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Re: Need Help setting the PA Bias Current for sBITX
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Aaron, like scotty said, you are doing great. I could probably put u up against a 1000 licensed hams and your new learned troubleshooting skils will probably out do 90% of them since most dont bother servicing their own equipment or experiment with their gear. Your mis identification of parts is not too big a deal for me since i dont own the SBITX and im not a self proclaimed expert.HI HI. This animal looks like it will help keeptrack of time and such, even? after the unit loses power because of a battery on the backside. So , i would still look for anyshorted semiconductors in the transmit path. something seems to me was shutting ur SBITX off when you tried to key up . Meaning it was possibly dragging ur voltage down past the proper operating range. Diode, Transistor, fet, or maybe a chip tied to the +12 v or any of the other source voltages on the board ,like 5vdc, 3vdc. If you have good receive, then get a notebook, and or print out the schematic large enough for you to follow easily. 1. you can go thru certain areas of the SBITX board and map out the voltages WHILE you are in receive mode. Then get some mini clips to attach to the board comonents and watch your voltmeter while you try to transmit. write all the voltages down for each section and or major part. 2. after you have this completed,LIke Scotty pointed out, you will have more insight into your SBITX than most have who never did this. You can then post your findings here to compare to others SBITX voltages and find the area thats causing the problem. I would, since the unit was reversed voltaged, and you now have it up and running with receive, ?check some of the voltages on and this will take some of your time D3,Q12,Q13,Q14,Q15, U3,,u1,u7.D2,Q5,Q3,Q9,Q10,Q11, Q16,Q17,Q18,Q19,D6 i looked at the Schematic and these "look like they are tied to the transmit section. These can be checked by measuring "idle voltage while in receive mode and make a chart. collector,emitter base, source drain gate, diodes measure voltage on anode versus the cathode end. also after voltage checks, unplug the unit and measure the resistance of the semiconductors with the ohmmeter section. Some prefer the diode setting ,some prefer the ohms setting . Just make a chart for each component and it might tell you whats not operating properly. having these measurements will tell others who have the SBITX whether or not you have proper voltages to these semidonductors or chips. having base? to collector or base to emitter voltages will tell if a semiconductor is properly operating with proper biasing The unfortunate thing about reverse polarity a unit, you sometimes have to do the shotgun troubleshooting method as i call it. whatever section is tied to the transmit, you shoot thru them to identify the possible bad items by voltage resistance testing. crossing each item off the list. Since your unit shut down trying to transmit, i would think you have a Semiconductor component internally shorted or failing under load.tied to the 12vdc rail.Maybe and this is a longshot, look for any deformed capacitors. I highly doubt there are any for the short period of time you stated it was reversed polaritied, but you never know. A shorted electrolytic cap would cause this too. after you get ur unit fully running, you might invest in a cheap priced but good reputation ESR meter to check ur electrolytic caps. preferably one that can check the "Equivilent Series Resistance "of the capacitors IN CIRCUIT. Just to make sure none have drastically changed values due to the incident. 80% of any of my service solves were capacitors that looked good but went bad for one reason or another.they mostly opened up or just plain shorted out. Even short application of too much reverse voltage can permanently
damage the capacitor. Sometimes the self healing abilities can reverse
this a bit over time, but there will be permanent damage. just my hillbilly take of it all.HI HI. 73 David ac9xh
On Wednesday, June 12, 2024 at 12:10:17 PM EDT, Scotty R <theroadcalls1@...> wrote:
Aaron, you are doing great! As a former RadioShack tech, I can tell you that getting stable power in the item is always first. Although inconvenient,? this radio troubleshooting will make you an expert on the design and components.? As a teen in late 70s,? I had almost memorized the schematics of the Heathkit hw101 transceiver.? So many modifications and repairs led to that. This radio is completely different,? but keep up the good work and efforts. Document everything and be proud of your achievements. Scotty WD4PYT On Wed, Jun 12, 2024, 11:13 AM Aaron K5ATG via <Aaron=k5atg.com@groups.io> wrote: CORRECTION: |
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