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uniden washington issue
- Thread starter kaos513
- Start date
There is more than one kind of failure that can shut down AM transmit only. The most common are in power stages. In this case, the AM modulator transistor TR34 and the driver feeding it TR35 are a common fault. Better than "checking" the transistor, a couple of DC-voltage measurements may narrow down the list of suspects.
The metal tab of the RF driver transistor TR39 should have about 6 Volts DC on it when you key the mike in AM mode. If you do, TR34 is not the problem.
If you *DON'T* show DC voltage on the collector tab of the RF driver transistor TR39, next probe the center pin of TR34. You should have the full power-supply voltage, 13.8 Volts DC or whatever the power supply is set for.
If you do, probe the front-most leg of TR34. If it shows zero, TR34 is breaking down under load.
And if you have that approx 6 Volts DC, this suggests wiring or a bad mode selector switch.
73
The metal tab of the RF driver transistor TR39 should have about 6 Volts DC on it when you key the mike in AM mode. If you do, TR34 is not the problem.
If you *DON'T* show DC voltage on the collector tab of the RF driver transistor TR39, next probe the center pin of TR34. You should have the full power-supply voltage, 13.8 Volts DC or whatever the power supply is set for.
If you do, probe the front-most leg of TR34. If it shows zero, TR34 is breaking down under load.
And if you have that approx 6 Volts DC, this suggests wiring or a bad mode selector switch.
73
Update: several caps where shorted around that circuit replaced all is great now
Can someone recommend a repair person for these radios? I am in the Bay Area of San Francisco if anyone is local. I can ship it if required. It needs a tune up. Been sitting in storage and it doesn't key up. Everything else is fine it appears.
Thanks for any recommendation...
Thanks for any recommendation...
We don't solicit shipped-in work, but the age of that radio may make it unreliable when it's put back into regular use. Mileage is also a factor, but there's no odometer to read for that. It's possible that "patching" just one or two failed parts in a 1980 radio will bring it back to life for a year or more. Electrolytic capacitors exhibit a limit on their lifetime. One or more of those caps will "remember" how old it is and fail in days or weeks in a high-mileage radio.
And the low-mileage radio may run for a year or more before that happens.
Just finding someone who can make a decent guess which category your radio falls into gets harder all the time. And if you hear the term "re-capping" associated with old electronics, this is a 'scorched earth' approach to the aluminum electrolytic caps in a CB, stereo, guitar amp etc. Replacing every last one of them. We do this routinely in tube-type radios. The higher temperatures inside the cabinet reduce the lifetimes of those parts. A solid-state radio is no different. In a Cobra 2000, the caps inside the clock/counter module often break down before the same size/type caps in the rest of the radio. The temperature inside the shielded metal box surrounding the clock/counter circuits is higher than inside the rest of the radio. Pretty sure this is why.
Re-capping isn't cheap, but changing them all at once takes less labor and should be cheaper than paying someone to do it two or three at a time over a couple of years.
We call that process "Electronic Whack-A-Mole".
At least for the tube-type radios.
73
And the low-mileage radio may run for a year or more before that happens.
Just finding someone who can make a decent guess which category your radio falls into gets harder all the time. And if you hear the term "re-capping" associated with old electronics, this is a 'scorched earth' approach to the aluminum electrolytic caps in a CB, stereo, guitar amp etc. Replacing every last one of them. We do this routinely in tube-type radios. The higher temperatures inside the cabinet reduce the lifetimes of those parts. A solid-state radio is no different. In a Cobra 2000, the caps inside the clock/counter module often break down before the same size/type caps in the rest of the radio. The temperature inside the shielded metal box surrounding the clock/counter circuits is higher than inside the rest of the radio. Pretty sure this is why.
Re-capping isn't cheap, but changing them all at once takes less labor and should be cheaper than paying someone to do it two or three at a time over a couple of years.
We call that process "Electronic Whack-A-Mole".
At least for the tube-type radios.
73
I have a Grant I bought along with it that is still working fine as well as a Cobra 2000 GTL bought a year later. They have both have stood time. The Washington lost its ability to key and I let it sit in the corner. I believe it is the keying circuit and I seem to recall the fix for that so I'll give it a go. I have a parts radio to pilfer the board it necessary. Very robust products.
Thanks anyway....cheers.
Thanks anyway....cheers.