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Cobra 2000GTL 10-Volt blues hit parade.

nomadradio

Analog Retentive
Apr 3, 2005
7,164
11,604
698
Louisville, KY
www.nomadradio.com
Today I stumbled across an abandoned project. A "10-Volt Blues" guide to the electrolytic caps we see cause the most-frequent mischief in the Cobra 2000GTL and similar radios.

What prompted it was changing all the electrolytics in a customer's 2000GTL clock/counter module. The displayed frequency was getting frisky, and the radio dated to 1983. Old enough.
Got some tips for that little chore, too. But in another post.

Later.

I always say that. While hunting the pics I use for that job, I found these. Had forgotten all about it.

First is C95. Usually a 47 uf cap, TEN VOLTS. When it shorts, there is no transmit. At all. Not even the tiniest whisper of an escaping low-level transmit signal when a sniffing antenna is waved around the transmitter parts, connected to a radio receiving the same channel.

xGL7aM.jpg


Yeah, there are other possible faults for this symptom, but if C95 shorts in its typical fashion, the 100-ohm resistor feeding into it will get right toasty. That would be R108. A shorted cap is easy to troubleshoot. Don't even need an ohm meter, just remove the cap. If it transmits, the cap was shorted. And don't forget to replace R108. It just can't be trusted after that kind of overload. Odds are it will look toasty enough you'll want to change it, too.

GkmV0E.jpg


Next on the hit parade is C175. This cap is between pin 9 of the TA7222P audio-power chip and the speakers. It serves to block DC current, allowing the audio to pass. When it shorts, it blows out the coil in the speaker, and the TA7222P, both. Usually.
Doesn't happen a lot, but we change this cap as cheap insurance. Sometimes the part will be marked 10 Volts, or 12, or even 16. Doesn't matter. We change this one on risk factor alone. Raises the customer's bill only the tiniest bit, saves me the time I would waste if his radio smokes the speaker and chip down the line.

OggI4d.jpg


This symptom has been popping up for years, frequently in a radio that ran for long years, was put up for a year or two and then put back in service. C186 is a 220uf part, typically ten Volt. When it shorts, you lose *ALL* receiver audio, AM and sideband. Transmit is NOT affected in any way, and above all the S-meter continues to kick about on channel noise and chatter as if nothing were wrong. Naturally if your S-meter has seized up and won't move because it's frozen in place this doesn't mean much. But again, if all you do is unsolder this cap and pull it out, the receiver audio will come back. It will sound terrible until a new cap goes back in, but you'll have an immediate indication that this was the culprit. Sure, you could put a DC meter onto the positive side of this cap, to see if you have the normal six Volts DC or so. And if not, check for a short from there to ground.

Or just pop it loose and see what difference it made. And if it made no difference, the root of the problem lies somewhere else. Just don't put the old cap back. Install a new one. It's just too old.

Next up is C18. It serves to filter the juice that powers the mike amplifier. This one will causes a radio to have *N0* mike audio at all, AM or sideband. You'll still have a dead AM carrier, but no PA, nothing from the mike. If it fails as a short, you lose the mike audio. If it fails as an open circuit, you get feedback squeal, typically when the carrier is turned down. We install a 470uf cap here. That's larger than original, but prevents the hinky behavior with a reduced AM carrier.

Nmq9wI.jpg


The schematic says "10 Volt" for C18 but we have seen a 6-Volt part here now and again.

The last two caps are also the least-likely to fail first, in our experience. But they cause trouble that can be frustrating to track down. C214 filters the power that feeds the bias trimpot for the driver transistor. C153 does this same filtering job for the final transistor's bias circuit. When C214 shorts, this will handicap the driver transistor in a big way. You might get a Watt or three out of the radio but no more. First hint to look at C214 is the bias test. If your current meter shows zero driver current no matter where the trimpot is turned, C214 may be shorted. Easy enough to test.

dg6ADY.jpg


When C153 fails, the transmitter power usually won't be as crippled as the driver would make it, but you'll never see more than 7 or 8 Watt peaks if C153 shorts. And again, checking the final bias adjustment will point you in this direction if the reading stays at zero no matter how the bias trimpot is turned.

So there are the most popular verses of the Ten Volt Blues. We first would see these symptoms in the 90s when the oldest Cobra 2000 was too young to vote. Mileage counts. A radio that ran 24/7 wears things out faster than the weekend-warrior radio. The lowest voltage rating we stock is 25 Volts. A higher rating is fine in any of these circuits. Higher voltage ratings also increase a capacitor's size, so if it fits you should be good to go.

Relating the symptom to the part causing the fault won't always be correct. Other failures can produce any of the above symptoms. These are just the most popular, the hit parade.

73
 


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