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Robyn 520D low receive / no TX modulation
- Thread starter RichardHartman
- Start date
David's right. A microfarad (uf) is way over the top too big.
The trimmer caps seen in the pic have a max 35 pf (picofarad) capacitance if memory serves. They tend to adjust for a peak near their minimum setting in a 35 MHz circuit. The peak will show with the cap set closer to halfway for a circuit tuned to 27 or 16 or Mhz range. And all the way full capacitance may or may not be enough for a 10.695 or 7.8 MHz circuit.
But measuring those internal capacitors for several dozen type numbers of slug-tuned cans is a job I would take if someone offered to pay me enough.
73
The trimmer caps seen in the pic have a max 35 pf (picofarad) capacitance if memory serves. They tend to adjust for a peak near their minimum setting in a 35 MHz circuit. The peak will show with the cap set closer to halfway for a circuit tuned to 27 or 16 or Mhz range. And all the way full capacitance may or may not be enough for a 10.695 or 7.8 MHz circuit.
But measuring those internal capacitors for several dozen type numbers of slug-tuned cans is a job I would take if someone offered to pay me enough.
73
check the 2 - 9 volt zeners near the relays, and the drop down resistors near them, clean and check the relay. also, make sure the tantalum cap in the tank circuit has been replaced... its the tiny blue one.
All good advice. And the tiny tantalum capacitor is C179. Not sure if it's always blue, but it's always tiny. It **MUST** be replaced or at least removed any time the AM modulation gets turned up, even a little. When C179 fails as a short, damage follows. This capacitor has the proper 25-Volt breakdown-voltage rating for legal 100% modulation ONLY. But a 35-Volt (or higher) rated cap lasts longer, and won't kill the relay, power supply etc.
73
Last edited:
Yes I replaced c179 when I did the recap. I have also tried 3 different relays still the same issue. I will check on those mentioned diodes and resistor. Going to replace L8 for some reason that slug will not turn. Could tr18 and tr19 show correct voltage but have issue with modulation?
73
Richard
Update...
Replaced L8 and retuned RX. I now have sufficient audio and the s meter is working on both AM and SSB but still not quite right. If its below S5 it doesn't have any meter movement. The VR1 is working for adjustment and I connected a new meter to verify it wasn't just the old meter not working.
After replacing L16 and VR8 I was able to align the TX. AM max out at 4 watt it will not increase anymore than that and will swing 8-10w with modulation. Not looking to increase it more just wanted the behavior to be noted. On SSB I can only achieve 4 watts at high peak. This has also made RF meter start working properly and VR12 is working now for adjustment.
So at this point low TX on SSB and week S meter is what's left to figure out.
Thanks again for the continued support and as always any further help is appreciated.
73
Richard
Replaced L8 and retuned RX. I now have sufficient audio and the s meter is working on both AM and SSB but still not quite right. If its below S5 it doesn't have any meter movement. The VR1 is working for adjustment and I connected a new meter to verify it wasn't just the old meter not working.
After replacing L16 and VR8 I was able to align the TX. AM max out at 4 watt it will not increase anymore than that and will swing 8-10w with modulation. Not looking to increase it more just wanted the behavior to be noted. On SSB I can only achieve 4 watts at high peak. This has also made RF meter start working properly and VR12 is working now for adjustment.
So at this point low TX on SSB and week S meter is what's left to figure out.
Thanks again for the continued support and as always any further help is appreciated.
73
Richard
up near the pll chip is a 5 volt regulation circuit that feeds the chip, check transistor and drop down resistors there also.
Another update...
So I did check the for mentioned regulators and resistors and all checked good. While I was looking everything over on the top side I did see a resistor soldered on top of R202. So I thought I would unsolder it and low and behold TX on SSB come back in full. Not sure why they had an extra resistor there?
So the only issue now at this time is the S meter. Its like it doesn't register anything under an S7. I replaced c35 and c98 twice and vr1 and still the same. I have also checked it with a new meter and also still the same.
Hopefully we can get this last part figured out.
73
Richard
So I did check the for mentioned regulators and resistors and all checked good. While I was looking everything over on the top side I did see a resistor soldered on top of R202. So I thought I would unsolder it and low and behold TX on SSB come back in full. Not sure why they had an extra resistor there?
So the only issue now at this time is the S meter. Its like it doesn't register anything under an S7. I replaced c35 and c98 twice and vr1 and still the same. I have also checked it with a new meter and also still the same.
Hopefully we can get this last part figured out.
73
Richard
Last update...
I was looking over the schematic again focusing on the S meter and BAM!!! it hit me. I changed out L8 earlier. L8 with a slight ,very slight adjustment brought the s meter back into full operation. Now onto the next Robyn, lol.
Thanks again everyone.
73
I was looking over the schematic again focusing on the S meter and BAM!!! it hit me. I changed out L8 earlier. L8 with a slight ,very slight adjustment brought the s meter back into full operation. Now onto the next Robyn, lol.
Thanks again everyone.
73
Way, way cool! Congratulations for sticking with it. And for not getting discouraged when the first fault found proves it's not the only one in the radio. The older they get, the longer the laundry list will be to make it function again.
73
73
