FL2100B untuned on 40 meters - Amplifiers Forum at eHam.net

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eHam.net Forum : Amplifiers : FL2100B untuned on 40 meters

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FL2100B untuned on 40 meters	Reply
by W9PDS on October 29, 2009	Mail this to a friend!
Hi Folks, I picked up an FL2100B at a hamfest a few months back at a pretty decent price, and have just gotten around to making a relay interface for it for my rig. So far so good, the amp seems to be working pretty well, and I've been having a lot of fun running a little bit of power on 15, 20 and 80 meters. I'm typically seeing about 500 watts peak on SSB, and I drive about 300 on CW. The amp is in great cosmetic condition both inside & out. However on 40 meters, the amp just won't give a good SWR to my rig. No matter where I tune it to, the rig complains of high SWR (about 5) and I won't be able to get any power out to drive it. Where do you guys suggest I start troubleshooting this? I've taken the cover off & checked for signs of a burned out bandswitch (known defect with this model), but it looks pretty clean. However, I did not take the bandswitch off yet & fully inspected it, I just took a quick peek with the cover off. I suspect these are the original tubes, could one band not tuning up be a sign that it's time to get some replacements? Any pointers would be very much appreciated. I know my way around a VOM, I'm aware of and always obey Ohm's law, but I do not have a scope or any advanced test gear here. Thanks in advance! W9PDS

RE: FL2100B untuned on 40 meters	Reply
by K4DPK on October 29, 2009	Mail this to a friend!
No, it's not the tubes. Look for something wrong with the bandswitch or the 40m input circuit components. Phil C. Sr. k4dpk

RE: FL2100B untuned on 40 meters	Reply
by K4DPK on October 29, 2009	Mail this to a friend!
You said "no matter where I tune it ..." and I assumed you meant the input network, but I may have been wrong. Have you tried adjusting the 40m input network coil slug? Phil C. Sr. k4dpk

RE: FL2100B untuned on 40 meters	Reply
by W9PDS on October 29, 2009	Mail this to a friend!
Phil, Yes, I mean no matter where I tune the input knob. I'll try the 40 coil adjustment. I assume I just follow the wire off the switch & find the first coil? Thanks. W9PDS

RE: FL2100B untuned on 40 meters	Reply
by K4DPK on October 29, 2009	Mail this to a friend!
OK.... If you have the manual, find L-203. If you don't have the manual, Google and download FL-2100B Schematic. Then locate L-203. It is the 40m input coil, and it is accessible from the bottom. It's the second inductor over from the side the RF output connector is on. Tune the thing up at a very low drive and power level, then adjust the slug in L-203 to see if you can reduce the SWR. If you can't, you probably have a bad component, like one of the pi-net capacitors in the input circuit. There is one other thing you might try. Make the coax between the radio and the amp a few feet longer or shorter, just to make sure you don't have some funky, unlucky tuned stub for a jumper cable. Phil C. Sr. k4dpk

RE: FL2100B untuned on 40 meters	Reply
by W9PDS on October 29, 2009	Mail this to a friend!
Phil, Thanks for the reply. I tried the quick coax trick but no such luck.. I'll take it apart tomorrow and try the slug. I do have an MFJ Antenna Analyzer. Can I use that to check the input SWR while turning the slug with the amp powered off & make short work of finding the right spot? Thanks again, sorry for the ignorant questions, but just want to make sure I don't do anything dumb. W9PDS

RE: FL2100B untuned on 40 meters	Reply
by K4DPK on October 29, 2009	Mail this to a friend!
When you get into it, examine all the solder joints and components associated with L-203, including the bandswitch connection, to see whether anything looks awry. You could use the MFJ-259, but it would involve putting a carbon resistor from filament to ground on the output side of the input network (value would be about 130 ohms), and then having a way of holding the relay in the amp closed while making the adjustment. I think it would be faster to see if you can make a difference just adjusting it on the fly, but at LOW POWER. Don't drive the amp with more than about ten watts while doing this, and make sure the plate circuit is tuned correctly. Phil C. Sr.

RE: FL2100B untuned on 40 meters	Reply
by W8JI on October 30, 2009	Mail this to a friend!
"no matter where I tune the input knob" What input knob?

RE: FL2100B untuned on 40 meters	Reply
by W9PDS on October 30, 2009	Mail this to a friend!
Well, I grabbed the manual and followed the input circuit tuning procedures. It was exactly as you described Phil, other than the manual requested that you disconnect the HV jumpers from the transformer before starting. I ran the input slug the entire way up & down and the SWR on the radio did not change one bit. While I was in there, I checked the input circuit with a flashlight & double-checked the band-switch, finding nothing visibly wrong. I guess I'll have to start looking at some of the individual components, and possibly pull out the band-switch for a closer inspection, however that looks like it will be a pretty tough job. If you have any last ideas, I would appreciate it, but thanks for all your help so far. 73, W9PDS

RE: FL2100B untuned on 40 meters	Reply
by K4DPK on October 30, 2009	Mail this to a friend!
No, nothing other than to make sure the exciter is actually putting out power on 40m. It seems to me it has to be either a connection, a component or the bandswitch gripper, and these have to be the ones associated with L-203 and 40m. Remember if you change a component, you'll need to re-adjust L-203. Maybe someone else has some other idea? Phil C. Sr. k4dpk

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