I recently acquired a Uniden 78 that has had the traditional cap\resistor swing mod put in it. When I pulled the covers off I was surprised to find the 220uf 35v cap had the negative side with the stripe facing the front of the radio. I was always under the impression that it had to face the final. It was installed by cutting the jumper, lifting it, and soldering the cap with the resistor across the legs of the cap to the factory jumper. All the work was done on the component side. How can it be that the radio is still keying and swinging normal? I had it put on a meter a few weeks ago to make sure it is working, but I always thought it’d mess up the cap or radio if installed this way.
-
You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
Swing kit question
- Thread starter Easyrider
- Start date
Check the traces below it. That JP36 "Mod"...
Since you're affecting the Driver, verify that the Trace that Jumper Mod is in, goes to the Driver.
But, yes, if it's not installed the right way, you would have heard a loud bang, followed by some smoke leaking out any holes and gaps back there, and upon opening - it looks like someone had a party back there and forgot to clean up afterwards - leaving it for several weeks - you'd know it by stench alone..
Plus if extensive (bigger caps give bigger bangs) the confetti of the after party clean up would have shorted out several components possibly further damaging the radio so it won't work anymore.
They may have not soldered the cap onto the part properly and so you're on borrowed time until that part finally makes contact with the trace and - see, and imagine - the above...(IT would not suprize me if THEY FORGOT TO REMOVE THE JUMPER - so the question of reversed leads is solved thru the fact that the jumper is less ohmic value than the cap - so it shorts across - they did something else inclusive to it. )
There are several jumpers in that area - so verify that the trace that mod uses - heads to the Driver on that Caps CATHODE (Negative Band) lead - else they may have turned or twisted the cap to let it rest with torque, so it didn't shake loose.
It may also use a different jumper than you're used to - you can post of photo if you're wondering about the physics of the problem...
Since you're affecting the Driver, verify that the Trace that Jumper Mod is in, goes to the Driver.
But, yes, if it's not installed the right way, you would have heard a loud bang, followed by some smoke leaking out any holes and gaps back there, and upon opening - it looks like someone had a party back there and forgot to clean up afterwards - leaving it for several weeks - you'd know it by stench alone..
Plus if extensive (bigger caps give bigger bangs) the confetti of the after party clean up would have shorted out several components possibly further damaging the radio so it won't work anymore.
They may have not soldered the cap onto the part properly and so you're on borrowed time until that part finally makes contact with the trace and - see, and imagine - the above...(IT would not suprize me if THEY FORGOT TO REMOVE THE JUMPER - so the question of reversed leads is solved thru the fact that the jumper is less ohmic value than the cap - so it shorts across - they did something else inclusive to it. )
There are several jumpers in that area - so verify that the trace that mod uses - heads to the Driver on that Caps CATHODE (Negative Band) lead - else they may have turned or twisted the cap to let it rest with torque, so it didn't shake loose.
It may also use a different jumper than you're used to - you can post of photo if you're wondering about the physics of the problem...
Thanks Handy Andy for the detailed response. I checked to make sure the cap is soldered in place where the jumper was removed on JV101. The jumper was in fact completely removed and the legs of the cap soldered through the holes. All the solder points look to be soldered good and check out good with a digital VOM.
I'm guess in this - but if the value of the resistor is low enough, the drop across it is not that great - but these caps do not take kindly to reversed bias for extended lengths of time.
If the value of the resistor is pretty low, like less that 50 ohms. I can see this survive - but you're on borrowed time here...
The low value resistor beneath it lessens the audio capacity the cap has to carry - Raises carrier to around 2W or so. And the Driver doesn't take a lot of power to feed it. So current draw is low to begin with, the ohmic value of the resistor below it allows the caps' charge on it's plates to recover easily enough.
Although I can see the soldering done, only hope the resistor and any mods - even tuning - has not placed this teeter-totter over the tipping point.
Eventually that cap will dry out from the gassing events the reverse draw does on the plates. Then it acts as a dead short brining up the wattage back to nearly full voltage with little ohmic intervention for the plates are pretty much welded being wrapped foiled together and their anodizing surface film is what keeps them separated.
When these dry out rapidly (used like this) the innards are forced to heat up and gasify their electrolyte - they will explode.
Its why "slots" are there, to allow the gasses built up inside, to press pass the shear leaf and release pressure from the inside. Too much gas building too quickly, the foil leaf panels shear but too late, too much pressure - boom.
There's holes in the bottom for the same reason - vented the gasses under a heating event.
I see this is a 35V rated and if it's rated 85 to100C. IF you're using a higher grade cap value, good! To a point, those can withstand some reverse polarity draw, but in this case, it is not a full supply-demand computer power supply rail reversal install this thing is installed in - because in power supplies, under those conditions these do explode.
If it was 16V we may have had a different story ending here...
Again, you're on borrowed time...
If the value of the resistor is pretty low, like less that 50 ohms. I can see this survive - but you're on borrowed time here...
The low value resistor beneath it lessens the audio capacity the cap has to carry - Raises carrier to around 2W or so. And the Driver doesn't take a lot of power to feed it. So current draw is low to begin with, the ohmic value of the resistor below it allows the caps' charge on it's plates to recover easily enough.
Although I can see the soldering done, only hope the resistor and any mods - even tuning - has not placed this teeter-totter over the tipping point.
Eventually that cap will dry out from the gassing events the reverse draw does on the plates. Then it acts as a dead short brining up the wattage back to nearly full voltage with little ohmic intervention for the plates are pretty much welded being wrapped foiled together and their anodizing surface film is what keeps them separated.
When these dry out rapidly (used like this) the innards are forced to heat up and gasify their electrolyte - they will explode.
Its why "slots" are there, to allow the gasses built up inside, to press pass the shear leaf and release pressure from the inside. Too much gas building too quickly, the foil leaf panels shear but too late, too much pressure - boom.
There's holes in the bottom for the same reason - vented the gasses under a heating event.
I see this is a 35V rated and if it's rated 85 to100C. IF you're using a higher grade cap value, good! To a point, those can withstand some reverse polarity draw, but in this case, it is not a full supply-demand computer power supply rail reversal install this thing is installed in - because in power supplies, under those conditions these do explode.
If it was 16V we may have had a different story ending here...
Again, you're on borrowed time...
Once again, thank you for clearing all this up. The small resistor is a 68ohm 1/4 watt. I’ve done this mod myself in the past in a 29, and knew this looked wrong when I took the covers off this 78. It’s crazy it lasted this long. I’m gonna get a fresh capacitor and resistor and re install it correctly. Thank you for all the help.
- No one is chatting at the moment.
-
-
dxBot:63Sprint has left the room.
-
dxBot:kennyjames 0151 has left the room.
-
-