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Post by husker on Dec 30, 2008 22:19:49 GMT -5
I replaced the "ping" cap, now I get a great ping, but i can't control the volume! It's acts like it is all the way up and the knob doesn't do anything. I have to use the RF Gain to control the volume level...any ideas.
Checked tubes, they are all very good...
HELP
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Sandbagger
Administrator/The Boss
Posts: 6,245
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Post by Sandbagger on Dec 30, 2008 22:49:38 GMT -5
I replaced the "ping" cap, now I get a great ping, but i can't control the volume! It's acts like it is all the way up and the knob doesn't do anything. I have to use the RF Gain to control the volume level...any ideas. Checked tubes, they are all very good... HELP The ground side of the volume control pot has likely opened up, or the ground connection to the pot has been cut somehow. I've had this happen on a few older radios, and without the ground side the series resistance in the volume control alone is not enough to make much difference and the volume runs close to max. It's easy enough to check with an ohm meter......
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Post by husker on Dec 31, 2008 0:34:50 GMT -5
Sounds like a good place to look!
Thanks and I will let you know if it works
Bob
oh and it also popped the "ping" resistor...I had replaced that one as well ( the 10ohm one) as the old one looked cooked, I ran the radio for 15 minutes then the sound stopped..completely. Looked under and saw that new resistor had a little hole in it..guess it wasn't happy being there. replaced it and back to the orginial problem again...but didn't run it as long..didn't want to waste another resistor..
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Post by FIXR on Dec 31, 2008 6:44:17 GMT -5
Bob,
If you are calling the "ping" resistor the one on the tiestrip that also has the red wire from pin 6 of the 6AQ5, then it should be a 2.2K. The positive of the "ping" capacitor would go there also.
Alan
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Post by husker on Dec 31, 2008 12:42:13 GMT -5
Yea Alan, it is that resistor...it connects to the same bridge as the pos side of the cap..so you are telling me that maybe it was changed out as I replaced what was there... interesting..what would be the reason to change the value??
Also I am putting in a 1 watt...should I find a 2 watt?
thanks
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Post by mark4 on Dec 31, 2008 13:03:29 GMT -5
The reason for changing the value would be... some hacker didn't have a clue. This resistor will over heat with a long ping cap. Don't go over 80uf. A 1 watt will get you by.
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Post by husker on Dec 31, 2008 13:53:19 GMT -5
I have a 33 in there now and it is perfect....but the volume issue happened after I did this. And when the first resistor went *pop* the volume went too....just wondering if I may have done something stupid yet again..I am getting good at that!
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Post by husker on Jan 2, 2009 22:32:15 GMT -5
Before I start re-hacking, I want to be sure we are talking the same resistor..here is the pic.
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Post by BionicChicken on Jan 2, 2009 22:38:40 GMT -5
As Fixr said, that should be a 2.2K resistor.
BC
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Post by husker on Jan 2, 2009 22:44:32 GMT -5
Just making sure, i have screwed up ENOUGH stuff without checking first!!! ;D
Thanks again!
I don't have any 2.2k's, so I am going to use a 2.4k...
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Post by 2600 on Jan 3, 2009 1:53:13 GMT -5
A 2.4k is only 10% larger than the factory value. Won't make any difference you can hear, and should work fine. Here's the pic we use for this question. The resistor in your pic has a black third color band. I'll guess that when the old R56 burned up, the red bands in the middle turned black or brown. Whoever replaced it didn't consider what color they USED to be, and just copied what he saw. Jumping this resistor or making it too small, like the 20-ohm part in your pic creates the risk of damage to the receive side of the relay in the transmitter. Bad idea. 73
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Post by husker on Jan 3, 2009 2:38:17 GMT -5
If there was damage, any ideas on how if would present it's self? This could be what I have been chasing for 7 months......Foe example, tonight I couldn't receive much ( it was very quiet also) so I keyed a walkie-talkie and it received that and I got some feedback as well..but I have had issues really receiving. If I pull out the squelch knob it gets better, but it just doesn't seem to work right.
Thanks guys!!!
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Post by 2600 on Jan 5, 2009 0:08:26 GMT -5
If that resistor damaged the relay's receiver-mute circuit, this would cause the receiver to drop out, or "forget" to come back when you unkey. Tapping on a relay with this kind of damage almost always causes the receiver to drop in and out, or at least make some "scratch" noises. Try removing the transmitter's receiver coax from the receiver's antenna socket, and plug your antenna coax directly to the receiver's antenna socket. If this improves the receiver sensitivity, the antenna circuit of the relay may be bad. Or a wire on the relay socket could be loose.
A weak Mark III receiver can have a handful of different causes.
Tubes are the single most common cause of any problem, whatever that may be.
Checking the 105-Volt zener diode is worth doing first. It doesn't go bad often, but can disrupt the receiver when it goes bad.
Checking DC voltages against the ones shown on the schematic is a good place to start. But beware of the points in the radio where a modern DVM will disrupt the circuit when you touch the probe tip to them. The voltage readings on the schematic were made with a VTVM, a Vacuum-Tube Volt Meter. The "Vacuum" part isn't all that meaningful, but the standard design of the DC-Volts probe is different. The meter they used for the measurements in the factory diagram had a 1-Megohm resistor inside the tip of the probe. Your modern digital meter does not. Neither will the passive "VOM" type analog multi meter, like a Simpson 260.
That resistor serves to isolate a sensitive signal circuit from the capacitance of the probe wire.
Best rule is that if there is a bypass cap, like a .01 or .005 uf in the circuit where you touch the probe, you probably won't disrupt the circuit making a DC-Voltage measurement. The screen-grid and cathode pins of a tube socket probably won't see any disruption from touching a meter probe. But touching it to the control-grid or plate pin of a tube socket will often make that tube begin to oscillate. That kind of disruption to the circuit makes your DC-Volts measurement meaningless.
There's one issue where some experience is helpful. The "peak" on the tuning slugs is sharper on some of them, and very broad on others. But a coil or IF transformer that should have a sharp peak offers a clue when turning that slug has little or no effect. At least this will point to a problem in the same circuit as the coil with that slug inside it. Doesn't mean the coil/transformer is necessarily bad. Just means that you're close to the cause of the trouble.
Now I can't remember if I suggested looking closely at the antenna coil for any sign of overheating. Gotta remember to open the reply on a separate tab when using this forum. That way I can switch back to see what I already suggested.
But there is no easy "They All Do That" kind of answer for a weak Mark III receiver. And if the antenna coil looks darker near the middle of the coil, this is a hint that the receiver absorbed some kind of surge down the antenna coax.
73
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