A little repair help please: diagnosing bad component

Discussion in 'The Projects Forum' started by Veracohr, Jun 29, 2011.

  1. Veracohr

    Thread Starter Well-Known Member

    Jan 3, 2011
    A friend of mine has an old video game system called Vectrex that hasn't been working lately and I told him I'd see if I can figure out what the problem is and fix it.

    I think I've narrowed it down to a power supply problem. When I had it open and was checking various points with the scope, it turned on a couple of times but most of the time it wouldn't. I found that when it wasn't working, both the input and output of the LM340 positive voltage regulator were around 1-1.2V. It should be 9V in and 5V out. The times it turned on correctly verified this.

    So my initial thought was a bad regulator, but I want to make sure before I go desoldering things. I'm unsure because the while the 5V regulator output goes to power the logic board, the 9V input is branched off to power things like audio opamp, video circuit and CRT. I don't know if something down that 9V path might be the problem.

    The power board schematic is on page 32 of this manual:


    What do you think? Should I try replacing the regulator?
  2. #12


    Nov 30, 2010
    For other helpers, the PS drawing is on P32.

    First answer: No

    Work on why the regulator is only getting 1.2 volts from the rectifiers.

    Those regulators are picky about their capacitors and capacitors normally fail from being old. I say start with the capacitors around the regulator chips.
  3. KJ6EAD

    Senior Member

    Apr 30, 2011
    In addition to electrolytic capacitors; connectors, fuse holders and any moveable control cables are all likely failure points. Sometimes on games of this sort, a +5 to ground short in the player controls will cause an intermittent symptom like you've described.
  4. SgtWookie


    Jul 17, 2007
    Along with old caps being problematic, solder joints can become cracked over time due to heating and cooling causing physical stress on the connections, particularly on PCB's. Cracked solder joints can be very difficult to see without a microscope, and are a very frequent cause of intermittent operation. When caps go bad, they usually cause a "hard failure" - that is, the thing just stops working completely. Cracked solder joints cause intermittent operation.

    Another item is the main power switch; if it's been turned on and off many times, the contacts may be pitted enough to cause an intermittent high resistance connection, resulting in very low output voltage from the supply transformer.
  5. Veracohr

    Thread Starter Well-Known Member

    Jan 3, 2011
    That's why I asked here, I can't figure it out myself.

    I don't think this is the issue. The transformer output before the rectifiers is correct, and the negative side of the rectifiers and regulator displayed no problems.

    When I was testing, it turned correctly on a couple of times, and at those times the positive side also showed up correctly: 9V out of the rectifiers and 5V out of the regulator.

    I'd definitely call it "intermittent", not "hard" failure. At one point it turned on when I touched the scope probe to one of the regulator solder joints. I'll go with that idea for now and try redoing a few solder joints. If that doesn't work I'll go for the caps next.

  6. someonesdad

    Senior Member

    Jul 7, 2009
    Intermittent behavior almost certainly means a poor connection somewhere. Solder joints and screw terminals are the usual suspects. Sounds like you're trying to drain the right swamp. Theoretically, it could also be inside of an IC and difficult to find, but I'd take odds on the solder/terminals.

    Senior Member

    Jul 1, 2008
    My bet is on a cold or halo joint but I've always avoided just willie nillie re-soldering joints. This is because when I handed the customer's property back to him I didn't want to see the darn thing on my bench again. I would scan each joint, in the suspected section of the circuit, with a good lens, while gently wiggling the component. Intermittents are a royal peta but it's unsettling when you're not absolutely sure that you fixed it.
  8. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    1v on the Vin and Vout pins of a LM340 5v regulator is very likely an over load condition, something after the 5v output may be shorted or partially shorted. It's pretty easy to test by removing the load after the 5v Vout pin (and see if 5v is restored), or cutting the track and putting a 1 ohm resistor there to measure the output current to the load during testing.

    Photos would help, does anything look cooked?
  9. KJ6EAD

    Senior Member

    Apr 30, 2011
    I alluded to that notion back in post #3.
    A divide and conquer approach may be effective. Unplug the connectors to peripheral boards one at a time until the problem disappears. When it does, you've located the peripheral, if any, that's causing it.
    Last edited: Jun 30, 2011
  10. Veracohr

    Thread Starter Well-Known Member

    Jan 3, 2011
    That sounds like a bad idea to me. A recipe for screwing it up even more.

    Nothing looks cooked, that was the first thing I looked for.

    There's an idea! I can unplug the logic board and CRT easily.

    I don't know why I didn't mention it before, but I did check continuity of many of the traces around the power supply, and they all seemed good, even when it wouldn't turn on. That seems to reduce the likelihood of a cracked solder joint.

    I haven't tried anything yet, I've been procrastinating.