Robyn 520D low receive / no TX modulation

[RichardHartman]

Try 10 or 12pF. 10 or 12uF is way too much.

May be the meter is "hanging up" mechanically??

73
David

I'm sorry I stated the value wrong...it is a 12pF capacitor I put in.


73
Richard

 

[nomadradio]

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

 

[johnnytek]

I have a Robyn 520D that I am working on now. The gentleman stated that he put it up 12 years ago working and when he fired it up recently it didn't work. So when I started here is what I got..

Extremely low receive-AM/LSB/USB but you can hear the hiss increase with the volume. I have to turn the volume all the way up to hear a station that is 2 miles away that normally hits with +10. Also checked with external speaker with same results. There is no reading on the S-meter, meter does work.

Transmit-
AM -1.5w on external meter will not increase or decrease when adjusting vr for AM stays the same. With modulation into the mic it has a slight negative swing. I can talk to station that is about a half mile away and he can hear and understand me.

LSB/USB - no power on key up, extremely low output less than half a watt with modulation. RX is better than AM but still extremely low.

So I started with a recap..

I recapped all the electrolytic caps and c179.
Pulled and checked pre driver/driver/final...all check good.
Rx volume is still the same and Tx modulation still the same. When checking on an external modulation meter I am getting around 20%

I am newer at this and I can go thru the schematic and check each part one by one but if I was more seasoned I would know where to start looking first. What I think if I am correct the audio chip is ok its voltage are consistent with the book and the PA section works and is very loud. The low receive and no Tx modulation I am assuming to be related?
Any help on where to start would be greatly appreciated.

73
Richard

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.

 

[nomadradio]

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

 

[RichardHartman]

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

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

 

[RichardHartman]

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

 

[johnnytek]

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

up near the pll chip is a 5 volt regulation circuit that feeds the chip, check transistor and drop down resistors there also.

 

[RichardHartman]

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

 

[RichardHartman]

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

 

[nomadradio]

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