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Post by 238 on Aug 17, 2021 14:52:33 GMT -5
Got a transmitter that someone had put a jumper across R6/2.2k resister. The minus voltage on 6GH8 & 5763 are way off. -1 on 6GH8 & little less on the 5763. I know you adjust L1 & L2 but i have never fooled with that before. Was wondering if something else might cause that? Jim/238
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Post by 2600 on Aug 17, 2021 23:43:53 GMT -5
I'll guess that when you press the "Spot" button that you won't hear anything in the receiver.
Until the selected channel crystal oscillates, you'll get nothing, kinda like your measurements show.
The resistor got jumpered by someone who wanted more output. But it's a good way to clobber the 6GH8 oscillator/driver tube. You really should make sure that tube isn't flat. And if you don't have a spare tube that works in another radio, or a tube tester this becomes an open question.
L1 and L2 have a tendency to interact a little bit. L1 will show an abrupt behavior and cause the crystal to start oscillating or to stop, if you turn the slug either side of this "cliff" threshold.
If you have another radio you can set to the same channel, see if it can hear the spot carrier. Peaking L1 and L2 for the S-meter on the monitor receiver will allow you to cross them off the list of culprits.
Never assume you have only one fault in a radio this old. Getting the 6GH8 tube to run and deliver drive to the final tube is where I would start.
73
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Post by 238 on Aug 18, 2021 0:46:50 GMT -5
I have a spot tone. When do you check the - voltage? Do you have to mash the spot button to check it? Jim/238
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Post by 2600 on Aug 18, 2021 23:38:46 GMT -5
There is a bit of fine print around the negative grid voltages shown in the schematic. They are supposed to be made using a tool called a "VTVM", or Vacuum Tube Volt Meter. Doesn't sound like a big deal. A meter is a meter, right?
Not in this case. A VTVM would use a shielded wire for the DC Volts probe. And most important, there is a 1-Megohm resistor just inside from the probe's tip.
Naturally, if you just wrap one wire of a 1-meg resistor around the tip of your everyday digital voltmeter and probe with the other end of the resistor this will ruin its accuracy, to some degree.
The 3-foot long unshielded wire attached to the probe on every digital meter is going to act as if it were a huge capacitor to the circuit where you touch the probe tip.
Huge enough to shut down a quartz crystal, and suppress the negative voltage you would otherwise be able to measure with the antique meter used to produce the numbers on the diagram.
If the crystal doesn't run, that negative voltage won't get generated. Best way around for me is to use a 'scope probe set for "times 10". This reduces the load the probe tip puts onto the crystal and the oscillator circuit. Usually doesn't shut down the crystal.
Somebody needs to design a probe you can put on your digital meter and still make this kind of measurement in a tube circuit.
Somebody else.
One last idea. If you can borrow a second radio, like a mobile and power pack and put it on the same channel as the Browning. Try peaking L1 with the spot button pushed, for max S-meter on the monitor radio.
Can't remember trying this. Might work.
73
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mattbee
Mudduck
Tram & Browning Enthusiast :)
Posts: 38 
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Post by mattbee on Aug 19, 2021 6:18:02 GMT -5
Chris, Unfortunately, it can be very trying to diagnose some issues with these old tube type radios without test equipment  Mattbee |
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Post by 238 on Aug 19, 2021 8:02:42 GMT -5
I hooked the transmitter up & it will talk now. I have a B&K 177 VTVM & a DVM. I try both i get the same results. I have a another MKll hooked up. When i hit the spot button, it shows on the other radio that i run all the time. So i am going to try your method & see what happens. If it peaks on my other receiver the voltage should be close. I wanted to get this figured out before i change any thing else. Thanks Jim/238
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Post by 2600 on Aug 19, 2021 23:31:38 GMT -5
I have a B&K 177 VTVM & a DVM. That is WAY cool! 73 |
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Post by 238 on Aug 21, 2021 14:42:33 GMT -5
Well 2600 i triad the spot tone too my other receiver, it did show some movement when i adjusted L1 & L2. So i got to thinking, dangerous for me, i hooked my meter & load back up. Plugged in the mike & keyed up an adjusted L1 & L2 that way. Ended up with 2 1/2 watts & 1 1/2 swing on my Siltronix 1000 meter. Hooked to a MKlll receiver an talk on it, everyone said it was sounding good. Thanks Jim/238
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Post by 2600 on Aug 21, 2021 23:23:27 GMT -5
Rock on! Good to hear it worked out.
73
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