"Gene McCalmont" <[EMAIL PROTECTED]> made an utterance to the drakelist gang ---------------------------------------------------------------------- Hello Joe; As you know, the L4B is a fairly straight forward grounded grid linear amplifier design. I've used several of these amps, including an L4 with 3-400 tubes, for many reliable years. As you suspected, the excessive grid drive is indicative of other problems and I would suspect from your description that the root cause is the lack of HV applied to the tubes, rather than the RF deck itself. The lack of HV applied to the tube plates will cause the grid to conduct excessive current. Although there are other failure modes that can present the same result, lack of HV is the most common. Please be very careful testing your amp as excessive grid will ruin your 3-500Zs. Of course, I'm sure you already know most of this.
I would suggest that you check these things in order of the most simple first: 1. Do you have the HV connection to the back of the RF deck firmly fixed in place. Is this cable and connector in good shape? These components will age and although they may look good from the outside, there may be hidden problems. If all looks good, proceed to the next step. 2. Remove the top cage of the L4-PS (check your user manual for proper instructions) and inspect the "fuse resistor" (a small .82 ohm component) on the deck's top side, to determine if the resistor is OK. This resistor is mounted to an insulated post on the top of the PS and is not easily confused with the big "power dropping resistors" mounted directly to the PS transformer. Also check to confirm that the HV cable to the RF deck is properly soldered. The resistor is a common source of problems for the L4-PS and will deteriorate with age. It is designed to "self sacrifice" should the HV short in the cable or at the RF deck. If it is cracked, burnt, blistered or slightly swollen, replace it. Replacement resistors are readily available and this is an easy job. The HV cable is generally very stiff and the wire within does not take solder readily. If the PS has been worked on, it is possible that the cable has become separated from the HV supply. If everything looks good, proceed to the next step. 3. Visually inspect the diode "daughter boards" to determine if the diode stack was properly replaced and installed. It is very easy to mount these daughter boards "backwards" such that the PS HV is actually cut off on one or each cycle rather than conducting. Check the circuit diagram for your PS to determine how the cathode should be connected on each bank. It is a common failure mode to connect these daughter boards with opposite polarity. You may have to remove the two capacitor boards to adequately inspect the diode boards. If everything looks correct, proceed to the next step. 4. Visually inspect the two banks of HV capacitors for problems such as electrolyte leaks, swelling, any unusual discoloration, burn marks from excessive heat or other signs of deterioration and open leads or cold solder joints. Problems with the HV caps is the most common failure mode for the HV power supply and they are difficult to replace because axel lead HV caps of the proper size and rating are not usually available. Any observed problem means that all HV capacitors must be replaced. There is a thread on this list that addresses a new HV capacitor replacement board that would be worth looking into, however, the replacement job is not complicated and there are many threads in the list archives that explain how to do this job. If everything looks correct, proceed to the next step. 5. Walk to the fridge and open the door by grasping the handle usually located on one side of the door opposite the hinges. Pull firmly and swing the door out of the way. Inspect the contents of the fridge and find a small container, usually brown glass, narrow at the top and with a bright label affixed to the side. Remove the container from the fridge and secure the fridge door by pushing it shut. Locate an appropriate opener for the container and pry the top closure loose from the container to expose the contents within. Note: your container may be different and require a different opening procedure. Please consult the instructions specifically written for your type of container for proper procedures. Find a comfortable place to sit and enjoy the contents of the container with perhaps a nice cigar. Just maybe you will think of something that will solve your problem. Best regards; Gene W5DDW Gene McCalmont 270 Oak Ridge Ln. Argyle Texas 76226 -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Joe Roth Sent: Monday, November 07, 2005 7:01 PM To: drakelist@www.zerobeat.net Subject: [drakelist] L4B help "Joe Roth" <[EMAIL PROTECTED]> made an utterance to the drakelist gang ---------------------------------------------------------------------- I was fortunate to find an L4B to complete my Drake C-line station. The tubes are supposed to be fairly new. The power supply just had diodes replaced. I carefully (slowly) turned the amp on and everything came quietly to life. Those 3-500Z's are the prettiest things next to a Christmas tree! Here is the trouble. I tried to drive it with 20 watts (T4XC). The grid current is off the scale. A mere 10 watts drive will bring it to full scale. Plate voltage and amps read zero no matter what. The amp's T/R relay engages but no RF out at all. I tried this on 20 & 40 meters with the same result. What should I check next gang? Bad tubes? Bad L4-PS? Thanks!! Joe, N4ARI ---------------------------------------------------------------------- Submissions: drakelist@www.zerobeat.net Unsubscribe: [EMAIL PROTECTED] - unsubscribe drakelist in body Hopelessly Lost: [EMAIL PROTECTED] - help in body of message Zerobeat Web Page: www.zerobeat.net - sponsored by www.tlchost.net ---------------------------------------------------------------------- ---------------------------------------------------------------------- Submissions: drakelist@www.zerobeat.net Unsubscribe: [EMAIL PROTECTED] - unsubscribe drakelist in body Hopelessly Lost: [EMAIL PROTECTED] - help in body of message Zerobeat Web Page: www.zerobeat.net - sponsored by www.tlchost.net ----------------------------------------------------------------------
