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Am hum in receive

Dr. Frankenstein said:
As has already been said ..... it's time to recap the radio due to its age.

We haven't been told if the hum is also heard on the receiving end by the people that are listening to the transmitted signal, but at this point, it doesn't make a difference.

No matter who does the work it would be interesting to find out exactly which capacitor(s) caused the hum.

This would involve replacing all the capacitors one by one until the hum goes away, then continuing to replace the rest of the capacitors.

But some people just don't care and want to get the job done ...... with the answer being ......
"Who cares, it's fixed!"

I'm sure we would like to know.......................

73
Click to expand...
I will order the recap kit from Klondike and start the process. I will take notes and let you guys know. This will be my first cap replace. I also would like to add a carrier control along with new final and driver. 25 watts would be nice. Does anyone know the transistors that are a replacement? Is there other work that needs to be done to upgrade final and driver?
 
MadisonMod said:
That is the tone
Click to expand...
154 Hz isn't a harmonic of 60 Hz. I would guess it's an internal oscillation somewhere. One of Klondike Mike's cap kits may do the trick.

There's guys here that are much smarter than me that can help you track down the source of the hum if recapping doesn't do it.
 
A 1978 radio that's full of 1978 aluminum electrolytic capacitors dern well ought to have some quirky habits. Those parts are long past their "use-by" dates.

Back in the day, a tech with a lot of experience on a particular model could tell you which bad cap caused which common fault. The caps that routinely failed at the ten, 20 or 30-year mark were a matter of experience, once you had seen a few hundred of a particular model. This led to a "mandatory" list of five caps we change on any of the post-1979 Uniden-made SSB CB radios made in Taiwan, for example.

Just which one is causing your unwanted audio-oscillator feature is not clear to me. Just the same, if you replaced ALL of the aluminum electrolytic caps and the problem remains, that would be a surprise. It's the only way you'll find out.

As for upgrading the final and driver? Consider that if you want more RF power from the radio you need a bigger power supply. If you want more wattage, get an amplifier.

The carrier-control circuit in this radio does not lend itself to a low-carrier/big swing result. Turning down the carrier trimpot also turns down the modulated peak power as well.

There is a mod to get around this, published on Jim's Wix site. It involves removing the entire factory circuit that sets the carrier and replacing the power transistor with a different type. Not for the faint of heart. But it will swing its ass off once that's installed.

There is another transmit-side 'squeal' issue in that radio that is caused by the current from the relay's coil passing down and back up the mike cord when you key the mike. A small PNP transistor is all it takes to cure that. This is posted on this forum, somewhere.

The older this model gets, the longer the laundry list will be to make it a daily driver again. Should be possible, just the same.

73
 
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nomadradio said:
A 1978 radio that's full of 1978 aluminum electrolytic capacitors dern well ought to have some quirky habits. Those parts are long past their "use-by" dates.

Back in the day, a tech with a lot of experience on a particular model could tell you which bad cap caused which common fault. The caps that routinely failed at the ten, 20 or 30-year mark were a matter of experience, once you had seen a few hundred of a particular model. This led to a "mandatory" list of five caps we change on any of the post-1979 Uniden-made SSB CB radios made in Taiwan, for example.

Just which one is causing your unwanted audio-oscillator feature is not clear to me. Just the same, if you replaced ALL of the aluminum electrolytic caps and the problem remains, that would be a surprise. It's the only way you'll find out.

As for upgrading the final and driver? Consider that if you want more RF power from the radio you need a bigger power supply. If you want more wattage, get an amplifier.

The carrier-control circuit in this radio does not lend itself to a low-carrier/big swing result. Turning down the carrier trimpot also turns down the modulated peak power as well.

There is a mod to get around this, published on Jim's Wix site. It involves removing the entire factory circuit that sets the carrier and replacing the power transistor with a different type. Not for the faint of heart. But it will swing its ass off once that's installed.

There is another transmit-side 'squeal' issue in that radio that is caused by the current from the relay's coil passing down and back up the mike cord when you key the mike. A small PNP transistor is all it takes to cure that. This is posted on this forum, somewhere.

The older this model gets, the longer the laundry list will be to make it a daily driver again. Should be possible, just the same.

73
Click to expand...
I did a Google search for Jim's Wix site , but it only comes up with ( wix build your own free website) can you give me the link for the site you're talking about?
 
Dr. Frankenstein said:
As has already been said ..... it's time to recap the radio due to its age.

We haven't been told if the hum is also heard on the receiving end by the people that are listening to the transmitted signal, but at this point, it doesn't make a difference.

No matter who does the work it would be interesting to find out exactly which capacitor(s) caused the hum.

This would involve replacing all the capacitors one by one until the hum goes away, then continuing to replace the rest of the capacitors.

But some people just don't care and want to get the job done ...... with the answer being ......
"Who cares, it's fixed!"

I'm sure we would like to know.......................

73
Click to expand...
Dr. Frankenstein said:
As has already been said ..... it's time to recap the radio due to its age.

We haven't been told if the hum is also heard on the receiving end by the people that are listening to the transmitted signal, but at this point, it doesn't make a difference.

No matter who does the work it would be interesting to find out exactly which capacitor(s) caused the hum.

This would involve replacing all the capacitors one by one until the hum goes away, then continuing to replace the rest of the capacitors.

But some people just don't care and want to get the job done ...... with the answer being ......
"Who cares, it's fixed!"

I'm sure we would like to know.......................

73
Click to expand...
Hello everyone. So I had to move 2 capacitors to read the uf and voltage to complete schematic with board # and values. c111 .1uf 16v and c93 .1uf 16v capacitors. I plugged the radio back in and turned unit on, the hum is gone. I will start the process of replacing each cap and checking one by one . I will update you when complete. But for now its possible that one or both of those are the problem. Maybe Coincidence ?
 
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MadisonMod said:
Hello everyone. So I had to move 2 capacitors to read the uf and voltage to complete schematic with board # and values. c111 .1uf 16v and c93 .1uf 16v capacitors. I plugged the radio back in and turned unit on, the hum is gone. I will start the process of replacing each cap and checking one by one . I will update you when complete. But for now its possible that one or both of those are the problem. Maybe Coincidence ?
Click to expand...
 

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