Ameritron AL-572 loss of HV reading question

Discussion in 'Amateur Radio Amplifiers' started by N0OHL, Sep 7, 2020.

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  1. N0OHL

    N0OHL Ham Member QRZ Page

    Your ahead of me --- need to watch your two new videos --- will report back in AM
  2. W1QJ

    W1QJ Ham Member QRZ Page

    Does this amp still not show any HV on the meter? Has the HV panel meter been checked? Do we know if the meter went open? Has an external meter been put on the B+ to check if there is in fact HV?
  3. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    While you're inside the amp measuring things, check that RFC1 isn't open. You should measure about 2 ohms from the bottom of the HV capacitor stack to chassis ground.

  4. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    I'm trying to have the OP avoid putting power on the amp open. With the cover and power on, he did move meter M1B by adjusting the ALC SET pot while the MULTIMETER switch was in ALC SET. This exonerates the meter movement and the +- 12 volt supplies.

    With the amp unplugged and open, he should hook an ohmmeter up to R26 (47k) HV meter multiplier while the MULTIMETER switch is set to HV and watch the meter movement deflect.

    I think it will. This would prove the HV meter circuit to be OK... working at least, if not accurate.

    After looking at the PS board schematics again on several Ameritron amps, I've been bothered by the drawing's lack of B- return. Here is a corrected schematic showing the B- return and the proper RF connections to the T/R relay.

    572 schemes_Page_1.jpg

    ... gonna study OP's earlier pictures again. I think RFC1 opened up during the arc event.
    Last edited: Sep 30, 2020
  5. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Pictures at 11:00.

    Make a couple of resistance measurements while you're in there. I made a couple of measurements while simulating my theory of what's going on in your amp.

    In a nutshell, tube arc shorts D17 and the current surge takes out RFC1. MULTIMETER selected to HV in this condition takes out C24.
  6. N0OHL

    N0OHL Ham Member QRZ Page

    OK -- May have something -- let me go thru tests.....
    1) left unplugged overnight - used stick - no charge --safe
    2) measured power diodes d4-D13- all look good .55 and open
    3) measured power resisters --look good-- r8-r9 750 ohms r10-r11 500 ohms
    4)Did meter check with HV selected in front and with diode check on meter - ground on r26 and pos on -meter OK (see video below)
    5)!!!! not sure if I understood the RFC1 check method... When I measure from the bottom of r26 I read 2.3 ohms to the cabinet.
    6) when I measure RFC1 itself.. I get .3 ohms when going into RFC1 and out to c7 and open when going other direction -- not sure what this means
    7) wanted to mention d17 tested good after I lifted it out of board and tested it
    8) also wanted to ask about C24 which you indicated may be bad ?

    Here's a video of the meter test

    Thanks again for all this help !!!!!!!!!!!!

    Attached Files:

  7. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    OK, take a soldering gun and lift one side of RFC1 and measure it with an ohm meter. It should measure as a short (0.3 ohms with meter leads).

    I'm sorry for my Brain-O-Graphic error, C34 was the one that blew it's bottom out waaaay back in an early post. That's the -12 volt supply filter capacitor.

    The R/G lead from the HV transformer secondary is used for testing as described on W8JI's website. The Red and Red/Yellow leads are hooked up as they should be.
  8. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    You are welcome. We're trying! :p
    I couldn't play the video link.
    Basically, I want to know if the HV MULTIMETER circuit is working from R26's point of view.
    If you put an ohm meter across R26 and the MULTIMETER switch on the front panel is set to HV, you should be able to deflect HV meter on the front panel.

    Use Rx10 on an Olde Fashioned ohm meter, or Diode Check on a new fangled DVM. Black lead near the big caps, red on the other side of R26. If the meter moves and the resistors measure OK, you should have HV meter action. If the meter doesn't move, switch the leads and try it again.

    At this point, this problem is starting to look a bit odd to me. If you lift one side of RFC1, you should measure 47k ohms from the bottom of the HV cap stack to the chassis. We're looking for an open B- line (which RFC1 completes the B- circuit to the shunt resistors).

    It has to be something here. I doubt the transformer secondary is open but you could measure resistance between the Red and Red/Yellow transformer wires and see perhaps 100 ohms. If this winding is open, that would explain everything!
  9. N0OHL

    N0OHL Ham Member QRZ Page

    Looks good :

    1) Meter works when low voltage(diode check) connected around R26
    2) RFC1 lifted -- 1.5 to 2 ohms measured thru RFC1
    3) with RFC lifted Bottom of HV Cap ( C17 or R26) to chassis OPEN
    with RFC1 connected 3.7 OHMS ( should be 47 ohms ?)
    4) **** Resistance between RED and RED/Yellow wires OPEN **** no connection No OHMS !!!!! RIMG0006.JPG RIMG0007.JPG ****
  10. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Did you dig the test lead points in when measuring the Red and Red/Yel wires? If so, the HV winding or a termination at the transformer is open. We'll send up a smoke signal to one of the guys here who fixes amps for a living to see if he's ever seen this one with this series of amps.

    Follow the wires from the board to the transformer and see if it's broken anywhere. The transformer has a termination strip. Check the connections there. Measure each wire from the terminal strip on the tranny to the board. If one of those wires popped off of the terminal strip, you should be able to resolder the connection.

    Whaddaya think, Lou @W1QJ?

    Solder everything else back in where it goes. Fix the bad termination and you're golden. If you measure open right at the transformer's terminal strip, you may have an open winding.

    I think you're closing in on the problem. It's always the last thing you look for! :p
    Last edited: Sep 30, 2020

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