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Robyn SB-520 (858 board) no transmit power

Lkaskel

Lkaskel

Well-Known Member
Aug 4, 2017
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Hi Gang,
Well, I'm at it again. I have on the bench an SB 520 that had no transmit. The owner said he had high SWR's and it quit. I checked the final with my transistor tester and it was bad so I replaced the driver and final with brand new (not fakes) 2166 driver and 1307 final. I also replaced the tantalum cap (179) with a 25 volt version. It still did not transmit. I had .7vdc on both bases and 9 volts on the collectors. The collectors are supposed to be 4.7vdc based on the schematic so 9 volts was wway too high. I went to set the bias on them and was able to set the driver to 60 ma and when I went to set the final to 50 ma it initially set for a second then drifted to over 400ma and will not come back down. Any thoughts?
 

Robb said:
Yeah; where did you get the driver and final from?
Click to expand...
I bought 2166's and 1969's from RF Parts and 1307's from Barkett Electronics. I have used them in other gear that has been waiting for "real" parts and they are working fine so I know that I now have a trusted source. I did buy many parts from E-Bay and none of them worked so......
 
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Well first the collectors are set at what the FCC requirements were at the time so they are not too high remember on SSB the collector voltage will be 13.8 give or take a few tenths. So that being said it's time to start looking at the pre-stages for signs of no RF working your way back from the driver with a scope should expose a lose of RF especially if all else is working on the radio as it should. You may check for a very low-level signal on a monitor radio this would help tell you how far back RF is being lost if you can HEAR/SEE a signal on a monitor radio or again a scope.
 
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Sure makes it sound as if you replaced two perfectly-good parts.

First thing to do is set another radio to the same channel and attach a coax jumper to it. Thread the shell of the plug at the far end back over the coax. The center pin now becomes a 'sniffing' antenna.

If you can hear the SB520 key up this way, it tells us that the transmit signal is being generated, but not making the whole trip downstream to the antenna.

And if you can't hear anything, even poking the 'sniffer' around the 520's transmit circuits, it's time to find out why there is no transmit signal being generated.

73
 
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There you go you are now on the way to fixin that Robyn, Robin, Roybin you know what I mean..
 
Lkaskel....If your able to set the bias on the driver at 60mA...and you mentioned the adjustment on the final was 50mA before it started to drift up to 400mA..I would look at VR16 first and make sure it is good (300ohms). If that VR checks out fine, then I would look at the bias diode (MV1Y) I believe is D51 in that radio. You could also try replacing VR 16 with a 1k (102) variable resistor to see if that adjusts the bias current back into spec. Post back with your findings.
-75
 

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