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Drats! Bad modulation transformer BW-5100

Discussion in 'Amplitude Modulation' started by W1BR, Jun 9, 2020.

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  1. W1BR

    W1BR Premium Subscriber QRZ Page

    What is my best bet to replace a bad modulation transformer in a BW-5100? Apache or DX-100
    parts mule? Just fired mine up and discovered the primary winding was open.

  2. W1NB

    W1NB XML Subscriber QRZ Page

    Or a Viking 1 or Viking II. The Apache used EL34s. Looks like the P-P load R might be a bit higher and max Ip a bit lower. I would think a 32v-3 would also work well. I’m not sure if the V2 and V1 used 807s.
  3. N2DTS

    N2DTS Ham Member QRZ Page

    I have one out of a viking 2 if that would help.
  4. W1BR

    W1BR Premium Subscriber QRZ Page

    That would likely work, but a friend thinks he has a parts mule 5100B in his storage unit. I'll let you know either way..
    he was going to check to verify whether he has it or not.

    Thanks for the reply!
    KP4SX likes this.
  5. K5UJ

    K5UJ Ham Member QRZ Page

    Those smaller mod. transformers may not be extremely expensive to have re-wound. There are videos on YouTube of guys making their own transformers. They get the lams, a plastic bobbin and wiring jig, spool of enameled wire, some insulation tape and shrink wrap and get to work. There are computer programs that give you the number of turns for whatever you want to make. Interesting viewing.
  6. W1BR

    W1BR Premium Subscriber QRZ Page

    Mike KE0ZU has offered me a replacement from a B&W 5100B parts mule hiding in his tractor shed. I think I got lucky.

    W1NB likes this.
  7. W1BR

    W1BR Premium Subscriber QRZ Page

    The replacement modulation transformer from a deceased 5100B arrived from Mike, and I did some small physical changes to get it mounted on the 5100 mod deck.
    I was able to fire it up last night and do some checks. First, almost all of the tubes are NOS replacements. I ran some quick checks using an audio generator and scope
    to check the modulation depth. The results were a bit disappointing.

    The modulator struggles to make 100% positive modulation, and there is some distortion on the peaks when 100% is reached. This is with the transmitter loaded
    to factory manual specs. Lowering the RF and plate current improves matters; but the modulator does not seem right. It is pretty hard to scope the stages due to
    the modular system used in the B&W, but that will be my next step. Have any 5100 owners run modulation checks who may have seen what I am seeing, or
    have seen suggested modifications that will improve the lack of mod power? I'm sure if the 6146's are running out of scrot, or if the problem is in the
    6AQ5 driver stage. I suspect the issue is when going into AB2, but haven't confirmed it.


    new rion.jpg
  8. W1BR

    W1BR Premium Subscriber QRZ Page

    Just tried pulling the 6U8 and injecting a signal into pin 9, which feeds G1 on the 6AQ5. Perfect modulation, except the lows roll off faster than I would like to see. Might be the cathode bypass on the driver?

    Running a high transconductance 6U8 that is tamed down using a filament dropping resistor seems to be a bit odd, I am tempted to go over to a simple dual triode? 6U8 tubes are cheaper, but the extremely
    low plate currents they are running those stages at (1/2 mA) also seems odd.
  9. W1BR

    W1BR Premium Subscriber QRZ Page

    Found it. RCA jack had a high resistance contact off of the mike gain control. I added RCA connectors to the flying shielded modulator leads that were previously soldered to the mic connector and mike gain pot to make removal of that deck easier. Self inflicted injury. Modulation is fine now, but I still give a bigger edge to my modified Ranger I. Now to figure out how to tame the excessive drive conditions on all bands due to the high line voltages.
  10. K5UJ

    K5UJ Ham Member QRZ Page

    There are a number of possible problems. Obviously if it is easily clipping the carrier then the problem might have to do with audio phase, but I'm guessing you're feeding a sine wave into the rig and it's symmetric up and down and not getting to 100%.
    Check: bias voltages (bias supply), look at the screen v. on the modulator tubes (807s? I am not extremely familiar with that rig).
    Check the h.v. and low B+ voltages under load.

    If the h.v. is okay but the carrier power is low, the final may not be getting enough grid drive or the screen v. on the finals is too low.

    The final PA has to have enough grid drive, screen and plate v. to sustain the sine wave at peak positive. Try new final PA tubes. Maybe the heaters and cathodes on the old ones are tired and not giving enough emission. The heater v. may be low. Some of this applies to modulator tubes. Try a different pair of 807s.

    If possible get rid of those POS ceramic plate grips and use some spring clips like National Grid Grips if you have any. Do this before you tear off a plate cap by accident.
    Here's a source for useful substitutes: take a look at for example (CLP) LF.25. It might fit an 807.
    I appologize: I just read that you have NOS tubes.

    I'm assuming you're looking at the modulation envelope with a scope when you wrote that. I'd focus on the final PA and look at its voltages and make sure it can put out full power. Put it in CW and see if you can load it to full CW output power.

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