Discussion in 'Amateur Radio Amplifiers' started by W2PPJ, Nov 3, 2017.

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  1. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    You didn't really answer the question with this reply.

    It doesn't matter what you did when you first purchased the amp, and it doesn't matter than you're using the FRB757 interface.

    Did you completely disconnect the RY and E wires from the back of the amplifier NOW? (That is, completely disconnect the FRB757 so there is nothing connected to the amp at all except the power cord for AC.) And with everything disconnected, is the amp still "keying" (relay clicks, idling current about 0.1A, etc)?

    If so, something inside the amp is shorting one end of the relay to ground.

    If not, then the problem is external to the amp and possibly the FRB757 has failed.
    WA7PRC likes this.
  2. W2PPJ

    W2PPJ Ham Member QRZ Page

    everything is DISCONNECTED and has been from the start. when i encountered this problem i disconnected EVERYTHING to start my diagnostics. it seemed logical . so now we know the short is inside go right to the relay and do an inspection? what am i looking for and where? nothing looks burnt blown broken etc etc etc. thanks peter
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Nothing needs to be burned out or broken. It's an extremely simple circuit.

    One end of the relay coil has 14Vdc or so on it all the time when the amp is powered ON, whether it's in standby or operate. The other end of the relay coil is normally ungrounded so until you switch the amp to OPERATE and also close the connection between RY and E on the rear terminal strip, it will also measure 14Vdc or so (same as the first end).

    With the amp switched to OPERATE and RY and E connected together, the "cold" end of this coil is grounded, so one end will have 14Vdc (or so) and the other end be at ground potential, thus activating the relay.

    If the relay engages when you turn on the amp and do absolutely nothing else, "something" is causing the relay to activate, which would mean "something" is grounding one end of the relay coil: Stray wire, a bridged connection at the STBY/OPR switch, a pinched wire, something. Doesn't mean anything burned out.
    WA7PRC likes this.
  4. WA7PRC

    WA7PRC Ham Member QRZ Page

    It's time to have someone else look at it.
  5. W2PPJ

    W2PPJ Ham Member QRZ Page

    im doing the best i can. its obvious i know nothing but i want to learn. i will just keep at it till i fix it there is alot of good people on here willing to help me. if i sent out everything that was or was ever broke i would be broke. not to long ago people at home before the internet people put these things together on a bench at home. i still think i can fix it with some help. elmer of the year award goes to wa7prc thanks for the support 73
    Last edited: Nov 5, 2017
  6. WA7PRC

    WA7PRC Ham Member QRZ Page

    Yep. Defective parts aren't always charred. They often LOOK fine.
    According to the schematic I provided in an earlier post, PL1 only indicates the unit is ON. If the unit was not ON, nothing would happen.

    One half of S4 (a DPST switch) operates PL2 (the lamp that indicates transmit mode). The other half of S4 operates the T/R relay and removes the cutoff bias to the tubes during transmit. IOW, PL2 operating correctly means little.

    Instead, a good tech will focus on what's happening on the OTHER side of S4. Something on the low side of RL1 coil is pulling it to/toward ground. All it takes is to follow that node, and start checking wiring and parts. R4, C208, and C209 are closest to RL1 coil.
  7. W1GCI

    W1GCI Ham Member QRZ Page

    I also have a FL2100B. It's a nicely built amp. I'd like to suggest that you do some simple resistance measurements - with the amp completely disconnected from power. The relay circuit is pretty straight forward and shouldn't take too long to meter out. Starting with all the switches off - power and standby. There should be no resistance from the RY connection on the back to ground. If there is, there's a short. Next, with the amp still unplugged and power switch off, engage the standby switch and measure the RY to ground again. There may be some resistance from the path through the bias pot and the relay coil, but it should not be zero. If it's zero, look there for a short to ground. Next measure the resistance across the bias pot. The schematic says 30 ohms, 10W. It's configured to go from zero to 30 ohms as the pot is turned. Note the resistance across the pot before tuning. Then turn the pot and see that it is moving from 0 to 30 ohms across it. After verifying that the pot is working correctly, rest the pot to the resistance value you first noted. That should be your proper bias point. Measure next from the relay side of the pot to ground. If it's ground, then your problem is in that wiring.

    The more likely fault is at the junction of the bias pot and the grids of the tube. If one of the caps at that point is shorted, then the relay will engage, even in standby and you will draw current. If you plug the amp back in, turn it on and observe the plate current - you said earlier I believe it was around 100 ma - then if there is a short from a bad cap of something at the wiper of the bias pot, the idling current will not vary as you vary the bias pot.

    Hope this helps,

    arnie - W1GCI
  8. W2PPJ

    W2PPJ Ham Member QRZ Page

    WIN_20171105_19_13_17_Pro.jpg WIN_20171105_19_13_07_Pro.jpg WIN_20171105_19_13_17_Pro.jpg WIN_20171105_19_13_07_Pro.jpg all my tests lead me to this small board located behind the tubes. slowly but surely . thanks for all the help. can i test everything on the board or will i have to remove everything. keep in mind im not a pro at this at all and im doing all i can do to make this easy for me and you. i willing to learn what you guys are willing teach me. w1gci arnie your post lead me to the back just like you said thank you very much.
    KA9JLM likes this.
  9. W2PPJ

    W2PPJ Ham Member QRZ Page

    is what i took a pic of c209 c208 r203 l210 im having a hard time with the diagram, i think im in the right spot.
  10. G3YRO

    G3YRO Ham Member QRZ Page

    You have a Multimeter, to measure resistance, right?

    So with the Amplifier off, do some resistance checks.

    The simplest one to do first is to measure between the RY (relay) screw terminal on the back and chassis . . . what is the resistance? Measure it with the OPERATE switch up, and then down.

    Report back with the results.

    By the way, people are telling you to adjust the "bias pot" . . . there is no bias pot !

    R4 is just a large green ceramic wirewound resistor with a clamp round it. This sets the bias, but in practice NEVER needs adjusting.

    Roger G3YRO
    Last edited: Nov 7, 2017

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