Alpha 374A or 76 series power switch

Discussion in 'Amateur Radio Amplifiers' started by AD0AC, Sep 8, 2019.

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  1. AD0AC

    AD0AC Ham Member QRZ Page

    I have an Alpha 374A on the bench that our club received from an SK estate. It will not power on and I’m wondering if the power switches are bad. The fuses are all good and the tubes are not shorted. When I push the On button, the switch does not latch in the pressed position. The Off button does the same thing. The other switches in the switch bank all latch in position. I need to know if this is normal operation for these switches. If not, I will replace the switches, if normal, I guess the next step is to check the relays and timer circuit.

    The amp has definitely been run hard. There is a nice burn mark on the 80 meter coil, so that may need to be rewound at some point. RF Parts still carries replacement PS caps, so that may be an eventual rebuild as well. Right now I just want to see if it will turn on at all.
  2. NU4Y

    NU4Y Ham Member QRZ Page

    The power switch is momentary "ON" and "OFF". Switch throws relay.
    There is a 1.25 amp fuse "F3", to the left of the Plate Current meter, if it is open no power on.
    AD0AC likes this.
  3. W1QJ

    W1QJ Ham Member QRZ Page

    Yup, it is a press on, press of circuit. Probably a latching relay circuit. Those amps were prone to having trouble with the contactor on/off relay and the SS relay. Check for open SS resistors.
    AD0AC likes this.
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The switches (at least on my old 374) are Schadow switches, made in Germany. Very high quality switches and they're not switching much current at all so they normally last forever; but the colored flag indicator buttons don't.

    The "colored flag" buttons are mechanical, and in front of the switch mechanism itself; they turn from red to green or whatever (2-colors) when you push the button to activate the switch.

    Those used to be really common 35-40 years ago but I haven't seen them on the market at all lately -- might be difficult to find, if you need one.
  5. W9GB

    W9GB Ham Member QRZ Page

    Steve is CORRECT.
    I refurbished an Alpha 76PA in 2005 (knew all former owners) and “flag” buttons were Rare.

    Schadow was first acquired by ITT. C&K acquired the ITT Switch Division in April 2000.
    Schadow Colored Flag buttons, 7/8” square, were popular in broadcasting, military, and electronics from 1960s to 1980s. This BUTTON Size was discontinued about 20 years ago.
    C&K makes a smaller sized “flag” button (~5/16” square) today, Style F07

    The Switch Mechanism itself, the C&K “F-series” was stocked by Alpha (spare parts) and
    a popular production series (current) by C&K.

    F-series C&K Push Buttons

    C&K Components
    15 Riverdale Avenue
    Newton MA 02458
    Tel: +617.969.3700
    Last edited: Sep 8, 2019
  6. W9GB

    W9GB Ham Member QRZ Page

    START with the Alpha 374A Product Page
    I think Glenn Pladsen (Service) is still at Alpha, for your questions.

    ONLY 366 units of the Alpha 374A were produced from 1/1979 to 3/1988

    ALPHA 374A HF Amplifier MANUAL

    Replacement Bulbs: Readily Available from Mouser or Digi-Key
    BULB CM85 : 28V G.E. (PANEL METERS) (Vertical mount)
    Replacing the Mallory (now CDE) Aluminum Electrolytic Capacitors (SEMS Mount) : EASY.
    ALWAYS Double-Check : Height, Diameter, and Terminal Spacing (Screws) for
    CORRECT FIT of Replacements.

    Mallory / CDE “CGS” series, 210 uFd, 450 VDC, Part number: CGS211T450R4C
    The Part Number for those HV Caps can easily be read on the Side of the Capacitor Bank.
    Early Alpha models (76A/374A series) used 180 uFd, but the 210 uFd size is standard and used on later models.

    Harbach Electronics (sells in sets of 6)μf-450vdc-filter-capacitors-with-sems-screws/

    Digi-Key PN: 338-1248-ND
    CDE: CGS211T450R4C CGS211T450R4C
    Last edited: Sep 8, 2019
    AD0AC likes this.
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I never had a 374A, but did own a 374 for several years. That was a 3-tube model; I think the 374A is only two tubes, but otherwise very similar.

    In the 374, the power transformer "plugged in" via mating connectors on power cable harnesses. If the plug/socket connection failed in any way, or was simply unplugged, no power.
  8. AD0AC

    AD0AC Ham Member QRZ Page

    Thanks everyone. I’ll post some pictures of the amp later. I pulled the transformer and HV board out so I could get to the back of the switches and check out the relays. Those all connect with quick connectors. I’ll see if I can clean up those connectors a bit and cycle the relays without power on. The main contactor doesn’t seem to be mechanically frozen, so I’ll test the coil too.

    I just wanted to check the operation of the switches before going that far. If they’re just momentary for power, then those should be fine. I also pulled the tubes just to run basic continuity tests, and they tested fine for that.

    We have an original 374 at the shack that’s been running since about 1976 on the original tubes, three 8874s. This amp would give us two spares at the very least, but I’d like to get it running if economically feasible. It’d give us a whole spare amp and even a second operating station with the Pro III that came with it.
  9. AD0AC

    AD0AC Ham Member QRZ Page

    Well, the switches definitely work. I was able to remove the transformer, power supply board, capacitor bank, and the HV relay to get to the switches and SS relays. I continuity tested the switches and they worked fine. The relays are a different story. The 120 VAC coil relay works when I apply 120 VAC to the coil contacts. All of the 24 VDC relays will not cycle when I test them with 24 VDC, including the second SS relay, HV relay, and the TR relay. I have a feeling the answer is going to be an LV supply malfunction sent excess voltage to the relay coils.

    The capacitor bank is toast. All of the caps had signs of overheating and were leaking from their vents. The 80 meter coil has a large burn from some serious overheating. This amp was run hot and hard.
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Well, it's also pretty old! I had a 374 "new" in the late 70s, so that was >40 years ago. The 374A is slightly newer, but still old.

    Sounds like you're on the path for de-bugging and will get it going. Hopefully the tubes are still good...if not, they're expensive! "Not shorted" doesn't mean much with 8874s; usually it's grid damage that kills them (overdrive or improper loading, or both). If not abused, they can last almost forever.

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