AL 811H intermittant.

Discussion in 'Amateur Radio Amplifiers' started by K5WP, Dec 22, 2010.

Thread Status:
Not open for further replies.
ad: L-HROutlet
ad: l-rl
ad: Left-2
ad: l-BCInc
ad: Subscribe
ad: L-MFJ
ad: Left-3
ad: abrind-2
ad: l-gcopper
  1. K5WP

    K5WP Ham Member QRZ Page

    When I run the amp every few days, all is well. Power on, switch meter from Ep to Ip, and plate and grid mA=0 in standby. If amp is unused for week or two, I will show irratic Ip about 75 mA in standby which reduces to 0 mA in about 15-20 seconds. Then all is well again. Not aware of any arcing.
    Visual inspection of tubes and internals revealed nothing except wire to plate choke was connected but unsoldered. Amp over a year old but above irregularity fairly recent. Tube getting leaky and self healing? Thoughts?
    Thanks, Glenn
  2. K5DBX

    K5DBX Ham Member QRZ Page

    Front panel switch would be my first suspect. When you see this anomaly, touch the switch, or flip it's position and watch your meter. Second, would be the 1A diode for meter protection. (D16) These are the first 2 things I would investigate.

    (Ameritron says check it by putting the radio in SSB and keying it up on low power and see if the grid meter moves, if it does D16 is bad.)
    Last edited: Dec 22, 2010
  3. W8JI

    W8JI Ham Member QRZ Page

    I designed that amp.

    It can be three things.

    1.) The HV / plate current switch can be bad. Flip it back and forth several times when acting up and see if it cures it while it is acting up..

    2.) It can be a shorted tube, filament to grid. This would also cause the grid meter to deflect slightly negative. See if the grid meter goes negative.

    3.) make sure the PS board screws are all tight.

    It can't be a diode with the symptoms of what you described.
  4. K5WP

    K5WP Ham Member QRZ Page

    Thanks for responding David and Tom. The PS screws were tight and the grid meter doesn't deflect. I do toggle the switch when the problem occurs but can't say that is it. The thing that stumps me about suspecting the switch is time dependancy (phantom or not). Only occurs after one-two week layoff, not on 2-3 days use, and goes away in 15-20 secs of power on. Needle wobbles around 75mA and slowly goes to 0 as I watch (still in standby). Too much regularity to my irregularity. I'll have to wait about a week before I can try another power on test.

    (edit: Mic key on ssb doesn't produce grid meter deflection)
    Last edited: Dec 22, 2010
  5. W8JI

    W8JI Ham Member QRZ Page

    It is **almost** certainly the meter switch. The original switches used had much better contact resistance than the newer switches. Like all things, they have moved off shore and contact materials are not as good.

    It could be something else, but not likely something else if your description is accurate.

    Disconnect the cables and CAREFULLY lay the amp backward on a thick cushion, and get a tube on a can of WD40. Carefully and slowly flood the switch from the front panel side with a little WD40 while clicking it back and forth many times. This will wet the contact paste inside the switch and wipe the contacts clean.

    See if that cures it.

    73 Tom
  6. K5WP

    K5WP Ham Member QRZ Page

    I'll certainly work on the switch next occurance Tom, and maybe aquire a spare. Thanks for the expert help. I was looking at the online Ameritron schematic yesterday and noticed the simplified single relay circuit redesign along with new part numbers for most components. I'm an avid believer in simplification for troubleshooting and access. Do you know if the redesign was primarily cost cutting, and/or early component failure reduction? I'm curious if the older circuit that I have provides greater protection whether really needed or not.
  7. AE5JU

    AE5JU Ham Member QRZ Page

    I'm glad the OP posted this question. My AL-811H has just begun to do the same thing.

    No negative voltage, but flip the switch back and forth a few times stops it, and voltage returns to 0.

    Tom, I have some Corrosion Guard. Would this be a good product for the switch?
  8. W8JI

    W8JI Ham Member QRZ Page

    I'm not sure what they are doing with relay systems in that amp now. All of the changes I'm aware of are because the open frame relays originally used have gotten too expensive, so the trend is to use two enclosed relays instead of one open relay.

    The last mods I was involved in are similar to these:

    The grids are now grounded directly without resistors and capacitors, and gas voltage clamps are added to the filaments like shown in the 572 link.

    I hand drew that schematic in the late 80's. My how time passes!!

    73 Tom
  9. W8JI

    W8JI Ham Member QRZ Page

    I don't know. All I know is WD40 works, and has a long repair life!!
  10. K5WP

    K5WP Ham Member QRZ Page

    If the new schematic is correct, looks like they retained the R-C off the grid. They went to an open 3PDT relay, bridge rectifier off the 12V tap, eliminated the Q1-Q2 relay circuit and filament center tap diode string. Sure got rid of a lot of parts. From all the part # changes, I'm guessing new suppliers.
Thread Status:
Not open for further replies.

Share This Page