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How to fix low receiving sensitivity of this transceiver?

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yo2550

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This is its circuit diagram
https://www.electro-tech-online.com/custompdfs/2012/07/TH7_X7A.pdf

Technical Description
https://www.electro-tech-online.com/custompdfs/2012/07/TH7_Technical_Description.pdf


The receiver circuit should have been distorted after I tested the radio with my DIY power amplifier. Once, the ANT was shorted to 12v. power from a battery. Not sure if the problem may also come from testing the radio with the PA many times with a high SWR.

This results in less sensitivity of signal receiving comparing to another same model of the radio. I tried adjusting the RV1, which is supposed to be a squelch but the problem still exists. I cannot receive weak signal while another same model one can. This radio is auto squelch, there is no SQ dial.

So, I'm thinking of changing the Q6, Q8, Q9 but I'm not sure if it would fix the problem.

Please tell me which components should I change to fix the receiver unit of this radio.

The Schematic Diagram is in the TH7_X7A.pdf file.

Thanks.
 
check your T/R switch diodes (looks like D7, 8, and 13) in and around the antenna circuit. D8 in particular looks like it's purpose is to mute any signals to Q6 while transmitting.
 
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ok, I check D8 and one of its leg was broken(connected to L3). I am afraid, I have used the radio to transmit for a while, the transmitted signal might go to Q6 and so on. Please advise, if the transmitted signal reach Q6 and other components, which components could be damaged?
 
Q6, and maybe R42, possibly the crystal filter, but the transistor going bad would likely have kept too much signal from going there for very long. it's a terrible copy of the schematic, a lot of lines don't show up. the only way i knew Q6 was the receiver front-end was the 220Mhz crystal filter.
 
You meant the SF1? I'm going to order the parts. Should I order all the parts in the receiving circuit such as C, R, or diode or just order the big parts first?

I checked R42 and it shows 47 Ohm. So it's working. I cannot check all the capacitors because I have a regular multimeter not c meter.
 
caps aren't likely. you can check to see if they're shorted. probably just Q6 and the broken diode.
if R42 is ok, then you probably damaged the B-E junction of Q6 instead of shorting the C-E junction.
 
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Q6, and maybe R42, possibly the crystal filter, but the transistor going bad would likely have kept too much signal from going there for very long. it's a terrible copy of the schematic, a lot of lines don't show up. the only way i knew Q6 was the receiver front-end was the 220Mhz crystal filter.

strange comment ... actually the schematic is perfect :) easy to read maybe update your PDF reader ;)

But I agree about the diodes and also include D1 in that list so D's 1,7,8,13 they are all likely to RF switching PIN diodes

here's the description of the TX/RX switching....

TRANSMIT/RECEIVE SWITCH
When the radio is in the transmit mode, pin diode switches D13 and D1 are both turned on (representing less than 0.7 ohms). D13 allows the transmit
signal to pass to the antenna and D1 shorts one leg of a T matching network (L3, L15 and C4) to ground in the receive path. This results in a parallel tuned
circuit high impedance being presented to the transmit signal so that the receive path does not load the transmit signal. In the receive mode, both D13
and D1 are off, resulting in the antenna signal being coupled into the receive LNA through the 50 ohm T matching network and the unwanted load of the
transmit final amplifier is reduced to less than 1 pF by D1.

Dave
 
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I agree. I tried removing D13 and D7 and turned on the radio, still get the problem of low sensitivity. Then I removed D1 and put at D8(removed) for testing, turned on the radio and still got low sensitivity. So, I think the problem might come from Q6 or other parts after the path of Q6. I will order the parts and replace them.
 
Anyone could tell me how T-Matching network works by two L and one C in the circuit. I know D1 shorts one leg of it to ground but not really understand how it works. (L3, L15, C4)

Is D8 always turned on when receiving and transmitting?
 
Problem solved: I replaced D8, and Q6 then sensitivity came back to normal comparing to another radio.
BS. I also replaced D13, D7, and D1 because I'm not sure whether I burned them with soldering iron after I removed them off the board for testing.
Thanks all you guys.


I lost R11 (560 ohm)when I removed D1. I should be stick to the soldering iron and I can't find it now.

Can I use a carbon film resistor instead of SMD resistor on the board?
 
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