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Power/Standby lights for Peavey Alpha Bass

Discussion in 'Amps and Cabs [BG]' started by Bruiser Stone, Apr 27, 2019.


  1. Bruiser Stone

    Bruiser Stone Supporting Member

    Dec 7, 2017
    Tennessee
    The power and standby lights recently went out on my Alpha Bass tube head. Everything else functions fine, but I ordered another couple of sets of lights from Peavey. Trouble is the new ones don’t work either, so I suspect something else is amiss. I have almost no electrical know-how, but a couple of friends of mine who do are very busy, and after a few weeks of waiting, I’m ready to roll up my sleeves and figure it out for myself. I’m sure a few of my fellow TB’ers could have this fixed in no time, so I thought I’d ask.

    Below are pics inside and out of the relevant areas. The wires from the lights run to the 4 pin connection. The second pic is before installing the new lights with a shot of the board under the connection, and the third is after installing the new lights and connecting the wiring. I’ve found a detailed Alpha Bass schematic off a forum and would be happy to email (it’s a large PDF) if anyone wants to take a look at it. I will be indebted to whoever can help me with this.
    A45113EB-9310-48C6-82F8-44508039045F.jpeg
    18944708-09B3-4D73-B971-B478635A88D2.jpeg
    4DB6424F-A9CC-491B-848C-4A6D44358E1F.jpeg
     
  2. Wasnex

    Wasnex

    Dec 25, 2011
    It took me awhile but I found the controlling circuit on the schematic. Look on page 3, schematic 1 of 3, grid coordinates B5. There is a note that says "TO LEDS". Since both LEDs have mysteriously stopped working, I suspect the problem is either a broken ground or the voltage is not getting to the terminal strip.

    I believe this is the way the circuit works...but I am an old hack tech so I could be wrong. When you turn on the power switch, a low level AC is supplied to J47 and J48 (B5). Basically this AC should appear at the terminal strips where both leads are wired in. There should also be a ground on the Power lead (applied through J58), so it should light. When the Standby switch is closed, (see switch at C7.5) a voltage is applied to the base of Q1 (B6.5). Q1 applies a ground to J57 and turns on the Standby LED.

    With an ohmmeter, you could check for the ground on the Power LED. With a voltmeter you could check for the low level AC when the power is turned on. After that it's a matter of tracking down the wires for either the missing ground or missing voltage. Good chance it's simply a broken solder joint somewhere in the amp...or something similar.

    There are lethal voltages present in the amp, so it's would be advisable to take the amp to a tech. It should be an easy repair, although removing and replacing the circuit cards may require substantial bench time.
     
    Big Shrek and Bruiser Stone like this.
  3. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    I believe it's even simpler than this, but I suspect that the LED is not a standard LED but one with an additional blocking diode built in (or rated to block ~25 volts, which most are not).

    With a scope, you should see ~20V AC at the left side of R29 & R31. If you don't have this, you won't have lights. Could also be the return ground, but with all those old style PCB connectors, it wouldn't surprise me if the problem was within the connectors.

    Disclaimer... you need to be careful poking around in there. It's always a good idea to ask for the number to 911 before you need it. ;)
     
    Bruiser Stone likes this.
  4. Bruiser Stone

    Bruiser Stone Supporting Member

    Dec 7, 2017
    Tennessee
    I’m glad you two were the first to respond. With the information you both provided, I could figure it out eventually, but unless either of you think the problem could be detrimental to other components apart from the inconvenience of not having lights (in the meantime I may mark the top of the switches fluorescent so at a glance I’ll know “off/standby”), I’m not sure its worth paying bench time or risking grill marks.
     
  5. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    My experience is that getting the amp apart and troubleshooting can involve a bit of time, though it's not complicated electronically, it is somewhat time consuming and there's the risk of causing more damage. Those adapter "bracket board" PCB's are a great way to assemble something the first time, but are more time consuming to troubleshoot if the signal passes through or around them.
     
    Bruiser Stone and Wasnex like this.
  6. Wasnex

    Wasnex

    Dec 25, 2011
    The actual repair should be easy, but it might involve quite a bit of disassembly which may be relatively difficult...can't say for sure.

    If the amp is otherwise functional, you may consider leaving it alone. There's always risk of causing unintended damage when you disassemble an old amp. That being said, I would eventually attempt to fix it, so YMMV.
     
    agedhorse and Bruiser Stone like this.
  7. Wasnex

    Wasnex

    Dec 25, 2011
    @Bruiser Stone please look at page 4, top image, lower right quadrant and find the text "J40 holes". Above this is where the LED circuitry appears to run. If you go straight up, two traces can be followed. I believe Q1 is at the right edge of the board just beyond the midway point going up. This is a guess, but the right trace is the return through Q1 and the left trace should be the AC. If you have access to this part of the board, you could take some readings with your multimeter. Additionally, R78 and R79 are centered (left/right) in the upper part of the image between V3, V4 and the main capacitor banks (C3, C3, C5, C6). You can follow a trace from R79 over to a termination point by Q1 and R36 on the right side of the image.

    Based upon this, I suspect the Power LED gets it's ground locally. So I think the problem is most likely continuity along the AC circuit.

    Hopefully this gives you enough markers to figure it out.
     
    Bruiser Stone likes this.
  8. Bruiser Stone

    Bruiser Stone Supporting Member

    Dec 7, 2017
    Tennessee
    I really appreciate you going the extra mile. I will at least wrap my head around it, but I may leave the actual intervention to my retired-Marine/radar tech buddy when his schedule simmers down.
     
    agedhorse and Wasnex like this.
  9. Primary

    Primary TB Assistant

    Here are some related products that TB members are talking about. Clicking on a product will take you to TB’s partner, Primary, where you can find links to TB discussions about these products.

     
    May 11, 2021

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