D16 and D17 should not be testing as zeners, those are regular switching diodes. At what voltage do they zener? The datasheet shows a reverse voltage rating of 30v.
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Another 2517 low receive issue
- Thread starter Hawkeye351
- Start date
I still do not have 8v on the RF gain control. Most I ever see on the RF gain control on this radio is 3.36v on any lead.
R233 shows 3.36v on one end and 0v on the other, when I turn the RF gain down then the control has no voltage on any lead. Shows as a 1k ohm (in circuit).
Replaced Q17 with a higher HFE of 82.
Replaced L5, can now see a little change when adjusting L5 but still not where it should be.
Checked R72 (I think it's R72) at D16/D17, shows as a 1k (in circuit).
Still lost
R233 shows 3.36v on one end and 0v on the other, when I turn the RF gain down then the control has no voltage on any lead. Shows as a 1k ohm (in circuit).
Replaced Q17 with a higher HFE of 82.
Replaced L5, can now see a little change when adjusting L5 but still not where it should be.
Checked R72 (I think it's R72) at D16/D17, shows as a 1k (in circuit).
Still lost
There will NEVER be 8v on the gain pot. The three components circled in green form a voltage divider. In RX, there should always be 8v on the left side of R233 and never more than 3.2v on the right side of it. That means there will never be more than 3.2v on the gain pot.
Here is a DC simulation with the pot maxed (counterclockwise). Here, it is putting the entire 1k in parallel with R291. The wiper and high side should both be at 2v.
If we then swing the pot all the way down (fully clockwise in the radio), the wiper is at ground and the high side sees 3.2v
And if I set the pot half way, I get this (pointless adding these pics because the images are the same besides the voltages on the left, I know)
So now we have a voltage control between 0v and 2v on the wiper. Take a look at the bias resistors at Q17 (circled in blue in the schematic), they are another voltage divider. That divider puts 2.1v at the base. When the RF gain pot is turned all the way up (wired for MINIMUM wiper voltage), the base of Q17 has the most current through it. When the gain pot is turned down (increasing the wiper voltage), it will cause less voltage drop across R72 meaning there is less bias current and less gain.
For example, with no voltage at the wiper, the base voltage (2.1v) minus the Vbe drop puts 1.4v across the 1k resistor, so 1.4mA of bias. If the pot is set to half way, there is 2.1v-0.7v-1.2v= 0.2v across R72 (0.2mA of bias).
edited to make schematic pic smaller so it loads faster, my bad lol
Here is a DC simulation with the pot maxed (counterclockwise). Here, it is putting the entire 1k in parallel with R291. The wiper and high side should both be at 2v.
If we then swing the pot all the way down (fully clockwise in the radio), the wiper is at ground and the high side sees 3.2v
And if I set the pot half way, I get this (pointless adding these pics because the images are the same besides the voltages on the left, I know)
So now we have a voltage control between 0v and 2v on the wiper. Take a look at the bias resistors at Q17 (circled in blue in the schematic), they are another voltage divider. That divider puts 2.1v at the base. When the RF gain pot is turned all the way up (wired for MINIMUM wiper voltage), the base of Q17 has the most current through it. When the gain pot is turned down (increasing the wiper voltage), it will cause less voltage drop across R72 meaning there is less bias current and less gain.
For example, with no voltage at the wiper, the base voltage (2.1v) minus the Vbe drop puts 1.4v across the 1k resistor, so 1.4mA of bias. If the pot is set to half way, there is 2.1v-0.7v-1.2v= 0.2v across R72 (0.2mA of bias).
edited to make schematic pic smaller so it loads faster, my bad lol
Last edited:
I don't have 8v on either side of R233. I get 8v on the collector of Q17, but not on either end of R233. This is a different Q17, the old one had an HFE of 34, which I figured was too low, so I replaced it with another one with an HFE of 82.
Unhook one end each of D16 and D17. See if L5 exhibits a peak now. L5 has no internal capacitor to go bad. If it won't show even a shallow peak there's a fault of some sort on its input side, most likely.
Or maybe the antenna was connected during a storm when a neighbor's tree was stuck? Burned insulation on L5 will reduce sensitivity, and turning the slug won't affect the signal level.
73