Gates of Hell - Posting for K3FEF.

Discussion in 'Amplitude Modulation' started by KA4KOE, Jun 7, 2017.

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  1. KA4KOE

    KA4KOE Ham Member QRZ Page

    Since Michael K3FEF doesn't seem to visit QRZ often, he emailed me with an issue with my former BC1-T, "The PhiliBuster". We discussed his issues in initial power up. The transmitter has low hour RF Parts 833As in the RF deck and a pair of 833C's in the modulator. He has tested all of the iron and it is good, per him. I brought the transmitter up initially on low power (1/2 plate voltage) with no issues except a bad 833 in the final was heating up (one of the originals that came with the transmitter). The RF driver board looks rough. I advised him to check/replace the components on that board as needed. He tells me that the tank circuit has already been adjusted to resonance. I advised him to check his neutralization and ALL the parasitic suppressors as well as the phasing on the feedback network.

    Here is his communication:

    "I’m having no luck bringing up the HV (he is on full Plate Voltage not reduced) – it immediately goes off on overload (as in a quick click of the relay!). I see the meters jump up, but the plate current meter also jumps real high. At first I thought I just did something real silly so I re-checked all my wiring but it all looks fine. Then I looked at the overload circuit to determine possible causes – and there are a few possibilities but the most obvious was to check the grid. Right away something isn’t right – I found (with LV contactor on, HV contactor off) -350V at the grid (yes, negative). And I can’t get grid current below ~225mA. So I’m thinking RF IPA somewhere but I could be wrong. The manual (under “Typical Voltage Chart”) shows Bias Volts = 360 DC. Did they leave out the minus sign? Am I barking up the wrong tree? Did you have the HV fully functional and coming on? Any thoughts?"

    I'll pass your theories on to Michael and maybe he'll get off his duff and visit us here on QRZ. I gave up on Buster as it had too much broken and required more patience than I had on reserve.

    Thanks.

    Fleep
     
  2. K3FEF

    K3FEF Premium Subscriber QRZ Page

    Well that's no fair! I was having fun troubleshooting this! :)
    More info: With one of the HV rectifier caps off, the contactor holds in without tripping the overload relay. However, it's not right.
    Plate current tops out at 625 mA, HV drops real low to about 450V, and the grid current is also real high at about 260 mA (with or without HV). Disconnecting the caps on the RF driver 807's gets things into normal ranges but of course there's no drive.
    Still working on it and testing - but I'm going slow so as not to make any mistakes. :) You know, BAD mistakes.
     
  3. KA4KOE

    KA4KOE Ham Member QRZ Page

    Now why would the B+ drop that much?
     
  4. KA4KOE

    KA4KOE Ham Member QRZ Page

    Tanya knock you off the tube anodes with a 2x4 type of mistakes?
     
    KD2ACO and K3FEF like this.
  5. K3FEF

    K3FEF Premium Subscriber QRZ Page

    Well, with only one rectifier going, I'd expect around 900-1000. But it's drawing a lot of current, and only have wave rectified, so the voltage is coming WAY down. Disconnecting the grids on the 833A's actually leaves me with about 1200V on the plate. SO....I think I have some oscillations going on. Time to move the scope up from the shop. I'll be able to troubleshoot more after work.
     
  6. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    With one rectifier plate disconnected, the half-wave rectified DC can be expected to drop drastically with a load on the power supply. Does the grid current reading change when you tune the PA grid/driver plate tank circuit? The 807 current should be around 200 mA, and the grid current around 150 mA. If you are getting 260 mA at the operating frequency, that's an awfully efficient driver stage, so something else is likely going on. Is the grid leak resistor for the 833As the proper value? What about the bias voltage on both the 833A and 807s? 807 plate and screen voltages?

    625mA is awfully high for running the transmitter at half voltage on the plates; that's more like the normal plate current at full +HV adjusted for 1000w output. Are you sure the final is tuned to resonance? Try rotating the Plate Tuning control to see if the current varies. You should get a sharp dip at resonance. If adjusting the tuning coil from one end to the other has no effect on plate current, then it is not tuning to resonance. Either a defective component or the tubes are getting grid drive at some frequency far removed from the operating frequency.

    One way to test at low power with good HV regulation would be to temporarily re-connect one of the primary leads to the HV plate transformer to neutral, instead of to the other leg of the 240v a.c. mains. That's the way the rig is run at 250 watts if equipped with a 250/1000w switch.

    One modification I would highly recommend would be to lift the a.c. neutral off the transmitter chassis/cabinet. Grounding the neutral to the cabinet can cause problems with the transmitter, and besides, it's potentially a safety hazard to anyone working on the rig with power on, and is a violation of present-day electrical codes.
     
  7. KA9JLM

    KA9JLM Ham Member QRZ Page

    Is this third party traffic ? o_O
     
    K3FEF likes this.
  8. W7TFO

    W7TFO Platinum Subscriber Platinum Subscriber QRZ Page

    Check the disc bypass cap next to the 1" dia. choke on the 833A shelf....

    73DG
     
  9. K3FEF

    K3FEF Premium Subscriber QRZ Page

    I'm definitely making headway here. I have everything back into normal ranges (a few issues on the driver board resolved) EXCEPT for the final grid current which is still way to high. With one rectifier disconnected, I've got 550 mA plate current and about 200 mA grid current. That's way out of line. Plate should be around 200 mA!
    Now, regarding tuning, I've tuned it OVER and OVER. LOL. But with the HV off and while tuning, I can't get the grid current down into the range they say I should be able to hit (120-160). So I've used a grid-dip meter and then checked it using my antenna analyzer backwards into the top. Neutralization is a bit weird. It's not doing what I'd expect it to do. The dips don't seem as big as I'd expect - barely noticeable actually - which I would think is good if it was set right. But I can't mess with the neutralization enough to see a big dip. That seems weird.
    W7TFO - Are you referring to this cap? I've got 650V on the driver side and about -350 on the grid side. That seems normal and fits with the voltage chart in the manual. Though I agree it could be suspect. I took it out though, and it's not shorted, but I'm only using a VOM. That's not going to reveal a leak at higher voltages.
    [​IMG]
     
  10. K3FEF

    K3FEF Premium Subscriber QRZ Page

    Now that I've got meters reading a bit more normal I'm going to go through again and take a new set of measurements. I'll report back soon. :)
    And I like the idea of running the HV transformer at 120V for now. I might do that first. (The lower power switch and contactor were all bypassed to run on high all the time).
     

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