Re: Need Help setting the PA Bias Current for sBITX
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Aaron,
Sounds like ur making headway. reverse polorizing a unit sometimes takes patience to work out all the weakened components or some that wont fail unless its under load.
I dont own the SBITX yet, but it seems from the info you provided its now isolated to the transmit sections.
I would start by looking at the si5351 area and where the Pi connect since you actually witnessed some smoke. where there is smoke, there is char, or holes in something or burnt traces. check driver transistors, and anything feeding the finals. Also check your interconnecting wires for any burn marks or punch thru holes .Since you are actually receiving stuff, i would say your si5351 and PI are good, unless the si5351 had one osc crap out .It has 3 outputs.
order a si5351board asembly which are cheap, and replace the one in your Sbitx. . Also look for any interconnecting components leading to these items on the board such as traces burnt, resistors discolored, etc. You are doing great. you got the receive up and running!!!!!! . recheck ur fets and also any driver transistors and pre amp transistors.Also check ur biasing resistors for opens.
Good luck on ur repair Aaron. You have made alot of headway since u are receiving stuff and power is flowing back into the unit.
might check your voltages on your finals , and anything up stream from the finals. Sounds like ur getting very very close.
GOOD JOB!!!!!!
73 David ac9xh
On Tuesday, June 11, 2024 at 10:42:15 PM EDT, Aaron K5ATG <aaron@...> wrote:
Hello All, Yup still trying to get my sBITX working again. Here is a recap.? Back in March I accidently cross polarized the transceiver First thing I did was to replace both IRF510's? Q16 & Q17 That didn't work so I found that D7 Needed to be replaced. I replaced it with another 1N4007 Diode ( I think that this is intended to protect the circuit in case of cross polarization. It didn't work that in my case.? I then replaced the voltage regulator with the suggested Drok Voltage Regulator I then would turn it on and would get wisps of smoke and the smell of burning electronics. I could not locate the source of the smoke but it was coming from the area around the SI5351 and RPI. A couple of days later I pulled the SI5351 out to look at it and returned it and powered up the transceiver and there was no smoke or smell of electronics burning.? I hooked it up to an antenna and it was receiving very good, 'Next I hooked up a dummy load to it and put it in transmit and it would completely shut off immediately Next I was told that I need to set the PA Bias since I replaced the IRF510's? in order to do that I had to follow these steps:
- Set mode to USB
- Set MIC to 1
- Set Drive to 1
- Set PA-BIAS1 to Zero (Fully Counter Clockwise
- Note DC current on PTT
- Slowly increase the bias current until the total current consumed by the radio is increased by approximately 250 mA
No idea on how to measure the current with a digital multimeter so I had to do some cinematic research on how to do it. I basically had to put the DMM probe in the right socket on the DMM and put the DMM in the circuit. Here is a pic of what I did (i apologize for the crudeness of the image, I just rushed through it)?  When the sBITX is in Receive the DMM displays a 0.43, when it goes into transmit it jumps to 0.86. Before I hooked up the DMM to start adjusting the? ? ? ? ? ?PA-BIAS1 I set it to zero (full counter clockwise.) When I adjust PA-BIAS1 there is no change to the DMM readings in either receive or transmits.? This is where I'm at now and I'm unsure what tunnel in this never ending maze of tunnels to take in eventually getting this radio back on the air. I'm starting to wonder if it ever worked in the first place and my recollection of me enjoy using it is just an illusion within the confines of my cerebral carnival.?? -- '72
Aaron?
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