when probing the chassis as high and board as low, I get an oscillation between inf and 13 to 1.3k ohms.
 
I figured out the bouncing has something to do with the 34mhz/vco J412 input to the counter. With the counter completely disconnected and isolated from radio chassis, no bouncing between counter chassis and counter board grounds. With only counter chassis and radio chassis connected, no bounce. With only carrier/vco input J412 connected and counter/radio chassis's connected, I get the bounce. No other combinations produce this condition without those 2 connections. I feel I'm taking the long way to the issue, or located a different issue. Anyways, thanks to Chris for pointing out a possible grounding issue. I have another counter I can sub to check, if symptoms persist then the counter isn't at fault and I need to check into the radio. Maybe should've done that first but I didn't want to take out the counter from a good radio.
 
I found an isolation cap that was sometimes leaky, sometimes not. Weird. Replaced it and the bounce was gone. Still had noise creeping onto the mic circuit so I resoldered all connections around the shield, the transistors, the headers, etc and cleaned the corrosion off the brass standoffs to bring their connection from 20+ ohms to <1 ohm. That seemed to lick it, so the corrosion of the posts(which are the only chassis to chassis connection points I realized), some loose soldering and a faulty cap a headache do make. Thanks!
 


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